Modeling the Blanco Watershed

Joanna C. Curran

Basin Planning with Computer Models

Models can integrate available data, help clarify and help quantify primary watershed processes, and guide further data collection

Models can be used to help predict the effect of changing management practices by simulating the essential watershed processes

Model = simplification of reality

Model results = hypotheses

Best interpreted in terms of relative changes, not absolute amounts

SWAT: Soil and Water Assessment Tool

• A watershed scale model intended to model the effects of land management practices on large complex watersheds

• Uses basin-specific physical data to predict the impacts of topography, soils, land use, management decisions, and weather on water, sediment, nutrient, and agricultural chemical yields for large ungaged watersheds

• Operates on a daily time step and is able to provide a continuous temporal simulation of physical watershed processes

• Able to simulate hundreds of years, thereby predicting the long-term impacts of management decisions.

• A long term yield model that is not designed to model single event flood routing

SWAT Components

– Canopy storage

– Infiltration

– Redistribution

– Evapotranspiration

– Lateral subsurface

flow

– Surface flow

– Tributary channels

– Return flow

– Plant Growth

• Climate

– daily precipitation, air temperature, solar radiation, wind speed and relative humidity are input from records of observed data or simulated

Watershed Delineation from Digital

Elevation Models at 30-meter resolution

Blanco River flows southeast for 140 km

Blanco Watershed area is 1365 km2

USGS seamless data website

Subbasins are defined according to drainage

USGS National Hydrography Dataset

By partitioning the watershed into subbasins, different areas of the watershed can be referenced to one another spatially

STATSGO Soil Layers, USDA-NRCS)

Records were obtained from the National

Climatic Data Center

The precipitation gage stations used for

the Blanco River basin model were:

Blanco, San Marcos, Fischer Store, and

Wimberley 1 NW

Solar radiation, wind speed, and relative

humidity were simulated by the model

using the WEXGEN weather generator

model and monthly climate statistics

calculated from long-term measured data

to simulate weather data

Precipitation and Temperature

Land use and soil data combine to define HRUs

Together, HRUs and the stream network data are used to model the

watershed

HRUs cross sub-basin boundaries, instead following combinations of

land use and soil type.

Precipitation and flows are calculated first for each HRU, and then for

each sub-basin based on the results from the HRUs

Discharge from each HRU are summarized in each subbasin and then routed through the stream network to the watershed outlet

Water balance is the governing equation for the model

Hydrological Response Units (HRUs)

• Surface runoff is predicted through a modification of the SCS curve number with daily rainfall records

• Volume of the runoff from a given precipitation event is calculated using the Green-Ampt equation

• Flow is routed through the tributaries and main channel using the Muskingham method. Gains and losses are accounted for due to soil water content, surface runoff, evapotranspiration, precipitation, water lost by seepage into the soil, and the amount of return flow. The amount of flow moving through the sub-basins is calculated using a daily time step.

• Potential evapotranspiration is calculated by the Priestly-Taylor method using temperature, solar radiation, and relative humidity data

• Measured air temperature data are used to compute a daily water temperature for a well mixed stream

Modeling Hydrologic Processes

Modeling Scenarios

1. 1990-2006 Precipitation with current land use

2. 1945-1960 Precipitation with current land use

3. 1990-2006 Precipitation with approximately 13%

urbanization

4. 1945-1960 Precipitation with approximately 13%

urbanization

5. 1990-2006 Precipitation with approximately 78%

urbanization

6. 1945-1960 Precipitation with approximately 78%

urbanization

Current Land Use

~2% urbanized

~13% urbanized

~80% urbanized

(From GIRAS, USGS)

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• Flow profiles illustrate how flow rates change in the Blanco River when traveling from the headwaters to the confluence with the San Marcos River. The trends in all of the simulation results show increasing flow rates in the downstream direction. This is to be expected as the main stem receives input from overland flow, tributaries, and possibly groundwater.

• Separate profiles are presented for each modeling scenario and for each of the 50%, 75%, and 90% flows, so for each scenario there are three profiles

• It is the difference in the flow profile trajectories that illustrate the effects of land use management and climate scenarios

• Flow within the tributaries is analyzed separately. The majority of the inputs are minor, making the inputs from Cypress Creek, the Little Blanco River, and Sink Creek easily discerned.

• The major tributaries to the Blanco River are the Little Blanco River, Cypress Creek, and the Sink Creek. They are located at 60 km, 92 km, and 142 km downstream respectively.

Longitudinal Flow Profiles

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2% urban; 13% urban; 78% urban

Blanco River: Rainfall

from 1990-2006

Q50

Q10

Under all scenarios, the flows are similar for

the first 60 km

Flows separate at 60 km and 100 km

downstream, corresponding to inputs from

the Little Blanco River and Cypress Creek.

Flows are consistently low at 13% urban

area

Tributaries have more influence at low or

high amounts of urban land use

Flows are consistent between the tributaries

under all urbanization scenarios

Downstream of 60 km, the trajectory of the

fully urbanized scenario increases

dramatically

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Tributaries: Rainfall

from 1990-2006 Q75

2% urban; 13% urban; 78% urban

Q50

Q10

Little Blanco River has a dramatic effect under the 78% urbanization, increasing main channel flow by approximately 50%

When the urbanization of the watershed is 13%, the Little Blanco input has little effect on the flow in the main channel.

Cypress Creek inputs cause a noticeable increase in Blanco River flow only under 78% urban and only during flood flows

Some tributaries are predicted to be dry under both today’s land use condition and when 13% of the watershed is urban

Those tributaries not predicted to be dry contribute 3 cfs or less to the Blanco during at low flows

Tributaries cannot be relied upon to keep the Blanco River flowing

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Blanco River: Rainfall

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2% urban; 13% urban; 78% urban

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Predicted flow regimes are almost all lower than those predicted under 1990’s rainfall

The difference is most dramatic at the lowest flows, when the 78% urbanized scenarios predicts the least flow

78% urban area leads to the lowest predicted flows, with the River almost dry at many locations

Increasing urban area from 2% to 13% does not distinctly alter the lowest flows

The flow expected half of the time for the Blanco River is consistently higher when impervious cover is increased to 78%

Increasing the urban area of the watershed to 13% deceases river flows by ~ 10 cfs from what is predicted under current land use conditions. High flows are predicted to be similar regardless of the rainfall scenario modeled

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Q75 Tributaries: Rainfall

from 1945-1960

2% urban; 13% urban; 78% urban

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Tributary influence under drought conditions is similar to that under normal precipitation, and the majority are minor contributors to the main stem

At the lowest flows, all of the tributaries have predicted flow rates of less than 10 cfs

The exceptions occur at 78% urban land cover, when flow rates in both the Little Blanco River and Cypress Creek exceed 10 cfs. The Little Blanco River is predicted to contribute over 20 cfs, although this is a small fraction of the main stem flow.

At the highest predicted flows, contributions from the Little Blanco and Cypress Creek are noticeable under all urbanization scenarios

• 13% urbanized watershed predicts the lowest flows

• 78% urbanized watershed has consistently the highest flows

• Current land use condition of 2% urban area falls in the mid-range flows

• The only exception to this trend is the predicted lowest flows under drought conditions

• The consistency of this trend provides a strong indication of expected changes to stream flow as urbanization progresses

• As the watershed area grows in population and infrastructure, the amount of flow reaching the river will decrease.

• As watershed urbanization continues, the amount of impervious cover and hardened land surface will reach a point where the majority of the precipitation becomes run-off and direct recharge to the Blanco River.

Common Features to the Modeled Scenarios

• A total of six different scenarios were modeled using the SWAT model with the BASINS interface and ArcView3.1. The scenarios modeled flows under current land use, 13% urban land use, and 78% urban land use against rainfall patterns from 1990-2006 and 1945-1960 in 38 38 separate sub-basins within the larger Blanco watershed.

• Model runs predict an initial decrease in flows as the watershed population grows and the urban area approaches 13% of the total land use. Under low, mid, and high flow rates, an expansion of urban area first decreases river flows.

• As the amount of impervious cover due to the infrastructure that accompanies urbanization increases to cover the majority of the watershed area, the flows in the river increase dramatically.

• When drought conditions are simulated, there occurs an expected drop in predicted flows for all but the most infrequent events. During a drought, the Blanco River is predicted to maintain only limited flow with almost negligible contribution from tributaries.

• Under drought, low flows continue until the confluence of the Blanco with the Little Blanco River which adds significant flow only during extremely high flow event. Under normal climate conditions, the contribution of the Little Blanco River is significant to both the mid and high flow regimes. In all scenarios, the only tributaries of significance are the Little Blanco and Cypress Creek.

Summary of Modeling Results

The case of 78% urbanization of the watershed area represents an extreme, but distant possibility.

In contrast, the watershed is well on its way to becoming 13% urbanized.

This research indicates flows in the Blanco River and its tributaries will decrease consistently as urbanization progresses, and some of the tributaries will have negligible flow rates.

The lowest flows are expected over the first 60 km of the river, before the addition tributary flow. This corresponds to the region of the headwaters and spring systems.

Summary of Modeling Results

Acknowledgements

Texas River Systems Institute and the Texas Nature

Conservancy for providing the funding to make this

project possible

Dept. of Biology for their participation and partnership

Nathanael Banda for his help with building the model

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% Impervious Cover

% Runoff

% Infiltration

Example of the effects of impervious cover on both rainfall runoff

and infiltration – data monitoring sites are from Austin, 1996