application of a coupled surface water-groundwater model ... · figure 47 simulated spatial...

Anthony Barr and Olga Barron

October 2009

Application of a coupled surface water–groundwater

model to evaluate environmental conditions in the

Southern River catchment

Water for a Healthy Country Flagship Report series ISSN: 1835-095X

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Citation: Barr, A. and Barron, O. 2009. Application of a coupled surface water-groundwater model to evaluate environmental conditions in the Southern River catchment. CSIRO: Water for a Healthy Country National Research Flagship

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Cover Photograph:

File: DSCN3152.JPG Description: Southern River in January 2006. This image was taken during the installation of the Greenspan mini analyser to monitor high resolution water quality in the river. © 2009 CSIRO

Application of a coupled surface water groundwater model to Southern River Page iii

CONTENTS Acknowledgments ....................................................................................................... ix

Executive Summary...................................................................................................... x

1. Introduction ......................................................................................................... 1

2. Description of the catchment............................................................................. 3 2.1. Topography, geology, hydrogeology and land use................................................... 3 2.2. Climate...................................................................................................................... 6 2.3. Groundwater ............................................................................................................. 8 2.4. Surface waters .......................................................................................................... 8

3. Data availability and limitations....................................................................... 11 3.1. Time series data ..................................................................................................... 11

3.1.1. Flow data.............................................................................................................12 3.1.2. Groundwater data................................................................................................13 3.1.3. Wetlands water level data ...................................................................................13 3.1.4. Climate ................................................................................................................13

3.2. Spatial data ............................................................................................................. 14 3.2.1. Lithology ..............................................................................................................14 3.2.2. Ground surface....................................................................................................14

4. Model description.............................................................................................. 15 4.1. Model domain and discretisation ............................................................................ 18 4.2. Vertical discretisation.............................................................................................. 19 4.3. Temporal discretisation........................................................................................... 20 4.4. Subsurface flow ...................................................................................................... 22

4.4.1. Subsurface boundary conditions .........................................................................22 4.4.2. Subsurface saturated flow ...................................................................................23 4.4.3. Subsurface unsaturated flow ...............................................................................23

4.5. Overland surface flow ............................................................................................. 24 4.6. Channel flow ........................................................................................................... 25 4.7. Water allocation and use ........................................................................................ 25 4.8. Rainfall Interception ................................................................................................ 27 4.9. Transpiration ........................................................................................................... 28 4.10. Evaporation............................................................................................................. 28 4.11. Recharge ................................................................................................................ 28

5. Uncertainties and limitations ........................................................................... 29 5.1. Evaporation from channels ..................................................................................... 29 5.2. Temporal discretisation........................................................................................... 29 5.3. Overall simulation period ........................................................................................ 30

6. Model calibration and validation...................................................................... 31 6.1. Saturated flow: calibration and validation ............................................................... 32

6.1.1. Calibration ...........................................................................................................32 6.1.2. Validation.............................................................................................................32

6.2. Validation of the channel flow in the simulation...................................................... 34 6.3. Validation of water level dynamics in the wetlands ................................................ 36 6.4. Summary................................................................................................................. 37

7. Modelling results............................................................................................... 38 7.1. Catchment water balance ....................................................................................... 38

Application of a coupled surface water groundwater model to Southern River Page iv

7.2. Groundwater discharge to the channel system ...................................................... 41 7.3. Channel flow hydrographs ...................................................................................... 42 7.4. Depth to watertable................................................................................................. 46 7.5. Infiltration and recharge .......................................................................................... 47 7.6. Wetlands water balance ......................................................................................... 51 7.7. Evaporation and evapotranspiration....................................................................... 53

8. Effect of climate variation on the Southern River hydrological regime ....... 56 8.1. Predictive rainfall sequences .................................................................................. 56 8.2. Catchment water balance ....................................................................................... 57 8.3. Subsurface water balance ...................................................................................... 59 8.4. Wetlands ................................................................................................................. 59 8.5. Channel flows ......................................................................................................... 61 8.6. Groundwater levels ................................................................................................. 65 8.7. Groundwater recharge............................................................................................ 69

9. Summary and Conclusions .............................................................................. 71 9.1. Identified characteristics of the hydrological and hydrogeological processes in the

catchment ............................................................................................................... 71 9.2. Effect of climate variability on the hydrological and hydrogeological processes in

the catchment ......................................................................................................... 72 9.3. Specifics in process-based coupled surface water-groundwater modelling........... 73

References .................................................................................................................. 75

Appendix A – Summary of MODHMS model ............................................................ 77

A.1. Introduction ....................................................................................................... 77

A.2. Governing processes and equations .............................................................. 78 A.2.1 Subsurface flow...................................................................................................79 A.2.3. Overland flow.......................................................................................................79 A.2.4 Channel flow and surface-water features ............................................................80 A.2.5. Treatment of depression storage and storage exclusion .....................................80 A.2.6. Treatment of interaction terms among domains ..................................................80 A.2.7. Treatment of small surface-water bodies at large simulation scales....................81 A.2.8. Treatment of boundary conditions .......................................................................81 A.2.9. Treatment of interception and evapotranspiration ...............................................81

A.3. Numerical solution ............................................................................................ 82 A.3.1. Domain discretisation ..........................................................................................82 A.3.2. Coupling, time-stepping and other solution considerations .................................83

A.4. References ......................................................................................................... 84

Application of a coupled surface water groundwater model to Southern River Page v

LIST OF FIGURES Figure 1 Map of the Southern River catchment with inset showing location of catchment in broader geographical context......................................................................................................3

Figure 2 Ground surface elevation for modelled area ................................................................4

Figure 3 Surface geology of model domain showing Guildford Formation-Bassendean Sands dividing line................................................................................................................................5

Figure 4 Land cover categories over the Southern River catchment..........................................6

Figure 5 Annual rainfall for Armadale (1890-2007) with long-term average and 5 year moving average ..........................................................................................................................7

Figure 6 Observed groundwater heads in deep and shallow levels of a nested bore (SRJN03) in the superficial aquifer and daily rainfall.................................................................................8

Figure 7 Observation points and hydrological features............................................................12

Figure 8 Observed monthly fluxes with accuracy bounds at Anaconda Drive, close to the outlet of the Southern River......................................................................................................13

Figure 9 Schematic of MODHMS model showing the four major model components and climatic forces, indicating the flux directions ..........................................................................16

Figure 10 Schematic diagram of canopy interception of rainfall and potential evaporation in MODHMS: blue indicates rainfall; pink indicates evaporation ...............................................16

Figure 11 Schematic of fluxes for surface domain in MODHMS including both overland and channel flow: blue indicates gains to surface water; pink indicates losses from surface water17

Figure 12 Schematic of fluxes for the subsurface domain in MODHMS: blue indicates gains to subsurface water; pink indicates losses from subsurface water ...........................................17

Figure 13 Model horizontal discretisation and boundary conditions .......................................18

Figure 14 Distribution of relief depression size and minimum and maximum grid size adopted in the model ..............................................................................................................................19

Figure 15 West-east cross-section through the middle of the model domain (row 93, 6443470 mN)...........................................................................................................................................20

Figure 16 The three types of time-dependent stresses used in the MODHMS model .............21

Figure 17 Example of time stepping in the MODHMS model using a 10-iteration convergence limit...........................................................................................................................................22

Figure 18 Daily channel fluxes over one event (14.8 mm on day 1662) for two scenarios of rainfall duration: six-hourly rainfall event and daily rainfall event..........................................30

Figure 19 Comparison of evaporative losses for two scenarios of rainfall duration: six-hourly rainfall event and daily rainfall event .......................................................................................30

Figure 20 Model validation: observed and simulated water levels at (a) bore T80; (b) bore JM16 and (c) bore T75 .............................................................................................................33

Figure 21 Observed and simulated watertable variation at BRM10 and cumulative rainfall ..34

Figure 22 Observed (with accuracy bounds) and simulated monthly flow rates at Anaconda Drive .........................................................................................................................................35

Figure 23 Comparison of simulated and observed monthly fluxes at Anaconda Drive...........35

Application of a coupled surface water groundwater model to Southern River Page vi

Figure 24 Comparison of observed and simulated baseflow (2000-2001 summer) (note the logarithmic scale on the vertical axis) ......................................................................................36

Figure 25 Model validation: observed and simulated water levels in Forrestdale Lake ..........37

Figure 26 Inundation duration curves for Forrestdale Lake for simulated and observed data.37

Figure 27 Sub-catchments within the Southern River catchment used for the modelling .......38

Figure 28 Simulated average annual water balance for the Southern River catchment ...........39

Figure 29 Simulated average annual water balance for the Southern River catchment excluding rainfall and evaporation ...........................................................................................40

Figure 30 Simulated monthly fluxes between channel and groundwater within the Forrestdale Main Drain catchment ..............................................................................................................41

Figure 31 Simulated monthly fluxes between channel and groundwater within the Southern River catchment excluding Forrestdale Main drain catchment ................................................41

Figure 32 Simulated daily, monthly and annual fluxes for Forrestdale Main Drain at Holmes St...............................................................................................................................................43

Figure 33 Simulated daily, monthly and annual fluxes for Wungong Brook at Armadale Rd 44

Figure 34 Simulated daily, monthly and annual fluxes for the Southern River at Anaconda Drive .........................................................................................................................................45

Figure 35 Comparison of monthly fluxes at Forrestdale Main Drain, Wungong Brook and Southern River..........................................................................................................................45

Figure 36 Simulated average annual maximum depth to water for the Southern River catchment..................................................................................................................................46

Figure 37 Simulated average annual minimum depth to water for the Southern River catchment..................................................................................................................................47

Figure 38 Simulated average annual recharge rates .................................................................48

Figure 39 Comparison of recharge and infiltration to rainfall on an (a) annual and (b) monthly basis ..........................................................................................................................................49

Figure 40 Comparison of recharge to infiltration on an (a) annual and (b) monthly basis ......50

Figure 41 Simulated average monthly ratio of infiltration and recharge to rainfall.................51

Figure 42 Simulated cumulative recharge and cumulative rainfall for 2004 ...........................51

Figure 43 Simulated watertable level and surface inundation with observations for Harrisdale Swamp: the watertable immediately west (Watertable (W)) of the swamp is also shown. .....52

Figure 44 Simulated watertable and surface inundation at Lake Balannup: the watertable east (Watertable (E)) of the lake is also shown ...............................................................................52

Figure 45 Simulated water levels and watertable in Armadale Road wetland.........................53

Figure 46 Simulated spatial distribution of annual canopy evaporation (mm/annum) ............54

Figure 47 Simulated spatial distribution of annual evapotranspiration from subsurface due to vegetation (mm/annum)............................................................................................................54

Figure 48 Simulated spatial distribution of annual evaporation from land surface and subsurface including the watertable (mm/annum)....................................................................55

Application of a coupled surface water groundwater model to Southern River Page vii

Figure 49 Simulated spatially averaged monthly evaporative fluxes.......................................55

Figure 50 Average annual water balance components for three climate scenarios: (a) total water balance; (b) minor components of the water balance, excluding rainfall, evaporative losses and channel flow ............................................................................................................58

Figure 51 Inundation duration curves for four wetlands for three climate scenarios...............60

Figure 52 Annual inflow to channels from various sources as fraction of total inflow (symbol size is scaled to total annual inflow).........................................................................................62

Figure 53 Simulated average monthly fluxes at three locations for three climate scenarios ...63

Figure 54 Simulated average monthly groundwater discharge for three climate scenarios to: (a) Forrestdale Main Drain (FMD); (b) Southern River upstream of confluence with FMD; and (c) Southern River downstream of confluence with FMD ................................................64

Figure 55 Fraction of the simulated channel network where flow is not detected by the model..................................................................................................................................................65

Figure 56 Differences (in m) between simulated annual average minimum watertables for current and drier climates (average minimum watertable current climate - average minimum watertable drier climate)...........................................................................................................66

Figure 57 Differences (in m) between simulated average maximum watertable levels for wetter and current climates (average maximum watertable wetter climate - average maximum watertable current climate) .......................................................................................................67

Figure 58 Simulated groundwater levels over a ten-year period at three points for three climate scenarios; the locations of the points are shown in Figure 7 .......................................68

Figure 59 Difference in simulated annual average recharge (in mm) between the current and drier climates ............................................................................................................................69

Figure 60 Difference in simulated annual average recharge (in mm) between the wetter and current climates ........................................................................................................................70

LIST OF TABLES Table 1 Annual average rainfall and evaporation (in mm/annum) for Armadale and Jandakot Aero stations over different periods (base values for the comparisons with long-term rainfall and evaporation are highlighted) ................................................................................................7

Table 2 Temporal discretisation for external stresses to model ...............................................22

Table 3 Hydrogeological parameters for the superficial aquifer..............................................23

Table 4 Unsaturated flow parameters used in model ...............................................................24

Table 5 Parameters for transpiration and overland flow for different land uses and for channel flow...........................................................................................................................................25

Table 6 Categories of groundwater abstraction and assumed groundwater use adopted in the model ........................................................................................................................................26

Table 7 Monthly distribution of abstracted groundwater used for irrigation in the model ......27

Table 8 Monthly variations in vegetation parameters ..............................................................27

Table 9 Evaporation extinction depths .....................................................................................28

Application of a coupled surface water groundwater model to Southern River Page viii

Table 10 Statistical measures achieved during model validation at selected bore locations ...34

Table 11 Statistical measures adopted for model result validation for modelled flow at the Southern River gauging station ................................................................................................35

Table 12 Average annual water balance for the Southern River catchment calculated over ten years of simulation ...................................................................................................................39

Table 13 Annual average water balance for the subsurface model component within the Southern River catchment calculated over the 10-year simulation ..........................................40

Table 14 Annual rainfall (mm) for three climate scenarios......................................................57

Table 15 Subsurface water balance for three climate scenarios...............................................59

Application of a coupled surface water groundwater model to Southern River Page ix

ACKNOWLEDGMENTS We would like to thank the CSIRO National Research Flagship Water for a Healthy Country for the funding provided to the project through the Swan Futures program and the Western Australian Water Foundation for the funding for the project ’Investigation of Techniques to Better Manage Western Australia’s Non-Potable Water Resources’. The activity reported in this document was undertaken under these two initiatives.

We would like to thank Mr Phil Wharton of Rockwater for developing a MODFLOW model of the same area that was initially used as a basis for the MODHMS model development. We would also like to thank members of the Wungong Water group for their assistance and feedback in the development of the MODHMS model, and Dr Sorab Panday, Dr Vivek Bedekar and Mr Ted Lillys of Hydrogeologic for help in applying MODHMS to the modelling of the Southern River catchment.

We would also like to thank the Western Australian Department of Environment and Department of Water and the Water Corporation for providing monitoring and spatial data, the Bureau of Meteorology and Queensland Environmental Protection Agency for meteorological data. We would also like to acknowledge the contribution of Mr Daniel Pollock of CSIRO who was in charge of database management and spatial analysis within the project and Mr Warrick Dawes of CSIRO for advice on many aspects of the modelling. We also appreciate the assistance from Dr Kresho Zic (KBR) in development of the post-processing routine for MODHMS outputs in MATLAB.

We would like to thank our reviewers: Dr Sylvain Massuel, Dr Syed Mahtab Ali and Ms Sonja Chandler for their comments which improved this report.

Application of a coupled surface water groundwater model to Southern River Page x

EXECUTIVE SUMMARY The Southern River catchment is one of the fastest urbanising areas of Perth, the capital city of Western Australia. The catchment is located on the south-east margin of the Perth metropolitan area and has a low-lying flat landscape where surface and groundwater interactions dominate the hydrology and influence environmental flows.

Local urban development is challenged by groundwater levels, which are within one or two metres of the soil surface, and multiple wetlands, some of which have high conservation value. It is expected that urbanisation will significantly alter the hydrological cycle due to the introduction of impervious surfaces and shallow watertable control measures.

As a part of a CSIRO Water for a Healthy Country Flagship project and a project funded by the Western Australian Water Foundation entitled ’Investigation of Techniques to Better Manage Western Australia’s Non-Potable Water Resources‘ a detailed hydrological model was developed of the Southern River catchment.

A coupled surface water-groundwater model, MODHMS, was used to simulate surface and groundwater interactions at a catchment scale. This process-based model allowed simultaneous simulation of water interception and evaporation from vegetation, infiltration, runoff, drainage and groundwater flows.

The main objectives of the modelling were:

identifying key hydrological and hydrogeological processes in the catchment

quantifying the contributions of surface water and groundwater to runoff and wetlands in the catchment

investigating the effect of climate change on the groundwater regime, runoff and wetlands in the catchment

The project gathered multiple data sets, including Southern River outflow; Neerigen Brook discharge measured in the foothills which characterises hills rainfall-runoff characteristics; and Western Australian Department of Water records of groundwater levels in a number of observation bores and wetlands across the catchment. Additionally, substantial data was gathered through the project with instrumentation installed and maintained by CSIRO. These data were used to create a calibrated and validated MODHMS model of the Southern River catchment.

The hydraulic conductivity of the aquifers was estimated using three sub-models in areas that were identified as representing different hydrogeological conditions in the catchment. These areas were the western part of the Bassendean Sands, the eastern part of the Guildford Formation and the central part of the catchment. The hydraulic conductivity for the remainder of the catchment was interpolated between these values. Recharge was not a calibration parameter – the model allowed partitioning of rainfall into canopy interception, infiltration, overland storage and overland flow.

The model was also validated against a range of criteria:

runoff at the outlet of the catchment

groundwater level, both its magnitude and the patterns of rising and falling

water levels observed in the wetlands.

It was identified that the major processes governing the hydrological and hydrogeological conditions in the catchment were closely dependent on soil type, slope and the seasonality of the rainfall.

Application of a coupled surface water groundwater model to Southern River Page xi

According to the modelling results at the end of the wet season, about 0.7% of the Southern River catchment (0.14 km2) is inundated as surface expressions of the watertable and 29% of the area has a watertable within 1.5 m of the surface.

By the end of the dry period there is very little surface water in the catchment. The model estimates that about 85% of the channel network does not flow in February and March. The exception to this is the deeply incised Southern River channel upstream from the confluence with the Canning River, which receives groundwater discharge throughout the year. At the end of the dry season only 9% of the Southern River catchment area has a watertable within 2.0 m of the ground surface.

The MODHMS model also quantifies the major components of the Southern River water balance, including evaporative losses, groundwater recharge, seasonal sub-catchments contribution to the river flow, including baseflow, and the water regime in a number of wetlands. The main findings concerning evaporation, recharge, seasonal sub-catchments’ contributions to river flow, and environmental flow are summarised below.

1. Evaporation

Evaporation includes losses from shallow watertables, the unsaturated zone, inundated areas, and vegetation canopies. It was estimated that over 90% of the rainfall in the catchment is lost through one of these components. The dominant evaporative component during winter is from the canopy, while in summer evaporation is mainly associated with transpiration. Overall 41% of the evaporation occurred from the canopy, 36% was transpiration and 23% was evaporation from the surface and soil.

Evaporative loss from the subsurface is approximately 93% of the infiltration, indicating that the vertical movement of water in the subsurface is a dominant groundwater flux.

2. Recharge

In the Mediterranean-type climate with long, hot dry summers and short, cool wet winters, 80% of the catchment’s rainfall occurs between June and September, when most groundwater recharge occurs.

The catchment has two main Quaternary landforms: the dunal Bassendean Sands in the west and the clayey Guildford Formation in the east. Sandy soil types dominate in the flat catchment, leading to up to 70% of rainfall infiltrating over winter:

a. average annual groundwater recharge on the Bassendean Sands was estimated to be 400 to 500 mm or 50 to 65% of average annual rainfall

b. average annual groundwater recharge on the Guildford Formation was estimated to be 100 to 300 mm or 13 to 40% of average annual rainfall.

3. Seasonal sub-catchments’ contributions to river flow

The runoff estimated by the model showed different seasonal characteristics depending on topography and soil type. Limited connectivity between sub-catchments reduces runoff because most water ponds on the flat surface without flowing to the drainage lines.

The flatter, sandy western part of the catchment has a delayed contribution of flow to the catchment outlet compared to the higher-gradient clayey-sand eastern part of the catchment. This is because the high infiltration capacity on the sandy soils prevents ponding and overland flow. Channel flow starts when groundwater levels rise above the invert of the channels. The western part of the catchment contributed less than 10% of the catchment outflow in June and July, over 20% in August and October and almost 30% in September.

In the clayey soils in the eastern part of the catchment, the infiltration rates are lower and the topographic gradients are greater which results in overland flow occurring earlier in the winter.

Application of a coupled surface water groundwater model to Southern River Page xii

In the parts of the channel system that are deeply incised into the landscape, the watertable at the end of summer can be above the base of the channel system, and thus groundwater discharge is possible throughout the year. In some other parts of the channel network, the watertable may fall to just below the base of the channel, and when recharge reaches the watertable, the watertable rises above the base of the channel and groundwater discharge is again possible. In parts of the channel system where the channels are not deeply incised in the landscape, the watertable may be greater than 1-2 m below the base of the channel. A considerable amount of recharge is needed before the watertable rises above the base of the channel and groundwater discharge into the channel can commence. Depending on the recharge rate, the rise of the watertable above the base of the channel may occur late in the wet season, and thus discharge into the channel system at this location may only occur for a short period of the year.

4. Environmental flow

The model estimated groundwater discharge to the channels and wetlands, which was considered to be an environmental flow. Groundwater discharge to the surface water network consisted of an average 3 GL/yr or 20% of annual river discharge.

The model was also used to estimate the effect of groundwater inflows and outflows on the water balance of wetlands. It was found that:

a. Forrestdale Lake receives very little groundwater inflow and, due to the low conductivity of the bottom sediments in the lake, infiltration to the underlying aquifer is not a large component of the water balance. Forrestdale Lake receives most of its water from rainfall on the lake and the surrounding dunes.

b. The Armadale Road wetland depends on flows in Wungong Brook, but has sufficient storage to remain wet throughout summer.

c. Lake Balannup and Harrisdale Swamp are both formed on lacustrine sediments within the Bassendean Sands. Although groundwater pressures in the aquifer directly beneath these wetlands rarely rise above the water level in the wetland, groundwater levels in sands adjacent to the wetlands do rise above the water-body water levels and seepage from these add to winter inflows.

The Southern River MODHMS model was also used for investigating the impact of climate variability on the catchment hydrological and hydrogeological cycles, wetlands water regime, runoff, recharge and groundwater levels. Scenarios in which rainfall was assumed to be either 10% less or 10% more over the last ten years indicated that

1. Inter-annual variations in watertable levels, including annual maxima and annual minima, are mainly influenced by the distribution of rainfall within the year rather than by long-term rainfall patterns. Groundwater levels appear to be resilient to climate variation within the timeframe and assumed climate variations.

2. The effect of climate change on the Southern River outflow is greatest during the winter (15% less for a 10% drier climate and 11% more for a 10% wetter climate), where the change in flow is mainly related to storm runoff. However, there was very little change in baseflows at the end of summer. The change in runoff during the summer was limited (20% less flow for a 10% drier climate), whilst the flow remained the same for a 10% wetter climate.

3. The estimated effect of climate change on wetlands depends on the type of wetland

a. Channel wetlands such as Wungong Brook at Armadale Road are strongly dependent on rainfall distributions rather than on long-term rainfall patterns.

Application of a coupled surface water groundwater model to Southern River Page xiii

They are strongly influenced by winter flows, but have enough storage to maintain water even in the driest year modelled.

b. Forrestdale Lake is predominantly surface water fed from rainfall over the immediate lake catchment and is therefore strongly dependent on rainfall. The lake is currently dry 99 days each year on average. In the model an increase of 10% in the average annual rainfall decreased the number of dry days by 21. A decrease of 10% in the average annual rainfall resulted in an increase of 25 days in the average time the lake was dry.

c. Groundwater-dependent wetlands such as Lake Balannup and Harrisdale Swamp are also strongly dependent on climate.

i. A 10% drier climate resulted in an average decrease of 62 days in the time Lake Balannup was inundated to depths in the range 10-20 cm. A 10% increase in the rainfall increased the average inundation time by over 40 days for depths in excess of 20 cm.

ii. Similarly, in Harrisdale Swamp an increase of 10% in rainfall increased the time of inundation for depths greater than 10 cm by 34 days, whilst a 10% decrease in rainfall reduced the inundation time by over 20 days for all depths greater than 10 cm.

Overall results indicate that the MODHMS model of the Southern River catchment successfully reproduces the main governing processes controlling catchment water regime. Further development of the model could investigate water quality and the pathways of solutes in the system to the waterways. The model will also be applied to predict the impact of urbanisation on river discharge and water quality.

Application of a coupled surface water groundwater model to Southern River Report Title Page 1

1. INTRODUCTION

The modelling of a catchment where the interaction between surface and groundwater is the dominant feature of the hydrological regime is a challenge: neither groundwater models nor hydrological models in isolation allow adequate simulation of the natural water regime (Freeze and Harlan, 1969). The Swan Coastal Plain part of the Southern River catchment close to Perth, which is characterised by shallow groundwater tables and multiple wetlands, is such a catchment. Large areas of the catchment are inundated during the wet season, often as an expression of the local watertable. This, in turn, influences the catchment runoff characteristics. Conversely, during the dry season only the lower reaches of the channel system, which continue to receive regional groundwater discharge, contain water.

The catchment is currently subject to urbanisation. The land use alteration will significantly affect the current hydrological cycle as inundation and shallow watertables are incompatible with most urban infrastructure. However, some of the existing wetlands are of high conservation value and thus urban development options are constrained in their vicinities.

In order to predict the variation in catchment water regime under the new land use, it is important to develop an accurate model of the current hydrological and hydrogeological conditions. Such a model is also required to enable simulation of alternative land use.

Process-based coupled surface water-groundwater models meet these criteria. Models describing the governing hydrological and hydrogeological processes in a particular catchment can simulate the complex interaction between surface and subsurface water, which can be non-linear as well as spatially and temporally variable. These models are also suitable for simulation of land use changes within the catchment, enabling adjustments of the model scenarios without having to recalibrate the model. Freeze and Harlan (1969) discussed the possibilities of such an integrated system, but only recently have models been available for simulating fully coupled surface water-subsurface water systems: the European MIKE SHE (DHI Water & Environment, 2007), and from North America InHM (VanderKwaak, 1999), MODHMS (Hydrogeologic Inc., 2006), MODFLOW WHaT (Thoms, 2003), HydroGeoSphere (Therrien et al., 2005) and more recently GSFLOW (Markstrom et al., 2008).

The reported application of these models has been restricted to small catchments (Maneta et al., 2008) or topographically diverse catchments (Jones et al., 2008). Werner and Gallagher (2006) used a multi-layer MODHMS model with density dependence to investigate the seawater intrusion into the Pioneer Creek catchment in Queensland, Australia, which includes an ephemeral channel network and a number of wetlands. However, they did not simulate the unsaturated zone in the model.

The challenge in using a physically-based distributed coupled surface water-groundwater model is the large data requirement. Each individual process: channel flow, overland flow, unsaturated zone and saturated flows, requires a specific set of parameters that have to be distributed over the model domain. The coupling of surface water and subsurface water processes is also computationally expensive as the spatial and temporal scales for the individual components are different, requiring either simulating all processes at the smallest relevant scale (fully coupled) or simulating each component individually and subsequently integrating them (iterative).

The MODHMS model was selected due to the ability to build on a pre-existing MODFLOW model, the inherent stability in its automatic time-stepping algorithm, and its capability in handling surface water-groundwater interaction including unsaturated fluxes using a fully coupled method.

This report describes the application of MODHMS modelling to the Southern River catchment. The focus of this report is on model development and its application to quantification of the most important environmental flow parameters – baseflow and wetlands water balance as well as their dependence on climate variability. These are considered as major environmental water requirements and constraints for urban development. The current

Application of a coupled surface water groundwater model to Southern River Page 2

model was expanded to incorporate the land use changes and alternative catchment management related to catchment drainage, introduction of hard surfaces and local groundwater use for non-potable water use in new urban developments. This expansion was reported in Barron et al. (2009).

The main objective of the Southern River catchment modelling was to provide base hydrological and hydrogeological data in order to assess future changes. The project included work to:

identify and quantify the key hydrological and hydrogeological processes in the catchment

quantify the contributions of surface water and groundwater to the channels and wetlands in the catchment

investigate the effect of climate change on the groundwater, channel flows and wetlands in the catchment.

This report presents catchment characteristics (Section 2), historical data availability and associated limitations to model development (Section 3), model description (Section 4), model uncertainties and limitations (Section 5), model calibration and validation (Section 6), results from the modelling (Sections 7). It also discusses climate effects on the environmental flow in the Southern River catchment (Section 8).

The research was undertaken within the Swan Futures project (funded by CSIRO National Research Flagship Water for a Healthy Country) and the project ’Investigation of Techniques to Better Manage Western Australia’s Non-Potable Water Resources‘ (funded by the Western Australian Water Foundation).


2. DESCRIPTION OF THE CATCHMENT The Southern River catchment on the Swan Coastal Plain is the southernmost catchment for the Swan-Canning Estuary (Figure 1). The Southern River network includes tributaries such as Forrestdale Main Drain, which is a man-made channel, and Neerigen and Wungong Brooks, both of which originate on the Darling Scarp. The name Southern River applies to the channel system between the confluence of Neerigen Brook and Wungong Brook and its outflow into the Canning River.

Figure 1 Map of the Southern River catchment with inset showing location of catchment in broader geographical context

2.1. Topography, geology, hydrogeology and land use The surface elevation of the modelled catchment is shown in Figure 2. The part of the catchment included in the modelling has topographic highs in the east adjacent to the Darling Scarp (up to 65 metres Australian Height Datum (mAHD)) and some high dunes in the west (up to 48 mAHD). The central part of the catchment is mainly flat (~25 mAHD) and contains the major drainage channel with the Southern River draining to the north and the Birrega Main Drain flowing to the south. The largest wetland within the catchment is Forrestdale Lake, a RAMSAR site, which topographically lies almost at the saddle point between the Birrega Main Drain and the Southern River.


Figure 2 Ground surface elevation for modelled area

The catchment is located within two geological units in the superficial formation (Figure 3). To the east there is the Guildford Formation, which is predominantly of fluvial origin and consists of clayey-sands and clays with lenses of coarse sands particularly at the base (Davidson, 1995). There are also extensive areas of sands overlying the Guildford Formation. The western part of the catchment consists of Bassendean Sands, which are predominantly medium-grained, moderately sorted, quartz sands, and commonly contain a layer of weakly limonite-cemented sand at or about the watertable (Davidson, 1995). There are also peaty deposits associated with inter-dunal wetlands, major rivers and drainage channels and the Canning Estuary.


Figure 3 Surface geology of model domain showing Guildford Formation-Bassendean Sands dividing line

The superficial formation overlies Mesozoic sedimentary formations. The thickness of the superficial formation varies between 10 m in the Wungong Urban Water (WUW) area and in the vicinity of the Canning River, to 70 m in the vicinity of the Jandakot Mound. Hydrogeologically, the Guildford Formation consists of a number of layers with varying hydraulic properties:

a thin layer of seasonally saturated clayey-sand as a top layer

an aquitard consisting of a clay layer of variable thickness and sand content, which often provides localised confinement to the deeper water-bearing strata

the main productive layer of the superficial aquifer (known as the regional aquifer), which may be less than 10 or more than 30 m thick.

Similarly, the Bassendean Sands consist of a highly conductive, seasonally-saturated surface layer of sand overlying the weakly limonite-cemented sand layer, which acts as a minor aquitard, which in turn overlies the superficial aquifer.

The current land use in the catchment, as shown in Figure 4, consists of urban areas, grassland and native vegetation. The urban areas are located on the eastern margins close to the Darling Scarp and in the north where the Southern River joins the Canning River. The remaining area consists of a mosaic of grassland and native vegetation, with the native vegetation centred on wetlands.


Figure 4 Land cover categories over the Southern River catchment

2.2. Climate The climate on the Swan Coastal Plain is of a Mediterranean type. Up to 80% of annual rainfall is recorded during the winter months from May until September. On an annual basis, potential evaporation is greater than rainfall, but monthly potential evaporation can be lower than monthly rainfall during winter months.

Figure 5 shows annual rainfall data for Armadale (Bureau of Meteorology (BOM) station 009001) over the period of meteorological record, along with the long-term average and 5-year moving average annual rainfall. The trend in 5-year moving average annual rainfall has been towards lower rainfall, with the moving average being below the long-term average since the late 1950s except for two brief periods in late 1960s and early 1990s. Table 1 contains a summary of the average annual rainfall for two stations, Armadale and Jandakot Aero (BOM Station 009172). The average rainfall for the period 1975-2007 is 10% below the average prior to 1975 for Armadale, whereas for Jandakot Aero, the average is 3% less for the same period. However, for both stations the average rainfall has fallen by 15% and 13% over the period 2001-2007 compared to the long-term average.


500

750

1000

1250

1500

1890 1910 1930 1950 1970 1990 2010

Year

Ra

infa

ll (

mm

/an

nu

m)

AnnualAverage5 year average

Figure 5 Annual rainfall for Armadale (1890-2007) with long-term average and 5 year moving average

Table 1 Annual average rainfall and evaporation (in mm/annum) for Armadale and Jandakot Aero stations over different periods (base values for the comparisons with long-term rainfall and evaporation are highlighted)

Period Armadale Jandakot Aero

Start Year

Final Year

Rain Fraction of long term

rainfall

Evapo-ration

Fraction of long term

evaporation

Rain Fraction of long term

rainfall

Evapo-ration

Fraction of long term

evaporation

1889 2007 890.3 862.4

1889 1974 915.6 1.03 870.40 1.01

1970 2007 833.4 0.94 1748.9 846.8 0.98 1817.7

1975 2007 824.3 0.93 1747.4 1.00 841.5 0.98 1819.0 1.00

1990 2007 835.7 0.94 1757.0 1.00 806.8 0.94 1834.7 1.01

1997 2007 787.9 0.88 1758.3 1.01 784.8 0.91 1832.1 1.01

2001 2007 757.9 0.85 1739.5 0.99 754.2 0.87 1805.5 0.99

A summary of the potential evaporation for the two sites is also given in Table 1. Although annual evaporation data is only available since 1970, it shows that average annual potential evaporation has remained almost constant at both sites. Evaporation at Jandakot Aero is around 4% greater than at Armadale.

Figure 6 shows the average monthly rainfall and potential evaporation for Armadale during the period 1997-2006. This shows that there is on average only the three winter months have rainfall that greatly exceeds the potential evaporation in the catchment.


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300

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

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ma

te (

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/mo

nth

)

Rain Potential evaporation

Figure 6 Average monthly rainfall and potential evaporation during the period 1997-2007 for Armadale

2.3. Groundwater The average watertable level in the catchment closely reflects the land surface. Regional groundwater gradients indicate the groundwater flows from the topographic highs of the Darling Scarp in the east and Jandakot Mound in the west towards the channel system in the centre of the catchment, and then to the north in the centre of the catchment. The seasonal dynamics of the watertable depend on the proximity of the land surface. In areas where the groundwater is close to the land surface, the watertable response to the winter rainfall is proportional to the rainfall until the watertable reaches a maximum level (see shallow bore in Figure 7). The watertable remains at about this level for the remainder of the rainy season before gradually dropping during summer. In areas where the watertable is deep, the seasonal response to rainfall is delayed, but the watertable dynamics overall mirror the rainfall patterns. The response in the deep part of the superficial aquifer can be delayed due to the presence of the overlaying aquitard (Figure 7).

22

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Jul-2006 Aug-2006 Sep-2006 Oct-2006Month

Wa

ter

leve

l in

sh

all

ow

b

ore

(m

AH

D)

0

100

200

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400

500

Cu

mu

lati

ve R

ain

fall

(m

m)shallow deep Rainfall

Figure 7 Observed groundwater heads in deep and shallow levels of a nested bore (SRJN03) in the superficial aquifer and daily rainfall

2.4. Surface waters The hydrological condition of the catchment has greatly changed since European settlement. The network of drains in the area is of generally anthropogenic origin, with only the lower and upper reaches of the Southern River consisting of natural channels. In addition to Neerigen Brook and Wungong Brook, the two other manmade streams flowing from the Darling Scarp into the Southern River catchment are the Brickworks A and B Drains (Figure 2).


The establishment of Wungong Dam in the Darling Scarp for the metropolitan water supply has greatly reduced the flow in Wungong Brook. It was reported that there was a 93% reduction in median annual flow when brook flow was compared for the period 1961-1975 (prior to Wungong Reservoir completion) to the period 1977-1996 (post completion) (ENV Australia Pty Ltd, 2007). However, there are dry season releases from Wungong Dam of approximately 1 GL to maintain environmental flows within Wungong Brook.

The average annual river discharge of the Southern River at the outflow from the catchment is approximately 13 GL/year. A monthly water balance was undertaken to evaluate changes in river flow in response to rainfall, and some observations can be summarised as follows (Barron et al., in prep.):

A monthly water balance indicates that the autumn and early winter do not result in a significant increase in river flow, with a monthly volumetric runoff coefficient less than 2%. It is believed that during this period the substantial storage capacity of the deep sandy soils and wetlands allows the accumulation of rain water with little stormwater yields. This period is referred to as the ‘storage recovery stage’.

The Southern River flow increases significantly during late winter and early spring (monthly volumetric runoff coefficient >25%). The analysis of the river discharge data for 1997-2006 shows that the increase in the flow rate occurs when the cumulative rainfall reaches 360-400mm. At this stage the subsurface storage in the catchment is largely filled and the stormwater yield increases. This period is referred to as the ‘storage deficiency stage’.

During summer, when evaporative losses exceed rainfall, the ‘storage depletion stage’ occurs.

Groundwater contributes to the channel flow within a number of sub-catchments. There is a permanent baseflow occurring in the lower reaches of the Southern River where the channel is deeply incised in the landscape. In the remaining areas however, baseflow is seasonal. In the Forrestdale Drain, tributary on the Bassendean sands, the groundwater contribution to the drainage network occurs late in the wet season when the watertable becomes higher than the bottom levels of the channels.

In addition to the surface water channel network there are many wetlands occurring in the catchment where free water occurs in winter and in some cases all year around. These include:

RAMSAR listed Forrestdale Lake

a number of wetlands located along flow channels (such as Wungong Brook)

perched wetlands in the part of the catchment within the Guildford Formation occurrence

groundwater-dependent wetlands in topographic depressions in the western part of the catchment.

In summary, and based on the available information, the key catchment characteristics which influence the Southern River water regime are:

Mediterranean climate with typically contrasting seasons: high rainfall and low evaporation during winter and low rainfall and high evaporation during summer

reduction in annual rainfall over last few decades

two major topographic features: hills and a central flat area with low surface gradient

two geological units within Quaternary deposits influencing hydrogeological conditions: Bassendean Sands in the west and Guildford Formation (clayey sands and clays) in the east

superficial aquifer consists of a conductive surface layer underlain by an (often localised) aquitard and further underlain by a regional aquifer


being close to the surface, the groundwater level is mainly affected by seasonal rainfall and evaporation

high level of interaction between groundwater and surface water

continuous baseflow into lower (incised) reaches of the Southern River

numerous wetlands: some are groundwater dependent, others are isolated from groundwater.


3. DATA AVAILABILITY AND LIMITATIONS As mentioned above the MODHMS modelling environment selected for the Southern River water regime simulation is largely process based. When models are developed based on understanding the physical processes and their modelling conceptualisation, the modelling leads to a robust representation of the catchment and is capable of providing a valuable insight into the behaviour of the catchment under a range of spatial and temporal variations. Such a modelling approach allows for a confident assessment of the likely impact of changes in water management practices on the governing processes of both surface water and groundwater, including scenarios that are outside of the features observed in the historical data used to validate the model.

However, such models require substantial data. Therefore it is important to review the available data to assess their efficiency for the selected model in terms of accuracy, spatial and temporal coverage, and in relation to the accuracy of the model’s predictions. The accuracy of the field data (and accordingly the model results) may be less critical at different times of the year. The understanding of the catchment processes, gained through the field data and further expanded through the modelling of the catchment provides an excellent guide to judge on how important the model’s accuracy can be for some particular event or a particular water management scenario.

Therefore data availability and accuracy are reviewed in the following sections for time series data and spatial data.

3.1. Time series data There is considerable time series data available in the catchment which can be grouped as follows (refer to Figure 8):

fluxes in the channels are observed on the Southern River, close to the confluence with the Canning River, and in Neerigen Brook close to the Darling Scarp

the Western Australian Department of Water (DoW) monitors surface water levels in Forrestdale Lake and other wetlands

longstanding groundwater observation bores in the catchment are used for monitoring the water levels in the aquifer systems (Figure 8 shows only selected bores for which the water level records have been used for calibration or validation)

high-frequency groundwater level observations are taken from a number of observation bores within the current project.


Figure 8 Observation points and hydrological features

3.1.1. Flow data

An inspection of the available Southern River hydrographs indicated that the river discharge reflects a complex interaction between runoff from the adjacent Darling Scarp, direct surface runoff from inundated areas, leakage to and discharge from the groundwater and the retardation of the surface flows by multiple wetlands of various sizes. The magnitude of these complex interactions in the natural environment varies seasonally and spatially.

The river flow data were recorded at the gauging station at the outflow from the catchment during the period 1997-2007. The gauging station was not equipped with a structural weir, and as such the water level records have associated uncertainty. The likely magnitude of these uncertainties has been evaluated in consultation with WA Department of Water. The accuracy estimated for the instantaneous flow measurement at the gauging station is ±25% for flow rates greater than 500 L/s, ±10% for the flow range 50-500 L/s and for low flows (<50 L/s) accuracy may be extremely low (up to ±500%) due to siltation and plant growth. The latter corresponds to summer baseflow.

When the total monthly discharge is considered, the uncertainty of the monthly flow estimation accumulates the uncertainty of the instantaneous flow measurements as the accuracy is associated with systematic limitations in the observations. Monthly river discharge (with accuracy bounds ) over the period March 1997 to December 2006 is shown in Figure 9.


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4

6

8

10

12

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Flu

x (G

L/m

on

th)

Upper accuracy boundObservedLower accuracy bound

Figure 9 Observed monthly fluxes with accuracy bounds at Anaconda Drive, close to the outlet of the Southern River

The flow data at Neerigen Brook was observed within the urban area of Armadale on the Swan Coastal Plain. The accuracy in the flow observations is around 10% (Allan Dean, Water Corporation, personal communication). The annual flow in Neerigen Brook is an average of 22% of the flow in the Southern River at Anaconda Drive.

3.1.2. Groundwater data

Observations of groundwater levels were extracted from the Water Resources Information Catalogue (WRIC), a collation of water quality, flows and water levels across Western Australia, maintained by the Western Australian Department of Water (DoW). In addition to existing monitoring sites, CSIRO has installed a number of water level loggers, amongst other instrumentation, in the catchment. This data is presented in greater detail in Barron et al. (2007). The potential errors in the actual observations (such as a drift in automatic sensors) are likely to be small. There may also be potential for systematic error where observation bores are located close to influential local features (such as abstraction bores or sumps) and as such may give a false indication of regional hydraulic heads. Only bores remote from locations of licensed bore-water abstraction were selected for further analysis.

3.1.3. Wetlands water level data

The major wetlands, including Forrestdale Lake, were monitored and lake water levels data stored in the WRIC. The accuracy of water level recording in large shallow lakes is often affected by the position of the gauging station in relation to the lowest point in the lake basin and the effects of wind on the immediate water level reading. In Forrestdale Lake the observations were taken manually at two-week intervals by a local resident. Whilst the actual accuracy of these data is not easy to estimate, the data are stored in the state data base. For the purpose of this study the data was used as an indication of the water level changes in the lake rather than as absolute values.

3.1.4. Climate

Climate data, and in particular rainfall data is collected at a number of meteorological stations within and nearby the catchment. Data for Armadale and Jandakot Aero climate stations (Figure 8) were obtained from the SILO database (Jeffrey et al., 2001) for the period 1889-2007. As discussed above, Table 1 contains average annual rainfall and potential evaporation for two climate stations over a number of different intervals.

The variation in the daily rainfall data collected at two locations within the catchment is noticeable. Standard deviation between the long-term daily rainfall records at the two local meteorological stations is consistent over the entire rainfall range and equal to 5 mm.


However, the monthly rainfall recorded at both stations was characterised by a smaller variation.

There are also some variations in the potential evaporation. The evaporation recorded at Armadale is about 4% less than that recorded at Jandakot Aero with the difference consistent over the whole record. The data from the Armadale site, which is located within the catchment, is used for the whole domain.

Analysis of the river discharge record indicated that rainfall events less than 5 mm produce low runoff. A review of historical rainfall data showed that on average there are 52 rainfall events greater than or equal to 5 mm during each year out of an average of 107 events per year. Comparison of the rainfall frequency with a sub-hourly rainfall recorded at Jandakot indicated that on average daily rainfall duration is approximately 6 hours. This will be further discussed later in relation to the time step used in the modelling of rainfall/runoff.

3.2. Spatial data

3.2.1. Lithology

The WRIC database of the DoW contains lithological records for 1647 bores within the model domain. The quality of these lithological records is highly variable due to differences in the purposes of the drilling and the experience of the drillers. There are also variations in the depth intervals over which the lithology is described.

This data was used to define the extent of superficial aquifer layers and as such data uncertainty may affect the estimated aquifer transmissivity values and their spatial distribution. However, the large number of available lithological records and simultaneous analysis of multiple neighbouring bores reduce the uncertainty level (Pollock and Barron, in prep.).

3.2.2. Ground surface

The ground surface elevation for the model was interpolated from:

a triangular irregular network (TIN) based on specified point elevations, 1 m contours, a 25 m digital elevation model (DEM) (ARMY, personal communication; Western Australian Department of Land Administration (DOLA), personal communication)

a 1:2000 digital topographic dataset (Western Australian Department of Land Information, personal communication).

The DEM accuracy is much greater than the spatial model discretisation so this data is unlikely to affect model accuracy. This data was used to establish the average elevations over the individual computational model’s cells.


4. MODEL DESCRIPTION The MODHMS model consists of four major components (Figure 10). The major components are:

Canopy (Figure 11): the canopy intercepts rainfall and has its own storage (which is specified as a depth multiplied by the leaf area index (LAI) for the cell), which must be full before it allows through-fall to the ground surface. The canopy storage is subject to evaporation.

Ground surface (Figure 12): at the ground surface, the through-fall is partitioned into infiltration into the ground system and surface inundation. The surface inundation initially comprises of local storage in rills and local depressions. Once these local depressions become full, overland flow of water can result. Evaporation occurs from all water remaining on the surface. Groundwater seepage may also occur where the subsurface watertable intersects with the ground surface. The overland flow is modelled in MODHMS using the vertically averaged Saint Venant equations.

Channel systems (Figure 12): the channels convey water through the model. They receive water from overland flow and groundwater discharge from the subsurface. Water from the channel can infiltrate into the subsurface and, when the channel overflows, spread across the ground surface. Rainfall is applied to the channel proportional to the area of channel. MODHMS uses the diffusion wave approximation to the 1-D Saint Venant equations to calculate the flow in the channels. The MODHMS model does not dynamically calculate evaporation from channels.

Subsurface consisting of saturated and unsaturated flow (Figure 13): the subsurface receives infiltration from the ground surface and leakage from the channels. Seepage occurs when the watertable level exceeds the ground surface, and groundwater discharge occurs to the channels when the Piezometric heads exceed the water level in the channels. Evapotranspiration takes place from vegetation with root systems above the watertable level, and evaporation takes place from all unsaturated layers as well as from the top saturated layer (watertable). MODHMS models the subsurface using a three-dimensional variably-saturated flow formulation.

The relevant sets of adopted mathematical models within the MODHMS environment are given in Appendix A.

External forcing includes: daily rainfall; potential evaporation; specified boundary conditions for both surface and subsurface systems; and groundwater abstraction. Their treatment in this study is described in greater detail in the following sections.


Figure 10 Schematic of MODHMS model showing the four major model components and climatic forces, indicating the flux directions

Size of cell

LAI of cell Overflow from canopy storage becomes through fall to land surface

Depth of canopy interception

Rainfall falls directly on land surface

Rainfall falls on canopy

Land Surface

Potential evaporation acts on canopy storage

Potential evaporation less any canopy evaporation is applied to land surface

Figure 11 Schematic diagram of canopy interception of rainfall and potential evaporation in MODHMS: blue indicates rainfall; pink indicates evaporation


Channel

Groundwater discharge

Seepage


Infiltration

Channel leakage

Evaporation

Overland flow to channel

Overland flow from channel

Ponding on surface in rills

Through fall (rain)

Watertable

Figure 12 Schematic of fluxes for surface domain in MODHMS including both overland and channel flow: blue indicates gains to surface water; pink indicates losses from surface water

Groundwater discharge Channel

Seepage


Infiltration

Channel leakage

ET

Evaporation

Abstraction

Layer 1

Layer 2

Layer 3

Layer 4

Figure 13 Schematic of fluxes for the subsurface domain in MODHMS: blue indicates gains to subsurface water; pink indicates losses from subsurface water


4.1. Model domain and discretisation The model domain includes the surface water and groundwater catchments of the Southern River on the Swan Coastal Plain. The domain extends to the Jandakot mound in the west and to the Canning Estuary in the north. It covers an area of 282 km2 (12 km by 23.5 km).

Figure 14 Model horizontal discretisation and boundary conditions

The model domain includes areas outside the Southern River surface water catchment as the groundwater flow system does not coincide with the surface water catchment. This also ensures that the boundaries do not influence the modelled fluxes within the Southern River catchment.

The model contains 123 columns and 163 rows in a rectangular finite-difference grid (Figure 14). The dimensions of the cells range from 500 m in the north, south and west of the domain to 83.33 m in the vicinity of the Southern River/Wungong Brook catchment where urbanisation will occur. This grid is used for both the overland flow and subsurface flows in the model.

The model discretisation was aligned with specifics of the Southern River landscape. In low-laying areas the catchment is formed by multiple chains of relief depressions which developed as a result of wind erosion. Detailed analysis of the catchment DEM (Barron et al., in prep.) calculated the statistical distribution of the size of relief depressions as shown on Figure 15.

The minimum grid size of the model (83.33 m by 83.33 m) is the dominant cell size within the Southern River catchment. This is less than the size of the majority landscape depressions (> 92%), which means that the model adequately represents the catchment landforms.


0%

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100 1000 10000 100000 1000000

Area (m2)

Cu

mu

lati

ve fr

eq

ue

nc

y Relief depression size

Minimum model grid size

Maximum model grid size

Figure 15 Distribution of relief depression size and minimum and maximum grid size adopted in the model

4.2. Vertical discretisation The superficial aquifer is described in the model using a six-layered hydrogeological system. The lowest layer represents the deep superficial regional aquifer, the second-lowest layer the superficial aquitard, and the top four layers are assigned to the upper-superficial layer.

The top and bottom of the aquitard layers were defined from analysis of 1647 bore lithological logs held in the WRIC database (DoW). These lithological records were processed using a key word analysis to identify clay (Guildford Formation) and indurated sands (Bassendean Sands) sections from the log and to find the highest (top of aquitard layer) (160 sites) and lowest (bottom of aquitard layer) (121 sites) levels. This method is an approximation of the aquifer structure as the lithological logs were produced by different organisations and thus there may be differences in interpretation and in the quality of the logging. The analysis method combines multiple layers of clay or indurated sands, inter-lain with more hydraulically conductive material into the aquitard.

The highest and lowest levels were interpolated as depths and then subtracted from the ground surface to create the aquitard levels for the model. The bottom of the model used an interpolation of values used for the Perth Regional Aquifer Modelling System (PRAMS) (Davidson and Yu, 2006).

The four layers in the upper-superficial layer are of the same thickness if the total thickness of the upper superficial is less than 4 m, or the upper three layers are of 1 m thickness and the thickness of the fourth layer consists of the remaining thickness (>1 m) of the upper superficial.

Figure 16 shows a west-east cross-section (from left to right) through the model in the middle of the catchment (see Figure 8). The western part of the cross-section is part of the Bassendean Sands and shows a considerable thickness of both surface and deeper aquifer layers and a relatively thin aquitard layer. In the east, the aquitard layer is a much bigger proportion of the aquifer thickness and the surficial layer of sand is thin.

Vertical spacing varied from 0.224 m to 59.4 m.


Figure 16 West-east cross-section through the middle of the model domain (row 93, 6443470 mN) (100x vertical exaggeration)

4.3. Temporal discretisation An important consideration in the catchment modelling is the selection of the computational time step. A balance has to be reached between the actual time scale of the governing processes and the anticipated computational time which is a function of the selected time step.

Typically, when modelled in isolation, groundwater processes are modelled with time scales of months and surface water processes in days or hours, while the unsaturated zone processes may require sub-hourly time steps. Most fully-coupled process-based models use the same time scale for all processes. Small time steps are generally needed when there are rapid changes occurring in modelling some of the considered hydrogeological processes. These changes may be due to variations in external forcing such as high intensity rainfall, large changes in groundwater abstraction, or when the transitions occur between two hydrogeological processes. The latter can be demonstrated when the inundation level increases from below the rill height to above the rill height in a cell, then the overland flow part of the model becomes active. The occurrence of this phenomenon cannot be predicted so the model has to be able to adjust its time step according to the need of the model.


Figure 17 The three types of time-dependent stresses used in the MODHMS model

The majority of stresses in the model (e.g. abstraction, potential evaporation) were specified as constant values for each month (which was the major time step used in the model; or ‘stress period’ in MODFLOW terminology); shown as a pink line in Figure 17. The specified head boundary condition in the south of the domain used linear interpolation between monthly heads (green line in Figure 17). However, MODHMS includes the capability of varying some input parameters such as climatic forcing (rainfall) and flow and head boundary conditions (FHB package) within a model stress period (blue line in Figure 17). For the modelling described here, the rainfall and channel inflows were specified daily. The selection of these time intervals for the model was a balance between the frequency of available data records and the requirement to minimise the computational resources (data storage and simulation time). The implications of this time discretisation on some of the processes will be discussed later.

The internal time steps used in MODHMS can be much smaller than the discretisation of the inputs. The adaptive time-stepping algorithm used in the model refines the time step depending on the ease or difficulty to achieve the convergence towards the specified accuracy. This progression, i.e. convergence, to the specified accuracy, is a balance between the size of the time step and the number of iterations allowed to reach this desired accuracy. An example of the time stepping is given in Figure 18. In this example, the maximum iteration limit is set to 10 iterations. For iterations less than 4 (green zone) the internal time-step size increases by a specified percentage, in this case 20%. For iterations between 4 and 6 inclusive, the internal time step remains the same, and for iterations 7 and above the internal time step is reduced by a factor of 5. The internal time step is also reduced by a factor of 5 when the iteration fails to converge. Note the steady increase in time step between times 10 and 11 due to the low number of iterations, the steady time-step size between times 11 and 12 as the iterations are in the range 4-6, and the large decrease in the time-step size at times 12 and 13.

The adaptive time-stepping method is ideal for simulating the complex water regime in the domain as the time step can be small when processes require high temporal discretisation, and the time step may be large when only small changes are occurring through the domain. Thus in the example in Figure 18, the internal time steps are relatively large between times 10 and 12 as only small changes are occurring in the external forces. However, at the start of times 12 and 13, some major changes to the external forcing occur and the internal time-step size reduces until the iterations converge. The maximum internal time step allowed in the model was daily, whilst the minimum was of the order of seconds. Table 2 contains a


summary of the temporal discretisation for various boundary conditions in the Southern River model.

0.0001

0.001

0.01

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Tim

e s

tep

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er

of i

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tio

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Time step Iterations

Figure 18 Example of time stepping in the MODHMS model using a 10-iteration convergence limit.

Table 2 Temporal discretisation for external stresses to model

Process/External stress Specification period

Abstraction Monthly

Drainage network Constant

Constant head (north) Constant

Specified head (south) Monthly

Channel inflow Daily

Rainfall Daily

Potential evaporation Monthly

Vegetation (LAI, root depth etc) Monthly

Irrigation application Monthly

4.4. Subsurface flow

4.4.1. Subsurface boundary conditions

The eastern and western groundwater boundaries are no-flow boundary conditions in the superficial aquifer. The western boundary coincides with a divide in the Jandakot Mound and in the east with the Darling Scarp as indicated in Figure 14.

Along the northern boundary, the western section has a constant head of 0.1 mAHD corresponding to the average water level in the Canning Estuary (for locations see Figure 14). The influence of this boundary condition is to allow northerly flow in the north-west part of the model towards the Canning Estuary and also create conditions for the upwelling of water from the deep part of the superficial aquifer into the Canning Estuary. The remainder of


the northern boundary is a no-flow boundary along a flow line from the Darling Scarp to the estuary. The eastern part of the southern boundary is also a no-flow boundary corresponding to a flow line from the Darling Range to Birrega Drain. The remainder of the southern boundary of the model are specified heads interpolated from historical data at three bores (T200, T210 and T220, see Figure 14). These heads are used for all simulations, including climate change simulations, as the effect of these heads on the Southern River catchment is negligible. This is because the southern boundary is remote from the Southern River catchment and the influence of these specified heads is insignificant compared to the influence of climate within a few rows.

4.4.2. Subsurface saturated flow

The aquifer is parameterised based on available lithology description of the Quaternary deposits shown in Figure 3. For a specified lithology, it is assumed that the hydrological parameters in the upper-superficial layers and aquitard vary in an east-west direction from the boundary of the model to the Guildford-Bassendean dividing line (GBDL) (see Figure 3), and are constant on the opposite side of the dividing line to the dominant lithology. Thus the parameters associated with the Guildford Formation vary linearly between the eastern boundary of the model and the GBDL, and have a constant value to the west of the GBDL. Similarly, the parameters for the Bassendean Sand vary linearly between the western boundary of the model and the GBDL, and are constant to the east of the GBDL. The hydrogeological parameters in the lower superficial vary linearly across the entire model domain. The parameters for each layer (as calibrated in the model, see Section 6) are given in Table 3. The hydrogeological parameters for the lacustrine sediments are constant across the whole domain.

Table 3 Hydrogeological parameters for the superficial aquifer

Parameter Layer Bassendean Guildford Lacustrine

west east west east

Horizontal 1-4 9.0 5.0 5.0 3.0 0.1

hydraulic 5 0.5 0.3 0.3 0.1

conductivity (m/day) 6 10 1

Vertical 1-4 0.1 0.1 0.1 0.1 0.001

hydraulic 5 0.05 0.03 0.03 0.01

conductivity (m/day) 6 0.1 0.1

1-4 0.25 0.25 0.25 0.25 0.05

Specific yield (-) 5 0.1 0.1 0.1 0.1

6 0.2 0.2

1-4 10-4 10-4 10-4 10-4 10-5

Storativity (-) 5 10-4 10-4 10-4 10-4

6 10-3 10-3

4.4.3. Subsurface unsaturated flow

The unsaturated zone in the model generally occurs in the top four layers of the model. The model initially had a single layer for the upper part of the superficial aquifer but the results indicated that recharge to the saturated zone was much faster, resulting in greater groundwater levels than observed in the field. The partitioning of this upper-superficial layer led to a much closer agreement with the monitoring data.

The parameters for the unsaturated zone are derived from the Rosetta software package (Schaap, 2000) for sand and sandy-clay and are shown in Table 4. These parameters are


distributed within the model domain using the same method as for the subsurface saturated parameters.

Table 4 Unsaturated flow parameters used in model

Soil Layer Van Genuchten α

(m-1)

Van Genuchten β

Residual saturation

Brooks-Corey

exponent

Bassendean Sand

Upper (layers 1-4)

3.5 3.2 0.1 1.5

Guildford Formation

Upper (layers 1-4)

3.3 3.0 0.1 1.5

Lacustrine sediments

Upper (layers 1-4)

3.5 1.7 0.1 1.5

Bassendean Sands

Aquitard (layer 5)

3.3 3.0 0.1 1.5

Guildford Formation

Aquitard (layer 5)

3.2 2.0 0.1 1.5

Regional aquifer

Lower (layer 6)

3.3 3.0 0.1 1.5

4.5. Overland surface flow When the rate of through-fall (rainfall and applied irrigation less canopy interception) to the ground surface exceeds the total infiltration and surface evaporation rates, water pooling occurs on the ground surface. Before any overland flow can occur, the surface roughness, represented by a rill height, must be exceeded. Overland flow is simulated in the model using the Manning equation with a condition that the flow is retarded if the water level is below a specified obstruction height representing obstacles to the flow. The selection of rill height, obstruction height, and Manning coefficient was based on land cover information and Arcement and Schneider (1989), as listed in Table 5. The infiltration rate is equal to the vertical hydraulic conductivity in the topmost layer of the subsurface.

The use of the parameters in Table 5 is reasonable where there are no additional controls on water. However, in urban areas the road and roof runoff can be directly discharged to the channel system. In the Southern River catchment, an analysis of the pixels in the land use raster (Figure 4) found that road and roof areas combined constitute 11% of the land area. It is assumed that rainfall on these surfaces is conveyed via a drainage system directly to a waterway. Allowing a 2 mm daily threshold before runoff occurs (i.e. only rainfall above the threshold produces runoff from these surface), and best-fit analysis of 50% of road runoff and 70% of roof runoff, the annual average volume of rainfall transported directly to the channel system is 14.9 GL. This water, including the threshold amount, is removed from the model and the runoff is added in post-processing to the channel flow.


Table 5 Parameters for transpiration and overland flow for different land uses and for channel flow

Land use Rill height

(m)

Obstruction height

(m)

Manning coefficient

(s/m1/3)

LAI

Root depth

(m)

Interception storage

(mm)

Unirrigated grassland

0.1 0.01 0.06 seasonal seasonal 0.2

Irrigated grassland

0.1 0.1 0.17 seasonal 0.5 0.2

Roads 0.005 0.01 0.011 na na na

Roofs 0.002 0.005 0.011 na na na

Trees 0.2 0.1 0.135 1.0 3.0 2.0

Lakes (open water)

0.001 0.001 0.01 na na na

Channel 0.05 0.01 0.04 na na na

4.6. Channel flow The drainage system for the Southern River/Wungong Brook has been modelled as 36 channel reaches consisting of 1020 segments. The channel dimensions were based on survey data (FUGRO Spatial Solutions Pty Ltd, personal communication; Jim Davies and Associates Consultant Hydrologists, personal communication). These surveys and site inspections included only discrete points along the network. These points were then interpolated along the channels ensuring each surface cell the channel passed through had at least one segment associated with the cell.

The leakage parameter for each segment was specified to be the same as the hydraulic conductivity of the underlying subsurface cell. The channel roughness parameters (Manning coefficient, rill and obstruction heights) were set to constant values along the entire network of channels using the values in Table 5. These values are typical for clean winding streams on a plain (Gregory and Walling, 1973).

Since the Darling Scarp was not included in the model, channel flow boundary conditions were defined as inflows to the model at three locations: where Neerigen Brook, Wungong Brook and the Brickworks A Drain intercept the model boundary. The specified flows are derived from the observations at Neerigen Brook with the flow proportional to the effective catchment areas of the channels. These catchments are similar in location and land use, with the catchments of Wungong Brook and Brickworks A Drain being 47% and 14% respectively of the size of the Neerigen Brook catchment.

The outflow from the network is assumed to be a zero gradient flux in the final segment of the Southern River channel. Similarly, the outfall from the drainage network above Forrestdale Lake has a zero gradient thus allowing channel flow into the lake.

4.7. Water allocation and use There is significant groundwater use in the catchment (WA Department of Water (DoW), personal communication). As of December 2006 there were 1322 active licenses for abstraction from the superficial aquifer within the model domain, with a total annual licensed abstraction of 15.8 GL. The annual individual licensed abstraction volumes range from 10 m3/annum to 2890000 m3/annum (or 2.89 GL/annum).


Although the information supplied by the DoW does not include the intended use for the water, the usage was inferred from the licensed quantity. As a result the model includes four different classifications of licenses (Table 6):

an annual allocation less than 500 kL/annum is associated with indoor household use

an allocation between 500 and 2000 kL/annum is associated with a small property, with 500 kL/annum assigned to indoor household use and the remainder being used for irrigation

an allocation in excess of 2000 kL/annum is associated with large business or a local council and is assumed to be for irrigation

the allocation for the water utility (this water is removed from the model)

Based proportionally on average monthly potential evaporation the seasonal component of the licensed abstraction is assigned for each month of the year according to the fractions in Table 7.

Table 6 Categories of groundwater abstraction and assumed groundwater use adopted in the model

Description Abstraction license (m3/annum)

Constant component

Seasonal component

Household Less than 500 all none

Small property Greater than 500 but less than 2000

500 m3/annum

remainder

Large business/local council Greater than 2000 none all

Water utility all all none

The calculated irrigation water is returned to the model through the recharge package in MODHMS. The water abstracted by the utility was considered to be part of the potable water supply and accordingly was removed from the model. The area for recharge was calculated combining the license information, which included coordinates with a cadastre of the region. The area of the cadastral cell containing the license was converted to an equivalent radius of a circle of the same area. The area, where additional recharge was supplied, was calculated as a buffer zone with the equivalent radius calculated above and centred in the model cell attributed with the groundwater allocation.


Table 7 Monthly distribution of abstracted groundwater used for irrigation in the model

Month Fraction of seasonal allocation

January 0.15

February 0.14

March 0.10

April 0.06

May 0.04

June 0.04

July 0.04

August 0.04

September 0.06

October 0.08

November 0.10

December 0.15

4.8. Rainfall Interception As described earlier, the model allows interception of rainfall and applied irrigation by the vegetation canopy where a certain volume may be stored before it is evaporated (Figure 11). The storage capacity per unit area of the vegetation is specified as the product of the leaf area index (LAI) and the interception storage as given in Table 5 and Table 8. The LAI for is calculated using areal averages based on the land cover categories (Figure 4) within that cell. This canopy interception and subsequent evaporation reduces the quantity of rainfall that falls onto the surface and the evaporation from the remainder of the soil profile.

Table 8 Monthly variations in vegetation parameters

Month Unirrigated grassland Irrigated grassland

LAI Root depth (m) LAI

January 0.0 0.0 0.7

February 0.0 0.0 0.7

March 0.0 0.0 0.7

April 0.1 0.1 0.8

May 0.4 0.4 0.9

June 0.8 0.5 1.0

July 1.0 0.5 1.0

August 1.0 0.5 1.0

September 1.0 0.5 1.0

October 0.9 0.5 1.0

November 0.5 0.5 0.9

December 0.1 0.5 0.8


4.9. Transpiration The model calculates the transpiration by vegetation, evaporation from any inundated surfaces, evaporation from the unsaturated zone and finally evaporation from the saturated zone. The total potential evaporation available for these processes is the potential evaporation supplied as the input parameter less any evaporation that has occurred from the canopy. The parameters governing the transpiration are calculated for each cell using the root depth layer thicknesses and assuming a conic downward shape to the roots. The root depth is an average depth based on the land cover within the cell (Figure 4). The maximum root depths used here are 3.0 m and 1.0 m, for trees and irrigated pasture, respectively, whilst the monthly variations in root depths used for non-irrigated pasture are given in Table 8.

The MODHMS model uses four key saturations for the transpiration process: (in order of increased saturation): wilting point (25% saturation), field capacity (50% saturation), oxic limit (90% saturation) and anoxic limit (95% saturation) (Feddes et al., 1978; Or and Wraith, 2000). Full transpiration is assumed to occur between field capacity and oxic limit and no transpiration occurs for saturations below wilting point or above anoxic limit.

4.10. Evaporation Evaporation occurs from the ground surface and subsurface when the LAI for the cell is less than 1 and the potential evaporation has not been exceeded by the combined canopy and vegetation transpiration. The evaporation preferentially occurs from the ground surface, and if the potential evaporation is not used up, it works through the unsaturated profile. The evaporation from a layer is the product of the available potential evaporation, an evaporation distribution function and a wetness factor. The wetness factor is a fraction between 0 and 1 indicating the availability of the water for evaporation. It is 1 for saturations above an energy limited saturation (specified as 0.6 in the model), and zero below a limiting moisture content (specified as 0.1 in the model). The evaporation distribution function is specified based on the surface geology and assumes a linear decline from a maximum at the surface to zero at the extinction depths as shown in Table 9.

Table 9 Evaporation extinction depths based on Shah et al. (2007)

Soil type Extinction depth (m)

Guildford Formation 2.0

Bassendean Sands 2.0

Lacustrine sediments 0.5

4.11. Recharge The MODHMS model, when simulating unsaturated flow, does not store the information on the recharge to groundwater. In order to quantify this important groundwater balance parameter, a post-processing methodology of model results was developed within the project.

The post-processing was based upon a purpose written Fortran 90 program which reads the simulation results for each stress period. For each output time step and for each computational cell, the program finds, starting from the bottom layer, the first layer that is not fully saturated. A bottom-up approach is used as it minimises the effects of any perched watertable in the model. The flux from the unsaturated layer into the underlying saturated layer is recorded as the recharge. However, if the top layer is saturated, then the recharge is equal to the sum of the infiltration and leakage from any channel in the particular cell. The model records the daily recharge for each output time and for each computational cell.


5. UNCERTAINTIES AND LIMITATIONS While the model has been set up based on considerable spatial and temporal data uncertainty and limitation exist in all modelling studies. A clear identification of these facilitates an adequate interpretation of modelling results. Furthermore alternative treatment of selected processes can be used to minimise the possible adverse impact of identified uncertainties and limitations.

5.1. Evaporation from channels A potential limitation of the MODHMS model is that evaporation is not calculated from the surface of channels. In most cases this does not cause significant error as the potential volume of evaporation is greatly exceeded by the flow through the system. However, this limitation may affect the modelled flow where a wetland is located along a channel and maintains water even when the channel flow has ceased. To circumvent this for the major identified wetlands along the channel, such as the Armadale Road wetland, the surface and channel elevations in the model were adjusted so that the channel flow can spill out onto the adjacent land surface and create surface inundation, which is subject to evaporation in the model. This method of adjustment is appropriate where the pooled water in a channel constitutes a wetland with a large surface area, but for the case where the water is backed up along a small width channel, no adjustments are made as evaporation is likely to be negligible.

The impact of this assumption can be quantified as follows:

The total length of the channel system is 173 km with a total area of 36.1 ha if the channels contain water. For an average evaporation rate of 10 mm/day in the summer, this would amount to 0.36 ML/day. However, during the summer, up to 80% of the channel system (by length) becomes dry due to leakage to groundwater and thus total evaporative losses from the channels in the summer are estimated to be below 0.07 ML/day. This composes 7% of the catchment daily baseflow during the summer of around 1 ML/day and as such falls below the accuracy of flow estimation.

5.2. Temporal discretisation The adopted daily time step for rainfall to the model, reflecting the available data, poses a challenge related to the overland and channel flow estimation.

As the rainfall events get smoothed out over the day and do not reflect variability in the storm intensity the surface runoff is likely to be underestimated during individual storm events. In addition the canopy in the model intercepts rainfall throughout the day and can thus continuously evaporate the rainfall, decreasing the amount of through-fall to the land surface.

As discussed in Section 3.1.4, the approximate length of time for a daily rainfall event in excess of 5 mm was 6 hours. The sensitivity of the model to the duration of rainfall was assessed using six-hourly and daily applications of the rainfall:

It was found that for the six-hourly rainfall, there was an initial increase in the channel flow compared to the daily rainfall, but the flow over the following days was smaller than the response to daily applied rainfall (Figure 19). After this period, the flow rates for both scenarios remain the same until the next rainfall event. The net change in the monthly and annual flow in the channel system was negligible (<1%).

The use of the six-hourly rainfall event reduced the amount of canopy interception and canopy evaporation by an average of 30%. However, when total evaporative losses from canopies, surface and subsurface were compared on monthly basis the differences were less than 3% (Figure 20).


5000

10000

15000

20000

1661 1662 1663 1664 1665 1666 1667 1668 1669

Day of simulation

Flu

x (m

^3

/da

y)

-500

0

500

1000

Flu

x D

iffe

ren

ce

(m

^3

/da

y)

Daily rainfall event

6 hourly rainfall event

Flux difference

Figure 19 Daily channel fluxes over one event (14.8 mm on day 1662) for two scenarios of rainfall duration: six-hourly rainfall event and daily rainfall event

y = 1.0041x - 0.1377

R2 = 0.9979

y = 0.7162x - 0.5353

R2 = 0.8450

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140Daily event (mm)

6 h

ou

rly

eve

nt

(mm

)

Total EvaporationCanopy EvaporationLinear (Total Evaporation)Linear (Canopy Evaporation)

Figure 20 Comparison of evaporative losses for two scenarios of rainfall duration: six-hourly rainfall event and daily rainfall event

Based on the above, whilst it was recognised that the employed time step may lead to an underestimation of channel peak flows during short duration rainfall events it is believed that these shorter events will not significantly affect the characterisation of catchment-wide processes and associated long-term water resources assessment in the catchment.

Based upon the application of the daily rainfall, a choice driven by the excessive computational resources that would be required to undertake sub-hourly rainfall/runoff aspects of the simulations, peak flow rates will be underestimated at times with higher rainfalls. However, these aspects are of lower importance for the objectives of this study compared to the well-captured surface groundwater interaction, the main objective of this work.

5.3. Overall simulation period Due to constraints associated with the duration of each scenario simulation, only a ten-year period was modelled. As a result, the predictive climate scenarios are only able to define variations in the catchment water regime within the 10 years. The modelling outcomes cannot therefore identify processes which may occur over a longer time scale.


6. MODEL CALIBRATION AND VALIDATION Although a large data set was available for the catchment, not all parameters used in the model have been measured or could be derived from published information.

Only the hydraulic properties of aquifers (saturated flow module) were adjusted in the model calibration. The remaining parameters were validated using comparisons between modelled and observed data. The emphasis was on the model reproducing the observed responses rather than absolute values of observations. The validation was undertaken for the following:

river discharge at the outflow from the catchment

baseflow as groundwater contribution to the channels

groundwater levels dynamics

water level dynamics in wetlands

Unsaturated zone fluxes were not directly validated, but indirectly the validation was encompassed in the groundwater levels dynamics analysis.

Model calibration and validation commonly include combinations of stresses (rainfall, land use types and others) that could be encountered during the predictive model application. To achieve this, the available data, both input stresses and observations, should be adequate in both temporal and spatial scales. In the considered case, the observation data was available only for a low rainfall period (see Figure 5). It has been therefore assumed here that the calibration and validation undertaken over this period provides a sufficient basis to account for alternative rainfall patterns. It is worth remembering that the main advantage of process-based models is their ability, once the main processes have been validated, to capture, with the appropriate confidence, the hydrogeological processes outside of the data used for their validation.

The model results were evaluated with a number of statistical methods for groundwater results (Murray-Darling Basin Commission (MDBC), 2001):

Scaled Mean Sum of Residuals (SMSR)

Scaled Root Mean Fraction Square (SRMFS)

Scaled Root Mean Square (SRMS)

Standard correlation between two time series (r)

These measures reflect a composite approach to model validation. Apart from the coefficient of determination and standard correlation, where a perfect model outcome approaches a value of unity (MDBC, 2001), the remainder of the measures should be minimal, indicating that the ratio of differences between observed and simulated are smaller than the variations in the observed values.

SMSR, SRMFS and SRMS are expressed as percentages such that:

an excellent result is less than 5

a good result is less than 15

an acceptable result is less than 30.

Standard correlation between two time series shows:

an excellent result when r is greater than 0.95

a good result when r is greater than 0.85

an acceptable result when r is greater than 0.70.

It is noted that the maximum value attainable by the standard correlation is 1.0.


The channel flows were validated using the Nash-Sutcliffe measure of model efficiency (Nash and Sutcliffe, 1970). The efficiency coefficient (ε) can theoretically vary from minus infinity to plus one. A value above 0.7 is considered good model performance.

6.1. Saturated flow: calibration and validation

6.1.1. Calibration

As discussed above one of the model limitations is associated with the duration of a model simulation. The full model takes approximately 28 days to run a 10-year simulation on a 3.0 GHz dual quad-core Intel Xeon. These long computational times prevented an automated calibration process using software such as PEST (Watermark Numerical Computing, 2003). Instead, a method of sub-model calibration was adopted where small areas of the larger domain were selected as representing different hydrogeological conditions in the catchment and these areas were calibrated independently. The boundary conditions for these areas were taken from an initial full-catchment simulation. The results from these sub-models were subsequently checked against the full-catchment simulation to ensure boundary fluxes and groundwater heads were consistent with the larger domain. The three selected catchments are shown in Figure 8.

The first sub-catchment was close to the top of the Jandakot Mound in the vicinity of the observation bore WR2C. This sub-catchment was used to calculate parameters for the western margin of the Bassendean Sands. This sub-model has an area of 5.4 km2 (2.5 km by 2.2 km).

A second sub-model was in the proximity of observation bore T115. This is located in the area between the Forrestdale Drain and Wungong Brook and is used to calibrate the parameters at the interface between the Guildford Formation and the Bassendean sands. This sub-model has an area of 1.6 km2 (1.9 km by 0.8 km).

The third sub-model was located in an area to the south of Wungong Brook centred on observation bore T170. This location was used to calculate the parameters for the eastern margin of the Guildford Formation. This sub-model has an area of 18.3 km2 (7.3 km by 2.5 km).

The initial conditions for the calibration runs were created by running the full model using the 1996 climate data with flow into the channels from the Darling Scarp specified using the 1997 data. The calibration simulations were run for 10 years.

The calibration for the three sub-models varied the vertical and horizontal hydraulic conductivities in the three layers using PEST. Table 3 in Section 4.4.2 shows the parameter values derived for the three areas, which are in agreement with previously reported values of Davidson (1995) and Rockwater Pty Ltd (2005). The values of parameters from these small-scale models were up-scaled to parameterise the catchment.

6.1.2. Validation

Groundwater hydrographs from three bores, T80, JM16 and T75 (shown in Figure 8) were selected for the validation, following analysis of the groundwater hydrographs within the catchment. These bores have records with at least two observations per year.

The validation used the parameters derived in the calibration of the sub-models for the catchment model. The validation simulation ran for a 10-year period between 1997 and 2006 inclusive.

The observed and simulated groundwater heads in bores T80, JM16 and T75 are shown in Figure 21(a) to (c) respectively.


24.0

24.5

25.0

25.5

26.0

26.5

27.0

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

He

ad

s (m

AH

D)

Observed Simulated(a) Bore T80

24.0

24.5

25.0

25.5

26.0

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

He

ad

s (m

AH

D)

Observed Simulated

(b) Bore JM16

17.5

18.0

18.5

19.0

19.5

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

He

ad

s (m

AH

D)

Observed Simulated(c) Bore T75

Figure 21 Model validation: observed and simulated water levels at (a) bore T80; (b) bore JM16 and (c) bore T75


The validation statistics, shown in Table 10, indicates that although some of the criteria for groundwater models are not met, the statistical results are very close to the guidelines.

An additional validation of the model’s ability to simulate groundwater processes in the catchment is provided by comparing the modelled and observed heads. Figure 22 shows the observed and simulated groundwater responses at a bore located within the Wungong Urban Water (WUW) precinct together with the relevant rainfall. The behaviour of the watertable shows the initial fast filling of the aquifer between days 80 and 160 (April-June), then the watertable remains at roughly the same level for the remainder of the winter before dropping slowly at around day 270 (end of September), with the rate of drawdown increasing through to December.

23.0

23.5

24.0

24.5

25.0

0 61 122 183 244 305 366Day

He

ad

s (m

AH

D)

0

200

400

600

800

Cu

mu

lati

ve r

ain

fall

(m

m)

Simulated

Observed

Cumulative Rainfall

Figure 22 Observed and simulated watertable variation at BRM10 and cumulative rainfall

Table 10 Statistical measures achieved during model validation at selected bore locations

Statistical measure Forrestdale Lake JM16 T80 T75 T85

SMSR (%) -0.76 -3.61 -0.21 -2.40 -2.71

SRMFS (%) 16.48 16.96 13.68 23.66 17.68

SRMS (%) 16.61 17.12 13.81 23.92 17.61

r 0.84 0.78 0.85 0.86 0.93 Bold indicates an excellent result Plain text indicates a good or acceptable result

6.2. Validation of the channel flow in the simulation Figure 23 shows the monthly observed river discharge at the outflow of the catchment (with accuracy range as already shown in Figure 9), and simulated flow. The simulated fluxes show the same patterns as the observed data for the high flows; however, the maximum simulated flow underestimates the observed flow. The main reason for this is the underestimation of overland flow and discharge to the drainage network during high intensity rainfall events due as a result of time step for rainfall as discussed in Section 5.2.

Table 11 shows the statistical measures for the river flow on a daily, monthly and annual basis. The differences in the model efficiency between daily, monthly and annual discharges indicate that although the shape of the simulated response is similar to the observed responses, there are differences in the magnitude of the flows. This is shown in the comparison of the simulated and observed monthly fluxes in Figure 24. The differences between the observed and simulated average daily, monthly and annual flows for the whole simulation are less than 1%.


0

2

4

6

8

10

12

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Flu

x (G

L/m

on

th)

Upper accuracy boundObservedLower accuracy boundSimulated

Figure 23 Observed (with accuracy bounds) and simulated monthly flow rates at Anaconda Drive

Table 11 Statistical measures adopted for model result validation for modelled flow at the Southern River gauging station

Statistical measure Daily Monthly Annual

ε 0.34 0.82 0.72 Bold indicates an excellent result Plain text indicates a good or adequate result Italics indicates a poor result

y = 0.7365x + 0.3236

R2 = 0.8319

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10Observed monthly flux (GL)

Sim

ula

ted

mo

nth

ly fl

ux

(GL

)

Figure 24 Comparison of simulated and observed monthly fluxes at Anaconda Drive


The model also calculates groundwater discharge to the channels, considered here as the baseflow in the channel system. Figure 25 shows a comparison between the observed and simulated baseflow for the period October 2000 to May 2001. The observation data includes the accuracy band of the flow measurement. The minimum observed accuracy band becomes zero flow if the observed flow drops below 3000 kL/day.

100

1000

10000

100000

1000000

Oct-00 Nov-00 Dec-00 Jan-01 Feb-01 Mar-01 Apr-01 May-01 Jun-01

Month

Flo

w (

kL/d

ay)

Upper accuracy boundObservedLower accuracy boundSimulated

Figure 25 Comparison of observed and simulated baseflow (2000-2001 summer) (note the logarithmic scale on the vertical axis)

The simulation results are in agreement with the observed values up to January 2001, and again in late April 2001. However, between January and April, the simulation shows a slight reducing trend in the flows and observations suggest a steady or slight increase in river discharge, with both observed and simulated river flows being within the accuracy bounds. Since during this period no rainfall was observed, the increase in baseflow is likely to be artificial and possibly arises from the steady clogging of the channel in the proximity of the gauging station by the growth of water plants during the summer.

The validation statistics show that the behaviour of the model closely approximates the observations at Anaconda Drive for the daily, and to a lesser extent, the monthly discharge. The poor annual results for the coefficient of determination indicate that although the behaviour of the catchment is captured, the magnitude of the fluxes for individual high flow events associated with high intensity rainfall was not successfully simulated.

6.3. Validation of water level dynamics in the wetlands Figure 26 shows the observed and simulated water levels in Forrestdale Lake for the period 1997-2006. Table 10 contains the statistical measures for model effectiveness in the simulation of the lake level. These show excellent results for the scaled mean sum of residuals (SMSR) and coefficient of determination (CD), with acceptable results for the other statistical measures. These statistics give an indication of the closeness of the simulation results to the observations, but, as discussed above, the observations also have some unquantified uncertainty associated with them.

The model reproduces well the seasonal changes in the water level in the lake with some over-prediction in the maximum level in the lake. The agreement between observed and modelled data related to the time of the rise and fall in the water levels allows suggesting that processes which lead to these changes in water levels are also well replicated by the model.

Figure 27 shows the inundation duration curves for Forrestdale Lake for both simulated and observed data over the simulation period (1997-2006). The model results show good agreement with the observed data.


21.0

21.5

22.0

22.5

23.0

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Wa

ter

leve

l (m

AH

D)

Observed Simulated

Figure 26 Model validation: observed and simulated water levels in Forrestdale Lake

21

22

23

0 0.2 0.4 0.6 0.8 1Fraction above level

Wa

ter

leve

l (m

AH

D)

Observed (1997-)

Simulated

Figure 27 Inundation duration curves for Forrestdale Lake for simulated and observed data

6.4. Summary Overall model results indicate that the MODHMS model of the Southern River catchment successfully reproduce the main governing processes controlling catchment water regime. These processes include

fast watertable recovery during recharge period

timing and rates of watertable drawdown during summer

monthly and daily channel flow at catchment outlet, at times with moderate rainfall

water level dynamics in lakes and wetlands

summer baseflow to the channel system.

As expected, based upon the application of daily rainfall, a choice driven by the excessive computational resources that would be required to undertake sub-daily rainfall/runoff aspects of the simulations, peak flow rates have been underestimated at times with higher rainfalls.


7. MODELLING RESULTS The validated model was used to quantify fluxes in the catchment and to examine the spatial and temporal distribution of these fluxes. For some of these analyses, the Southern River channel system within the model domain was further divided into sub-catchments (Figure 28). The sub-catchments of Wungong Brook, Southern River and Forrestdale Main Drain all contribute to the outflow from the catchment. A sub-catchment upgradient from Forrestdale Lake does not contribute to the Southern River, with Forrestdale Lake acting as a terminal lake.

Figure 28 Sub-catchments within the Southern River catchment used for the modelling

All results discussed below are for the current climate and land use.

7.1. Catchment water balance A mass or water balance of the catchment specifies the magnitude of external fluxes both into and out of the system. An average annual Southern River catchment water balance based over the ten years of simulation is presented in Table 12, Figure 29 and Figure 30. These show that the largest components of the catchment water balance are the rainfall and evaporative losses. The remaining inflows to the catchment are channel flow from the hills and some lateral groundwater flow around the edges of the catchment. Additional to evaporative losses from the catchment are the Southern River discharge and some groundwater outflow influenced by abstraction immediately adjacent to the catchment and groundwater fluxes towards the Canning River in the east and north.


Table 12 Average annual water balance for the Southern River catchment calculated over ten years of simulation

Flux In (GL/annum) Out (GL/annum) In (%) Out (%)

Rainfall 86 93

ET (total) 84 90

Evaporation (canopy) 27 29

ET (vegetation) 23 24

Evaporation (inundation, soil) 35 37

Channels 5 8 5 8

Abstraction# 0.2 0.2

Subsurface 1.3 1.0 1.4 1.1

Overland flow 0.04 0.2 0.04 0.2

Total* 92 94 100 100

# This amount is different from the 15.8 GL for the whole region as (i) the abstraction reported here is for the Southern River catchment as coloured in Figure 28 and (ii) this is the net amount removed from the model and does not include any irrigation water which is reapplied within the catchment * The discrepancy may have occurred due to variation in soil moisture content, decrease in groundwater storage (particularly as the final year of simulation was a very low rainfall year) or groundwater abstraction outside the Southern River catchment boundary but within the groundwater catchment

Rain93.20%

Surface flow0.04%

Channel5.32%

Subsurface Flow

1.44%

(a) Inflows

Subsurface Flow1.1%

Channel8.5%

Abstraction0.2%

ET90.1%

Surface flow0.2%

(b) Outflows

Figure 29 Simulated average annual water balance for the Southern River catchment


Channel78%

Surface flow1%

Subsurface Flow21%

(a) Inflows

Abstraction2%

Surface flow2%

Subsurface Flow11%

Channel85%

(b) Outflows

Figure 30 Simulated average annual water balance for the Southern River catchment excluding rainfall and evaporation

A water balance was also calculated for the subsurface model component including both the unsaturated and saturated zones of the aquifer (Table 13). Similarly to the overall catchment water balance, the major components of the subsurface balance are the infiltration and the evaporation and evapotranspiration. In comparison, the groundwater discharge to the channel system (baseflow) is only 5% of the total losses.

Table 13 Annual average water balance for the subsurface model component within the Southern River catchment calculated over the 10-year simulation

Flux In (GL/annum)

Out (GL/annum)

In (%)

Out (%)

Surface infiltration (includes rainfall and irrigation)

54 97

Groundwater discharge to surface 0.2 0.3

ET (total) 55 92

ET (vegetation) 21 36

Evaporation (inundation, soil)@ 33 56

Channel leakage 0.6 1.1

Groundwater discharge to channel 3 5

Abstraction# 0.5 0.9

Lateral subsurface flows 1.3 1.0 2 2

Total* 56 59 100 100

@ This term of the water balance is an overestimate as it includes surface evaporation. * The discrepancy occurs due to the inclusion of surface evaporation, variation in soil moisture content, decrease in groundwater storage (particularly as the final year of simulation was a very low rainfall year) or groundwater abstraction outside the Southern River catchment boundary but within the groundwater catchment. # The difference between the subsurface balance and the overall balance is that the overall balance does not include the abstraction that is re-applied within the catchment. In the subsurface balance, this amount is applied as recharge to the canopy


The water balance analysis shows that vertical fluxes in the aquifer prevail due to the fact that climatic forcing (rainfall/infiltration and evaporation/evapo-transpiration) is much greater than lateral flows (baseflow and subsurface inter-catchment flows). This also means that any solutes added to the subsurface with recharge are likely to remain within the catchment for a considerable period of time.

7.2. Groundwater discharge to the channel system The simulated fluxes between the channel system and the groundwater are shown for the Forrestdale Main Drain (FMD) and the remaining Southern River (SR) sub-catchments in Figure 31 and Figure 32, respectively.

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1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

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x (

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/mo

nth

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Channel leakage to groundwater

Groundwater discharge to channel

Figure 31 Simulated monthly fluxes between channel and groundwater within the Forrestdale Main Drain catchment

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on

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Channel leakage to groundwater


Figure 32 Simulated monthly fluxes between channel and groundwater within the Southern River catchment excluding Forrestdale Main drain catchment


In the FMD sub-catchment, the maximum channel leakage to groundwater (Figure 31) generally occurs early in the wet season. During this period, the generated overland flow entering the channels seeps into the groundwater as the watertable is below the water level in the channel. The groundwater discharge to the channels increases as the watertable rises. This is associated with two processes:

The first process is related to the start of a recharge event, when the watertable rises faster than the water level in the channels.

The second process is that the length of the channel system receiving the groundwater discharge increases as the watertable rises above the base of the channel in additional areas. However, increases in the discharge to the channel system in upstream areas can increase the water level in the downstream parts of the channel system, and this increase in level may be greater than the local rise in the watertable, resulting in a simultaneous increase in leakage from the channel system. This is more likely to occur in the flatter catchments of the Forrestdale Main Drain.

In the remaining Southern River sub-catchments the channel leakage to groundwater shows two maxima. The first occurs early in the wet season as the flow from the hills increases with the onset of rainfall, but the watertable remains below the level of water in the channels. In the middle of the winter, the channel leakage decreases as the watertable in a majority of areas at this stage is greater than the water level in the channel and groundwater discharge is occurring. Then in spring watertables start to fall due to the effect of increasing evaporative losses and decreasing rainfall. However, channel flow (which continues from the hills, though at a lesser rate) and leakage from the channels recharge the aquifer again. This results in a second peak in the hydrograph.

7.3. Channel flow hydrographs The daily, monthly and annual discharge from sub-catchments is shown in Figure 33 for the FMD, Figure 34 for the Wungong Brook (WB) and Figure 35 for the entire catchment. All the flux distributions show a slight delay in the onset of flow at the start of the wet season whilst the relevant catchment wets up, with a slightly greater delay in the FMD sub-catchment. The channel flow in all sub-catchments responds quickly to rainfall events, with the quickest return to baseflow observed in the FMD sub-catchment and the slowest at the end of the Southern River catchment at Anaconda Drive, where discharge is influenced by quick flow from current urban area. Figure 36 shows the monthly discharge for the three locations (Southern River, Wungong Brook and Forrestdale Main Drain) on a single plot. At all locations the peak monthly flows occur during the same month, but the flows in the FMD may start later than the flows at the other locations.


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ay)

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1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

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00.20.40.60.8

11.21.41.61.8

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

(c) Annual

Flu

x (G

L/a

nn

um

)

Figure 33 Simulated daily, monthly and annual fluxes for Forrestdale Main Drain at Holmes St


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Figure 34 Simulated daily, monthly and annual fluxes for Wungong Brook at Armadale Rd


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Figure 35 Simulated daily, monthly and annual fluxes for the Southern River at Anaconda Drive

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Southern River Wungong Brook Forrestdale Main Drain

Figure 36 Comparison of monthly fluxes at Forrestdale Main Drain, Wungong Brook and Southern River


7.4. Depth to watertable Figure 37 shows the simulated average maximum depth to the watertable in the Southern River catchment. For the majority of the catchment area the watertable is below 2.0 m at the end of summer. However, under Forrestdale Lake and along the Wungong Brook-Southern River channel, the depth to water is shallower in the range 1.5-2.0 m.

The average annual minimum depth to water illustrated by Figure 38 occurs at the end of winter, with 0.7% of the catchment inundated due to the watertable being at or above the ground surface and 29% of the catchment having watertables within 1.5 m of the ground surface. The percentage of area inundated does not include the areas with perched water bodies.

Figure 37 Simulated average annual maximum depth to water for the Southern River catchment


Figure 38 Simulated average annual minimum depth to water for the Southern River catchment

7.5. Infiltration and recharge The distribution of simulated annual average recharge to the aquifer is shown in Figure 39. The recharge is calculated according to the method outlined in Section 4.11. The analysis indicates that there is an average of 400-500 mm/annum of recharge in the area of Bassendean Sands (or 50-65% of annual rainfall) and 100-300 mm/annum in the area of the Guildford Formation (or 13-40% of annual rainfall). The areas of low recharge at Forrestdale Lake and other wetlands on the Bassendean Sands (yellow and orange in Figure 39) have low-permeability lacustrine sediments in the surface layer and thus a low rate of infiltration and recharge. Extremely high rates (>500 mm) of recharge were found in low-lying areas of Bassendean Sands adjacent to lacustrine deposits, indicating some overflow from the wetlands on these deposits.


Figure 39 Simulated average annual recharge rates

On average, the calculated annual recharge rate is less than 50% of the total rainfall within the Southern River catchment (Figure 40(a)), whilst the infiltration rate is less than 70% of the annual rainfall. The correlation between infiltration and rainfall is closer than the correlation between rainfall and recharge. This is because infiltration occurs close to the rainfall event, but recharge may be considerably delayed. Similarly, for the monthly data shown in Figure 40(b), infiltration is again closely correlated with rainfall, whereas recharge vs rainfall has a much greater scatter. This is because infiltration takes time to reach the watertable and become recharge, so even in dry (no rain) months, there may be recharge occurring in areas with deep watertables.

Figure 41(a) shows that there is a strong correlation between annual rates of recharge and infiltration with recharge about 68% of the infiltration. There is considerable more scatter between the recharge and infiltration on a monthly basis in Figure 41(b). This is a result of the recharge to the watertable not necessarily occurring in the same month as the infiltration.


y = 0.4751x - 100.95

R2 = 0.8509

y = 0.7135x - 52.946

R2 = 0.9324

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Linear (Recharge)

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y = 0.001x2 + 0.0902x + 8.7531

R2 = 0.7609

y = 0.7159x - 4.7568

R2 = 0.9748

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Figure 40 Comparison of recharge and infiltration to rainfall on an (a) annual and (b) monthly basis


y = 0.6751x - 70.373

R2 = 0.9379

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R2 = 0.7357

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Re

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Figure 41 Comparison of recharge to infiltration on an (a) annual and (b) monthly basis

The simulated average monthly ratio of infiltration to rainfall (Figure 42) has a subtle maximum in June and then slowly decreases for the rest of the winter. This is because the watertable reaches the surface in the low-lying areas, either stopping the infiltration altogether or reducing the infiltration rate to the same rate as leakage to the deeper parts of the aquifer. This may be classified as a ‘rejected recharge’.

The monthly ratio of recharge to rainfall has a minimum in late autumn (May) and then increases through the winter-spring period. The gradual increase in the recharge occurs because infiltration associated with the early winter rain takes time to infiltrate through the soil profile to recharge the deeper watertable areas, and because the rise in the watertable during winter reduces the time taken for the infiltration to reach the watertable. The anomalous high ratios for February and December occur because of the low average rainfall in these months compared to the other summer months. Although irrigation is applied to the model in these months, the amount of irrigation is small compared to the summer rainfall (abstraction of 0.5 GL/annum is equivalent to 5 mm rainfall over the whole domain) however, it may have a local effect where it is applied.


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tio

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infa

ll (

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Infiltration Recharge Rainfall

Figure 42 Simulated average monthly ratio of infiltration and recharge to rainfall

The simulated annual cumulative daily rainfall and recharge for the Southern River catchment (Figure 43) show that recharge occurs throughout the year, with the highest rates of recharge in the winter rain period, indicated by a steep rise of the cumulative recharge. The delay at the start of the winter wet period relative to the increasing rate of rainfall is due to the infiltration slowly percolating through the unsaturated zone to deep watertables.

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Cu

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Rainfall Recharge

Figure 43 Simulated cumulative recharge and cumulative rainfall for 2004

7.6. Wetlands water balance A number of wetlands located within the Southern River catchment are of conservation value including Harrisdale Swamp and Lake Balannup (Figure 8). The Armadale Rd wetland is currently listed as a resource enhancement wetland.

Harrisdale Swamp is located in the area of lacustrine sediments in an inter-dunal depression in the Bassendean Sands. The simulated variation in watertable and water level in the swamp are shown in Figure 44 together with the observed watertable within the swamp, and the simulated watertable level 83 m west of the observation model cell within the swamp. Due to the low conductivity in the lacustrine sediments, the infiltration rate is small, and the water level in the swamp rises with increasing rainfall. In the sands adjacent to the swamp, the conductivity of the soil is much higher, and the majority of the water applied at the surface infiltrates into the subsurface. Thus the watertable under the lacustrine sediments responds slowly, and that in the sands responds quickly to rainfall. In the middle of the wet season the watertable level in the cell immediately west of the swamp is generally higher


than the inundation water level in the swamp, indicating that groundwater contributes to the surface water in the swamp. The groundwater levels fall far faster than the levels in the swamp which means that in the summer the swamp water may infiltrate towards the groundwater, but as the sediments have low conductivity, it is likely that the water loss occurs as evaporation, until the swamp dries out.

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vati

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(m

AH

D)

Watertable Surface inundation Observed (WT) Watertable (W)

Figure 44 Simulated watertable level and surface inundation with observations for Harrisdale Swamp: the watertable immediately west (Watertable (W)) of the swamp is also shown.

Lake Balannup is also located within lacustrine sediments on Bassendean Sands. The modelled surface water inundation levels, underlying watertable and watertable adjacent to the lake are shown in Figure 45. The watertable underneath the lake is below the lake level for all but the highest watertable level, but the watertable adjacent to the lake rises before the lake water level, again indicating the lake could be groundwater fed.

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Figure 45 Simulated watertable and surface inundation at Lake Balannup: the watertable east (Watertable (E)) of the lake is also shown

The wetland at Armadale Road is located in a surface depression along the Wungong Brook channel. Water is observed in this wetland throughout the year, but downstream of the wetland the Southern River is periodically dry at the end of summer. During the wet winter period, the wetland water level rises and falls during rainfall events, with the base level during this period increasing with greater rainfall (Figure 46). The watertable in winter is close


to the wetland water level, meaning potential groundwater interaction with the wetland could be both groundwater discharge to the wetland, or the wetland recharging the groundwater. However, in summer, the watertable falls faster than the water level in the wetland, which still receives inflow from the hills including additional environmental releases from Wungong Dam of about 1 GL per dry season.

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Wa

ter

leve

l (m

AH

D)

Wetland Watertable

Figure 46 Simulated water levels and watertable in Armadale Road wetland

7.7. Evaporation and evapotranspiration One of the most significant components of the water balance of the Southern River catchment is evaporation, which can vary both spatially and temporally. The following figures (Figure 47 to Figure 49) show the distribution of the individual elements of the average annual catchment evaporative losses over the model domain. These evaporative losses include canopy interception and evaporation, evapotranspiration losses from vegetation, and evaporative losses from the ground surface and subsurface including the watertable.

Spatial variation in the simulated average annual canopy interception and evaporation distribution is shown in Figure 47. The areas where interception is smaller are related to cleared land or open water such as in Forrestdale Lake.

The simulated average annual evapotranspiration from the subsurface due to vegetation is shown in Figure 48. Similar to the losses from canopies, the higher rates of vegetation evapotranspiration occur in the treed areas, with lower rates occurring for open water (Forrestdale Lake) and unirrigated grasslands. Particularly high losses are related to the areas along the channels and wetlands where water is available for transpiration over a longer period of the year.

The evaporation from the ground surface and subsurface is demonstrated in Figure 49. The higher evaporation rates for areas such as Forrestdale Lake indicate the areas of longer inundation. Other areas of high evaporation correspond to low-lying sub-catchments prone to surface inundation. The evaporation from the watertable is greatest where the watertable is close to the surface and the surface itself is not inundated. Such areas occur in the vicinity of the channel system and around the edges of wetlands and lakes.


Figure 47 Simulated spatial distribution of annual canopy evaporation (mm/annum)

Figure 48 Simulated spatial distribution of annual evapotranspiration from subsurface due to vegetation (mm/annum)


Figure 49 Simulated spatial distribution of annual evaporation from land surface and subsurface including the watertable (mm/annum)

The monthly variations in evaporative fluxes over the modelled period are given in Figure 50. The canopy evaporation is greatest during the wetter winter months, while the vegetation evapotranspiration increases during the winter months and peaks in late spring/early summer. This is the period when unirrigated grasslands are degrading due to lack of water and as a result the vegetation cover and thus evapotranspiration is reduced.

The evaporation from the surface and subsurface has a maximum in spring and occasionally a second maximum in autumn. The maximum in spring occurs as the evaporation rate increases whilst the surface and upper profile of the soil remain wet. During the dry season this water percolates deeper into the profile or evaporates, thus, due to the decrease in water content in the profile, the amount of surface and soil evaporation falls. The maxima in soil and surface evaporation in late autumn occurs after early rain has wet the upper soil profile and created small local areas of inundation and before the evaporation rate decreases to its winter minimum.

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Figure 50 Simulated spatially averaged monthly evaporative fluxes


8. EFFECT OF CLIMATE VARIATION ON THE SOUTHERN RIVER HYDROLOGICAL REGIME

The model developed in this study was further used to investigate variations in the hydrological cycle of the catchment due to climate change. The analysis included wetlands water balance and river flow which are considered to be the major environmental constraints to urban development of the catchment.

The environmental cycle in wetlands can be represented by the period of inundation for various standing water levels, and the intervals between the inundation events. The period of inundation for a habitat at a particular water level can be used as an indicator for habitat usage as a breeding site. For instance, if an area habitat in a wetland has been identified as a breeding site of a particular water-dependent species then the requirement is that the area remains inundated to a specified level for a given number of days.

The MODHMS model provides extensive information about levels and fluxes within the catchment hydrological cycle. The important quantities used for the environmental analysis of the hydrological regime are the components of evaporation/evapotranspiration, the water levels in the lakes and wetlands, and the baseflow in the channels.

Groundwater discharge maintains the baseflow in the river close to the outlet of the catchment. However, during the summer a large proportion of the drainage channels may become dry. This may also be an important part of the environmental cycle at the site in terms of ecosystem function.

Three rainfall scenarios and their impact on environmental fluxes in the catchment were used in predictive model scenarios, as described below.

8.1. Predictive rainfall sequences The different rainfall sequences used in the model examine the natural variability in the catchment. Three different rainfall scenarios are simulated:

drier than the current climate

the current climate

wetter than the current climate.

The climate scenarios were defined to represent a potential change in annual average rainfall in a range of ±10% changes when compared with the annual average rainfall over the ten-year period from 1997-2006.

As described in Section 4.6 the model does not spatially cover the hill sub-catchment of the Southern River catchment and as such cannot be used for the prediction of rainfall-runoff relationship in these catchments under various climate scenarios. However, the discharge from four streams was used in the model as a channel flow boundary condition. This model setup limits the options for adaptation of synthetic rainfall sequences outside of rainfall data and therefore stream flow in the hills catchments available over the specified period.

In order to overcome this limitation, different combinations of the observed annual rainfall data were used to produce the required average rainfall. The data from the chosen years was distributed randomly over a ten-year period to create the sequences used in the modelling (Table 14). The fluxes from the scarp and potential evaporation were specified using the same sequence.


Table 14 Annual rainfall (mm) for three climate scenarios

Year Current Drier Wetter

1 719.1 786.1 994.8

2 786.1 666.6 866.0

3 862.0 516.1 786.1

4 994.8 719.1 862.0

5 666.6 866.0 866.0

6 806.9 786.1 719.1

7 914.0 719.1 914.0

8 735.6 516.1 914.0

9 866.0 719.1 866.0

10 516.1 786.1 866.0

Average 786.7 708.0 865.4

The reduction in rainfall for the drier climate scenario is of the order of the change adopted in a similar study undertaken in the Perth region (Gnangara Sustainability Strategy (GSS), 2007). The wetter scenario for the GSS study (2007) has an increase of 21% in the average rainfall. For this catchment, this would result in an average annual rainfall of 951.9 mm. With only one year (2000) in the observation data set having rainfall in excess of this average, there would be a high repetition of that year in the wetter climate scenario. Therefore, due to data limitations, a more modest wetter climate increase of 10% is adopted.

It is important to mention that the current climate data over the ten-year period (1997-2006) are among the driest period over the century of observation.

8.2. Catchment water balance The variation in the Southern River catchment water balance for the three rainfall scenarios is given in Figure 51. Similarly to previous discussion in Section 7.1, the water balance includes rainfall over model domain, runoff from the Darling Scarp, lateral groundwater flow, evaporation/evapotranspiration, groundwater abstraction, and discharge through the channel system. For all three scenarios the major sources of water flowing into the catchment are precipitation (93%) and runoff from the Darling Scarp (5-6%).

The major outflow of water from the catchment is evaporation (86-87%). The channel discharge from the catchment is 12-13% of the total outflow from the system. The minor components of the water balance are the groundwater abstraction within the catchment, overland flow and the subsurface flow, including the groundwater flow exchange between the surface water in the catchment and also with the surrounding region.


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Figure 51 Average annual water balance components for three climate scenarios: (a) total water balance; (b) minor components of the water balance, excluding rainfall, evaporative losses and channel flow


8.3. Subsurface water balance The subsurface water balance for the three rainfall scenarios is presented in Table 15. The surface infiltration varies in the range 95-96% of the inflows to the subsurface, and the evaporation comprises 93-94% of the losses from the subsurface. Groundwater discharge to the channels is 3-4% and abstraction is about 1% of the losses. The lateral groundwater fluxes constitute about 2% of both the inflows and outflows.

Table 15 Subsurface water balance for three climate scenarios

Drier Current Wetter

Flux In (GL/yr)

Out (GL/yr)

In (GL/yr)

Out (GL/yr)

In (GL/yr)

Out (GL/yr)

Surface infiltration (includes rainfall and irrigation)

49 55 62

Groundwater discharge to surface

0.06 0.09 0.13

Evaporation (total) 45 51 58

Transpiration (vegetation) 21 23 25

Evaporation (soil) 24 28 33

Channel leakage 0.8 0.8 0.8


1.7 2 3

Abstraction# 0.5 0.5 0.5

Lateral subsurface flows 1.2 0.9 1.3 1.0 1.3 1.0

Total* 51 48 57 55 64 62

* The discrepancy occurs due to the variation in soil moisture content, decrease in the groundwater storage (particularly as initial conditions were taken at the end of a very low rainfall year) # The difference in abstraction between the subsurface balance and the overall balance is that the overall balance does not include the abstraction that is re-applied within the catchment. In the subsurface balance, this amount is included in the surface infiltration

8.4. Wetlands The variation in wetland hydraulic cycle was studied in four wetlands: Forrestdale Lake, Armadale Road wetland, Harrisdale Swamp, and Lake Balannup. Figure 52 shows Inundation Duration Curves (IDCs), similar to Flow Duration Curves frequently used in hydrology, for these wetlands.


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Figure 52 Inundation duration curves for four wetlands for three climate scenarios


The IDC for Forrestdale Lake shows that the water level in the lake is very sensitive to rainfall. Over the ten-year simulation period, the lake level is essentially dry 32% (116 days), 26% (94 days) and 21% (76 days) of the time for the drier, current and wetter scenarios respectively. There are greater differences at the higher lake levels. For the drier climate scenario the lake rarely exceeds 0.7 m depth, whereas it exceeds this level for 10% and 20% of the simulation time for the current and wetter climates. This suggests a significant level of lake water balance dependence on rainfall.

The IDC for the Armadale Road wetland shows very little difference between the various climates. This is because the water level in the wetland is controlled by the upstream and downstream flows. The wetland becomes disconnected from the downstream flows when the level drops below 0.35 m depth (21.84 mAHD).

The effects of different rainfall regimes on the IDC for Harrisdale Swamp are shown in Figure 52. The wetland is dry for 37% (135 days), 28% (102 days), and 17% (62 days) of the year for the drier, current and wetter rainfall sequences respectively. The maximum level in the swamp is controlled by a drain providing an outlet from the wetland. As a result IDC for the high water levels show little dependency on climate variability.

The Lake Balannup IDC shows distinct differences between the periods of inundation for the different rainfall regimes. There is about a month increase in the inundation period between the current and wetter scenarios for any given level, and a months decrease in the inundation period between the drier and current rainfalls.

Apart from the wetland on Wungong Brook close to Armadale Rd, which is mainly a flow-through wetland, Forrestdale Lake, Harrisdale Swamp and Lake Balannup exhibit water levels that are strongly dependent on climate.

Though both Harrisdale Swamp and Lake Balannup are shallower than Forrestdale Lake their IDCs indicate a longer duration of inundation, which suggests that they are more dependent on the groundwater inflow.

8.5. Channel flows The fractional contributions to the total annual flow in channels of the flow from the Darling Scarp, overland flow into the channels and groundwater discharge into the channels is presented in Figure 53 in the form of a Piper diagram. This figure includes data from the three climate scenarios. The groundwater contribution varies between 20 and 40% of the annual total inflow, the Darling Scarp contribution varies between 40 and 75%, and the overland flow varies between 5 and 30%. In general the larger the total flux, indicated by a larger symbol size, the smaller the proportion of the Darling Scarp flow into the model, and the bigger the contributions of groundwater and overland flow.

However, there is a distinct change in the trend for two annual results from the current and wetter climates. This is likely to indicate that there is a threshold value above which the overland flow contribution increases, which is likely to be related to a high inundation rate in the catchment and greater runoff.


Figure 53 Annual inflow to channels from various sources as fraction of total inflow (symbol size is scaled to total annual inflow)

The flow in the channel system is highly seasonal. Figure 54 shows the average monthly contribution from the two major drainage networks in the catchment, the Wungong Brook system at Armadale Road and the Forrestdale Main Drain system at the confluence with the Southern River (see Figure 28). The difference in the shape of the contributions from the two sub-catchments in Figure 54 is due to the different patterns of rainfall-runoff relationship in those catchments. The Wungong Brook catchment receives flow from the Darling Scarp and urban areas along the base of the scarp. These sources respond quickly to rainfall and thus flow in Wungong Brook increases rapidly with the start of the wet season.

The Forrestdale Main Drain, on the other hand, does not receive any external inflows. The channel network consists of a large proportion of shallow channels, the catchment consists mainly of sands with high infiltration rates and is characterised by a low regional gradient. This means that the rainfall generally infiltrates the soil, raising the watertable. The channel flow from this catchment consists of local runoff to the channels early in the wet season (‘storage recovery stage’ in Section 2.4), followed by an increase in flow when the watertable reaches the base of the channel and groundwater starts to discharge into the channel (‘storage depletion stage’ in Section 2.4). The discharge from this catchment is dominated by groundwater discharge to the channels.

The simulated average monthly groundwater contribution to the channel network under three climate scenarios is shown for three sub-catchments in Figure 55 (FMD, WB-SR above the confluence with FMD and Southern River downstream of confluence with FMD). All catchments show that the maximum groundwater discharge occurs in the winter and the discharge decreases from September onwards. In the FMD sub-catchment the discharge is negligible during the summer months, and the overall decline in groundwater discharge is the most rapid.


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Figure 54 Simulated average monthly fluxes at three locations for three climate scenarios


In the Southern River downstream of the confluence with the FMD, the minimum monthly groundwater discharge for each climate scenario is around 15% of the maximum groundwater discharge. The differences in groundwater discharge between the various rainfall regimes are greatest in the wetter winter months, and become relatively small in the summer months. This indicates that the major impacts of climate change on the groundwater contribution to the channels will occur during the winter months.

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Figure 55 Simulated average monthly groundwater discharge for three climate scenarios to: (a) Forrestdale Main Drain (FMD); (b) Southern River upstream of confluence with FMD; and (c) Southern River downstream of confluence with FMD


The areas of channels that remain flowing during dry spells in the summer are those that receive flow from the Darling Scarp, or those in lower parts of the catchment that receive groundwater discharge. Figure 56 shows the average fraction of channel segments in the entire catchment where the channel flow is zero for each month over the ten years of the simulation. It shows that over 80% of the channel segments indicate zero flow during the summer, and between 5 and 30% of the segments, depending on the rainfall, are not flowing during winter. These non-flowing segments are generally in the upper reaches of the Forrestdale Main Drain network. However, whilst there may not be any flow in the segments, there may be standing water in the channels.

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8.6. Groundwater levels The model results indicate that the changes in average watertable levels between rainfall scenarios are smaller than the inter-annual variation in watertable levels. However, changes to the maximum and minimum watertable levels indicate possible changes in the watertable range under different climates. Changes in the maximum watertable levels influence the extent of the areas affected by inundation and the area of shallow watertable occurrence, and also have an effect on the water levels in the groundwater-fed wetlands. Changes in the minimum watertable level influence the baseflow (groundwater discharge) to the channel system during the summer.

The difference in average annual minimum water table levels between current and drier climate are presented in Figure 57, while Figure 58 illustrates the difference between average maximum annual water levels in the wetter and current climate regimes. The difference between minimum levels is in the range of 0.2 to 0.5 m for the majority of the Southern River catchment, with greater changes occurring in the area adjacent to the Darling Scarp and smaller changes in the vicinity of channels and also wetlands.

The changes in the average maximum groundwater levels are small (<0.2m) in the western part of the Southern River catchment. However, in the area adjacent to the Darling Scarp there are increases in the average maximum watertable of greater than 0.5 m. There are also increases in the average maximum watertable of up to 0.5 m in the centre part of the catchment between Forrestdale Main Drain and the Southern River. These large increases occur in areas where there is low transmissivity in the superficial aquifer, either through the presence of the Guildford Formation (see Figure 3) or a thick aquitard layer (see centre of cross-section in Figure 16).


The lower average minimum watertables in a drier climate are likely to decrease summer groundwater discharge to the channel system. As the discharge is small at the end of summer when the minimum groundwater levels occur (Section 8.5) the effect on the annual flow from the catchment is likely to be small. Similarly, a wetter climate will increase the watertable levels in the catchment, but in the majority of the low-lying area in the centre of the catchment, the changes in maximum watertable level are small due to the high transmissivity and storage of the system.

Figure 57 Differences (in m) between simulated annual average minimum watertables for current and drier climates (average minimum watertable current climate - average minimum watertable drier climate)


Figure 58 Differences (in m) between simulated average maximum watertable levels for wetter and current climates (average maximum watertable wetter climate - average maximum watertable current climate)

Figure 59 shows time series of hydraulic heads at selected locations in the catchment as shown in Figure 8. Along the channel, in the upper and middle reaches where the channel flow is intermittent, the minimum annual watertable level may be lower following low rainfall years, but leakage from the channel refills the aquifer in the following winter (Figure 59(a)).

Close to the outlet of the catchment at Observation Point C, the channel receives groundwater discharge for the majority of the year as the minimum watertable levels are above the invert level of the channel (Figure 59(b)).

In areas remote from the channel system, minimum watertable level is constrained by the root depth and the extinction depth for the evaporation, whilst the maximum level is constrained by the ground surface topography (Figure 59(c)).

The change in climate has a minimal long-term effect on the average watertable level in the catchment. However, there is significant inter-annual variability. Lower minimum watertable levels occur after low-rainfall years, but recover after average or high-rainfall years. Similarly, high watertable levels occur during high-rainfall years but in the following dry season, the watertable level variation is controlled by evaporation. This indicates that there are unlikely to be long-term trends in groundwater levels, as a year with average rainfall is likely to refill the aquifer and surface topography and dry summers ensure there are no upward trends for watertables.

The hydraulic characteristics of sandy deposits allow for greater potential recharge. When the water table is lowered by groundwater abstraction, the recharge increases (previously ‘rejected’ recharge or surface inundation). This is why there are only small groundwater level changes between the climate scenarios.


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Figure 59 Simulated groundwater levels over a ten-year period at three points for three climate scenarios; the locations of the points are shown in Figure 8


8.7. Groundwater recharge The distribution of changes in the average annual recharge between the current and the wet climate scenarios is presented in Figure 60. This average annual recharge to the watertable has fallen by 10-50 mm/annum in the eastern part of the catchment and between 20-100 mm/annum in the western part of the catchment. There are also some locations where the recharge has increased. These occurs in areas where the surface is inundated for part of the year under the current climate, with the period of inundation declining in the drier climate scenario due to lower water tables in the vicinity causing greater infiltration. Figure 61 shows the changes in recharge between the wetter climate scenario and the current climate scenario. The increase in recharge was greatest in the western part of the catchment being in the range 50-100 mm, whilst in the eastern part of the catchment the increase in recharge is between 20 and 50 mm/annum. The areas where the recharge decreased with the increase in average rainfall are similar to those that occur with the change between current and drier climates.

Figure 60 Difference in simulated annual average recharge (in mm) between the current and drier climates


Figure 61 Difference in simulated annual average recharge (in mm) between the wetter and current climates

The effects, within a ten-year timeframe, of wetter and drier climates compared to the current climate are as follows:

the modelled rainfall variability leads to major changes in overland flow, which decreased by 30% and increased by 40% for the drier and wetter climate scenarios respectively

groundwater discharge to the channel system decreased by 15% for the drier climate scenario and increased by 50% for the wetter climates scenario

the changes in subsurface storage for all climate scenarios over the ten-year period were similar indicating that although there are changes in the magnitude of the fluxes into and out of the subsurface, the subsurface storage is relatively stable

The implications of these results are that the major effects of a changing climate within the Southern River catchment are likely to be more significant on the surface water flow and inundation, including those in wetlands. The groundwater regime appears to be more resilient to climate variation within the modelled timeframe and adopted climate variation.


9. SUMMARY AND CONCLUSIONS

The undertaken analysis of the water balance of the Southern River catchment was one of the activities within the Swan Futures program (CSIRO Water for a Healthy Country Flagship) and the project ‘Investigation of Techniques to Better Manage Western Australia’s Non-Potable Water Resources’ (the Western Australian Water Foundation), aiming to:

provide baseline data for evaluation of future changes in the catchment water balance

identify the key hydrological and hydrogeological processes in the catchment

quantify the contributions of surface water and groundwater to the channels and wetlands in the catchment

investigate the effect of climate change on the groundwater, channel flows and wetlands in the catchment.

The project created a calibrated and validated MODHMS model of the catchment that allows simulation of the hydrological and hydrogeological processes. The model calibration and validation was based on monitoring data, both historic and generated within the project.

The results from the undertaken analysis fall into three categories, which are summarised individually below.

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9.1. Identified characteristics of the hydrological and hydrogeological processes in the catchment

Overall model results indicate that the MODHMS model of the Southern River catchment successfully reproduces the main governing processes controlling the catchment water regime. These processes include

fast watertable recovery during recharge period

timing and rates of watertable drawdown during summer

monthly and daily channel flow at catchment outlet, at times with moderate rainfall

water level dynamics in lakes and wetlands

summer baseflow to the channel system.

The major processes governing the hydrological and hydrogeological conditions in the catchment are highly dependent on catchment soil types, catchment relief and seasonality in rainfall. These lead to

The maximum infiltration rates, at 72% of rainfall, occur in early winter. Infiltration then declines over the remainder of winter to 65% of the rainfall as more of the catchment becomes inundated.

Watertable recharge rates, as a fraction of the rainfall, increase during the wet winters, from 17% of rainfall in May and 27% in June to 36-38% during July-August. This fraction remains high for spring and early summer even as monthly rainfall decreases and evaporation increases due to continuing infiltration from inundated areas and delayed recharge in areas of deep watertables.

Due to the occurrence of a shallow groundwater table, evaporative loss from the subsurface comprises around 93% of the infiltration. This means that the vertical fluxes in shallow groundwater are greater than lateral fluxes.

During the winter about 60% of evaporation occurs from the canopy, whilst during the summer, transpiration from vegetation accounts for around 60% of the total evaporation. High annual transpiration occurs in the treed areas in the eastern part of the catchment with some isolated vegetated areas in the west also having high


transpiration. High annual canopy evaporation also occurs in the treed areas as well as on the irrigated grassland and cleared areas. The lowest canopy evaporation occurs on the permanent water bodies such as Forrestdale Lake and in the urban areas. Evaporation from the surface and soil, including from the watertable, peaks in early spring, with a second smaller maximum occurring in late autumn after early rainfall but before evaporation rates have reached their winter minimum.

Limited connectivity between sub-catchments reduces runoff because most water ponds on the flat surface without flowing to drainage lines. The flatter, sandy western part of the catchment provides a delayed contribution of flow to the catchment outlet compared to the higher-gradient clayey-sand eastern part of the catchment. This is due to the high infiltration rates on the sandy soils reducing the overland flow to a negligible amount, with the channel flow starting once the groundwater level has risen above the bottom of the channels.

The environmental flow component considered within the project included the groundwater contribution to the Southern River baseflow and also the inflow to a number of the wetlands.

Groundwater discharge to the Southern River includes

o deep groundwater discharge, which occurs to the northern part of the Southern River where the channel is deeply incised. This part, however, only represents 35% of the total annual baseflow.

o shallow groundwater contribution to the channel system upstream from this river reach, which contributes 65% annual baseflow. The contribution varies seasonally from 0% during summer months to 80% during winter months.

The hydrological regime of a wetland is dependent on groundwater systems (to varying degrees), for example

o Forrestdale Lake receives very little groundwater inflow due to the low conductivity of the sediments in the lake, with rainfall contributing the majority of the lake’s volume

o Armadale Road wetland is dependent on flow along Wungong Brook

o Harrisdale Swamp and Lake Balannup are both dependent on rainfall and groundwater discharge during winter, and become isolated from the groundwater during the summer

9.2. Effect of climate variability on the hydrological and hydrogeological processes in the catchment

The effects of changing climate were investigated using the calibrated model and a variable rainfall sequence with 10% less than current rainfall to represent a drier climate and 10% more than current rainfall to represent a wetter climate. The modelling results demonstrate that

Inter-annual variation in watertable dynamics, including annual maximum and annual minimum water levels, is mainly influenced by the rainfall distribution within the annual hydrological cycle rather than long-term rainfall patterns. The groundwater regime appears to be resilient to climate variation within the modelled timeframe and adopted climate variation.

The effects of climate change on wetlands depends on the wetland type; these include wetlands located along the major surface water channels (in-stream), perched wetlands and groundwater-dependent wetlands:

o in-stream wetlands such as Wungong Brook at Armadale Road have hydrological regimes that are strongly dependent on the channel fluxes and thus the rainfall distribution within the annual hydrological cycle. The water


level variation in the wetland is mainly influenced by winter fluxes, but the wetland has enough storage to maintain some water even in the driest year

o perched wetlands such as Forrestdale Lake are predominantly surface water fed and the level in the lake is strongly dependent on rainfall. A 10% increase in rainfall increased the average level in the lake by 8 cm, decreasing the average time the lake was dry by about 21 days, and increasing the average time the lake depth was greater than 50 cm by 42 days. A decrease of 10% in the rainfall decreased the average lake level by 10 cm, increased the time the lake was dry by an average of 25 days a year and decreased the time the lake level exceeded 50 cm by 48 days

o groundwater-dependent wetlands such as Lake Balannup and Harrisdale Swamp are also strongly dependent on climate. The drier climate results in reduced levels and areas for the wetlands. A 10% increase in the rainfall increased the time Lake Balannup was inundated above a level of 21 cm by 31 days, whilst a 10% decrease in rainfall reduced the time of inundation for the same level by 62 days. Similarly, in Harrisdale Swamp, an increase of 10% in the rainfall increased the time of inundation above a level of 22 cm by 34 days, whilst a 10% decrease in rainfall reduced the inundation time over 22 cm by 28 days.

The effect of climate change on the flow at the Southern River outlet is

o greatest during the winter (15% less for a 10% drier climate and 11% more for a 10% wetter climate), where the change in flow is due to the change in rainfall

o limited during the baseflow at the end of summer (20% less for 10% drier climate, whilst the flow remained the same for a 10% wetter climate).

9.3. Specifics in process-based coupled surface water-groundwater modelling

Due to the high computational overheads of the model a multi-faceted calibration and validation procedure was adopted:

the hydraulic conductivity of the aquifers was calibrated using three sub-models in areas that were identified as representing different hydrogeological conditions in the catchment. These areas were the western part of the Bassendean Sands, the eastern part of the Guildford Formation and the central part of the catchment. The hydraulic conductivities from these sub-models were interpolated for use in the catchment model

the sensitivity of the channel flow at the catchment outlet to discretisation of the rainfall data was analysed. It was found that the smaller the interval for the discretisation of the rainfall, the more immediate the response to the rainfall event and the greater the peak flow. However, the average channel flow over several days showed negligible differences between the different rainfall interval discretisations

clarification of input data accuracy and associated errors explicitly recognised that there is inherent uncertainty in the observed data.

Additional constraints to the model were:

the observed channel flow at the outlet of the catchment

recharge was not a calibration parameter – the model partitioned the rainfall into canopy interception, infiltration and overland storage and flow

the response of the watertable within a wet season


the hydrological cycle in Forrestdale Lake.

The adaptive time-stepping algorithm used in the MODHMS model results in a large number of internal model time steps within each month (stress period) of the simulation. To ensure all fluxes in the model were included in the analysis, the fluxes at every step were saved. Keeping all such fluxes would require terabytes of storage. Instead, the model was halted after each stress period, the fluxes were processed and saved as average values over the period, and the model was restarted.

The subsurface component of the MODHMS model does not explicitly calculate the recharge to the watertable. To include the recharge in the analysis, a program to post-process the results of the simulation was developed. The program analysed the heads for each time step of the simulation to quantify the recharge and saved the results on a daily basis for later analysis.

Further development of the model will investigate water quality and the pathways of solutes in the system to the waterways. The model will also be applied for predicting the impact of catchment urbanisation on river discharge and water quality.


REFERENCES Arcement, G.J., Jr. and Schneider, V.R. (1989). Guide for selecting Manning's

roughness coefficients for natural channels and flood plains: U.S. Geological Survey Water-Supply Paper 2339, metric edition, 67pp.

Barron, O., Barr A. and Pollock D. (2009). Effect of urban development on water balance in the Southern River catchment. CSIRO: Water for a Healthy Country Flagship Report.

Barron, O., Pollock, D., Donn, M., Johnstone, C., Lambert, P. and Higginson, S. (2007). Groundwater monitoring in the Southern River Catchment. CSIRO: Water for a Healthy Country Flagship Interim Report.

Barron, O., Pollock, D. and Dawes, W. (in prep.). Evaluation of catchment connectivity and storm runoff in flat terrain subject to urbanisation.

Davidson, W.A. (1995). Hydrogeology and groundwater resources of the Perth Region, Western Australia. Western Australia Geological Survey, Bulletin 142, 257pp.

Davidson, W.A. and Yu, X. (2006). Perth Region Aquifer Modelling System – PRAMS, Hydrogeology and groundwater modelling. Western Australia Department of Water, Hydrogeology Report No. 202.

DHI Water & Environment. (2007). MIKE SHE User Manual: Volume 2: Reference Guide. DHI Software.

ENV Australia Pty Ltd. (2007). Wungong Urban Water Redevelopment Area: Foreshore Management Plan. Report prepared for Armadale Redevelopment Authority, October 2007, 102pp.

Feddes, R.A., Kowalik, P.J. and Zaradny, H. (1978). Simulation of field water use and crop yield. Centre for Agricultural Publishing and Documentation, Wageningen, The Netherlands.

Freeze, R.A. and Harlan, R.L. (1969). Blueprint for a physically-based digitally-simulated hydrologic response model. Journal of Hydrology, 9: 237–258.

Gnangara Sustainability Strategy. (2007). E-Bulletin 02, December 2007. http://portal.water.wa.gov.au/portal/page/portal/gss/Content/Latest%20News/GSS_EBulletin02.pdf

Gregory, K.J. and Walling, D.E. (1973). Drainage Basin Form and Process: A geomorphological approach. Edward Arnold (Publishers) Ltd., London.

HydroGeoLogic, Inc. (2006). MODHMS: A comprehensive MODFLOW-based hydrologic modeling system, Version 3.0. HydroGeoLogic Incorporated, Herndon, VA.

Jeffrey, S.J., Carter, J.O., Moodie, K.M. and Beswick, A.R. (2001). Using spatial interpolation to construct a comprehensive archive of Australian climate data. Environmental Modelling and Software, 16/4: 309–330.

Jones, J.P., Sudicky, E.A. and McLaren, R.G. (2008). Application of a fully-integrated surface-subsurface flow model at the watershed-scale: a case study. Water Resources Research, 44, W03407, doi:10.1029/2006WR005603.

McDonald M.G. and Harbaugh, A.W. (1988). A modular three-dimensional finite-difference groundwater flow model. US geological survey techniques of water-resources investigations, Book 6; 586 pp [chapter A1].


Maneta, M., Schnabel, S. and Jetten, V. (2008). Continuous spatially distributed simulation of surface and subsurface hydrological processes in a small semiarid catchment. Hydrological Processes, 22(13): 2196–2214.

Markstrom, S.L., Niswonger, R.G., Regan, R.S., Prudic, D.E. and Barlow, P.M. (2008). GSFLOW—Coupled ground-water and surface-water flow model based on the integration of the Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW-2005): U.S. Geological Survey Techniques and Methods 6-D1, 240pp.

Murray-Darling Basin Commission (2001). Groundwater Flow Modelling Guideline, Aquaterra Consulting Pty Ltd., 133pp.

Nash, J.E. and Sutcliffe J.V. (1970). River flow forecasting through conceptual models part I – a discussion of principles. Journal of Hydrology, 10 (3): 282–290.

Or, D. and Wraith J.M. (2000). ‘Soil water content and water potential relationships’, in Handbook of Soil Science, Sumner M.E. (ed), CRC Press, USA, ppA53–A85.

Panday, S. and Huyakorn, P.S. (2004). A fully coupled physically-based spatially-distributed model for evaluation of surface/subsurface flow. Advances in Water Resources, 27: 361–382.

Pollock, D.W. and Barron, O.V. (in prep.). Exploring raw lithology logs to aid hydrogeological assessment.

Rockwater Pty Ltd. (2005). Southern River Development Area – groundwater modelling to assess the effects of climatic variations, and planned development. Report for Water Corporation, November 2005.

Schaap, M.G. (2000). Rosetta Version 1.2, U.S. Salinity Laboratory ARS-USDA http://www.ars.usda.gov/Services/docs.htm?docid=8953

Shah, N., Nachebe, M. and Ross, M. (2007). Extinction depth and evapotranspiration from ground water under selected land covers, Ground Water, 45(3), 329-338.

Therrien, R., McLaren, R.G., Sudicky, E.A. and Panday S.M. (2005). HydroGeoSphere: A three-dimensional numerical model describing fully-integrated subsurface and surface flow and solute transport. Groundwater Simulations Group, University of Waterloo, Waterloo, Ontario, Canada, 322pp.

Thoms, R.B. (2003). Simulating fully coupled overland and variably saturated subsurface flow using MODFLOW. Oregon Health and Science University, Department of Environmental Science and Engineering, Portland, Oregon, Master’s Thesis, 141pp.

VanderKwaak, J.E. (1999). Numerical simulation of flow and chemical transport in integrated surface-subsurface hydrologic systems. University. of Waterloo, Waterloo, Ontario, Canada, Ph.D. thesis, 217pp.

Watermark Numerical Computing. (2003). PEST Model-Independent Parameter Estimation: User Manual: 5th Edition, 336pp.

Werner, A.D. and Gallagher, M.R. (2006). Characterisation of sea-water intrusion in the Pioneer Valley, Australia using hydrochemistry and three-dimensional numerical modelling. Hydrogeology Journal, 14: 1452–1469.

APPENDIX A – SUMMARY OF MODHMS MODEL

The brief summary provided here is based on Panday and Huyakorn (2004) and MODHMS manual (HydroGeoLogic, 2006).

A.1. Introduction MODHMS is a fully-coupled, physically-based, spatially-distributed model of conjunctive surface/subsurface flow. The model is designed to take into account all key components of the hydrologic cycle, shown in Figure A 1. For each time step, the model solves surface and groundwater equations and provides complete water balance. Referring to Figure A 1, these water budgets are presented in Table A 1.

Figure A 1 Regional hydrologic cycle (HydroGeoLogic, 2006)

The model includes a fully three-dimensional solution for saturated-unsaturated flow in the subsurface coupled with two- and one-dimensional solutions for surface-water flow; the two-dimensional solutions represent overland runoff, and the one-dimensional solutions represent flow within and through surface-water features such as rivers, canals, pipes, lakes and ponds that are of a much smaller dimension than that of the simulation.

Other scale-dependent features of the model include depression storage and obstruction storage exclusion which affect surface-water flow and storage and surface/subsurface interactions in a physically-based fashion. The model also incorporates control features for flow through rivers and streams to include the effects of bridges, culverts, manholes, weirs, and pump-stations in a simulation.


Table A 1 Simulated water budget equations (HydroGeoLogic, 2006)

Reference evapotranspiration estimates may be input from pan data or calculated from climatic and crop information for a fully mechanistic approach to hydrologic-cycle analysis. Several models for plant transpiration based on crop/land-cover and soil moisture information are provided in the model which, along with various evaporation models that are applicable to a variety of situations, allow for realistic estimates of long-term water budget evaluations.

Complexity of simulation may be incorporated on an as needed basis, even within a single model. Therefore, each part of the domain for a given simulation can be specified with its own degree of complexity, to allow for a wide range of applicability including flood forecasting, water resource assessment, watershed hydrologic analysis, and flood plain/fluvial hydraulic analysis.

The governing equations are solved using current state-of-the-art techniques designed for highly non-linear complex situations. The model can simulate fully dry regions, as well as regions with large surface, subsurface, or interaction fluxes in an efficient and robust manner. The fully coupled solution technique is supplemented with linked/iteratively coupled approaches to provide a framework for comparison between these solution techniques.

The domains are fully coupled, or may be linked in an iterative or time-lagged manner.

A.2. Governing processes and equations The equations governing flow of water for hydrologic-cycle modelling are presented below. Processes describing water movement in the subsurface and surface flow domains are first discussed, followed by interaction processes and processes related to evapotranspiration, which span all domains.


A.2.1 Subsurface flow

A rigorous approach satisfying flow continuity in three dimensions in both the saturated and unsaturated zones of the subsurface is required for robust and reliable solutions for subsurface flow under a variety of general flow conditions. This is provided by the mixed form of the Richard’s equation for variably saturated subsurface flow expressed as:

t

hSS

t

SqqW

z

hkK

zy

hkK

yx

hkK

xG

Sww

gcgG

rwzzG

rwyyG

rwxx

0 (1)

subject to evapotranspiration and various other boundary conditions, where:

x, y, and z are Cartesian coordinates (L)

Kxx, Kyy , and Kzz are the principal components of hydraulic conductivity along the x, y, and z axes, respectively (LT-1)

krw is the relative permeability which is a function of water saturation as provided by the relative permeability curve

hG is the hydraulic head of the subsurface flow system (L)

W is a volumetric flux per unit volume of the subsurface domain and represents sources and/or sinks of water (T-1)

qg0 is the flux per unit volume of subsurface from the two-dimensional overland flow domain (T-1)

qgc is the flux per unit volume of subsurface from the one-dimensional channel or surface water feature domain (T-1)

is the porosity

Sw is the degree of water saturation and is determined by the moisture retention curve as a function of the pressure head

Ss is the specific storage of the porous material (L-1)

t is time (T).

Dimensions of this equation are volumetric flux per unit volume (T-1).

A.2.3. Overland flow

Overland flow/runoff is characterised by the two-dimensional diffusion wave approximation to the St.Venant equations governing shallow-water flow:

0)()(

ocogo

yo

xo dqdq

y

hdk

yx

hdk

xt

h (2)

subject to precipitation, evaporation and various other boundary conditions, where:

ho=zo+d is the water surface elevation (L)

d is the depth of flow (L)

zo is the bed (land surface) elevation (L)

qog is the flux per unit volume of overland flow domain from the subsurface (T-1), where qgo=-qog

qoc is the flux per unit volume of overland flow domain from the channel (T-1).

The dimensions of this equation are volumetric flux per unit area (vertically integrated over the depth of the overland flow domain) (LT-1).


A.2.4 Channel flow and surface-water features

All features of the land surface can conceptually be represented by the overland flow surface, including surface-water features such as ponds, lakes, reservoirs, rivers, streams and canals, by use of a sufficiently small discretisation. However, there is a practical issue of scale in regional simulations, whereby a minimum limit must be set on the areal grid-block size of the overland flow surface as well as of the subsurface layers. Therefore, to accurately simulate surface-water features which are smaller than the associated grid-block dimensions, and to convey water through canals or conveyance structures (whose widths are much finer than the grid-block scale or whose orientation may not conform well with the finite difference structure), a surface-water features/channel flow layer is added to the surficial model layer. This layer is characterised by a network of one-dimensional channels/features which communicate water within the network, as well as between it and the overland flow and subsurface domains.

Flow through a network of rivers and channels is characterised by the one-dimensional diffusion wave approximation to the St. Venant equations:

0)(

cocgc

lC AqAq

l

hk

lt

hB (3)

subject to precipitation, evaporation and various other boundary conditions, where:

B is the top width (L)

hc=zc+d is the water surface elevation in the channel (L)

d is the flow depth in the channel (L)

zC is the channel bed elevation (L)

l is the length along the direction of flow (L)

kl‘ is the conductance term along the length of the channel (L3T-1) resulting from manipulation of the one-dimensional St. Venant equations

qcg is the flux per unit volume of channel flow domain from the subsurface (T-1), where qcg=-qgc

qco is the flux per unit volume of channel flow domain from the overland flow domain (T-1), where qco = -qoc.

Dimensions of this equation are volumetric flux per unit length of channel (L2/T), integrated over the channel’s cross-sectional area of flow, A(L2).

A.2.5. Treatment of depression storage and storage exclusion

To accommodate urban or agricultural settings at large scales of analysis, the above equations for the two-dimensional overland flow (runoff) domain and the one-dimensional channel (surface-water feature) flow domain are modified to include terms for depression storage and for obstruction storage exclusion.

A.2.6. Treatment of interaction terms among domains

Flows between the subsurface, overland, and channel domains of the system are represented by the unit interactive fluxes presented in equations. (1), (2) and (3). The overland/subsurface interaction term qgo is the unit flux across the ground surface which, for grid cell of dimensions x and y, is computed as

gooGgorgoGgo QhhKyxkVq )()( (4)

where:

Qgo (L3T-1) is the flux across the total area of the interface, from the overland flow domain to the subsurface (negative for flow out of the subsurface)


VG is the multiplier of subsurface elementary volume, and accounts for qgo being a flux per unit volume of subsurface

Kgo is the leakance across the ground surface to the modelled subsurface.

The channel/subsurface interaction term qgc is expressed as:

gccGgcupscrgcGgc QhhKPLkVq )()( (5)

where:

Qgc (L3T-1) is the flux across the total area of the interface from the channel to the subsurface (negative for flow out of the subsurface)

LC is the channel segment length (L)

Kgc is the net leakance across the channel bed to the subsurface system

Pups is the upstream wetted perimeter for the different segment geometries.

A.2.7. Treatment of small surface-water bodies at large simulation scales

Conceptually, all surface-water bodies such as lakes, ponds, or wetlands may be treated by the two-dimensional overland flow equations by providing appropriate topography and bathymetry. However, this is not possible when the scale of the surface-water body is on the order of (or smaller than) its associated overland flow grid-block. Such features are effectively handled without over-discretisation of the overland flow surface by assigning the appropriate channel node as a surface water body. The surface-water body node thus has flow connections along the channel dimension as well as to its appropriate overland flow node and subsurface node.

A.2.8. Treatment of boundary conditions

Boundary conditions to the subsurface include:

prescribed head or flux conditions

well conditions

recharge

general head boundaries.

River or drain boundary conditions are also provided, if rigorous modelling of the river/channel system is not warranted. Surface-water boundary conditions also include:

prescribed head or flux conditions

zero-depth-gradient

critical depth conditions.

A general outflow boundary condition (probably occurring over a structure) in a channel may also be provided by user-defined, tabulated Q–h relations at a channel outflow boundary.

A.2.9. Treatment of interception and evapotranspiration

Interception and comprehensive evapotranspiration are simulated as mechanistic processes governed by plant and climatic conditions (except wind). Interception is the process involving retention of a certain amount of precipitation on the leaves, branches, and stems of vegetation or on buildings and structures in urban areas. The interception process is simulated by the bucket model, wherein precipitation in excess of interception storage and evaporation from interception reaches the ground surface. This is efficiently computed at the


start of every time step of simulation by keeping track of Sint, the interception storage (L), for each areal model location.

The interception storage varies between zero and Smaxint, the interception storage capacity

(L), which depends on the vegetation type and its stage of development. It is evaluated based upon the leaf area index and the canopy storage parameter.

Evapotranspiration is rigorously modelled as a combination of plant transpiration and evaporation, and affects nodes in both surface and subsurface flow domains. Transpiration from vegetation occurs within the root zone of the subsurface which may be above or below the water table and may involve several nodal layers.

Two models are provided for evaporation. The first model assumes that evaporation occurs if the reference evapotranspiration has not been removed by the above processes of canopy evaporation and plant transpiration, Tp. Therefore, evaporation from the soil surface and subsurface soil layers is estimated as follows:

IpcanPISI EDFTEEE * (6)

where:

Ep is the reference evapotranspiration

Ecan is the canopy evaporation

Tp is the plant transpiration

EDFI is the evaporation distribution function among a vertical set of nodes that includes the overland and subsurface flow domains.

The second model assumes that evaporation occurs along with transpiration, resulting from energy that penetrates the vegetation cover and is expressed as:

IcanPISI EDFLAIfEEE 1* 1 (7)

where I* is the wetness factor given by

(8)

where:

e1 is the moisture content at the end of the energy-limiting stage (above which full evaporation can occur)

e2 is the limiting moisture content below which evaporation is zero

LAI is the leaf area index.

A.3. Numerical solution

A.3.1. Domain discretisation

The system of equations governing moisture movement through the hydrologic cycle has been discussed above. The spatial domains include the subsurface domain, the overland flow domain, and the channel flow domain. The subsurface domain is discretised using a block-centred finite-difference scheme, based on the framework of the popular groundwater flow model, MODFLOW (McDonald and Harbaugh, 1988). The subsurface grid may be distorted vertically to reflect hydrostratigraphic boundaries and may be orthogonal-curvilinear in its areal dimensions to follow complex boundaries or flow-lines. The overland flow domain


grid areally mirrors the subsurface grid as shown in Figure A 2, with overland flow node elevations corresponding to the land surface elevation.

The channel flow domain is discretised using a finite volume concept with the channel network superimposed on the overland flow and subsurface domains. The channel regime discretisation is completely independent of the areal grids, and there are no constraints on discretising the channel network that arise from discretisation considerations given to the subsurface or overland flow regimes. Further, the channel segments may be discretised in any order due to the general nature of the equations and solution schemes.

The channel network consists of interconnected reaches, each of which is divided into sequentially numbered segments with nodes at their centres.

A.3.2. Coupling, time-stepping and other solution considerations

The discretised system is solved by a fully implicit time-marching procedure. Alternatively, the subsurface flow equations may be linked (in a time-lagged manner) or iteratively coupled to a simultaneous solution to the overland and channel flow equations.

Adaptive time-stepping is provided such that the time-step size grows (up to a maximum) when the number of iterations required for convergence is low (solution is easy); reduces when the number of iterations is high; and is considered optimal with no change when the number of iterations required for convergence to a previous time-step is medium as compared to the prescribed maximum number of iterations. For linked/iteratively coupled solutions, the surface-water domain solution is also adaptive in time, where its time-step size is governed by the ease with which the surface-water domain is solved. However, this time-step size cannot be larger than the current time-step size used for the subsurface domain, which is governed by the ease with which the subsurface equations are solved. Additionally, for iteratively coupled schemes, the time-step size used for the subsurface domain is also governed by the ease with which the iterative coupling procedure converges.

Figure A 2 Spatial discretisation of the subsurface and overland flow surface (Panday and Huyakorn, 2004)


A.4. References Panday, S. and Huyakorn, P.S. (2004). A fully coupled physically-based spatially-distributed

model for evaluation of surface/subsurface flow. Advances in Water Resources, 27 361-382.

HydroGeoLogic, Inc. (2006). MODHMS: A comprehensive MODFLOW-based hydrologic modeling system, Version 3.0. HydroGeoLogic Incorporated, Herndon, VA.

McDonald M.G. and Harbaugh A.W. (1988). A modular three-dimensional finite-difference groundwater flow model. US geological survey techniques of water-resources investigations, Book 6; 586 pp [chapter A1].

application of a coupled surface water-groundwater model ... · figure 47 simulated spatial...

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