Science

A 2007 Model of Growth Impacts on the Flow of Texas's Blanco River

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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
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  • 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
  • 4. 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
  • 5. 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
  • 6. 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
  • 7. STATSGO Soil Layers, USDA-NRCS)
  • 8. 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
  • 9. 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)
  • 10. • 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
  • 11. 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
  • 12. Current Land Use ~2% urbanized ~13% urbanized ~80% urbanized (From GIRAS, USGS)
  • 13. 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 80 100 120 140 Jan-92 Sep-94 Jun-97 Mar-00 Dec-02 Sep-05 Q,cfs
  • 14. • 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
  • 15. 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 160 Distance Downstream (km) Q,cfs 0 50 100 150 200 250 300 350 0 20 40 60 80 100 120 140 160 Distance Downstream (km) Q,cfs 0 500 1000 1500 2000 2500 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
  • 16. 0 1 2 3 4 5 6 7 8 9 10 0 20 40 60 80 100 120 140 160 Distance Downstream (km) Q,cfs 0 5 10 15 20 25 30 35 40 0 20 40 60 80 100 120 140 160 Distance Downstream (km) Q,cfs 0 20 40 60 80 100 120 140 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
  • 17. 0 500 1000 1500 2000 2500 0 20 40 60 80 100 120 140 160 distance downstream (km) Q(cfs) 0 50 100 150 200 250 300 350 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
  • 18. 0 10 20 30 40 50 60 70 80 90 0 20 40 60 80 100 120 Distance Downstream (km) Q,cfs 0 5 10 15 20 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
  • 19. • 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
  • 20. • 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
  • 21. 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
  • 22. 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
  • 23. 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
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