surface water hydrology of the south esk river catchment

Water Assessment Branch

Department of Primary Industries and Water

Report compiled for Water Resources Division

Technical Report No. WA 07/02

June 2007

Surface Water Hydrologyof the

South Esk River Catchment

A report supporting the development of a water management plan for thecatchment

2

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in accordance with the Copyright Act 1968 of the Commonwealth Parliament, no part of this

report may, in any form or by any means, be reproduced, transmitted or used. This report

cannot be redistributed for any commercial purpose whatsoever, or distributed to a third

party for such purpose, without prior written permission being sought from the Department

of Primary Industries and Water, on behalf of the Crown in Right of the State of Tasmania.

Disclaimer:

Whilst DPIW has made every attempt to ensure the accuracy and reliability of the

information and data provided, it is the responsibility of the data user to make their own

decisions about the accuracy, currency, reliability and correctness of information provided.

The Department of Primary Industries and Water, its employees and agents, and the Crown

in the Right of the State of Tasmania do not accept any liability for any damage caused by, or

economic loss arising from, reliance on this information.

Preferred Citation:

DPIW (2007). Surface Water Hydrology of the South Esk River Catchment. Technical Report

No. WA 07/02. Water Assessment Branch, Department of Primary Industries and Water,

Hobart.

Cover Page Images: South Esk River Catchment.

The Department of Primary Industries and Water

The Department of Primary Industries and Water provides leadership in the sustainable

management and development of Tasmania’s resources. The Mission of the Department is to

advance Tasmania’s prosperity through the sustainable development of our natural resources

and the conservation of our natural and cultural heritage for the future.

The Water Resources Division provides a focus for water management and water

development in Tasmania through a diverse range of functions including the design of policy

and regulatory frameworks to ensure sustainable use of the surface water and groundwater

resources; monitoring, assessment and reporting on the condition of the State’s freshwater

resources; facilitation of infrastructure development projects to ensure the efficient and

sustainable supply of water; and implementation of the Water Management Act 1999, relatedlegislation and the State Water Development Plan.

3

Executive Summary

The catchment of the South Esk River upstream of Longford covers an area of approximately

3,350 km2. The catchment experiences widely varying climatic conditions with rainfall ranging

from 500 mm in the low lying areas to up to 1,500 mm in the highlands. With an annual

average rainfall of 835 mm, the total water input into the South Esk River catchment is

approximately 3,000 GL/year. The total catchment annual yield at Longford is around 900

GL/year and therefore comprises about 43% of the total annual water input. This simple water

budget indicates that the vast majority (57%) of total water input into the catchment is either

evaporated, transpired or moves into to the local and regional groundwater system.

The majority of the catchment rainfall and runoff occurs in the northern and eastern headwaters

and as a result maximum runoff is converted to river flows in these regions. Low rainfall and

higher evaporation in the southeast of the catchment has contributed to low conversion of the

runoff into the cumulative yields in the Lower South Esk regions. Considerable climate

variability within the catchment results in high spatial variability of runoff yield. The Upper Esk

subregion and Nile River catchment are identified as the most productive areas in relation to

converting rainfall into runoff and hence yield as river flow.

Average floods (1 in 2 year floods) in the lower reaches of the South Esk River catchment peak

at around 400 m3s

-1, while floods range from 100 to 150 m

3s

-1 in the major tributaries. At Perth

the observed streamflow recession after an average flood event is around 72 m3s

-1 per day and it

takes roughly five days for the peak flow to recede to an average river flow of 24 m3s

-1. Low

flow probability analysis indicated that within a given year the likelihood of occurrence of

average flows ≤ 1.0 m3s

-1 over a five day consecutive period is around 60%.

The current annual total allocated water in the catchment is around 44,415 ML. The subregion

breakdown of water usage shows that the bulk of the allocation is in the Lower Esk subregion.

The consumptive water usage for the entire catchment upstream of Longford is about 5% of the

total annual yield and this is reflected in the low hydrological disturbance index for the

catchment. Low water usage in the catchment is also a product of the seasonal flood extraction

rules that protect power generation at the Trevallyn power station. Strategies to balance the

consumptive and environmental uses of the water resources from the catchment must therefore

take into account the differences in flows, the variability of catchment yields and flood

extraction rules for the catchment.

Currently, surface water resources are allocated solely on surface water information, with only

cursory regard for groundwater influences. While it is recognised that river flow in this

catchment suggest a predominantly groundwater-driven system, there is little understanding of

groundwater in the catchment, and water allocation is presently based on the premise that water

is being allocated from surface water only, when in fact it may be drawing indirectly on the

groundwater resource. Conversely, the level of groundwater extraction is largely unknown and

may be having some impact on surface water. With increasing demand for better management

of the State’s water resources, greater emphasis must therefore be put into implementing an

holistic approach to understanding surface and groundwater connectivity within this and other

catchments in Tasmania.

4

Table of Contents

Executive Summary................................................................................................................................... 3

1.0 Introduction.......................................................................................................................................... 5

2.0 Catchment Description........................................................................................................................ 5

3.0 Catchment Hydrology ......................................................................................................................... 6

3.1 RAINFAL, RUNOFF AND EVAPORATION................................................................................................. 6

3.2 GAUGED FLOW MONITORING AND CHARACTERISTICS .......................................................................... 9

3.3 FLOOD FREQUENCIES ....................................................................................................................... 10

3.4 LOW FLOWS...................................................................................................................................... 12

3.5 FLOW RECESSION ............................................................................................................................. 13

3.6 WET AND DRY SEASON COMPARISON ................................................................................................. 15

3.7 HYDROLOGICAL CHARACTER OF THE SOUTH ESK RIVER CATCHMENT ................................................. 15

4.0 Catchment Water Balance Model .................................................................................................... 17

4.1 NATURAL FLOW ESTIMATION............................................................................................................. 18

4.2 HYDROLOGICAL DISTURBANCE INDICES ............................................................................................ 20

5.0 Flow Characteristics at Environmental Flow Assessment Locations............................................ 21

5.1 ENVIRONMENTAL FLOW ASSESSMENT LOCATIONS. ............................................................................. 21

5.2 NATURAL AND CURRENT FLOW CHARACTERISTICS ............................................................................. 21

5.3 FLOW DURATION ANALYSIS ............................................................................................................... 23

6.0 Catchment and Subregion Water Budget........................................................................................ 25

6.1 CATCHMENT WATER ALLOCATIONS.................................................................................................... 25

6.2 FLOOD TAKE RULES – SOUTH ESK BASIN .......................................................................................... 26

6.3 CATCHMENT WATER YIELD ............................................................................................................... 26

References................................................................................................................................................. 30

5

1.0 Introduction

This report provides relevant hydrological information that will support the development of a

water management plan for the South Esk River catchment. The key features of this report

include

• analyses of streamflow data collected at various locations around the catchment to

characterise the current hydrology of the catchment

• outputs from a catchment hydrological model to provide hydrological information for

environmental flow assessment, calculations of a catchment water budget and

assessment of yields for water usage and allocation decisions.

Six environmental flow assessment sites have been established in the catchment, and time series

flow data was extracted from the model and used to describe the flow at each of these sites.

Five water management subregions have been designated for the catchment, and modelled flow

data and catchment water allocations were analysed to provide water balances for these water

management subregions and the South Esk River catchment as a whole. From this water

balance approach, a view of the overall water budget is provided for each of the water

management subregions

2.0 Catchment Description

The South Esk River catchment (upstream Longford) is located in the northeast and midlands of

Tasmania and covers an area of approximately 3,350 km2 (Figure 2.1). The catchment rises in

the Fingal Tier in the East and is bounded by Ben Lomond and Mt. Saddleback to the North. Its

principal sub-catchments are drained by the Nile, St Pauls and Break O’Day Rivers.

Downstream of Longford, the South Esk River receives inflow from the Macquarie and

Meander Rivers before flowing into Tamar Estuary, thus forming part of the greater Tamar

River Basin.

6

Figure 2.1 South Esk River catchment.

The topography and the drainage pattern of the catchment are largely controlled by the local

and regional geology of the area. The upper South Esk catchment is characterised by

quartzwacke and mudstone whereas the middle parts of the catchment are dominated by

Jurassic dolerite. The lower part of the catchment is characterised by alluvial sediments derived

mainly from volcanic rocks. The drainage system generally follows a mixture of parallel to

dendritic pattern and has a density of around 2.0 km of river length per square kilometre of

catchment area. The mean elevation ranges from AHD 140 m at the Longford outlet to around

1,600 m at the top of the Ben Lomond Massif.

The South Esk River is essentially unregulated. There are no major storages within its

catchment and natural flows are altered primarily due to the combined influences of water

abstraction during the summer irrigation season and other land use practices. The primary land

uses in the catchment are agriculture and forestry. The lower part of the catchment is primarily

developed for grazing and irrigated agriculture, with the majority of the catchment cleared for

pasture.

3.0 Catchment Hydrology

3.1 Rainfall, Runoff and Evaporation

The South Esk River catchment is situated in one of the drier parts of eastern Tasmania, and

receives variable rainfall ranging from about 500 mm in the southwest to 1,800 mm in the

northeast (Figure 3.1). Higher rainfall generally occurs in the upper reaches of South Esk River

whereas the low lying valley and flatlands experience a drier climate and are prone to drought.

Spatial average of rainfall data indicates an average annual rainfall of 835 mm within the South

Esk River Basin for the period of 1960-2003. Some 15% of its 3,350 km2 catchment lies within

the high rainfall (≥800mm annual average) area of the Ben Lomond and Eastern Highlands

areas.

7

Figure 3.1 Distribution of mean annual rainfall in the South Esk River catchment.

Distribution of monthly rainfall varies widely across the catchment (Figure 3.2). Monthly

average rainfall in the south and west of the catchment is generally less than 50 mm. The

highest monthly rainfall generally occurs in the Ben Lomond area and as a result the Nile River

catchment receives comparatively greater runoff than the rest of the catchment. The Fingal and

Avoca region of the South Esk basin receive less than 100 mm of monthly rainfall.

0

50

100

150

200

250

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

Rainfall (mm)

Longford

Avoca

Mathinna

Ben Lomond

8

Figure 3.2 Mean monthly rainfall at selected regions of the South Esk River catchment.

Analysis of the mean monthly rainfall for the entire catchment indicates that rainfall exceeds

potential evaporation during the months of April to September when rainfall is highest (Figure

3.3). The average winter (May-Oct) rainfall is around 80 mm compared to 60 mm in summer

(Nov-April). The total average rainfall for the winter period is 478 mm while it is

approximately 357 mm in summer. The long-term average winter evaporation is 47 mm

compared to 117 mm during the summer. Evaporation is highest in the south and west of the

catchment. Low evaporation and maximum runoff occur at the head waters of the South Esk

River and its major tributaries.

0

20

40

60

80

100

120

140

160


Rainfall/Evaporation (mm)

Rainfall

Evaporation

Figure 3.3 Mean monthly rainfall / evaporation in South Esk River catchment.

A 5-year moving average analysis of annual rainfall data (1960-2003) indicates a roughly

decadal cycle of wet and dry years over the 43 years of the record period (Figure 3.4). The years

1972, 1982 and 1994 were identified as the driest years with annual average rainfall of

<600 mm across the catchment. The 1970's were the wettest years with peak annual rainfall in

the range 1,000-1,300 mm. Since 1975, the annual average rainfall in the South Esk catchment

has not exceeded 1000 mm.

9

500

600

700

800

900

1000

1100

1200

1300

1400

1960 1965 1970 1975 1980 1985 1990 1995 2000

Mean Annual Depth (mm)

Rainfall

5-year moving average

Evaporation

Figure 3.4 Variation in annual rainfall and evaporation data (1960-2003) superimposed with 5-

year moving average.

3.2 Gauged Flow Monitoring and Characteristics

There are currently five streamflow monitoring stations in the South Esk River catchment

(Table 3.1). Historical flow records also exist for an additional five streamflow monitoring sites

with various records available from 1937 to 1994.

Table 3.1 Streamflow monitoring stations in the South Esk River catchment.

Site_Name Easting Northing Area (km2) Period of Record

181_South Esk at Perth 516900 5394750 3280 19/12/1956 – present

150_South Esk at Llewellyn# 546850 5370340 2242 18/11/1952 – present

25_Nile at Deddington 538200 5397199 226 03/06/1982 – present

18311_St Pauls at Avoca 560400 5373500 495 04/05/1988 – present

191_Break O’Day at Killymoon 588000 5394500 240 04/11/1983 – present

# maintained by Hydro Tasmania.

The summary statistics for the streamflow monitoring sites are presented in Table 3.2. All the

streamflow monitoring sites exhibit a high variability in average flow from year to year. Flows

at Perth and Llewellyn are highly correlated with approximately 32% more gauged flow at

Perth. The area between Llewellyn and Perth contributes approximately 15% less runoff per

unit area than the catchment upstream of Llewellyn. This result is not surprising since the

average rainfall in the upper part of the catchment tends to be greater than that in the lower part.

10

Table 3.2 Summary statistics of stream flow (ML/day) at the selected streamflow monitoring

sites.

South Esk @ Perth (181) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean 2064.0 955.1 735.1 650.2 1284.3 1879.5 2753.7 3305.4 4145.7 3549.9 2382.5 1539.6 1492.3

Std.Dev. 1062.2 1180.2 1217.8 1442.0 1686.1 2140.2 3136.9 2644.8 3025.7 2479.3 2088.7 1521.7 2081.6

Minimum 4.9 14.5 4.9 10.9 11.2 48.4 68.7 218.9 363.3 192.2 143.1 48.9 15.1

Maximum 199715.0 81857.0 38596.0 114480.0 71818.0 199715.0 133193.0 48895.0 82230.0 58512.0 81989.0 38116.0 65169.0

South Esk @ Llewellyn (150) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean 1678.3 801.5 627.9 561.8 1257.1 1963.5 2344.0 2689.0 2951.1 2647.9 1699.7 1418.9 1385.7

Std.Dev. 974.0 922.5 1273.6 890.0 1895.0 3352.5 2954.4 2281.2 2123.1 2010.8 1153.8 1357.0 2011.0

Minimum 29.6 39.6 33.7 29.6 29.8 56.3 125.4 301.5 247.9 211.7 133.5 74.4 52.7

Maximum 203816.0 49938.0 42106.0 20678.0 61008.0 203816.0 104842.0 82131.0 41455.0 58271.0 22100.0 35496.0 54701.0

Nile at Deddington (25) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean 305.4 175.7 79.9 61.8 212.2 276.5 379.7 558.8 581.5 544.1 322.7 163.5 242.2

Std.Dev. 82.8 158.1 88.7 62.3 200.4 218.1 210.3 217.9 333.8 347.8 222.1 86.7 296.5

Minimum 3.3 7.1 5.5 4.1 3.3 11.7 18.0 36.9 36.8 34.8 20.9 13.8 7.5

Maximum 9743.8 6794.1 1926.0 2573.4 6874.6 7893.2 7798.5 6543.9 9743.8 5713.2 7863.2 2678.0 6250.1

St Paauls at Avoca (18311) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean 270.6 237.1 127.6 67.1 331.5 239.1 223.4 398.3 349.2 288.2 269.0 237.7 253.3

Std.Dev. 175.5 324.9 185.7 83.2 529.4 499.3 269.8 401.2 468.6 282.1 363.6 289.4 527.1

Minimum 0.1 0.1 0.1 0.3 0.8 0.8 1.5 9.5 13.2 7.5 1.0 0.9 0.0

Maximum 36095.0 36095.0 4197.8 2812.2 14081.0 12710.0 9063.4 9694.2 22333.0 5876.8 16132.0 6707.1 28274.0

Break O'Day at Killymoon (191) Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean 138.4 155.5 58.6 30.2 128.4 179.8 79.7 207.3 186.6 183.2 131.7 129.6 162.7

Std.Dev. 82.3 255.0 87.2 37.9 239.6 393.7 89.0 233.5 289.7 290.7 232.6 176.7 304.4

Minimum 0.3 0.4 0.3 0.4 1.0 3.4 3.2 6.7 4.3 5.7 2.6 1.6 0.6

Maximum 31008.0 21921.0 2132.2 2125.1 7772.4 31008.0 2623.2 12523.0 13299.0 6880.3 14773.0 6903.9 10158.0

One way of assessing the efficiency of a catchment to yield flow is by calculating the specific

yield based on the size of the catchment area. Specific yield is simply a flow volume (ML) per

unit pickup area (km2). This specific yield can also be used to determine the proportional

amount of runoff (mm) that is converted to flow. A summary of specific yields from the gauged

flows is presented in Table 3.3, and shows that the Nile River catchment has the highest yield

per unit area of catchment. Both the St Pauls and Break O’Day Rivers have low specific yields

indicating poor performance of these catchments to yield surface flow.

Table 3.3 Summary of catchment yields.

Area Flow Specific Yield

Site (km2) (ML/day) (ML/day/km

2)

South esk at Perth 3280 2064.0 0.63

South esk at Llewellyn 2242 1678.3 0.75

Nile at Deddington 226 305.4 1.35

St Pauls at Avoca 495 270.6 0.55

Break O'Day at Killymoon 240 138.4 0.58

The winter average rainfall and evaporation for the South Esk River catchment upstream of

Perth are approximately 500 mm and 300 mm respectively resulting in approximately 200 mm

of potential direct runoff. During winter the efficiency of the catchment pickup area to translate

rainfall into effective river flow at Perth is approximately 68%. This means that about 32% of

the potential winter runoff is taken by consumptive water storages, wetlands and groundwater

recharge.

3.3 Flood Frequencies

Flood frequency analysis of flows from selected streamgauge sites in the South Esk River

catchment was carried out for the duration of gauged flow. The results of this analysis are given

in Table 3.4 and a selected number are graphically represented in Figure 3.5. The average flood

(1:2 years) in the lower reaches of the South Esk River (Lower Esk) peaks at around 400 m3s

-1

11

while it is 98 m3.s

-1 in the Nile River at Deddington. Both the St Pauls and Break O’Day Rivers

show a similar average flood magnitude of around 150 m3.s

-1.

1

Table 3.4 Annual exceedance probabilities (AEP) and average recurrence interval (ARI, years)

of peak floods (m3.s

-1) at selected stream gauging sites in the South Esk River catchment.

AEP ARI South Esk at Perth South Esk at Llewellyn Nile at Deddington St Pauls at Avoca Break O'Day at Killymoon

1.00 1 35 27 30 5 5

0.50 2 431 404 98 154 150

0.20 5 909 851 139 298 292

0.10 10 1301 1202 164 400 394

0.05 20 1721 1564 186 500 493

0.02 50 2319 2057 214 630 623

0.01 100 2802 2437 233 729 722

The South Esk River is the main source of major flood flows affecting the low lying areas of the

Fingal Valley, Longford, Hadspen and Launceston. The areas around Gray near St Marys (the

upper reaches of the Break O’Day River) and near Mathinna are well known as locations of

high rainfall events and flash flooding.

1

10

100

1000

10000

1 10 100

Annual Exceedance Probailityl (1:X years)

Flood Peak (m

3s-1)

South Esk at Llewellyn

St Pauls at Avoca

Nile at Deddington

Figure 3.5 Frequency curves of annual floods at selected streamflow monitoring sites in the

South Esk River catchment.

1 The units used in the flood frequency analysis are cubic metres per second (Cumecs) as the flood

frequency represents a peak rate of discharge. If mega litres per day were used as the units this would

provide a volume and the results may be misleading or misinterpreted.

12

3.4 Low Flows

Low flow frequency curves have been derived for a range of durations from 5 days through to

90 days (Figure 3.6). The curves give the probability that any given minimum flow will occur

over various time periods. For example, for a 5-day period the probability that a minimum

average daily flow of about 100 ML will occur in any given year is approximately 60%, while

for a longer period such as 90-days this probability decreases to around 15% for the South Esk

River at Perth. South Esk at Llewellyn shows a similar low flow probability range while it

varies widely for the major tributaries, Break O’Day, Nile and St Pauls Rivers.

This information has implications for environmental water provisions, water quality monitoring

and for the assessment of risk in supply of water from the river for purposes such as irrigation

and domestic use. Such risks will need to be taken into account during the planning process for

the six management subregions within the catchment.

Figure 3.6 Frequency curves of low flows at selected streamflow monitoring sites in the South

Esk River catchment.

South Esk at Perth

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60 70 80 90 100

Probability (%) of flow less than that shown in any given year

Flow (ML/day)

Expon. (90-Day)

Expon. (60-Day)

Expon. (30-Day)

Expon. (5-Day)


0

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60 70 80 90 100


Flow (ML/day)

Expon. (90-Day)

Expon. (60-Day)

Expon. (30-Day)

Expon. (5-Day)

St Pauls at Avoca

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80 90 100


Flow (ML/day)

Expon. (90-Day)

Expon. (60-Day)

Expon. (30-Day)

Expon. (5-Day)

Break O'Day at Killymonn

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90 100


Flow (ML/day)

Expon. (90-Day)

Expon. (60-Day)

Expon. (30-Day)

Expon. (5-Day)

Nile at Deddington

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90 100


Flow (ML/day)

Expon. (90-Day)

Expon. (60-Day)

Expon. (30-Day)

Expon. (5-Day)

13

3.5 Flow Recession

A recession curve is a specific part of the flood hydrograph after the crest where streamflow

diminishes. The recession segment shows how the water storage in the river decreases over time

following a significant rain event. The recession curve basically reflects the baseflow

component of river flow and how groundwater storage influences and sustains flows in rivers.

Each recession segment of the flood hydrograph is described by a classic exponential decay

function of the form:

Qt = Qo e-αt

where Qt is the streamflow at time t, Qo is the initial streamflow at start of the

recession, e is natural logarithm and α is cut-off frequency constant.

Recession curves for peak flows in the South Esk River and its major tributaries are presented

in Figures 3.7 and 3.8. The recession equations for the South Esk River were:

Flow = 576 * e-0.0003*Time (Minutes)

South Esk River at Perth


South Esk River at Llewellyn

The upper part of the recession curve is dominantly surface water and as flow recedes the

surface flow contribution gradually diminish until the flow is comprised almost entirely of

baseflow (or groundwater discharge), depicted by the lower section of the curve. The curves in

Figure 3.7 show that it takes approximately 7000 minutes (approx 5 days) for the peak flow in

the South Esk River at Perth to recede from an average flood of 400 m3.s

-1 (during winter) to 50

m3.s

-1, which is a rate of decrease of about 72 m

3.s

-1 per day.

Flow Recession Curves

0

100

200

300

400

500

600

0 1000 2000 3000 4000 5000 6000 7000 8000

Recession Time (Minutes)

Flow (m

3s-1)

South Esk at Perth


Figure 3.7 Recession curves for peak observed flow in the South Esk River at Perth and South

Esk River at Llewellyn.

14

Recession curves for observed flows in the Nile, Break O’Day and St Pauls Rivers are shown in

Figure 3.8. The peak flow recession for sites in these rivers is described by the flowing

equations.


Nile at Deddington


Break O’Day at Killymoon


St Pauls at Avoca

The Nile and Break O’Day Rivers show a very similar recession rate of approximately 25 m3s

-1

per day, while the recession rate for St Pauls River is markedly steeper, at around 40 m3s

-1 per

day.

0

50

100

150

200

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Recession Time (Minutes)

Flow (m3s-1)

Nile at Deddington

Break Oday at Killymoon

St Pauls at Avoca

Figure 3.8 Recession curves for peak observed flow in the Nile, Break O’Day and St Pauls

Rivers.

15

3.6 Wet and Dry Season Comparison

Observed flow data (1960-2007) from the South Esk at Perth was examined to identify

relatively wet and dry years. This indicated that the years 1974 and 2006 represent years of

wetter-than-average conditions and drier-than-average. Figure 3.9 provides a comparison of the

hydrographs for these years plotted along with the monthly median flow for the record period

1960-2006. This has been included to demonstrate the degree of interannual variation that

occurs in runoff from this catchment.

Figure 3.9 Median monthly flow compared to wet and dry year hydrographs of flows from

South Esk at Perth.

3.7 Hydrological character of the South Esk River Catchment

The catchment experiences widely varying climatic conditions with rainfall ranging from 500

mm in the low lying areas to up to 1,500 mm in the highlands. With an annual average rainfall

of 835 mm, the total water input into the South Esk River catchment is approximately 3,000

GL/year. The total catchment annual yield at Longford is around 900 GL/year and therefore

comprises about 43% of the total annual water input. This simple water budget indicates that the

vast majority (57%) of total water input into the catchment is either evaporated, transpired or

moves into to the local and regional groundwater system.

The majority of the catchment rainfall and runoff occurs in the northern and eastern headwaters

and as a result maximum runoff is converted to river flows in these regions. Low rainfall and

higher evaporation in the southeast of the catchment has contributed to low conversion of the

runoff into the cumulative yields in the Lower South Esk regions. Considerable climate

variability within the catchment results in high spatial variability of runoff yield. The Upper Esk

subregion and Nile River catchment are identified as the most productive areas in relation to

converting rainfall into runoff and hence yield as river flow.

16

All flow gauge sites exhibit a high variability in average flow from year to year. Flows in the

South Esk River at Perth and Llewellyn are highly correlated with approximately 25% more

flow at Perth. The area between Llewellyn and Perth contributes approximately 40% less

runoff per unit area than the catchment upstream of Llewellyn over a year. This result is not

surprising since the average rainfall in the upper part of the catchment tends to be greater than

that in the lower part.

Perth and Llewellyn exhibit strong seasonal patterns generally peaking in the period July

through September. The Break O’Day and St Pauls River catchments display high variability

of flows. The Break O’Day River is dominated by very low and extremely high monthly flows.

The South Esk River is the main source of major flood flows affecting the low lying areas of the

Fingal Valley, Longford, Hadspen and Launceston. The areas around Gray near St Marys (ie.

in the upper reaches of the Break O’Day River) and near Mathinna are well known as locations

of high rainfall events and flash flooding.

Average floods (1 in 2 year floods) in the lower reaches of the South Esk River catchment peak

at around 400 m3s

-1, while floods range from 100 to 150 m

3s

-1 in the major tributaries. At Perth

the observed streamflow recession after an average flood event is around 72 m3s

-1 per day and it

takes roughly five days for the peak flow to recede to an average river flow of 24 m3s

-1. Low

flow probability analysis indicated that within a given year the likelihood of occurrence of

average flows ≤ 1.0 m3s

-1 over a five day consecutive period is around 60%.

17

4.0 Catchment Water Balance Model

Under the National Action Plan (NAP) a rainfall and runoff water balance model was developed

for the South Esk River catchment (HEC, 2005).

Whilst gauged data provides a good picture of the hydrology of a catchment, they are generally

limited or intermittent in the period of record at any one site. They are not able to provide a

clear picture of the catchment hydrology under natural2 conditions, nor can they provide a

picture of long term trends related to climate variability. A water balance model is used to

generate natural flow and current3 flow time series data over a much greater time period, using

rainfall and evaporation data. These models allow an assessment of changes in hydrology due

to current water abstraction, and allow catchment yields to be determined at several locations

within catchments so that various water allocation scenarios can be tested.

Calibration of the water balance model was achieved by adjusting catchment parameters (eg.

infiltration, baseflow, storage capacities) so that the modelled natural2 flow output best matches

the observed flow record at the calibration site. A detailed description of the development and

calibration of the South Esk River catchment model can be found in HEC (2005). The model

can generate a daily time-series of natural flow based on daily rainfall and evaporation records

over 100 years (1901-2003). The model can also generate a daily time-series of current flow

for the same period; that is, what flow would have been from 1901 to 2003 has current water

abstractions occurred in these years.

The catchment model has been used to generate natural flow time series data for the

environmental flow assessment locations and yield analysis for the water management

subregions. The model is also used to derive a daily time-series of current3 flow, which takes

into account water abstractions from the system and gives an indication of impact on the natural

streamflow and hence on the gross catchment yield. The impact of water abstractions from the

system has been used to derive the hydrological disturbance indices described in section 4.3.

Information on the current water abstractions in the catchment was obtained from entitlement

allocations in the DPIW Water Information Management System database (WIMS). The

allocations in the catchment are of a given Surety (from 1 to 8) and they have a specific period

of applicability. This period was used in the modelling to allocate an average daily abstraction

for each licensed abstraction. For example, a 184 ML allocation for the period May 1 to

October 31 (184 days) would have an average extraction of 1 ML/day in the modelling process

for May 1 to October 31. While it is realised that this even spread of abstraction may be

unrealistic (as abstractions will occur at different rates within the allocation period), there are

minimal historical records to assist in defining how each individual licence is utilised. This is

further discussed in the next section.

Furthermore the model does not explicitly account for changes in landuse and vegetation over

time within each of the subcatchments.

2 Natural flow: Flow that is expected to occur in a river where there is no water extracted for

consumptive use. This is generally produced from a hydrologic model for a catchment using rainfall and

evaporation as the primary input data. It does not take into account any changes in land-use that may

have occurred over the period of interest.

3 Current flow: Flow in a river where water has been extracted for consumptive use. For the purpose of

this report this involved subtracting the licensed water use for 2003 from the entire ‘natural’ flow record

produced by hydrologic catchment models.

18

4.1 Natural Flow Estimation

Natural flow was estimated for the environmental flow assessment locations and for the water

management subregions. This was done by calibrating the natural flow output with observed

winter flow at the South Esk River at Perth. Observed winter flow was assumed to be

representative of natural flow with minimal impact by water abstractions, although this may not

be strictly true. The result of the calibrated data is shown in Figure 4.1a. Time series plots

(Figure 4.1a) of the modelled and observed flow for an average rainfall year (1977) show that

the South Esk catchment model gives a fair representation of the natural flows at South Esk

River at Perth. The daily flow regression coefficient value (modelled flow vs observed flow) at

the calibration site was around 66%. The regression coefficient value is further improved to

85% if comparison is made between the monthly flow data. Overall, there is a general tendency

for the model to underestimate or overestimate flows depending on the state of catchment

saturation in any given year. The model predicts flows more accurately during wetter years,

when the losses mentioned in Section 3.3 are low.

Although modelled data from 1901 is available, data from 1960 onward was used for the

analyses. This period of data was used for the following reasons

• the rainfall and evaporation data prior to 1960 is less reliable

• the period 1960 to 2003 is more representative of the prevailing climatic conditions.

Prior to 1960 data reflects a wetter climate.

Duration analysis of the daily flow for the period 1960-2003 in Figure 4.1b indicates that at

exceedance greater than 60%, natural flow and observed flow correspond reasonably well.

Current flow is markedly less in this region of the duration curve, and this is likely to be a result

of the manner in which allocation data is used in the modelling process, as mentioned in Section

4.0. In addition, current flow is derived by subtracting water usage data for 2003 from the

entire period of the natural flow record (1960-2003). However, historical extraction from the

river would have increased from a low level in the 1960’s to the current level. Application of

the 2003 abstraction data to the entire record accentuates the differences between both natural

and current flow, and between current and observed flow.

In addition, the current quantity of water extracted from the catchment is unverified. It relies on

water licence information (WIMS, 2003) for estimates of extractions, and these may not

represent the true quantity of water being extracted. It also needs to be pointed out that the

modelled current flow does not take into account periods of water restriction, which will result

in observed flows during dry periods (exceedance greater than 90%) being closer to modelled

natural flow.

All of the issues highlighted above must be borne in mind when viewing the disturbance indices

in the next section and the plots in Figure 5.1.

19

Figure 4.1 Time series and flow duration analysis of the, current, observed and natural flow data

for South Esk River at Perth.

20

4.2 Hydrological Disturbance Indices

Indices of hydrological disturbance provide and assessment of the change in hydrology due to

water abstraction in the catchment. These indices were derived from a comparison of the natural

and current flows estimated from the NAP catchment model (Table 4.1). The indices were

calculated using the formulas stated in the Natural Resource Management (NRM) Monitoring

and Evaluation Framework developed by SKM for the Murray-Darling Basin (SKM, 2003).

Table 4.1 Indices of hydrological disturbance at selected water management subregions of the

South Esk catchment.

Hydrologic Disturbance Indices Longford Neck of Bottle Glen esk Ormley Malahide Glen Mavis Benham

Mean Annual Flow Index 0.96 0.96 0.99 0.99 0.99 0.95 0.99

Flow Duration Curve Difference Index 0.81 0.82 0.91 0.92 0.94 0.69 0.86

Seasonal Amplitude Index 0.94 0.94 0.98 0.98 0.99 0.86 0.98

Season Period Index 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Hydrological Disturbance Index 0.90 0.90 0.95 0.96 0.97 0.83 0.93

Index of Mean Annual Flow: This provides a measure of the difference in total flow volume

between current and natural conditions. It is calculated as the ratio of the current and natural

mean annual flow volumes and assumes that increases and reductions in mean annual flow have

equivalent impacts on habitat condition.

Index of Flow Duration Curve Difference: The difference from 1 of the proportional flow

deviation, averaged over p monthly flow percentile point. A measure of the overall difference

between current and natural monthly flow duration curves. All flow diverted would give a score

of 0.

Index of Seasonal Amplitude: The change in amplitude of the seasonal pattern of monthly

flows. It is defined as the average of two current: natural ratios, firstly, that of the highest

monthly flows, and secondly, that of the lowest monthly flows based on calendar month means.

Index of Seasonal Periodicity: The change in seasonal timing of flows. It is defined as the

difference from 1 of one twelfth of the sum of the absolute values of the differences between

current and natural of first, the numerical values of the months with the highest mean monthly

flows, and second, the numerical values of the months with the lowest mean monthly flows.

Hydrological Disturbance Index: This provides an indication of the hydrological disturbance

to the river’s natural flow regime. A value of 1 represents no hydrological disturbance, while a

value approaching 0 represents extreme hydrological disturbance.

21

5.0 Flow Characteristics at Environmental Flow Assessment Locations

5.1 Environmental Flow Assessment Locations.

Six sites were chosen in the South Esk River catchment for the purposes of environmental flow

assessment (shown in Figure 5.1, grid locations listed in Table 5.1). These sites were selected

as representative of the physical river characteristics of the main subcatchment areas. Natural

and current flow was generated from the hydrological model for the six environmental flow

assessment locations.

Figure 5.1. Environmental flow assessment locations in the South Esk River catchment.

Table 5.1 Approximate location of environmental flow assessment sites in the South Esk River

catchment.

Site_ID Easting Northing Upstream Area (km2)

South Esk @ Malahide (u/s Pig Creek) 583250 5399770 726

South Esk @ Ormley 568030 5380770 1370

South Esk @ Glen Esk 540550 5375230 2360

South Esk @ Neck of Bottle 521700 5394070 3165

Nile @ Glen Mavis 529350 5389930 255

St Pauls @ Benham 561900 5372400 293

5.2 Natural and Current Flow Characteristics

A summary of natural and current flow characteristics at the environmental flow assessmentlocations are presented in Table 5.2.

22

Table 5.2

Longte

rm a

ver

age

flow

(M

L/d

) ch

arac

teri

stic

s under

nat

ura

l an

d c

urr

ent

wat

er u

se c

ondit

ions

at t

he

six e

nvir

onm

enta

l fl

ow

asse

ssm

ent lo

cati

ons

in the

South

Esk

Riv

er c

atch

men

t. F

low

dat

a an

alyse

d f

or

the

reco

rd p

erio

d 1

960-2

003.

Neck of Bottle

Glen Esk

Ormley

Malahide

Glen Mavis

Benham

NaturalCurrent

NaturalCurrentNaturalCurrent

NaturalCurrent

NaturalCurrent

NaturalCurrent

Mean

2392

2312

1735

1711

1347

1329

962

952

317

302

180

177

Median

962

878

660

637

510

491

358

346

120

104

42

40

CV

2.1

2.2

2.4

2.4

2.4

2.4

2.4

2.4

2.1

2.2

3.9

4.0

Daily Minimum

7.7

0.0

5.4

0.0

4.1

0.0

2.3

0.0

0.6

0.0

0.4

0.0

Daily Maximum

154605

154499

149684

149659

116335

116310

88178

88163

14906

14889

24377

24375

10th%ile

146

83

100

80

71

58

45

40

14

17

4

30th%ile

426

356

298

277

221

206

149

142

45

31

19

17

90th%ile

5517

5418

3793

3766

3014

2992

2200

2185

769

752

331

328

Jan

763

702

581

563

427

416

264

260

72

59

86

84

Feb

536

476

416

397

302

291

170

167

49

37

60

58

Mar

649

589

539

520

396

384

228

224

56

44

91

89

Apr

1342

1282

1081

1061

875

863

576

572

159

147

133

131

May

2703

2617

2016

1990

1604

1582

1165

1150

357

342

223

220

Jun

3769

3677

2786

2760

2107

2085

1477

1462

489

472

334

331

Jul

4678

4583

3327

3301

2653

2630

1986

1971

671

654

318

315

Aug

5613

5517

3898

3872

3031

3008

2267

2252

829

812

348

345

Sep

3978

3882

2740

2713

2066

2043

1527

1512

537

520

248

245

Oct

2086

1988

1455

1429

1149

1126

854

839

289

272

111

109

Nov

1271

1179

946

920

758

735

517

502

151

134

75

72

Dec

1177

1114

935

914

722

710

450

446

125

111

120

118

Winter

3804

3711

2704

2678

2101

2079

1546

1531

529

512

264

261

Summer

956

890

750

729

580

567

368

362

102

89

94

92

23

5.3 Flow Duration Analysis

A flow duration curve (FDC) is a plot of discharge vs. percent of time that a particular

discharge was equalled or exceeded. An FDC represents the relationship between the magnitude

and frequency of daily, weekly, monthly or yearly streamflow for a particular river basin,

providing an estimate of the percentage of time a given streamflow was equalled or exceeded

over a historical period. The area under the flow duration curve gives the average daily flows

for the natural and current conditions, and the median daily flow is the 50% value. An FDC

provides a simple, yet comprehensive, graphical view of the overall historical variability

associated with streamflow in a river basin.

A series of FDCs have been generated for the six environment flow assessment locations in the

South Esk River catchment (Figure 5.1). These curves compare long-term duration

characteristics of natural and current flow conditions.

A FDC also characterizes the ability of the basin to provide flows of various magnitudes. Table

5.2 provides a summary of the extracts from the FDC showing thresholds of natural flow at

varying probability of exceedance. Information concerning the relative amount of time that

flows past a site are likely to equal or exceed a specified value of interest is extremely useful for

the design of structures on a stream. For example, a structure can be designed to perform well

within some range of flows, such as flows that occur between 20 and 80% of the time (or some

other selected interval).

Table 5.2 Probability of exceedance (POE) of natural flows (ML/day) at the six environment

flow assessment locations in the South Esk River catchment

POE (%) Neck of Bottle Glen Esk Ormley Malahide Glen Mavis (Nile) Benham (St Pauls)

5 9135 6516 5232 3981 1355 567

10 5517 3793 3014 2200 769 331

20 3027 2030 1511 1039 375 170

30 2022 1360 1010 697 239 100

40 1385 945 727 514 172 63

50 962 660 510 358 120 42

60 649 449 346 240 75 28

70 426 298 221 149 45 19

80 267 188 139 90 26 12

80 267 188 139 89 26 12

90 146 100 71 45 14 7

95 90 61 44 28 9 4

99 46 31 22 14 5 2

The shape of a FDC in its upper and lower regions is particularly significant in evaluating the

stream and basin characteristics. The shape of the curve in the high-flow region indicates the

type of flood regime the basin is likely to have, whereas, the shape of the low-flow region

characterizes the ability of the basin to sustain low flows during dry seasons. A very steep curve

(high flows for short periods) would be expected for rain-caused floods on small watersheds. In

the low-flow region, an intermittent stream would exhibit periods of no flow, whereas, a very

flat curve indicates that moderate flows are sustained throughout the year due to natural or

artificial streamflow regulation, or due to a large groundwater capacity which sustains the base

flow to the stream.

The general effect of water abstractions and various landuse practices in the catchment is to

depress the natural flow scenario at low flow end of the FDC. Figure 5.1 shows the impact on

natural flow by water abstraction alone. The FDCs for all the environment flow assessment

24

locations show depression of natural flows at probability of exceedance greater than 60%,

however the least impact is displayed at the South Esk at Malahide site.

This is typical of many unregulated rivers in Tasmania, where the impact of water abstraction is

mostly on the low flow part of the flow regime. This is not surprising as summer irrigation

coincides with the low flow period for most Tasmanian rivers. Generally, the flow regime in

most of Tasmania’s unregulated rivers is close to natural apart from the low flow component,

and hence the focus of management is on sharing of water between the environment and

consumptive use during drier times.

Figure 5.1 Flow duration curves for the six environment flow assessment locations in the South

Esk River catchment.

Flow Duration Curve for South Esk at Neck of Bottle (1960-2003)

1

10

100

1000

10000

100000

1000000

0 20 40 60 80 100

Percentage of time discharge was equalled or exceeded

Discharge (ML/d)

Natural Flow

Current Flow

Flow Duration Curve for South Esk at Glen Esk (1960-2003)

1

10

100

1000

10000

100000

1000000

0 20 40 60 80 100


Discharge (ML/d)

Natural Flow

Current Flow

Flow Duration Curve for South Esk at Ormley (1960-2003)

1

10

100

1000

10000

100000

1000000

0 20 40 60 80 100


Discharge (ML/d)

Natural Flow

Current Flow

Flow Duration Curve for South Esk at Malahide (1960-2003)

1

10

100

1000

10000

100000

0 20 40 60 80 100


Discharge (ML/d)

Natural Flow

Current Flow

Flow Duration Curve for Nile at Glen Mavis (1960-2003)

1

10

100

1000

10000

100000

0 20 40 60 80 100


Discharge (ML/d)

Natural Flow

Current Flow

Flow Duration Curve for St Pauls at Benham (1960-2003)

1

10

100

1000

10000

100000

0 20 40 60 80 100


Discharge (ML/d)

Natural Flow

Current Flow

25

6.0 Catchment and Subregion Water Budget

6.1 Catchment Water Allocations

The distribution of current consumptive water allocations in the South Esk River catchment is

shown in Figure 6.1. The bulk of the allocation lies within the Lower Esk subregion. The

current total annual consumptive water allocation in the South Esk River catchment is

44,415 ML. A summary of the water usage and subregion distribution of allocation is given in

Table 6.1, and shows that 55% of the volume of allocated water is used in the Lower Esk

subregion.

Figure 6.1. Distribution of water allocations in the South Esk River catchment.

Table 6.1. Summary of the water usage and subregion distribution of allocation

Subregions Area (km2) Allocation (ML/Year) Purpose Allocation (ML/Year)

Upper Esk 1016 6845 Irrigation 43002

Avoca-Break O'Day 514 3497 S & D 614

St Pauls River 524 1470 Water Supply 485

Nile River 318 7946 Industrial 314

Lower Esk 972 24658 Direct Take 21906

Total 3345 44415 Storage 22509Allocation excludes storage for aesthetic and recreation.

26

6.2 Flood Take Rules – South Esk Basin

Under an Act of Parliament, all of the water in the Tamar River Basin is managed to support

hydro-electric power generation by Hydro Tasmania. Under this management framework, some

water is made available for agricultural production. Through a memorandum of understanding

(MOU) developed in 2004 between Hydro Tasmania, the Tasmanian Farmers and Graziers

Association and the Department of Primary Industries, Water and Environment (now DPIW),

Hydro Tasmania recognised that other water users can take additional water from the South Esk

catchment during times of flood. In this MOU, Hydro Tasmania set out a number of rules by

which other water users in the catchment (mainly agricultural) can take water without affecting

the ability of Hydro Tasmania to maximise power production at Trevallyn Power Station.

These rules take the form of summer and winter ‘trigger flows’ at agreed locations, and in the

South Esk River, the trigger point is located at the ‘Llewellyn’ streamflow monitoring station

(Table 6.2). At this location, when flow at this station exceeds 20.3 m3.s

-1 (1,750 ML.day

-1) in

winter and 23.4 m3.s

-1 (2,450 ML.day

-1) in summer, flood extraction throughout the catchment

can occur for 5 days. This period can be extended based on the discretion of Hydro Tasmania,

and is dependant on continuing spill of Lake Trevallyn.

This management rule should be kept in mind when viewing the data regarding water use and

water availability.

Table 6.2. The “trigger flow” for the South Esk River at Llewellyn, which determines

when flood water can be extracted from the river system

Summer trigger Winter triggerSite Name

m3.s

-1ML.day

-1m

3.s

-1ML.day

-1

South Esk River at Llewellyn 23.4 2,450 20.3 1,750

6.3 Catchment Water Yield

The average annual natural flow yield of the South Esk River at Longford is 887,658 ML with

an average daily outflow of 2,430 ML/day. Table 6.3 shows a summary of the natural flow

yields for the South Esk River (at Longford) and its major tributaries. About 80% of the total

natural flow yield at Longford is contributed by the winter runoff mainly from the upper

reaches of the catchment. The major tributaries (Nile, St Pauls and Break O’Day Rivers)

together contribute about 28% of the total catchment yield.

Monthly flows are generally highest during August and lowest during February for all the

subregions. The Upper Esk shows the highest monthly and seasonal yields and this is expected

as maximum runoff occurs in this part of the region. Although the Nile River catchment has a

smaller catchment area (318 km2) than the St Pauls River (524 km

2), its winter yields are greater

by twice the amount.

27

Table 6.3 Distribution of average monthly, seasonal and annual yields (ML) in the South Esk

River catchment. Also included in the table is the annual allocation (ML).

Upper Esk Avoca-Break O'Day St Pauls River Nile River Lower Esk Catchment

Jan 11346 2896 2663 2265 8008 24515

Feb 7120 2125 1697 1409 4825 15479

Mar 10495 2591 2819 1704 5320 20109

Apr 23585 3910 3990 4635 7418 39549

May 44327 8499 6898 10873 20898 84598

Jun 53459 14992 10011 14695 31290 114436

Jul 72250 15678 9852 20733 38368 147029

Aug 80510 21727 10785 25654 48936 176827

Sep 52347 15846 7447 16337 38687 123217

Oct 30829 7868 3452 9097 18744 66538

Nov 20005 4514 2243 4556 9774 38850

Dec 19408 4501 3726 3864 8737 36510

Winter Yield (May-Oct) 333723 84610 48447 97389 196923 712646

Summer Yield (Nov-Apr) 91959 20538 17139 18434 44082 175013

Annual Yield 425682 105148 65585 115823 241005 887658

Annual Allocation 6845 3497 1470 7946 24658 44415

Figure 6.2 shows the catchment yields and relative distribution of water allocations within the

water management subregions. The Upper South Esk, Avoca-Break O’Day and St Pauls River

subregions show relatively lower water allocations compared to summer yields.

1000

10000

100000

1000000

Upper Esk Avoca-Break

O'Day

St Pauls

River

Nile River Lower Esk Catchment

Total Yield (ML)

Annual Yield Winter Yield (May-Oct) Summer Yield (Nov-Apr) Annual Allocation

Figure 6.2 Distribution of yields and current allocations in the water management subregions.

28

The seasonal and annual specific yields of the water management subregions are shown in

Figure 6.3 and compared to the whole of the catchment upstream of Longford. Both the Upper

Esk and Nile River subregions show significantly higher specific yields than the remainder of

the subregions. The Lower Esk subregion shows similar yield capability to that of the whole

catchment.

0

100

200

300

400

500

Upper Esk Avoca-Break

O'Day

St Pauls

River

Nile River Lower Esk Catchment

ML//day/km2

Summer Winter Annual

Figure 6.3 Distribution of summer, winter and annual specific yields in the water management

subregions.

The ability to convert maximum runoff to river flow by a catchment is best shown by analysing

the monthly specific yields (Figure 6.4). The figure shows that the Upper Esk and the Nile

River are the most productive subregions. Both these subregions show peak specific yields of

around 80 ML/day/km2 during mid-winter period. Winter specific yields are generally less than

20 ML/day/km2 for the St Pauls River subregion.

29

0

10

20

30

40

50

60

70

80

90


ML/day/km2

Upper Esk

Avoca-Break O'Day

St Pauls River

Nile River

Lower Esk

Figure 6.4 Monthly specific yields in the water management subregions.

The overall catchment water budget for the South Esk River catchment is given in Table 6.4.

The total average annual yield for the entire catchment up to Longford is approximately

887,658 ML with current annual water allocation of 44,415 ML.

Table 6.4 Distribution of annual yield (ML) in the water management subregions.

Subregions Annual Yield (ML) Annual Allocation (ML) Available Yield# (ML)

Upper Esk 425682 6845 418838

Avoca-Break O'Day 105148 3497 101651

St Pauls River 65585 1470 64116

Nile River 115823 7946 107877

Lower Esk 241005 24658 216347

Catchment 887658 44415 843244

#Yield subject to Hydro Tasmania's MOU on flood take rules.

Currently, about 5% of the total catchment yield is allocated water. Approximately

840,000 ML of catchment yield is available for resource allocation subject to licensing

agreements on flood takes rule for Trevallyn Dam. Future utilisation of water resources in the

catchment will need to focus on the development of management scenarios that include

groundwater resources in the catchment water budget.

Currently 44,415 ML allocated for agricultural and other consumptive uses out of a total of

887,658 ML. Hydro Tasmanias flood take rules stipulate that water cannot be taken until the

triggers have been crossed. This means that of the total annual yield from the catchment,

approximately 555,000 ML is protected for power generation, 44,415 ML is currently allocated

for agricultural and other consumptive uses and 287,000 ML contained in flows above the

trigger levels.

30

References

HEC 2005a. NAP Region Hydrological Model South Esk Catchment. Report 118783-2. Hydroelectric

Corporation: Hobart, Tasmania.

HEC 1999. South Esk – Great Lake Hydro Catchment

SRA 2003. Hydrology Theme Pilot Audit Technical Report – Sustainable Rivers Audit. MDBC

Publication 08/04. Murray Darling Basin Commission: Canberra, ACT.

SKM (2003) Sustainable Rivers Audit Hydrology Theme – Ovens River Basin Hydrology Report,

Murray-Darling Basin Commission. Sinclair Knight Mertz.

surface water hydrology of the south esk river catchment

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