In cooperation with the San Antonio Water System
Simulation of Runoff and Recharge and Estimation of Constituent Loads in Runoff, Edwards Aquifer Recharge Zone (Outcrop) and Catchment Area, Bexar County, Texas, 1997–2000
By Darwin J. Ockerman
U.S. Geological Survey
Water-Resources Investigations Report 02–4241
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Contents
Abstract
Introduction
Purpose and Scope
Description of Study Area
Description of Simulation Model and Modeling Process
Sources of Model Calibration Data
Simulation of Runoff and Recharge
Model Calibration and Testing
Helotes Creek Basin
East Elm Creek and West Elm Creek Basins
Panther Springs Creek Basin
Error Analysis
Sensitivity Analysis
Runoff From and Recharge to the Major Basins
Estimation of Constituent Loads in Runoff
Summary
References
Figures
| 1–2. | Maps showing: | |
| 1. | Edwards aquifer recharge zone (outcrop), catchment area, and associated stream basins, Bexar County, Texas | |
| 2. | Land use in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas | |
| 3. | Photograph showing floodwater retention behind Salado Creek structure, October 1998 | |
| 4–5. | Maps showing: | |
| 4. | Stream and reservoir segments in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas | |
| 5. | Data-collection stations used for Hydrological Simulation Program—FORTRAN model calibration | |
| 6. | Graph showing Salado Creek streamflow losses in the Edwards aquifer recharge zone (outcrop), Bexar County, Texas, October 1998 | |
| 7–9. | Hydrographs showing: | |
| 7. | Measured evapotranspiration, potential evapotranspiration, and rainfall, Seco Creek Basin, May 1–August 12, 1991 | |
| 8. | Measured and simulated streamflow, Helotes Creek at Helotes, October 15–31, 1998 | |
| 9. | Simulated and estimated evapotranspiration, Helotes Creek Basin, 1997–98 | |
| 10. | Graph showing measured and simulated storm runoff volumes, East Elm Creek and West Elm Creek Basins, 1976–78 | |
| 11. | Hydrograph showing measured and simulated streamflow, East Elm Creek Basin, September 28, 1976 | |
| 12–13. | Graphs showing: | |
| 12. | Measured and simulated storm runoff volumes, Panther Springs Creek Basin, 1973 | |
| 13. | Simulated monthly runoff from (streamflow exiting) and recharge to the study area, 1997–2000 | |
Tables
| 1. | Selected characteristics of stream basins in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas |
| 2. | Basin-related model parameters for the Hydrological Simulation Program—FORTRAN |
| 3. | Process-related model parameters for the Hydrological Simulation Program—FORTRAN |
| 4. | Pervious and impervious land segment areas by geologic and land use categories for the 10 major basins in the study area |
| 5. | Data-collection stations providing data for model calibration |
| 6. | Summary of model calibration results for Helotes Creek Basin, 1997–98 |
| 7. | Summary of model calibration and testing results for East Elm Creek and West Elm Creek Basins, 1976–78 |
| 8. | Summary of model testing results for Panther Springs Creek Basin, 1973 |
| 9. | Errors, mean absolute errors, and mean absolute runoff-weighted errors for storm runoff volumes used in model calibrations |
| 10. | Selected process-related calibration parameters used for basin simulations |
| 11. | Sensitivity of selected process-related parameters, Panther Springs Creek Basin |
| 12. | Annual rainfall and estimates of runoff from (streamflow exiting) and recharge to the 10 major basins in the study area, 1997–2000 |
| 13. | Simulated recharge to the Edwards aquifer in the study area by source, 1997–2000 |
| 14. | Median event-mean concentrations, by land use category, used to compute constituent loads |
| 15. | Estimates of annual loads, by land use, and yields for suspended solids in surface-water runoff for the 10 major basins in the study area, 1997–2000 |
| 16. | Estimates of annual loads, by land use, and yields for dissolved solids in surface-water runoff for the 10 major basins in the study area, 1997–2000 |
| 17. | Estimates of annual loads, by land use, and yields for dissolved nitrite plus nitrate nitrogen in surface-water runoff for the 10 major basins in the study area, 1997–2000 |
| 18. | Estimates of annual loads, by land use, and yields for total lead in surface-water runoff for the 10 major basins in the study area, 1997–2000 |
Abstract
The U.S. Geological Survey developed a watershed model (Hydrological Simulation Program—FORTRAN) to simulate runoff and recharge and to estimate constituent loads in surface-water runoff in the Edwards aquifer recharge zone (outcrop) and catchment area in Bexar County, Texas. Rainfall and runoff data collected during 1970–98 from four gaged basins in the outcrop and catchment area were used to calibrate and test the model. The calibration parameters were applied in simulations of the four calibration basins and six ungaged basins that compose the study area to obtain runoff and recharge volumes for 4 years, 1997–2000. In 1997, simulated runoff from the study area was 5.62 inches. Simulated recharge in the study area was 7.85 inches (20 percent of rainfall). In 1998, simulated runoff was 11.05 inches; simulated recharge was 10.99 inches (25 percent of rainfall). In 1999, simulated runoff was 0.66 inch; simulated recharge was 3.03 inches (19 percent of rainfall). In 2000, simulated runoff was 5.29 inches; simulated recharge was 7.19 inches (21 percent of rainfall). During 1997–2000, direct infiltration of rainfall accounted for about 56 percent of the total Edwards aquifer recharge in Bexar County. Streamflow losses contributed about 37 percent of the recharge; flood impoundment contributed 7 percent. The simulated runoff volumes were used with event-mean-concentration data from basins in the study area and from other Bexar County basins to compute constituent loads and yields for various land uses. Annual loads for suspended solids, dissolved solids, dissolved nitrite plus nitrate nitrogen, and total lead were consistently largest from undeveloped land and smallest from commercial land or transportation corridors. Annual loads and yields varied with rainfall, with the maximum loads produced in the wettest year (1998) and the minimum loads produced in the driest year (1999).
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