(Outcrop) and Catchment Area, Bexar County, Texas, 1997–2000

Cover: Salado Creek above Loop 1604, October 17, 1998. U.S. Department of the Interior U.S. Geological Survey

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

In cooperation with the San Antonio Water System

Austin, Texas 2002 U.S. DEPARTMENT OF THE INTERIOR Gale A. Norton, Secretary

U.S. GEOLOGICAL SURVEY Charles G. Groat, Director

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For additional information write to

District Chief U.S. Geological Survey 8027 Exchange Dr. Austin, TX 78754–4733 E-mail: [email protected]

Copies of this report can be purchased from

U.S. Geological Survey Information Services Box 25286 Denver, CO 80225–0286 E-mail: [email protected] ii CONTENTS

Abstract ...... 1 Introduction ...... 1 Purpose and Scope ...... 3 Description of Study Area ...... 3 Description of Simulation Model and Modeling Process ...... 4 Sources of Model Calibration Data ...... 10 Simulation of Runoff and Recharge ...... 12 Model Calibration and Testing ...... 12 Helotes Creek Basin...... 12 East Elm Creek and West Elm Creek Basins ...... 17 Panther Springs Creek Basin ...... 19 Error Analysis ...... 20 Sensitivity Analysis ...... 21 Runoff From and Recharge to the Major Basins ...... 22 Estimation of Constituent Loads in Runoff ...... 23 Summary ...... 30 References ...... 31

FIGURES

1–2. Maps showing: 1. Edwards aquifer recharge zone (outcrop), catchment area, and associated stream basins, Bexar County, Texas ...... 2 2. Land use in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas ..... 5 3. Photograph showing floodwater retention behind Salado Creek structure, October 1998 ...... 6 4–5. Maps showing: 4. Stream and reservoir segments in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas ...... 7 5. Data-collection stations used for Hydrological Simulation Program—FORTRAN model calibration ...... 11 6. Graph showing Salado Creek streamflow losses in the Edwards aquifer recharge zone (outcrop), Bexar County, Texas, October 1998 ...... 12 7–9. Hydrographs showing: 7. Measured evapotranspiration, potential evapotranspiration, and rainfall, Seco Creek Basin, May 1–August 12, 1991 ...... 13 8. Measured and simulated streamflow, Helotes Creek at Helotes, October 15–31, 1998 ...... 15 9. Simulated and estimated evapotranspiration, Helotes Creek Basin, 1997–98 ...... 16 10. Graph showing measured and simulated storm runoff volumes, East Elm Creek and West Elm Creek Basins, 1976–78 ...... 17 11. Hydrograph showing measured and simulated streamflow, East Elm Creek Basin, September 28, 1976 ...... 18 12–13. Graphs showing: 12. Measured and simulated storm runoff volumes, Panther Springs Creek Basin, 1973 ...... 19 13. Simulated monthly runoff from (streamflow exiting) and recharge to the study area, 1997–2000 ...... 23

TABLES

1. Selected characteristics of stream basins in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas ...... 3 2. Basin-related model parameters for the Hydrological Simulation Program—FORTRAN ...... 4

CONTENTS iii 3. Process-related model parameters for the Hydrological Simulation Program—FORTRAN ...... 8 4. Pervious and impervious land segment areas by geologic and land use categories for the 10 major basins in the study area ...... 9 5. Data-collection stations providing data for model calibration ...... 13 6. Summary of model calibration results for Helotes Creek Basin, 1997–98 ...... 15 7. Summary of model calibration and testing results for East Elm Creek and West Elm Creek Basins, 1976–78 ...... 18 8. Summary of model testing results for Panther Springs Creek Basin, 1973 ...... 19 9. Errors, mean absolute errors, and mean absolute runoff-weighted errors for storm runoff volumes used in model calibrations ...... 20 10. Selected process-related calibration parameters used for basin simulations ...... 20 11. Sensitivity of selected process-related parameters, Panther Springs Creek Basin ...... 21 12. Annual rainfall and estimates of runoff from (streamflow exiting) and recharge to the 10 major basins in the study area, 1997–2000 ...... 24 13. Simulated recharge to the Edwards aquifer in the study area by source, 1997–2000 ...... 25 14. Median event-mean concentrations, by land use category, used to compute constituent loads ...... 25 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 ...... 26 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 ...... 27 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 ...... 28 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 ...... 29

iv 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

Abstract solids, dissolved nitrite plus nitrate nitrogen, and total lead were consistently largest from undevel- The U.S. Geological Survey developed a oped land and smallest from commercial land or watershed model (Hydrological Simulation transportation corridors. Annual loads and yields Program—FORTRAN) to simulate runoff and varied with rainfall, with the maximum loads pro- recharge and to estimate constituent loads in duced in the wettest year (1998) and the minimum surface-water runoff in the Edwards aquifer loads produced in the driest year (1999). recharge zone (outcrop) and catchment area in Bexar County, Texas. Rainfall and runoff data INTRODUCTION collected during 1970–98 from four gaged basins in the outcrop and catchment area were used to The Edwards aquifer is one of the most produc- calibrate and test the model. The calibration param- tive carbonate aquifers in the Nation and is the major source of public water supply for San Antonio, Texas, eters were applied in simulations of the four cali- and Bexar County (fig. 1). In addition to providing bration basins and six ungaged basins that compose public water supply for more than 1 million people in the study area to obtain runoff and recharge vol- south-central Texas, the Edwards aquifer supplies large umes for 4 years, 1997–2000. In 1997, simulated quantities of water for agriculture, industry, military runoff from the study area was 5.62 inches. Simu- installations, and recreational activities. The aquifer lated recharge in the study area was 7.85 inches also is a source of water to major springs in the region. (20 percent of rainfall). In 1998, simulated runoff These springs supply flow to downstream users and was 11.05 inches; simulated recharge was 10.99 provide habitat for several threatened or endangered inches (25 percent of rainfall). In 1999, simulated species. runoff was 0.66 inch; simulated recharge was 3.03 Most recharge to the Edwards aquifer occurs west inches (19 percent of rainfall). In 2000, simulated of Bexar County by direct infiltration of precipitation runoff was 5.29 inches; simulated recharge was and by streamflow loss in the Edwards recharge zone. 7.19 inches (21 percent of rainfall). During 1997– After entering the aquifer, water moves from west to 2000, direct infiltration of rainfall accounted for east to points of discharge in Bexar County (mostly about 56 percent of the total Edwards aquifer public supply wells) and then northeastward into Comal and Hays Counties (fig. 1), where it is discharged by recharge in Bexar County. Streamflow losses withdrawals (wells) and two major springs. Additional contributed about 37 percent of the recharge; recharge to the Edwards aquifer occurs in the recharge flood impoundment contributed 7 percent. The zone in northern Bexar County and southern Comal and simulated runoff volumes were used with event- Hays Counties. mean-concentration data from basins in the study In northern Bexar County, residential and com- area and from other Bexar County basins to commercial development in the Edwards aquifer recharge pute constituent loads and yields for various land zone and catchment area is increasing. Urban develop- uses. Annual loads for suspended solids, dissolved ment can have an appreciable influence on the quality of

Abstract 1 4 22'30" o HAYS COUNTY HAYS 98 BEXAR COUNTY TEXAS LOCATION MAP LOCATION COMAL COUNTY 1:24,000 quadrangles Base modified from U.S. Geological Survey Mercator Projection, Zone 1 Universal Transverse ELM CREEK 6 MILES MUD CREEK 30' o 98 CREEK SPRINGS PANTHER CREEK CIBOLO San Antonio 024 BEXAR COUNTY Study area OLMOS/ CREEKS HUEBNER CREEK SALADO 37'30" o 98 LEON CREEK CREEK HELOTES 45' o 98 Catchment area Catchment Edwards aquifer recharge zone (outcrop) recharge aquifer Edwards Stream basin boundary CREEK CULEBRA EXPLANATION SAN CREEK GERONIMO Edwards aquifer recharge zone (outcrop), catchment area, and associated stream basins, Bexar County, Texas. 37'30" o 29 Figure 1. Figure

2 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 Table 1. Selected characteristics of stream basins in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas

Edwards aquifer Estimated Total area Basin outcrop area impervious fraction (square miles) (square miles) (percent) San Geronimo Creek (within Bexar County) 24.52 3.12 0 Culebra Creek 17.38 13.55 .5 Helotes Creek 27.41 5.23 3.9 Leon Creek 65.25 16.73 7.3 Olmos/Huebner Creeks 8.48 7.38 12.5 Salado Creek 30.68 6.61 2.3 Panther Springs Creek 22.26 14.45 7.2 Mud Creek 22.73 18.14 9.7 Elm Creek 30.07 25.33 4.1 Cibolo Creek (within Bexar County) 61.62 14.56 2.1 Total 310.4 125.1 4.6 surface water and on water that recharges the aquifer. Program—FORTRAN (HSPF) watershed model Impervious land cover in developed areas can result in (Bicknell and others, 1997) was used for the simula- increased stormwater runoff conveying contaminants tions. Rainfall and runoff data collected during 1970–98 from nonpoint sources to local streams or geologic from selected gaged basins of the Edwards aquifer features (caves, fractures) where infiltration into the recharge zone and catchment area were compiled and aquifer can occur. used to calibrate and test the model. The calibration Estimates of runoff, water-quality constituent parameters then were transferred to the largely ungaged loads in runoff, and recharge to the Edwards aquifer are study area, and simulations were done for the entire uncertain because of insufficient streamflow and water- study area to obtain runoff and recharge volumes. The quality data in the Edwards aquifer recharge zone and simulated runoff volumes were used with event-mean- catchment area and the complexity of the rainfall-runoff concentration data from basins in the study area and process. The effects of commercial and residential other Bexar County basins to compute constituent loads development on runoff quantity and quality and and yields for various land uses. recharge quantity are not well known. To learn more about these phenomena, the U.S. Geological Survey Description of Study Area (USGS), in cooperation with the San Antonio Water System (SAWS), began a study in 1996 to develop a The study area comprises the Edwards aquifer watershed model to simulate runoff and recharge and to recharge zone and the adjacent catchment area in north- estimate constituent loads in surface-water runoff in the ern Bexar County (fig. 1). The Edwards aquifer Edwards aquifer recharge zone and catchment area in recharge zone is approximately coincident with the Bexar County. Edwards aquifer outcrop. (The Edwards aquifer Purpose and Scope recharge zone is hereinafter referred to as the Edwards aquifer outcrop.) The catchment area consists of drain- This report describes the calibration, testing, age basins of streams that lose water to (recharge) the and use of a watershed model to simulate runoff and Edwards aquifer as they cross the outcrop. The 10 recharge, the results of which were used to estimate stream basins of the study area encompass about 310 constituent loads in surface-water runoff in the Edwards square miles that include about 125 square miles of aquifer recharge zone and catchment area in Bexar Edwards aquifer outcrop and about 185 square miles of County for 1997–2000. The Hydrological Simulation catchment area (table 1).

INTRODUCTION 3 Most of the land (about 72 percent) in the outcrop Description of Simulation Model and Modeling is undeveloped (fig. 2) (David Kruse, Alamo Area Process Council of Governments, written commun., 1999). In 1998, residential land use was about 10 percent of the The HSPF (Bicknell and others, 1997) is a Edwards aquifer outcrop area, although development continuous-simulation model using a conceptual framework to represent infiltration, evaporation, interception there is increasing. A comparison of 1996–98 land use storage, surface runoff, interflow (water that infiltrates data indicates that residential land use in the study area into the soil and moves laterally through the upper soil increased by about 9 percent from 19,260 acres in 1996 horizons until it returns to the surface, often in a to 20,970 acres in 1998. stream), and base flow on a pervious land segment The primary geologic formations that crop out in (PERLND) and to represent retention storage and sur- the study area are the Glen Rose Limestone and the face runoff on an impervious land segment (IMPLND). Kainer, Person, and Georgetown Formations (Stein and Each user-defined land segment represents its own Ozuna, 1995). The Glen Rose Limestone covers most unique hydrologic response system on the basis of soil of the catchment area, and the Kainer, Person, and type, land cover, basin slope, or other pertinent basin Georgetown Formations cover the Edwards aquifer out- characteristic. These land segments do not need to be crop. The Glen Rose Limestone is less permeable than contiguous within the model. The runoff from each land the formations of the Edwards aquifer outcrop and gen- segment is moved through a system of channels or res- erates more runoff per unit area. The Glen Rose Lime- ervoir reaches (RCHRES) using storage routing. stone also sustains base flow (ground-water inflow to The HSPF model uses input from three types of stream channels) for days or weeks after large rainfalls. data: time series, basin-related model parameters, and In contrast, the more permeable formations of the process-related model parameters. Continuous time outcrop allow streamflow losses to the aquifer, thus lim- series of precipitation and potential evaporation are iting streamflow after large rainfalls to very short dura- needed to run the model. Point-precipitation data, tion (often only a few hours). These streamflow losses measured by rain gages, are assumed to be uniform contribute recharge directly to the Edwards aquifer over a land segment. Potential evaporation data can be (Land and others, 1983). estimated from measured pan evaporation or computed from minimum and maximum daily temperatures. Time Vegetation on the undeveloped part of the Glen series of measured streamflow are used for model cali- Rose Limestone consists of moderate to dense stands bration and testing. of juniper, oak, and shrub. Vegetation on the Edwards The six basin-related model parameters (table 2) aquifer outcrop consists of open to moderately open define the areal extent of each land segment (AREA), oak, juniper, and shrub (University of Texas, Bureau of the reach length (LEN), and a table of values (FTABLE) Economic Geology, 1985). The soils in the study area of surface area (SAREA), volume (VOL), and discharge consist primarily of two associations: Tarrant-Brackett and Crawford-Bexar (U.S. Department of Agriculture, Soil Conservation Service, 1966). The Tarrant-Brackett Table 2. Basin-related model parameters for the soils account for about 80 percent of the study area and Hydrological Simulation Program—FORTRAN overlie most of the Glen Rose Limestone. Crawford- Bexar soils are more common on the Edwards aquifer [PERLND, pervious land segment; IMPLND, impervious outcrop. Both soil associations are primarily clay, land segment; FTABLE, table of depth, surface area, ranging from a few inches to about 16 inches deep. volume, and discharge for each stream reach] Twelve major flood-control structures are Parameter Description (units) located in the study area (San Antonio River Authority, AREA Drainage area of each PERLND or 1996); 10 are located in the Edwards aquifer outcrop. IMPLND (acres) During major runoff, the structures retain water so that LEN Stream reach length (miles) it has more time to infiltrate as ground-water recharge. DEPTH FTABLE depth (feet) Figure 3 is a photograph of water behind a flood-control SAREA FTABLE surface area (acres) structure in the upper Salado Creek Basin after a rainfall VOL FTABLE volume (acre-feet) in October 1998. DISCH FTABLE discharge (cubic feet per second)

4 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 22'30" o 98

Base modified from U.S. Geological Survey 1:24,000 quadrangles Mercator Projection, Zone 14 Universal Transverse Land use from Alamo Area Council of Governments digital data, 1998

6 MILES US 281 US 30' o 98

024 Loop1604 Stream basin boundary 37'30" o 98

I–10 EXPLANATION Military Industrial Commercial Residential Institutional/cultural/recreational Quarries Undeveloped Land use (1998) Edwards aquifer recharge zone (outcrop) recharge aquifer Edwards 45' o 98 State 16 Land use in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas. 37'30" o 29 Figure 2. Figure

INTRODUCTION 5 Figure 3. Floodwater retention behind Salado Creek structure, October 1998.

(DISCH), as a function of depth (DEPTH) for each ing. HSPEXP is a stand-alone program that incorporates reach or reservoir of the basin. These parameters repre- HSPF. The HSPEXP procedures consist of a set of sent the physical characteristics of each land segment or hierarchical rules designed to guide the calibration of reach of a basin and generally remain unchanged during the model through a systematic evaluation of the model calibration and testing of the model. parameters. Process-related model parameters (table 3) The first step in developing the HSPF model for represent the physical processes of infiltration, evapo- the study area basins was to subdivide the basins into transpiration (ET), interception, interflow, ground- segments (RCHRES), taking into consideration the water recession, and surface runoff for each land stream and reservoir (flood-control impoundment) con- segment. The process-related model parameters for figuration of each basin. After subdivision, each basin each land segment are adjusted to calibrate the model. contained from 1 to 12 RCHRES segments—63 stream The following parameters can be varied by month segments and 12 reservoir segments in all for the 10 to account for seasonal variations: interception storage major basins (fig. 4). FTABLE information for gaged capacity (CEPSC), interflow inflow (INTFW), interstream reaches was based on discharge measurements flow recession rate (IRC), lower-zone ET (LZETP), made at USGS streamflow-gaging stations in the study Manning’s n for assumed overland flow plane (NSUR), area. FTABLE information for ungaged reaches was and upper-zone nominal storage (UZSN). Because estimated from similar gaged reaches. FTABLE infor- the available data were insufficient to characterize mation for flood-control reservoirs was developed from monthly variations, the parameters were not varied by the dam and reservoir design specifications. month. The HSPF users manual (Bicknell and others, Pervious land segments (PERLNDs) and imper- 1997) provides a more complete description of each vious land segments (IMPLNDs) were configured parameter. according to geology and land use. The two geologic The HSPEXP (Lumb and others, 1994), a categories were based on the relative permeability of computerized expert system, was used to help adjust the rocks that compose the Edwards aquifer outcrop process-related parameters during calibration and test- (relatively high permeability) and the catchment area

6 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 22'30" o 98 ELM CREEK 1:24,000 quadrangles Base modified from U.S. Geological Survey Mercator Projection, Zone 14 Universal Transverse 6 MILES MUD CREEK 30' o 98 CREEK SPRINGS PANTHER CREEK CIBOLO 024 OLMOS/ CREEKS HUEBNER CREEK SALADO 37'30" o 98 LEON CREEK CREEK HELOTES 45' o 98 Stream in catchment area Stream in catchment Edwards aquifer recharge zone (outcrop) recharge aquifer Edwards Stream basin boundary Stream- or reservoir-segment boundary Stream- or reservoir-segment Stream in Edwards aquifer recharge zone recharge aquifer Stream in Edwards Flood-control impoundment in catchment area impoundment in catchment Flood-control Flood-control impoundment in Edwards aquifer recharge zone recharge aquifer impoundment in Edwards Flood-control Catchment area Catchment CREEK CULEBRA EXPLANATION SAN CREEK GERONIMO Stream and reservoir segments in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas. 37'30" o 29 Figure 4. Figure

INTRODUCTION 7 Table 3. Process-related model parameters for the Hydrological Simulation Program—FORTRAN [ET, evapotranspiration; --, none]

Default Minimum Maximum Parameter Description Units value value value AGWS Initial active ground-water storage -- 0 -- inches AGWETP Available ET satisfied by active ground water 0 0 1.0 -- AGWRC Active ground-water recession rate -- .001 1.0 per day BASETP Available ET satisfied by base flow 0 0 1.0 --

CEPSC Interception storage capacity 0 0 1.0 inches DEEPFR Fraction of inflow that enters inactive ground water 0 0 1.0 -- INFEXP Infiltration equation exponent 2.0 0 10.0 -- INFILD Ratio of maximum to mean infiltration capacities 2.0 1.0 2.0 --

INFILT Index to infiltration capacity of soil -- .0001 100.0 inches per hour INTFW Interflow inflow -- 0 -- -- IRC Interflow recession rate -- 0 1.0 per day KVARY Nonexponential ground-water recession rate 0 0 -- per inch

LSUR Length of assumed overland flow plane -- 1.0 -- feet LZETP Lower-zone ET 0 0 1.0 -- LZS Initial lower-zone storage -- 0 -- inches LZSN Lower-zone nominal storage -- .01 100.0 inches

NSUR Manning’s n for assumed overland flow plane .1 .001 1.0 -- SLSUR Slope of assumed overland flow plane -- .000001 10.0 feet per foot UZS Initial upper-zone storage -- 0 -- inches UZSN Upper-zone nominal storage -- .01 10.0 inches

(relatively low permeability). Land use categories used related parameters for geologically homogeneous in the model were undeveloped, residential, commer- basins facilitated the subsequent calibration of gaged cial, transportation corridor, and quarries. Military land basins that included both geologic categories. The trans- use largely is undeveloped rangeland. The remaining ferability of calibration parameters to ungaged basins land use categories (fig. 2) account for less than 5 per- was tested by simulating gaged basins with parameter cent of the overall land use and, therefore, were grouped sets developed from other gaged basins. with other categories that had similar hydrologic char- Because uncertainty in model parameters still acteristics. Table 4 lists the areas, in acres, of PERLNDs exists after calibration and testing, sensitivity of simu- and IMPLNDs for each basin. lated runoff and recharge to changes in selected process- Process-related parameters were calibrated for related input parameters was tested by altering values gaged basins. During the calibration process, parameter of selected parameters and evaluating the resulting sets first were developed for gaged basins that were changes in runoff and recharge. essentially undeveloped and consisted of a single geo- After calibration, testing, and sensitivity analysis, logic category—outcrop or catchment area. Because the the calibration model parameters were applied to both outcrop and catchment area have substantially different gaged and ungaged basins, and model simulations were rainfall-runoff characteristics, development of process- done to estimate runoff and recharge. The runoff results

8 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 Creek Cibolo Elm Creek Mud Creek Creek Panther Springs Creek Salado Basin Creeks Olmos/ Huebner Leon Creek Creek PERLND (pervious land segment) land PERLND (pervious Helotes IMPLND (impervious land segment) land (impervious IMPLND Creek Culebra San Creek Geronimo Category Pervious and impervious land segment areas geologic by and land use categories the 10 major for basins in the study area Outcrop–undevelopedOutcrop–residentialOutcrop–commercial 2,512Outcrop–transportationOutcrop–quarries 8,671Catchment area–undeveloped 2 3,158 0 area–residentialCatchment 55 12,878 7,610 area–commercialCatchment 0 2,498 2,222 area–transportationCatchment 102 0 0 174 0 10,962 area–quarriesCatchment 21 3,447 3 8 34 23,368 996 140 6,617 503 0 347Outcrop–residential 2,017 1,022 482 7,724 0 32 284 0Outcrop–commercial 140 4,332 232 13,688 14,625 124 94 382Outcrop–transportation 226 4,812 area–residential 52Catchment 14 1,087 8,703 564 686 0 86 120 area–commercial 2,108Catchment 1 26 1,715 188 0 562 372 area–transportationCatchment 61 55 41 1,645 11 1,073 326 87 0 2 135 0 472 34 26,634 12 36 0 24 0 160 494 672 372 0 0 41 360 0 32 24 1 6 285 5 108 139 174 34 764 0 100 504 0 17 16 2,247 345 562 758 340 685 139 5 0 106 31 232 60 128 40 0 52 99 0 216 280 86 425 2 191 46 35 171 372 469 342 2 92 6 472 209 0 159 18 368 81 17 55 31 33 105 248 0 277 215 Table 4. Table acres] [In

INTRODUCTION 9 were used to compute constituent loads for the entire downstream from one RCHRES to the next RCHRES. study area. However, water entering a RCHRES as runoff from a PERLND or IMPLND does not travel the entire length Sources of Model Calibration Data of a model RCHRES; therefore, the streamflow-loss relation must be scaled accordingly. The simplifying Sources of data (data-collection stations) for assumption was made that PERLND and IMPLND run- model calibration and development of process-related off within a RCHRES occurred at the midpoint of the parameter sets for the Edwards aquifer outcrop and RCHRES. Therefore, streamflow entering a RCHRES catchment area are shown in figure 5 and listed in from an upstream RCHRES incurs streamflow losses table 5. Some rainfall and runoff calibration data were along the entire RCHRES, and streamflow entering a obtained from a 1970–79 data-collection program in RCHRES as PERLND or IMPLND runoff incurs several undeveloped basins in the Edwards aquifer out- streamflow losses over one-half the RCHRES length. crop in Bexar County. Most of the stations from that Direct infiltration to the Edwards aquifer also program have been discontinued; land use changes, occurs from storm runoff detained by 10 flood-control including suburban development and construction of structures. The infiltration behind these structures was flood-control structures, have altered the basin charac- estimated on the basis of data collected at Parkers teristics. The 1970s data-collection program provided Creek Reservoir in Medina County (site no. 14, fig. 5). the only available data for characterizing the rainfall- This reservoir in the outcrop collects runoff from a runoff relation in the undeveloped Edwards aquifer out- 10-square-mile basin and is similar in size and design to crop in the study area. The rainfall and streamflow data the flood-control structures in the study area in Bexar from undeveloped basins are required for the model cal- County. The relation between reservoir impoundment ibration process, especially for selection of process- surface area and infiltration at Parkers Creek was used related parameters for PERLNDs associated with the in the model to simulate withdrawals on the basis of res- Edwards aquifer outcrop. ervoir elevation and surface area for the Bexar County Another important factor in the rainfall-runoff flood-control reservoirs. relation is streamflow losses in the outcrop area. ET is water transpired from vegetation and evap- Streamflow loss, as a function of streamflow, in Salado orated from land surfaces. In the HSPF model, rainfall 1 Creek after a major runoff during October 1998 is infiltrating into the ground is recharge or base flow that shown in figure 6. Streamflow losses are not accounted returns to the stream channel. The base-flow component for by the HSPF process-related parameters. Instead, can be calibrated using streamflow records. However, the channel routing process in HSPF was adjusted to to estimate the fraction of infiltration that becomes incorporate streamflow losses as channel withdrawals ground-water recharge, ET must be estimated. To cali- on the basis of streamflow in a reach. The relation brate the HSPF parameters affecting ET, actual ET data shown in figure 6 was used to obtain streamflow losses are needed. Actual ET data were not available for the for the major stream reaches crossing the Edwards aqui- study area but were collected (along with rainfall and fer outcrop in the study area. The relation was linearly potential ET [PET]) from the Seco Creek Basin in extrapolated to simulate streamflow losses for stream- Medina and Uvalde Counties (Dugas and others, 1998) flows greater than about 200 cubic feet per second. during the months of February–September2 from 1991 Streamflow losses are assumed to contribute directly to through 1995. The area of the Seco Creek Basin where ground-water recharge (Puente, 1978). the ET data were collected is undeveloped rangeland on The streamflow-loss relation is appropriate for Glen Rose Limestone, similar to that in the catchment computing losses of streamflow as the water moves area in Bexar County. To use the Seco Creek ET data for model calibra- 1 Streamflow-loss data (unpublished) were collected by the tion, an empirical relation between measured ET, PET, USGS along Salado Creek after a storm during October 17–19, and rainfall at Seco Creek was developed. A sample of 1998. Flood runoff stored by flood-control structures in the upper basin (Salado structures 1 and 2) was released over 2 weeks after the Seco Creek data is shown in figure 7. The empirical the storm, providing an opportunity to measure streamflow at ET relation then was used with PET from the National several locations along a 5.2-mile reach of Salado Creek in the Edwards aquifer outcrop and to compute streamflow losses per mile 2 ET data were not collected during November–February for a range of discharges. because the study area was inaccessible.

10 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 Dam 5 Canyon Area enlarged TEXAS Cibolo Creek LOCATION MAP LOCATION COMAL COUNTY 12 East 2 Elm Creek Creek West Elm West 3 Bulverde 19 Stone Mountain drainage channel Stone Mountain drainage Creek 4 Springs Panther BEXAR 98˚30' Salado Creek 6 COUNTY at Old Blanco Road 16 15 10 5 15 MILES 7 at Loop 1604 Salado Creek 8 Creek Huebner 9 Olmos Creek tributary 010 Huebner Creek 1 Leon Creek 18 17 COUNTY KENDALL 11 Creek Helotes Boerne Creek Sea World Govenment Canyon Govenment Pan evaporation Pan Evapotranspiration Number refers to table 5 to table Number refers 99˚ 17 20 KERR COUNTY Station description— Water quality Water Streamflow Reservoir Rainfall 14 7 12 14 15 COUNTY BANDERA Parkers Creek Parkers Reservoir

13 Parkers Creek Parkers MEDINA COUNTY MEDINA

Study area Study Gaged basin UVALDE COUNTY UVALDE EXPLANATION 20 Seco Creek Base modified from U.S. Geological Survey 1:24,000 quadrangles Mercator Projection, Zone 14 Universal Transverse 99˚30' Data-collection stations used for Hydrological Simulation Program—FORTRAN model calibration. 30˚ 29˚30' 29˚45' Figure 5. Figure

INTRODUCTION 11 24

Loss per mile = (0.062)streamflow + 8.3

Measured Regression

STREAMFLOW LOSS PER MILE, IN CUBIC FEET SECOND 4

0 0 2550 75 100 125 150 175 200 STREAMFLOW, IN CUBIC FEET PER SECOND

Figure 6. Salado Creek streamflow losses in the Edwards aquifer recharge zone (outcrop), Bexar County, Texas, October 1998.

Weather Service (NWS) station at Sea World of San Helotes Creek Basin Antonio and with rainfall data from the Helotes Creek About 98 percent of the gaged part (upper 9,500 at Helotes streamflow-gaging station to estimate actual acres) of the Helotes Creek Basin is undeveloped. The ET for the Helotes Creek Basin during 1997–98. This uppermost geologic unit is the Glen Rose Limestone. estimated actual ET then was used to calibrate the HSPF Data from six storms during April 1997–October 1998 parameters that influence ET. were used for calibration of the parameters related to runoff (table 6). SIMULATION OF RUNOFF AND Measured and simulated streamflow during the RECHARGE storm of Oct. 15–31, 1998, are shown in figure 8. The overall error in simulated runoff (computed as the sum Model Calibration and Testing of simulated runoff, minus the sum of measured runoff, divided by the sum of measured runoff, quantity times Calibration and testing of process-related 100) for the six storms at Helotes Creek was 4.4 percent. parameters involved four gaged basins in the study The mean absolute error in simulated runoff for individ- area: Helotes Creek, East Elm Creek, West Elm Creek, ual storms (average of the absolute values of all storm and Panther Springs Creek. Helotes Creek Basin prima- runoff percent differences) was 60 percent. This large rily is in the catchment area. East Elm Creek and West error is heavily influenced by the 220-percent difference Elm Creek Basins are in the Edwards aquifer outcrop. from the April 1997 storm. Generally, large runoffs are Panther Springs Creek Basin is partly in the Edwards simulated more accurately than smaller runoffs. There- aquifer outcrop and partly in the catchment area. fore, mean absolute error was recomputed to weight the

12 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 0.4 3.0

Measured evapotranspiration (Bowen ratio method) Potential evapotranspiration Rainfall

2.5

0.3

2.0

0.2 1.5 DAILY RAINFALL, IN INCHES 1.0 DAILY EVAPOTRANSPIRATION, IN INCHES 0.1

0.5

0 0 MAY JUNE JULY AUGUST 1991

Figure 7. Hydrograph showing measured evapotranspiration, potential evapotranspiration, and rainfall, Seco Creek Basin, May 1–August 12, 1991.

Table 5. Data-collection stations providing data for model calibration

[USGS, U.S. Geological Survey; FM, Farm Road; NWS, National Weather Service; SH, State Highway; NRCS, Natural TableResources 5. Data-collection Conservation Service] stations providing data for model calibration—Continued

Refer- Name and USGS no. Type Period of ence no. Comment (if applicable) of data operation (fig. 5) 1 USGS station 08181400 Helotes Rainfall/ 1997–2000 15-square-mile basin; data used for Creek at Helotes streamflow calibration of process-related parameters for catchment area 2 USGS station 08178645 East Elm Rainfall/ 1976 2.33-square-mile basin; data used for Creek at San Antonio streamflow calibration of process-related parameters for outcrop–undeveloped 3 USGS station 08178640 West Elm Rainfall/ 1976–78 2.45-square-mile basin; data used for Creek at San Antonio streamflow testing of process-related parameters for outcrop–undeveloped

SIMULATION OF RUNOFF AND RECHARGE 13 Table 5. Data-collection stations providing data for model calibration—Continued

Refer- Name and USGS no. Type Period of ence no. Comment (if applicable) of data operation (fig. 5) 4 USGS station 08178600 Panther Rainfall/ 1973 9.54-square-mile basin; data used for Springs Creek at FM 2696 near San streamflow testing process-related parameters for Antonio outcrop and catchment area 5 NWS station at Canyon Dam Pan evaporation 1973–78 Data used for calibration simulations of West Elm Creek, East Elm Creek, and Panther Springs Creek Basins 6 USGS station 08178595 Stone Rainfall/water 1997–2000 Rainfall data used for 1997–2000 Mountain drainage channel at quality simulations; water-quality data used to Granite Path, San Antonio characterize residential runoff 7 USGS station 08181420 Huebner Water quality 1996 Data used to characterize commercial Creek at De Zavala Rd., San runoff Antonio 8 USGS station 08181425 Cedar Elm Water quality 1996–97 Data used to characterize commercial outfall at Huebner Creek tributary, runoff San Antonio 9 USGS station 08181440 Leon Creek Water quality 1992–99 Data for 1997–99 used to characterize outfall at Ingram Rd. near SH 151, transportation runoff San Antonio 10 USGS station 08177600 Olmos Creek Water quality 1997–98 Data used to characterize residential runoff tributary at FM 1535 at Shavano Park 11 USGS station 08180940 Government Water quality 1997–98 Data used to characterize undeveloped land Canyon Creek near Helotes runoff 12 USGS station 08185000 Cibolo Creek Rainfall 1997–2000 Data used for 1997–2000 simulations at Selma 13 USGS station 08202790 Parkers Streamflow 1992–97 Measured inflow used to estimate water Creek Reservoir inflow near balance of Parkers Creek Reservoir and, D’Hanis in turn, to estimate infiltration at flood- control reservoirs in Bexar County 14 USGS station 08202800 Parkers Reservoir 1992–97 Measured infiltration rates used to estimate Creek Reservoir near D’Hanis elevation and infiltration at flood-control reservoirs in storage Bexar County 15 USGS station 08178592 Salado Streamflow 1998 Gain-loss measurement data used to Creek at Loop 1604 near San estimate streamflow losses in outcrop Antonio 16 USGS station 08178593 Salado Streamflow 1998 Gain-loss measurement data used to Creek at Old Blanco Rd. near San estimate streamflow losses in outcrop Antonio 17 NWS station at Sea World, San Pan evaporation 1997–2000 Data used for 1997–2000 simulations Antonio 18 NWS station near Boerne Rainfall 1997–2000 Data used for 1997–2000 simulations 19 NWS station near Bulverde Rainfall 1997–2000 Data used for 1997–2000 simulations 20 NRCS station in Seco Creek Basin, Evapotrans- 1992–96 Data used for calibration simulations, Medina County piration Helotes Creek Basin

14 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 1,400

1,200 Measured Simulated

1,000

800

600 STREAMFLOW, IN CUBIC FEET PER SECOND 400

200

0 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 OCTOBER 1998

Figure 8. Hydrograph showing measured and simulated streamflow, Helotes Creek at Helotes, October 15–31, 1998.

Table 6. Summary of model calibration results for Helotes Creek Basin, 1997–98

Rainfall amount Measured runoff Simulated runoff Percent Date (inches) (inches) (inches) difference1

04/01–12/1997 4.16 0.29 0.93 220

05/21–31/1997 .80 .52 .08 –85

06/21–30/1997 8.38 5.79 5.65 –2.4

02/25–03/03/1998 1.15 .42 .62 48

03/16–17/1998 1.54 1.02 1.01 –1.0

10/15–31/1998 10.11 7.16 7.58 5.9

Total 26.14 15.20 15.87 4.4

1 Percent difference between measured and simulated runoff computed as simulated minus measured, divided by measured, quantity times 100.

SIMULATION OF RUNOFF AND RECHARGE 15 4.0

Simulated Estimated

3.0

2.0 EVAPOTRANSPIRATION, IN INCHES

1.0

0 JAN JAN FEB FEB APR APR OCT DEC OCT DEC MAY AUG NOV MAY AUG NOV MAR MAR JULY JULY JUNE JUNE SEPT SEPT

1997–98

Figure 9. Hydrograph showing simulated and estimated evapotranspiration, Helotes Creek Basin, 1997–98. error of each storm by measured (gaged) runoff. The of annual rainfall. During 1997, 1998, and 2000, annual mean absolute runoff-weighted error of the six Helotes rainfall was greater than normal, and much of the rain- Creek storms was 12 percent. fall occurred in a few large storms that generated con- siderable runoff that exited the study area and was not Along with parameters related to runoff, param- subject to ET. The result was low ET as a percentage of eters related to ET also were calibrated. Rainfall from rainfall in 1997, 1998, and 2000. In contrast, 1999 was the Helotes Creek at Helotes gage and PET (from the dryer than normal with no major storms; ET as a per- Sea World NWS station) were used to estimate actual centage of rainfall was high. ET on undeveloped Glen Rose Limestone using the empirical relation between measured ET, PET, and rain- Helotes Creek annual rainfall and HSPF simulated fall from the Seco Creek Basin previously described. evapotranspiration (ET), 1997–2000 The model parameters (primarily UZSN, LZSN, and Glen Rose LZETP) were adjusted so that ET simulated by the Glen Rose Rainfall Limestone ET model approximated ET estimated from the empirical Year Limestone ET (inches) (percent of relation. Monthly simulated ET compared with ET (inches) estimated from the empirical relation is shown in annual rainfall) figure 9. 1997 36.75 18.93 51.5 The following table shows rainfall for the Helotes 1998 41.38 18.96 45.8 Creek Basin and simulated ET for the undeveloped 1999 15.72 15.32 97.4 Glen Rose Limestone and ET expressed as a percentage 2000 36.30 18.22 50.2

16 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 1.00

East Elm Creek West Elm Creek

0.10 SIMULATED STORM RUNOFF, IN INCHES

0.01 0.01 0.10 1.00 MEASURED STORM RUNOFF, IN INCHES Figure 10. Measured and simulated storm runoff volumes, East Elm Creek and West Elm Creek Basins, 1976–78.

East Elm Creek and West Elm Creek Basins in figure 10; a hydrograph showing measured and simulated streamflow for the East Elm Creek storm on Sep- Calibration and testing of East Elm Creek and tember 28, 1976, is shown in figure 11. West Elm Creek Basins were done to obtain process- related parameters for the undeveloped Edwards aquifer The overall errors in simulated runoff for the four outcrop. Rainfall and runoff data during May 1976– storms at East Elm Creek and the four storms at West September 1978 (when the basins were undeveloped) Elm Creek were 4.7 and -3.1 percent, respectively. The were used for the model calibration and testing. East mean absolute errors in simulated runoff for individual Elm Creek Basin was calibrated using data from four storms were 17 and 20 percent, respectively; and the storms during 1976. To test the calibration and also mean absolute runoff-weighted errors were 17 and 12 to test the transferability of model parameters to similar percent, respectively. ungaged basins, the calibration model parameters from Data to calibrate ET-related parameters in the the East Elm Creek Basin were applied to the West Edwards aquifer outcrop were not available. Although Elm Creek Basin in simulations of four storms during the infiltration capacity and permeability of the 1976–78. Simulated runoff volumes were 4.7-percent Edwards aquifer outcrop and Glen Rose Limestone larger than the gaged volumes for the East Elm Creek likely are different, other factors affecting ET, such storms and 3.1-percent smaller for the West Elm Creek as soil and vegetation, are similar. The ET-related storms (table 7). Simulated and measured runoff vol- parameters developed for the catchment area (Glen umes for the East and West Elm Creek Basins are shown Rose Limestone) in the Helotes Creek Basin calibration

SIMULATION OF RUNOFF AND RECHARGE 17 2.0 160

140

120 1.5 Measured streamflow Simulated streamflow Cumulative rainfall

100

1.0 80

Total rainfall - 1.67 inches CUMULATIVE RAINFALL, IN INCHES Measured runoff - 0.23 inch 0.5 Simulated runoff - 0.26 inch 40 STREAMFLOW, IN CUBIC FEET PER SECOND

0 0 0123456789101112131415 TIME, IN HOURS

Figure 11. Hydrograph showing measured and simulated streamflow, East Elm Creek Basin, September 28, 1976.

Table 7. Summary of model calibration and testing results for East Elm Creek and West Elm Creek Basins, 1976–78

Rainfall amount Measured runoff Simulated runoff Percent Date (inches) (inches) (inches) difference1 East Elm Creek (calibration) 05/26/1976 2.72 0.11 0.15 36 07/06/1976 2.08 .22 .18 -18 09/28/1976 1.67 .23 .26 13 10/23/1976 1.82 .08 .08 0 Total 8.29 .64 .67 4.7

West Elm Creek (testing) 05/07/1976 2.38 .27 .24 -11 09/28/1976 1.70 .14 .21 50 11/01/1977 4.56 .87 .84 -3.5 09/13/1978 2.71 .36 .30 -17 Total 11.35 1.64 1.59 –3.1

1 Percent difference between measured and simulated runoff computed as simulated minus measured, divided by measured, quantity times 100.

18 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 1.50

1.25

1.00

0.75

SIMULATED RUNOFF, IN INCHES 0.50

0.25

0 0 0.25 0.50 0.75 1.00 1.25 1.50 MEASURED RUNOFF, IN INCHES Figure 12. Measured and simulated storm runoff volumes, Panther Springs Creek Basin, 1973.

Table 8. Summary of model testing results for Panther Springs Creek Basin, 1973 Rainfall amount Measured runoff Simulated runoff Percent Date (inches) (inches) (inches) difference1 06/11–12/1973 4.96 0.45 0.38 –16 07/16/1973 3.29 1.14 .67 –41 09/26–27/1973 4.05 .85 1.45 71 10/11/1973 1.94 .27 .30 11 Total 14.24 2.71 2.80 3.3 1 Percent difference between measured and simulated runoff computed as simulated minus measured, divided by measured, quantity times 100. were used for calibration, testing, and simulations of Panther Springs Creek Basin. The gaged area of the basins in the Edwards aquifer outcrop. basin in the 1970s (about 6,100 acres) was about equally divided between catchment area and Edwards aquifer Panther Springs Creek Basin outcrop. The gaged area was largely undeveloped in The calibration parameters from Helotes Creek, 1973 when rainfall and runoff data were collected dur- East Elm Creek, and West Elm Creek Basins were fur- ing four storms (table 8). Simulated runoff during these ther tested by applying them in simulations of the storms is compared to measured runoff in figure 12.

SIMULATION OF RUNOFF AND RECHARGE 19 Table 9. Errors, mean absolute errors, and mean absolute runoff-weighted errors for storm runoff volumes used in model calibrations [In percent]

Number of Mean absolute Mean absolute runoff- Watershed Error storm events error weighted error Helotes Creek 6 4.4 60 12 East Elm Creek 4 4.7 17 17 West Elm Creek 4 –3.1 20 12 Panther Springs Creek 4 3.3 35 43

The simulations of the four storms for the Panther Table 10. Selected process-related calibration Springs Creek Basin yielded an overall error in runoff parameters used for basin simulations volumes of 3.3 percent. The mean absolute error for the individual storms was 35 percent, and the mean absolute [Parameter definitions in table 3; --, not applicable] runoff-weighted error was 43 percent. Unlike the simu- Parameter Value Unit lations for the other three basins, weighting the errors by runoff volume did not result in a smaller mean error. AGWRC–outcrop 0.50 per day The simulation errors for all four basins are sum- AGWRC–catchment area .98 per day marized in table 9. As a result of model calibration and AGWETP 0 -- testing with data for the four basins, a set of process- BASETP 0 -- related model parameters for simulation of all of the basins in the study area was obtained; those parameters CEPSC .1 inches are summarized in table 10. DEEPFR–outcrop .95 -- Error Analysis DEEPFR–catchment area .05 -- INFEXP 2.0 -- The types of error from the model calibration and testing can be classified as measurement errors or sys- INFILD 2.0 -- tematic errors (Raines, 1996). Measurement errors are introduced as a result of missing data or of inaccurate INFILT–outcrop .40 inches per hour data used in calibration or testing. For example, meas- INFILT–catchment area .15 inches per hour ured or gaged streamflow is subject to potential error in INTFW–outcrop .2 -- rating tables of stage and discharge; insufficient data also contribute to potential error. For example, relatively INTFW–catchment area 2.0 -- few gaged storms (from only four of the 10 basins) were IRC–outcrop .1 per day available for calibration, and the results (calibration IRC–catchment area .4 per day parameters) were transferred to ungaged basins. Possi- ble seasonal hydrologic factors could not be investi- LZETP .1 -- gated because of the small number of storms. ET is a substantial component of the hydrologic cycle, yet ET LZSN–outcrop 4.0 inches data were not available for the Edwards aquifer outcrop. LZSN–catchment area 5.0 inches Also, streamflow-loss data from one stream (Salado NSUR .15 -- Creek) was used to characterize losses for most of the UZSN–outcrop .20 inches major streams in the study area. Finally, the spatial vari- ability of rainfall in a basin might not be adequately rep- UZSN–catchment area .25 inches resented by the available network of rain gages. Systematic errors are associated with limited abil- 1 The users manual for Hydrological Simulation Program— ity of the simulation model to represent the hydrologic FORTRAN (Bicknell and others, 1997) provides a more complete processes of the basins in the study area. Limits exist on description of each parameter.

20 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 Table 11. Sensitivity of selected process-related parameters, Panther Springs Creek Basin

[Parameter definitions in table 3]

Change in Change in Change in Edwards Change in stream- Initial Adjusted Parameter parameter surface runoff aquifer recharge flow exiting basin value value (percent) (percent) (percent) (percent) CEPSC 0.10 0.15 50 –0.40 –4.9 –1.7 DEEPFR–outcrop .95 .85 10.5 0 –1.6 5.9 DEEPFR–catchment area .05 .10 100 0 –4.4 8.0 INFILT–outcrop .40 .30 –25 1.4 –4.2 8.0 INFILT–catchment area .10 .15 50 –.60 –2.6 –2.4 INTFW–catchment area 1.0 .8 20 .90 –.30 1.0 IRC–catchment area .50 .40 20 0 –.10 .30 LZETP .10 .15 50 –1.4 –9.1 –6.2 LZSN–outcrop 2.5 3.0 20 –.30 –.40 –1.8 UZSN–outcrop .50 .60 20 –.60 –.40 –3.5 how well the model parameters and equations describe storms (table 8), which featured less, but more intense, the physical properties of runoff. Also, the configura- rainfall. tion of the model segments (PERLNDs, IMPLNDs, and RCHRESs) and the selection of simulation time Sensitivity Analysis step (in this case, hourly) can only approximate the Panther Springs Creek Basin parameters were actual physical configuration and hydrologic response used in a sensitivity analysis of selected HSPF model of the basins. parameters to determine the effects of changes in the Despite incomplete data, the HSPF model pro- values of selected parameters on simulated runoff, vides reasonable simulations of runoff volumes com- recharge, and streamflow exiting the basin. (Streamflow pared with observed data. For each of the calibration exiting the basin is runoff minus streamflow losses and basins, total simulated runoff was within 5 percent of water lost to infiltration behind recharge dams.) Each measured runoff. For individual storms, mean absolute parameter was changed by a hydrologically reasonable runoff-weighted errors were within 20 percent for amount while keeping other parameters unchanged, and Helotes Creek, East Elm Creek, and West Elm Creek simulations were run. The resulting changes in surface Basins. runoff (surface plus interflow runoff), recharge, and streamflow exiting the basin are listed in table 11. Figures 8 and 11 show how the model simulations The parameters to which simulated model outputs represent the peak flow and runoff response of the were most sensitive were lower-zone ET (LZETP) basins. There is a characteristic difference in runoff and index to soil infiltration capacity (INFILT). A 50- response between basins in the catchment area and percent increase in LZETP resulted in a 9.1-percent those in the Edwards aquifer outcrop. Runoff for decrease in recharge, a 1.4-percent decrease in surface Helotes Creek (catchment area) lasted weeks, whereas runoff, and a 6.2-percent decrease in streamflow exiting runoff at East Elm Creek (outcrop) ceased within hours. the basin. Also, recharge was somewhat sensitive to Although the Panther Springs Creek storms exhibited interception storage capacity (CEPSC). A 50-percent the highest mean absolute error in runoff volumes, the increase in CEPSC resulted in a 4.9-percent decrease in HSPF model was able to account for differences in rain- recharge. fall intensity and antecedent conditions in runoff simu- In addition to the process parameters, the per- lation. For example, the June 1973 rainfall (4.96 inches) centage of impervious area in residential and commer- occurred after a relatively dry month (0.16 inch in May). cial land use categories is subject to some uncertainty. The June 1973 measured runoff of 0.45 inch is less than The effect of a percentage change in the effective the runoff that occurred during the July and September impervious cover for residential land use was simulated.

SIMULATION OF RUNOFF AND RECHARGE 21 The overall, effective impervious cover for residential In 1997, simulated streamflow exiting the 10 land use in the Panther Springs Creek Basin in 1998 was major basins (and thus runoff from the study area) was estimated to be 29 percent. A model simulation was 5.62 inches. Simulated recharge to the 10 major basins done with a residential impervious cover of 32 percent (essentially in the Edwards aquifer outcrop in the study (10-percent increase). The increase in overall basin area) was 7.85 inches (20 percent of rainfall). In 1998, impervious cover was from 7.2 to 7.6 percent. The simulated streamflow exiting was 11.05 inches, and resulting percentage changes in runoff and recharge are simulated recharge was 10.99 inches (25 percent of listed in the following table: rainfall). In 1999, simulated streamflow exiting was 0.66 inch, and simulated recharge was 3.03 inches (19 Results of sensitivity analysis of change in percentage percent of rainfall). In 2000, simulated streamflow exit- of residential land use effective impervious cover ing was 5.29 inches; simulated recharge was 7.19 inches (21 percent of rainfall). [In percent] During 1997–2000, simulated recharge in the Change in Change in Change in study area (essentially in the Edwards aquifer outcrop) residential Edwards volume of was about 22 percent of rainfall, simulated runoff from Scenario surface aquifer streamflow the study area was about 17 percent of rainfall, and sim- runoff recharge exiting basin ulated ET in the study area was about 51 percent of rain- 10-percent increase in 5.6 0.04 1.1 fall. The remaining 10 percent of rainfall during 1997– residential effective 2000 was accounted for by simulated changes in soil impervious cover and unsaturated-zone storages and by a small amount of simulated recharge in the catchment area. Figure 13 Increasing the residential effective impervious shows simulated monthly runoff from and recharge to cover from 29 percent to 32 percent resulted in a negli- the study area during 1997–2000. About 37 percent of gible increase in recharge of 0.04 percent. Although the recharge (177,000 acre-ft) and about 66 percent of recharge from direct infiltration decreased because of the runoff (248,000 acre-ft) occurred during four pervious area loss, recharge occurring from streamflow months—June 1997, October 1998, and October and losses increased, and the two changes effectively can- November 2000. Thus the proportions of rainfall celed one another. accounted for by recharge, runoff, and ET during 1997– 2000 might not reflect the proportions characteristic of Runoff From and Recharge to the Major long-term average hydrologic conditions Basins Simulated recharge and runoff during 1997–2000 varied widely among the 10 basins. The Leon Creek The process-related model parameters calibrated Basin (mostly Glen Rose Limestone) had 20.60 inches and tested in the Helotes Creek, East Elm Creek, West of recharge; the Elm Creek Basin (mostly Edwards Elm Creek, and Panther Springs Creek Basins were aquifer outcrop; includes four flood-control structures) applied to the 10 major basins in the study area to sim- had 49.30 inches of recharge. For the study area as a ulate runoff from (streamflow exiting) the basins and whole, recharge during 1997–2000 was 29.06 inches recharge to the basins during 1997–2000 (table 12). The Helotes Creek Basin had the least amount of runoff During 1997, 39.23 inches of rain (area-weighted (6.60 inches compared with 22.62 inches for the entire average of rain-gage data used for the simulations) fell study area) mostly because of relatively large stream- on the study area—about 35 percent above normal. In flow losses in a stream reach that passes through a June 1997, a major rainfall occurred in the western part quarry near the exit of the basin. Cibolo Creek had the of the study area where as much as 10 inches of rain fell largest amount of runoff (36.18 inches); it also received during a 2-day period. Rainfall in 1998 was 44.06 in, the largest amount of rainfall (150.86 inches compared about 50 percent above normal. Some areas in the north- with the area-weighted average of 133.80 inches for the eastern part of the study area received more than 20 entire study area). inches of rainfall in October 1998. In contrast, rainfall Primary sources of recharge to the Edwards in 1999 was 15.85 inches, about 45 percent below nor- aquifer are direct infiltration of rainfall in interstream mal; rainfall in 2000 was 34.66 inches, about 20 percent areas, streamflow losses in the outcrop, and infiltration above normal. of impounded runoff behind flood-control structures.

22 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 120,000

Streamflow 100,000 Recharge

80,000

60,000

40,000 STREAMFLOW OR RECHARGE, IN ACRE-FEET

20,000

0 1997 1998 1999 2000

Figure 13. Simulated monthly runoff from (streamflow exiting) and recharge to the study area, 1997–2000.

Simulated recharges, by primary sources, are listed in R = runoff volume, in acre-feet; table 13. During 1997–2000, direct infiltration of rain- EMC = median event-mean concentration during run- fall accounted for about 56 percent of the total Edwards off, in milligrams per liter or micrograms per aquifer recharge in Bexar County. Streamflow losses liter; and contributed about 37 percent of the recharge; flood impoundment contributed 7 percent. Cf = conversion factor, 2.718 for concentrations in milligrams per liter or 0.00272 for concentra- ESTIMATION OF CONSTITUENT LOADS IN tions in micrograms per liter. RUNOFF Constituent yield, a measure of the load- producing characteristics of a basin, is computed by the In this report a constituent load is the mass of a equation constituent moved past a point by water in a specified time. Loads for suspended solids, dissolved solids, dis- Y = L/DA, (2) solved nitrite plus nitrate nitrogen, and total lead in run- where off were computed using the equation Y = constituent yield, in pounds per acre; L = R x EMC x Cf, (1) L = constituent load, in pounds; and where DA = contributing drainage area of the basin, in L = constituent load, in pounds; acres.

ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 23 Total –2000 Creek Cibolo Elm Creek Mud Creek Creek Panther Springs Creek Salado Creeks Olmos/ Huebner Leon Creek Creek Helotes Creek Culebra San Creek Geronimo Annual and rainfall estimates of runoff from (streamflow exiting) and recharge to the 10 major basins in the study area, 1997 1997 Basin area, acresAnnual rainfall, inchesExiting acre-feet streamflow, Exiting inches streamflow, 9,410 15,690 acre-feet recharge, Edwards 36.75 2,940Edwards recharge, inches 3,110 11,121 7.20 36.751998 11,600 2,560 17,540Basin area, acres 3.17 2.38 36.75 29,600 41,759 9,960Annual rainfall, inchesExiting acre-feet streamflow, 12.52 40.05 1.75 18,800 1,690 5,426 16,800Exiting inches streamflow, 15,690 acre-feet recharge, Edwards 34.73 19,637 40.33 5,390 6.81 6,750 8.51 5,600Edwards recharge, inches 3,810 11,121 12.82 14,245 40.00 7,720 40.33 2,120 4,840 5.401999 14,900 3.74 17,540 14,546 6.04Basin area, acres 12,200 2.91 33.08 40.33 50,300 13,000 2,450 11.93 41,759Annual rainfall, inches 4.12 19,244 13,400 acre-feetExiting streamflow, 3.31 24,500 16.08 33.08 42.46 4,440 2,350 5,426 inches 4.72 39,438 streamflow, Exiting 21,200 1.79 935 15,690 15,300 14.45 7,060Edwards 36.31 33,100 recharge, 41.28 19,637 15.72 acre-feet 8.89 198,646 26,400 10.27 inches recharge, Edwards 11,121 10,000 1,210 2.02 .71 93,000 7,540 14,245 47.29 45.06 26 9.82 15.72 130,000 7.042000 11.08 17,540 21,200 14,546 4,160Basin area, acres 7,410 1.47 41.19 15.72 .93 .03 39.23 9.35 15.60 41,759 13.24 22,500Annual rainfall, inches 20 19,244 12,200 4,040Exiting acre-feet streamflow, 10.07 41.19 16.24 32,700 4.49 5,426 10,900 6.35 6.12 4,690 39,438Exiting inches streamflow, .01 8.04 59,000 7,900 15,690Edwards recharge, acre-feet 45.94 32,000 15.80 19,637 36.30 2,700 5.62 198,646 2.76 17.86 183,000 Edwards recharge, inches 6.11 1.33 11,121 440 3,310 2,300 8.33 7.85 182,000 14,245 51.01 15.87 36.30 2,240 18.53 17,540 12,500 2.27 7.59 2,500 14,546 150 .97 2.91 2.53 15.87 36.30 33,800 44.06 41,759 11,200 20.38 19,244 5.08 17.96 5,440 13.49 15.87 36.30 510 2,720 1.53 20,500 .09 5,426 39,438 9.73 6,120 1.53 11.05 14.74 37.03 8,240 19,637 6,100 7.66 9.71 730 198,646 .43 9,170 10.99 4.58 14,245 16.10 37.03 1,190 7,630 1,260 5.89 6.01 7,300 .60 14,546 11,200 5.05 29.00 1,510 15.85 2,090 13.50 5.03 19,244 50,100 12,200 29.00 1,320 .78 5.72 10,900 4.66 39,438 16,000 1.00 23,000 29.03 2.22 198,646 .64 18,300 9.44 1.24 87,600 36.46 119,000 10.07 3.03 .66 .83 34.66 9.96 6.99 5.57 5.29 7.19 Table 12. Table

24 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 Table 13. Simulated recharge to Edwards aquifer in the study area by source, 1997–2000

[In acre-feet]

Total, Source of recharge 1997 1998 1999 2000 1997–2000 Direct infiltration 75,400 94,200 31,400 68,500 270,000 Streamflow losses 48,200 67,300 15,500 45,300 176,000 Flood-control impoundment 6,140 20,000 3,240 5,040 34,400 Annual total 130,000 182,000 50,100 119,000 1481,000

1 Annual total does not equal 1997–2000 total because of rounding.

Table 14. Median event-mean concentrations1, by land use category, used to compute constituent loads

Transportation Constituent Undeveloped Residential Commercial corridor Suspended solids (milligrams per liter) 48 53 114 76 Dissolved solids (milligrams per liter) 119 54 52 42 Nitrite + nitrate nitrogen, dissolved (milligrams per liter) .56 .28 .32 .31 Total lead (micrograms per liter) 2.2 4.1 8.9 9.5

1 From Ockerman and others (1999) and B.L. Petri (U.S. Geological Survey, written commun., 2000).

Event-mean concentration (EMC) (Charbeneau structures for commercial land use in the Edwards and Barrett, 1998) represents a discharge-weighted aquifer outcrop designed to capture and isolate the average constituent concentration during storm runoff. first 0.5 inch of runoff during a storm to minimize pol- Two sources of EMCs for Bexar County were available lutants entering the aquifer (San Antonio Water System, for this study: (1) Data collected during 1996–98 for a 2000). The estimates of constituent loads in runoff water-quality study of five basins with specific land do not consider the effects of these pollution-control uses in the Edwards aquifer outcrop, Bexar County structures. (Ockerman and others, 1999); and (2) data obtained Estimates of annual loads and yields by land use from a study of the San Antonio National Pollutant Dis- for suspended solids, dissolved solids, dissolved nitrite charge Elimination System (B.L. Petri, U.S. Geological plus nitrate nitrogen, and total lead during 1997–2000 Survey, written commun., 2000). Median EMCs from for each of the 10 major basins are listed in tables 15– the two sources (table 14) were used to compute loads. 18. Annual loads for the four constituents were consis- The computed constituent loads in surface-water tently largest from undeveloped land and smallest from runoff are estimates of annual total loads originating commercial land or transportation corridors. Annual from pervious and impervious surfaces and transported loads varied with rainfall (table 12), with the maximum to streams in the study area. The transport and fate of loads for the study area produced in 1998 (wettest year) these constituent loads are not considered. For example, and the minimum loads produced in 1999 (driest year). some of the load will be retained behind flood-control For example, in 1998 when rainfall was about 50 per- structures; some of the load infiltrates the aquifer with cent above normal, nitrite plus nitrate nitrogen load streamflow lost to the aquifer; and some of the load is from undeveloped areas was 134 tons; in 1999, when transported from the study area in streamflow exiting rainfall was about 45 percent below normal, nitrite plus the study area. Constituents transported to streams by nitrate nitrogen load from undeveloped areas was 7.5 interflow or base flow are not included in the load tons (table 17). estimates. Also, basin protection ordinances in Bexar Because they are computed from loads, annual County require detention, sedimentation, and filtration yields for the four constituents also varied with rainfall.

ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 25 Total e studyarea, Creek Cibolo Elm Creek Mud Creek Creek Panther Springs Creek Salado Creeks Olmos/ Huebner Leon Creek Creek Helotes Creek Culebra San Creek Geronimo Estimates of annual loads, land by and use, yields suspended for solids in surface-water runoff the 10 for major basins in th Table 15. Table 1997–2000 Load (undeveloped), tonsLoad (residential), tonsLoad (commercial), tonstons corridor), (transportation Load 540acre per pounds Yield, 25 280 6.21998 1.0 11Load (undeveloped), tons 500 10Load (residential), tons 73 0Load 35 (commercial), tons 1,000 170tons corridor), (transportation Load 1,000acre per pounds 54Yield, 79 38 340 36 380 540 11 731999 90 530 490 2 14 72Load (undeveloped), tons 950 16Load (residential), tons 81 180 140 110 41Load 44 (commercial), tons 2,000 0 69 260tons corridor), (transportation Load 140 97 100 140 98 29 poundsYield, peracre 59 100 430 95 11 180 590 150 1,300 127 110 74 64 22000 130 11 660 130 1,600 .4Load (undeveloped), 720 tons 4 130 170 63 120 81Load (residential), tons 58 5,000 3 71 60 180 120 570Load (commercial), tons 0 9.1 13 120 150tons corridor), (transportation Load 66 78 41 180 260 620 45 980 110acre per pounds Yield, 1,000 160 3 29 68 130 200 120 210 48 1,300 3,200 280 11 290 160 4 100 31 17 2.5 190 220 24 11,000 1,200 130 110 170 580 28 160 18 34 39 89 0 130 14 36 130 25 1,200 360 85 160 180 18 1,400 75 47 36 60 10 80 210 2,200 79 350 200 410 44 200 1,500 53 100 35 60 530 680 200 8 160 61 130 220 120 39 130 98 53 28 150 260 33 100 84 70 32 92 35 79 29 640 100 110 43 63 130 400 110 60 26 50 110 1,200 380 70 430 57 5,000 12 52 86 64 180 1,200 20 37 1,400 140 1,200 81 88 1997

26 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 Total e studyarea, Creek Cibolo Elm Creek Mud Creek Creek Panther Springs Creek Salado Creeks Olmos/ Huebner Leon Creek Creek Helotes Creek Culebra San Creek Geronimo Estimates of annual land loads, by and use, yields dissolved for solids in runoffsurface-water the 10 major for basins in th Load (undeveloped), tonsLoad (undeveloped), tonsLoad (residential), tonsLoad (commercial), tons corridor), Load (transportation 1,300 acre per pounds Yield, 14 690 6.31998 1,200 .5 tonsLoad (undeveloped), 6 11 tonsLoad (residential), 2,600 170 tonsLoad (commercial), 0 19 170 tons corridor), Load (transportation 2,500 94 130 acre per pounds Yield, 36 20 190 1,300 390 1,200 170 111999 2,400 240 40 73 .7 450Load (undeveloped), tons 8 160 16 tons 4,900Load (residential), 330 37 350 tonsLoad (commercial), 0 23 70 24 90 360 260 tons corridor), Load (transportation 240 450 150 13 acre per pounds Yield, 3,300 54 46 97 240 130 6 600 4,000 310 1,800 2.02000 34 27 12,000 130 70 300 60 130 89 .2 tonsLoad (undeveloped), 1,400 2 290 tonsLoad (residential), 3.1 150 37 55 2,400 72 72 20 120 tonsLoad (commercial), 0 32 81 220 46 tons 8,000corridor), Load (transportation 7 1,500 270 260 36 180 19 acre per 66 pounds Yield, 28,000 83 13 6 65 350 38 1,300 100 720 10 11 72 290 96 550 55 110 300 230 1.1 25 1,400 13 87 29 170 17 530 97 18 120 150 260 59 3,000 200 0 16 20 26 370 18 180 180 200 290 69 8 140 59 2,200 36 26 190 4.6 130 45 1,700 430 420 770 190 26 96 110 240 16 13 320 320 200 320 62 180 690 33 100 54 250 18 230 38 150 58 220 1,600 29 32 320 46 29 80 34 44 200 3,000 19 29 59 12,000 27 110 410 46 67 220 32 210 29 33 17 59 15 180 48 24 41 1,400 64 610 170 550 150 1997 1997–2000 Table 16. Table

ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 27 Total Creek Cibolo e 10 major basins basins major e 10 Elm Creek Mud Creek Creek Panther Springs Creek Salado Creeks Olmos/ Huebner Leon Creek Creek Helotes Creek Culebra San Creek Geronimo Estimates of annual loads, land by use, and yields dissolved for nitrite plus nitrate nitrogen in surface-water runoff th for Load (undeveloped), tonsLoad (undeveloped), tonsLoad (residential), tonsLoad (commercial), tons corridor), (transportation Load 6.3 acre per pounds Yield, .10 .031998 0 3.3Load (undeveloped), tons .04 tonsLoad (residential), .06 5.8 .82 tonsLoad (commercial), 0 .14 tons corridor), (transportation Load 12 .90 12 1.4 acre per pounds Yield, .61 .15 .22 2.0 0.4 .061999 0 .30 .81 1.5 6.3 tonsLoad (undeveloped), .38 .06 5.7 tonsLoad (residential), .08 11 .17 .81 1.5 .23 tonsLoad (commercial), 0 .18 .36 2.1 tons corridor), (transportation Load 1.4 .50 .40 23 acre 1.8 per pounds Yield, .69 .08 1.2 .50 .04 1.6 .28 3.1 .52 .64 .012000 1.7 0 .21 .44 1.5 .69 1.9 2.1 .13 tonsLoad (undeveloped), .68 .02 .53 .45 tonsLoad (residential), 15 .23 1.4 19 .02 .24 .38 .70 .34 .09 tonsLoad (commercial), 0 .49 .05 .63 .42 tons corridor), (transportation Load 1.4 .22 1.2 .24 1.3 58 .61 8.4 7.2 acre per pounds 4.1 Yield, .12 .53 .95 .03 .34 .28 .59 .08 6.7 .53 1.7 .82 6.7 .05 .06 .34 .13 1.1 1.5 .01 3.4 .12 3.3 .53 11 .15 .10 .90 1.4 .46 .36 .10 6.8 .06 .90 .14 .73 0 .10 .97 38 .51 .15 1.3 .10 .86 1.9 .36 14 .95 .15 .19 1.4 5.6 134 .03 1.4 .22 .23 .64 11 .62 .59 2.2 .24 .06 .92 .10 1.5 .82 2.1 .92 1.5 .32 4.2 1.1 .22 .19 7.9 .11 1.6 .62 1.2 .40 .30 .91 .14 .77 .18 1.5 .18 1.3 .23 7.5 .34 .20 1.6 .42 .23 1.1 .17 2.1 .44 .18 .95 .09 .58 1.3 1.5 .22 .34 .20 14 .17 .13 .35 .09 .31 .95 58 4.5 .39 .21 7.2 3.4 .80 .74 1997 Table 17. in the study area, 1997–2000

28 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 Total y area, Creek Cibolo Elm Creek Mud Creek Creek Panther Springs Creek Salado Creeks Olmos/ Huebner Leon Creek Creek Helotes Creek Culebra San Creek Geronimo

Estimates of annual land loads, by and use, yields total for lead in surface-water runoff the 10 major for basins in the stud 1997 Load (undeveloped), tons (undeveloped), Load Load (residential), tons tons (commercial), Load tons corridor), (transportation 50Load 2.0 poundsYield, per acre 26 .96 .161998 .90Load (undeveloped), tons 1.6 46 0Load (residential), tons 2.9 .003Load (commercial), 26 tons 94 tons corridor), 28 (transportation Load 12 94 .002 2.9 poundsYield, per acre 59 1.7 .006 83 3.5 50 6.0 1.11999 .23 11 tons (undeveloped), Load .017 2.5 45 13 3.3 87 0 tons (residential), Load 3.6 .006 .002 11 tons (commercial), Load 40 17 8.0 180 tons 35 corridor), (transportation Load 4.5 16 5 .003 .004 poundsYield, per acre 15 .90 91 10 13 12 13 100 .009 .31 8.9 .003 1.0 .062000 .33 20 20 11 tons 120 17 (undeveloped), Load .03 13 .004 .46 19 4.7 0 tons (residential), Load 5.5 1.1 .0004 11 150 19 10 tons (commercial), Load .003 .004 7.0 66 12 12 10 tons corridor), 11 6.4 (transportation Load 57 .0001 4.5 .015 5.6 40 .01 460 poundsYield, per 15 acre 28 82 52 33 .001 16 19 1.7 30 27 2.5 .37 .005 .39 2.0 200 .008 43 .004 4.3 180 90 18 2.8 20 3.4 1.1 53 0 5.3 .008 2.9 .0008 .004 2.8 26 300 28 10 20 110 6.8 .01 3.8 .0006 1.6 29 12 .002 1,000 6.8 56 63 110 .002 33 4.9 .025 4.9 5.5 7.2 16 .006 83 9.4 330 .002 31 12 240 .008 62 4.3 6.1 .018 34 8.0 .002 8.8 22 10 .006 2.9 9.2 12 .002 11 6.8 6.6 .007 32 12 59 9.4 .001 8.4 8.7 62 10 .003 17 72 12 4.4 .003 11 110 5.1 .002 7.0 5.0 28 460 .011 90 22 210 .005 190 Table 18. Table 1997–2000

ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 29 Annual yields for suspended solids (table 15) ranged Elm and West Elm Creek basins are in the Edwards from 20 pounds per acre 1999 (driest year) to 160 aquifer outcrop. Panther Springs Creek Basin is partly pounds per acre in 1998 (wettest year). Among basins, in the Edwards aquifer outcrop and partly in the catch- the average annual yield for suspended solids in 1999 ment area. A computerized expert system was used to ranged from 3 pounds per acre in the Culebra Creek guide the adjustment of process-related parameters dur- Basin (mostly Edwards aquifer outcrop and one of the ing calibration and testing. least developed basins) to 36 pounds per acre in the For the Helotes Creek Basin, data from six storms Olmos/Huebner Creeks Basin (the basin with the high- were used for calibration of the parameters related to est estimated percentage of impervious area [table 1], runoff. The mean absolute runoff-weighted error for primarily related to residential development). calibration simulations of the six Helotes Creek storms Annual yields for dissolved solids (table 16) and was 12 percent. East Elm Creek Basin was calibrated dissolved nitrite plus nitrate nitrogen (table 17) showed using data from four storms. To test the calibration and variations with rainfall similar to those for suspended also to test the transferability of model parameters to solids—generally smallest in 1999 and largest in 1998. similar ungaged basins, the calibration parameters from Yields of these constituents among basins, as with sus- the East Elm Creek Basin were applied to the West Elm pended solids, varied widely. Creek Basin in simulations of four storms. The mean The annual yield for total lead (table 18) was absolute runoff-weighted errors for the calibration (East about eight times greater in 1998, the wettest year Elm) and testing (West Elm) simulations were 17 and 12 (0.008 pound per acre) than in 1999, the driest year percent, respectively. The calibration parameters from (0.001 pound per acre). The Culebra Creek Basin had Helotes Creek, East Elm Creek, and West Elm Creek the smallest annual yield of total lead (0.0001 pound per Basins were further tested by applying them in simula- acre in 1999), and the Leon Creek watershed (mostly tions of the Panther Springs Creek Basin. The mean Glen Rose Limestone and relatively greater residential absolute runoff-weighted error for the Panther Creek and commercial development) had the largest annual testing simulations was 43 percent. yield (0.018 pound per acre in 2000, a relatively wet The process-related model parameters calibrated year). and tested in the Helotes Creek, East Elm Creek, West Elm Creek, and Panther Springs Creek Basins were SUMMARY applied to the 10 major basins in the study area to simulate streamflow exiting and recharge to the basins dur- The USGS developed an HSPF watershed model ing 1997–2000. In 1997, simulated streamflow exiting to simulate runoff and recharge and to estimate constit- the 10 major basins (and thus runoff from the study uent loads in surface-water runoff in the Edwards area) was 5.62 inches; simulated recharge to the 10 aquifer outcrop and catchment area in Bexar County. major basins (essentially in the recharge zone in the Rainfall and runoff data collected during 1970–98 from study area) was 7.85 inches (20 percent of rainfall). In four gaged basins in the Edwards aquifer outcrop and 1998, simulated streamflow exiting was 11.05 inches; catchment area were used to calibrate and test the simulated recharge was 10.99 inches (25 percent of model. The calibration parameters were applied in sim- rainfall). In 1999, simulated streamflow exiting was ulations of the four calibration basins and six ungaged 0.66 inch; simulated recharge was 3.03 inches (19 per- basins of the study area to obtain runoff and recharge cent of rainfall). In 2000, simulated streamflow exiting volumes for 4 years, 1997–2000. The simulated runoff was 5.29 inches; simulated recharge was 7.19 inches volumes were used with EMC data from basins in the (21 percent of rainfall). study area and from other Bexar County basins to com- Primary sources of recharge to the Edwards pute constituent loads and yields for various land uses. aquifer are direct infiltration of rainfall in interstream Calibration and testing of process-related model areas, streamflow losses in the outcrop, and infiltration parameters (those that represent the physical processes of impounded runoff behind flood-control structures. of infiltration, ET, interception, interflow, ground-water During 1997–2000, direct infiltration of rainfall recession, and surface runoff) involved four gaged accounted for about 56 percent of the total Edwards basins in the study area: Helotes Creek, East Elm Creek, aquifer recharge in Bexar County. Streamflow losses West Elm Creek, and Panther Springs Creek. Helotes contributed about 37 percent of the recharge; flood Creek Basin primarily is in the catchment area. East impoundment contributed 7 percent.

30 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 Annual loads for suspended solids, dissolved Lumb, A.M., McCammon, R.B., and Kittle, J.L., Jr., 1994, solids, dissolved nitrite plus nitrate nitrogen, and total Users manual for an expert system (HSPEXP) for lead were consistently largest from undeveloped land calibration of the Hydrological Simulation Program— and smallest from commercial land or transportation FORTRAN: U.S. Geological Survey Water-Resources corridors. Annual loads varied with rainfall, with the Investigations Report 94–4168, 102 p. maximum loads produced in the wettest year (1998) and Ockerman, D.J., Petri, B.L., and Slattery, R.N., 1999, Stormwater runoff for selected watersheds in the the minimum loads produced in the driest year (1999). Edwards aquifer recharge zone, Bexar County, Texas, Because they are computed from loads, annual 1996–98: U.S. Geological Survey Fact Sheet yields for the four constituents also varied with rainfall. FS–172–98, 6 p. Annual yields for suspended solids ranged from 20 Puente, Celso, 1978, Method of estimating natural recharge pounds per acre in 1999 (driest year) to 160 pounds per to the Edwards aquifer in the San Antonio area, Texas: acre in 1998 (wettest year). Annual yields for dissolved U.S. Geological Survey Water-Resources Investigations solids and dissolved nitrite plus nitrate nitrogen showed Report 78–10, 34 p. variations with rainfall similar to those for suspended Raines, T.H., 1996, Simulation of storm peaks and storm solids—generally smallest in 1999 and largest in 1998. volumes for selected subbasins in the West Fork Trinity The annual yield for total lead was about eight times River Basin, Texas, water years 1993–94: U.S. Geologi- greater in 1998 (0.008 pound per acre) than in 1999 cal Survey Water-Resources Investigations Report (0.001 pound per acre). 96–4110, 41 p. San Antonio River Authority, 1996, Salado Creek watershed REFERENCES protection and flood prevention project: San Antonio River Authority Fact Sheet, 2 p. Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Donigian, San Antonio Water System, 2000, Outline of ordinance A.S., Jr., and Johanson, R.C., 1997, Hydrological #81491, aquifer recharge zone and watershed protection, Simulation Program—FORTRAN, user’s manual for San Antonio Texas, approved January 12, 1995: version 11: Athens, Ga., U.S. Environmental Protection Accessed January 10, 2000, at URL Agency, National Exposure Research Laboratory, http:www.saws.org/our_water/Source_Water_ EPA/600/R–97–080, 755 p. Watershed_Protection/aquifer_protection/ Charbeneau, R.J., and Barrett, M.E., 1998, Evaluation documents/4outline.pdf of methods for estimating stormwater pollutant Stein, W.G., and Ozuna, G.B., 1995, Geologic framework loads: Water Environment Research, v. 70, no. 7, and hydrogeologic characteristics of the Edwards p. 1,295–1,302. aquifer recharge zone, Bexar County, Texas: U.S. Geo- Dugas, W.A., Hicks, R.A., and Wright, P.W., 1998, Effect of logical Survey Water-Resources Investigations Report removal of Juniperus ashei on evapotranspiration and 95–4030, 8 p. runoff in the Seco Creek watershed: Water Resources U.S. Department of Agriculture, Soil Conservation Service, Research, v. 34, no. 6, p. 1,499–1,506. 1966, Soil survey of Bexar County, Texas: 126 p., Land, L.F., Boning, C.W., Harmsen, Lynn, and Reeves, 88 sheets. R.D., 1983, Streamflow losses along the Balcones fault University of Texas, Bureau of Economic Geology, 1985, The zone, Nueces River Basin, Texas: U.S. Geological Sur- Guadalupe-Lavaca-San Antonio-Nueces River Basins vey Water-Resources Investigations Report 83–4168, regional study, San Antonio East-Llano East sheets: 72 p. Austin, Tex., scale 1:250,000, 5 sheets.

REFERENCES 31