More on this study and Texas's Memorial Day week floods on Dot Earth: http://j.mp/dotTXfloods This is a presentation by hydrologist Joanna C. Curran describing her 2007 modeling study assessing how urbanization in the Blanco River watershed west of Austin, Texas, could affect the river under various scenarios for growth. Two of various conclusions: - 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. Curran was at Texas State University and the University of Virginia at the time, but is now at Northwest Hydraulic Consultants and is a researcher at the University of Tennessee. At the time, the research focus was more on drought than flood, but in developing the model of the river and surrounding basin, The paper: Modeling future flows in the Blanco River watershed under various development and rainfall scenarios Posted in full here: http://www.freepatentsonline.com/article/Texas-Journal-Science/176375390.html 08/01/2007 The Texas Journal of Science Texas Academy of Science Texas Academy of Science ISSN: 0040-4403 Date: August, 2007 Source Volume: 59 Source Issue: 3

1. Modeling the Blanco Watershed
Joanna C. Curran

2. 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

3. 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

5. 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

6. 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

7. 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

8. STATSGO Soil Layers, USDA-NRCS)

9. 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

10. 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)

11. • 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

12. 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

13. Current Land Use
~2% urbanized
~13% urbanized
~80% urbanized
(From GIRAS, USGS)

14. 0
15
30
45
60
75
90
105
120
135
150
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
%time flow exceeded
Q,cfs
Model results
are analyzed
through
hydrographs
and flow
duration curves
Flow expected
half the time
Q50 = 20 cfs
Flow exceeded
10% of time
Q10 = 45 cfs
Flow exceeded
75% of time
Q10 = 12 cfs
0
20
40
60
80
100
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140
Jan-92 Sep-94 Jun-97 Mar-00 Dec-02 Sep-05
Q,cfs

15. • 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

16. 0
20
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120
140
0 20 40 60 80 100 120 140 160
Distance Downstream (km)
Q,cfs
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0 20 40 60 80 100 120 140 160
Distance Downstream (km)
Q,cfs
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0 20 40 60 80 100 120 140 160
Distance Downstream (km)
Q,cfs
Q75
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

17. 0
1
2
3
4
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6
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10
0 20 40 60 80 100 120 140 160
Distance Downstream (km)
Q,cfs
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0 20 40 60 80 100 120 140 160
Distance Downstream (km)
Q,cfs
0
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140
0 20 40 60 80 100 120 140 160
Distance Downstream (km)
Q,cfs
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

18. 0
500
1000
1500
2000
2500
0 20 40 60 80 100 120 140 160
distance downstream (km)
Q(cfs)
0
50
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350
0 20 40 60 80 100 120 140 160
distance downstream (km)
Q(cfs)
0
20
40
60
80
100
120
0 20 40 60 80 100 120 140 160
distance downstream (km)
Q(cfs)
Blanco River: Rainfall
from 1945-1960Q75
2% urban; 13% urban; 78% urban
Q50
Q10
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

19. 0
10
20
30
40
50
60
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80
90
0 20 40 60 80 100 120
Distance Downstream (km)
Q,cfs
0
5
10
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25
0 20 40 60 80 100 120
Distance Downstream (km)
Q,cfs
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80 100 120
Distance Downstream (km)
Q,cfs
Q75 Tributaries: Rainfall
from 1945-1960
2% urban; 13% urban; 78% urban
Q50
Q10
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

20. • 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

21. • 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

22. 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

23. 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

24. 0
10
20
30
40
50
60
70
80
90
100
BearCreekatFM
1826
SlaughterCreekat
FM1826
BartonCreekat71
BartonCreekatLost
Creek
BullCreekat360
BartonCreekat360
WilliamsonCreekat
OakHill
WalnutCreekat
Webberville
ShoarCreekat12
WallerCreekat23
WallerCreekat38
BoggyCreekat183
Percentage
% Impervious Cover
% Runoff
% Infiltration
Example of the effects of impervious cover on both rainfall runoff
and infiltration – data monitoring sites are from Austin, 1996