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
ii

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: dc_tx@usgs.gov

Copies of this report can be purchased from

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225–0286

E-mail: infoservices@usgs.gov
CONTENTS iii

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
iv 

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 surfacewater 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
Abstract 1

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

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 eventmean-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).

INTRODUCTION

The Edwards aquifer is one of the most productive carbonate aquifers in the Nation and is the major 

source of public water supply for San Antonio, Texas, 

and Bexar County (fig. 1). In addition to providing 

public water supply for more than 1 million people in 

south-central Texas, the Edwards aquifer supplies large 

quantities of water for agriculture, industry, military 

installations, and recreational activities. The aquifer 

also is a source of water to major springs in the region. 

These springs supply flow to downstream users and 

provide habitat for several threatened or endangered 

species.

Most recharge to the Edwards aquifer occurs west 

of Bexar County by direct infiltration of precipitation 

and by streamflow loss in the Edwards recharge zone. 

After entering the aquifer, water moves from west to 

east to points of discharge in Bexar County (mostly 

public supply wells) and then northeastward into Comal 

and Hays Counties (fig. 1), where it is discharged by 

withdrawals (wells) and two major springs. Additional 

recharge to the Edwards aquifer occurs in the recharge 

zone in northern Bexar County and southern Comal and 

Hays Counties.

In northern Bexar County, residential and commercial development in the Edwards aquifer recharge 

zone and catchment area is increasing. Urban development can have an appreciable influence on the quality of 
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 

BEXAR

 COUNTY

Study area

Base mo

1:24,000 

Universa

Stream basin boundary

Edwards aquifer recharge zone (outcrop)

Catchment area

EXPLANATION

HELOTES

CREEK

SAN 

GERONIMO

CREEK

CIBOLO

CREEK

CULEBRA

CREEK

LEON

CREEK

SALADO

CREEK

PANTHER

SPRINGS

CREEK

MUD

CREEK

ELM

CREE

OLMOS/

HUEBNER

CREEKS

San Antonio

98o30'

29o37'30"

98o37'30"

98o45'

024 6 MILES

Figure 1. Edwards aquifer recharge zone (outcrop), catchment area, and associated stream basins, Bexar County, Texas
INTRODUCTION 3

surface water and on water that recharges the aquifer. 

Impervious land cover in developed areas can result in 

increased stormwater runoff conveying contaminants 

from nonpoint sources to local streams or geologic 

features (caves, fractures) where infiltration into the 

aquifer can occur.

Estimates of runoff, water-quality constituent 

loads in runoff, and recharge to the Edwards aquifer are 

uncertain because of insufficient streamflow and waterquality data in the Edwards aquifer recharge zone and 

catchment area and the complexity of the rainfall-runoff 

process. The effects of commercial and residential 

development on runoff quantity and quality and 

recharge quantity are not well known. To learn more 

about these phenomena, the U.S. Geological Survey 

(USGS), in cooperation with the San Antonio Water 

System (SAWS), began a study in 1996 to develop a 

watershed model to simulate runoff and recharge and to 

estimate constituent loads in surface-water runoff in the 

Edwards aquifer recharge zone and catchment area in 

Bexar County. 

Purpose and Scope

This report describes the calibration, testing, 

and use of a watershed model to simulate runoff and 

recharge, the results of which were used to estimate 

constituent loads in surface-water runoff in the Edwards 

aquifer recharge zone and catchment area in Bexar 

County for 1997–2000. The Hydrological Simulation 

Program—FORTRAN (HSPF) watershed model 

(Bicknell and others, 1997) was used for the simulations. Rainfall and runoff data collected during 1970–98 

from selected gaged basins of the Edwards aquifer 

recharge zone and catchment area were compiled and 

used to calibrate and test the model. The calibration 

parameters then were transferred to the largely ungaged 

study area, and simulations were done for the entire 

study area to obtain runoff and recharge volumes. The 

simulated runoff volumes were used with event-meanconcentration data from basins in the study area and 

other Bexar County basins to compute constituent loads 

and yields for various land uses.

Description of Study Area 

The study area comprises the Edwards aquifer 

recharge zone and the adjacent catchment area in northern Bexar County (fig. 1). The Edwards aquifer 

recharge zone is approximately coincident with the 

Edwards aquifer outcrop. (The Edwards aquifer 

recharge zone is hereinafter referred to as the Edwards 

aquifer outcrop.) The catchment area consists of drainage basins of streams that lose water to (recharge) the 

Edwards aquifer as they cross the outcrop. The 10 

stream basins of the study area encompass about 310 

square miles that include about 125 square miles of 

Edwards aquifer outcrop and about 185 square miles of 

catchment area (table 1). 

Table 1. Selected characteristics of stream basins in the Edwards aquifer recharge zone (outcrop) and catchment 

area, Bexar County, Texas 

Basin Total area

 (square miles)

Edwards aquifer

outcrop area

 (square miles)

Estimated

impervious fraction

(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
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 

Most of the land (about 72 percent) in the outcrop 

is undeveloped (fig. 2) (David Kruse, Alamo Area 

Council of Governments, written commun., 1999). In 

1998, residential land use was about 10 percent of the 

Edwards aquifer outcrop area, although development 

there is increasing. A comparison of 1996–98 land use 

data indicates that residential land use in the study area 

increased by about 9 percent from 19,260 acres in 1996 

to 20,970 acres in 1998.

The primary geologic formations that crop out in 

the study area are the Glen Rose Limestone and the 

Kainer, Person, and Georgetown Formations (Stein and 

Ozuna, 1995). The Glen Rose Limestone covers most 

of the catchment area, and the Kainer, Person, and 

Georgetown Formations cover the Edwards aquifer outcrop. The Glen Rose Limestone is less permeable than 

the formations of the Edwards aquifer outcrop and generates more runoff per unit area. The Glen Rose Limestone also sustains base flow (ground-water inflow to 

stream channels) for days or weeks after large rainfalls. 

In contrast, the more permeable formations of the 

outcrop allow streamflow losses to the aquifer, thus limiting streamflow after large rainfalls to very short duration (often only a few hours). These streamflow losses 

contribute recharge directly to the Edwards aquifer 

(Land and others, 1983). 

Vegetation on the undeveloped part of the Glen 

Rose Limestone consists of moderate to dense stands 

of juniper, oak, and shrub. Vegetation on the Edwards 

aquifer outcrop consists of open to moderately open 

oak, juniper, and shrub (University of Texas, Bureau of 

Economic Geology, 1985). The soils in the study area 

consist primarily of two associations: Tarrant-Brackett 

and Crawford-Bexar (U.S. Department of Agriculture, 

Soil Conservation Service, 1966). The Tarrant-Brackett 

soils account for about 80 percent of the study area and 

overlie most of the Glen Rose Limestone. CrawfordBexar soils are more common on the Edwards aquifer 

outcrop. Both soil associations are primarily clay, 

ranging from a few inches to about 16 inches deep. 

Twelve major flood-control structures are 

located in the study area (San Antonio River Authority, 

1996); 10 are located in the Edwards aquifer outcrop. 

During major runoff, the structures retain water so that 

it has more time to infiltrate as ground-water recharge. 

Figure 3 is a photograph of water behind a flood-control 

structure in the upper Salado Creek Basin after a rainfall 

in October 1998. 

Description of Simulation Model and Modeling 

Process

The HSPF (Bicknell and others, 1997) is a 

continuous-simulation model using a conceptual framework to represent infiltration, evaporation, interception 

storage, surface runoff, interflow (water that infiltrates 

into the soil and moves laterally through the upper soil 

horizons until it returns to the surface, often in a 

stream), and base flow on a pervious land segment 

(PERLND) and to represent retention storage and surface runoff on an impervious land segment (IMPLND). 

Each user-defined land segment represents its own 

unique hydrologic response system on the basis of soil 

type, land cover, basin slope, or other pertinent basin 

characteristic. These land segments do not need to be 

contiguous within the model. The runoff from each land 

segment is moved through a system of channels or reservoir reaches (RCHRES) using storage routing.

The HSPF model uses input from three types of 

data: time series, basin-related model parameters, and 

process-related model parameters. Continuous time 

series of precipitation and potential evaporation are 

needed to run the model. Point-precipitation data, 

measured by rain gages, are assumed to be uniform 

over a land segment. Potential evaporation data can be 

estimated from measured pan evaporation or computed 

from minimum and maximum daily temperatures. Time 

series of measured streamflow are used for model calibration and testing.

The six basin-related model parameters (table 2) 

define the areal extent of each land segment (AREA), 

the reach length (LEN), and a table of values (FTABLE) 

of surface area (SAREA), volume (VOL), and discharge 

Table 2. Basin-related model parameters for the 

Hydrological Simulation Program—FORTRAN

[PERLND, pervious land segment; IMPLND, impervious 

land segment; FTABLE, table of depth, surface area, 

volume, and discharge for each stream reach] 

Parameter Description (units)

AREA Drainage area of each PERLND or 

IMPLND (acres)

LEN Stream reach length (miles)

DEPTH FTABLE depth (feet)

SAREA FTABLE surface area (acres)

VOL FTABLE volume (acre-feet)

DISCH FTABLE discharge (cubic feet per second)
INTRODUCTION 5

I–10

State 16

Loop1604

US 281

98o30'

29o37'30"

98o37'30"

98o45'

024 6 MILES

Base mod1:24,000 qUniversaLand use of Govern

Land use (1998)

CommercialResidentialInstitutional/cultural/recreationalQuarries

Industrial

Military

Undeveloped

 Edwards aquifer recharge zone (outcrop) Stream basin boundary

EXPLANATION

Figure 2. Land use in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar County, Texas. 
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 

(DISCH), as a function of depth (DEPTH) for each 

reach or reservoir of the basin. These parameters represent the physical characteristics of each land segment or 

reach of a basin and generally remain unchanged during 

calibration and testing of the model.

Process-related model parameters (table 3) 

represent the physical processes of infiltration, evapotranspiration (ET), interception, interflow, groundwater recession, and surface runoff for each land 

segment. The process-related model parameters for 

each land segment are adjusted to calibrate the model. 

The following parameters can be varied by month 

to account for seasonal variations: interception storage 

capacity (CEPSC), interflow inflow (INTFW), interflow recession rate (IRC), lower-zone ET (LZETP), 

Manning’s n for assumed overland flow plane (NSUR), 

and upper-zone nominal storage (UZSN). Because 

the available data were insufficient to characterize 

monthly variations, the parameters were not varied by 

month. The HSPF users manual (Bicknell and others, 

1997) provides a more complete description of each 

parameter.

The HSPEXP (Lumb and others, 1994), a 

computerized expert system, was used to help adjust 

process-related parameters during calibration and testing. HSPEXP is a stand-alone program that incorporates 

HSPF. The HSPEXP procedures consist of a set of 

hierarchical rules designed to guide the calibration of 

the model through a systematic evaluation of the model 

parameters.

The first step in developing the HSPF model for 

the study area basins was to subdivide the basins into 

segments (RCHRES), taking into consideration the 

stream and reservoir (flood-control impoundment) configuration of each basin. After subdivision, each basin 

contained from 1 to 12 RCHRES segments—63 stream 

segments and 12 reservoir segments in all for the 10 

major basins (fig. 4). FTABLE information for gaged 

stream reaches was based on discharge measurements 

made at USGS streamflow-gaging stations in the study 

area. FTABLE information for ungaged reaches was 

estimated from similar gaged reaches. FTABLE information for flood-control reservoirs was developed from 

the dam and reservoir design specifications.

Pervious land segments (PERLNDs) and impervious land segments (IMPLNDs) were configured 

according to geology and land use. The two geologic 

categories were based on the relative permeability of 

the rocks that compose the Edwards aquifer outcrop 

(relatively high permeability) and the catchment area 

Figure 3. Floodwater retention behind Salado Creek structure, October 1998. 
INTRODUCTION 7

Base modified 

1:24,000 quadra

Universal Tran

EXPLANATION

Stream in Edwards aquifer recharge zone

Flood-control impoundment in Edwards aquifer recharge zone

Flood-control impoundment in catchment area

Stream in catchment area

Stream basin boundary

Stream- or reservoir-segment boundary

Catchment area

Edwards aquifer recharge zone (outcrop)

HELOTES

CREEK

SAN 

GERONIMO

CREEK

CIBOLO

CREEK

CULEBRA

CREEK

LEON

CREEK

SALADO

CREEK

PANTHER

SPRINGS

CREEK

MUD

CREEK

ELM

CREE

OLMOS/

HUEBNER

CREEKS

98o30'

29o37'30"

98o37'30"

98o45'

024 6 MILES

Figure 4. Stream and reservoir segments in the Edwards aquifer recharge zone (outcrop) and catchment area, Bexar Cou
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 

(relatively low permeability). Land use categories used 

in the model were undeveloped, residential, commercial, transportation corridor, and quarries. Military land 

use largely is undeveloped rangeland. The remaining 

land use categories (fig. 2) account for less than 5 percent of the overall land use and, therefore, were grouped 

with other categories that had similar hydrologic characteristics. Table 4 lists the areas, in acres, of PERLNDs 

and IMPLNDs for each basin. 

Process-related parameters were calibrated for 

gaged basins. During the calibration process, parameter 

sets first were developed for gaged basins that were 

essentially undeveloped and consisted of a single geologic category—outcrop or catchment area. Because the 

outcrop and catchment area have substantially different 

rainfall-runoff characteristics, development of processrelated parameters for geologically homogeneous 

basins facilitated the subsequent calibration of gaged 

basins that included both geologic categories. The transferability of calibration parameters to ungaged basins 

was tested by simulating gaged basins with parameter 

sets developed from other gaged basins.

Because uncertainty in model parameters still 

exists after calibration and testing, sensitivity of simulated runoff and recharge to changes in selected processrelated input parameters was tested by altering values 

of selected parameters and evaluating the resulting 

changes in runoff and recharge.

After calibration, testing, and sensitivity analysis, 

the calibration model parameters were applied to both 

gaged and ungaged basins, and model simulations were 

done to estimate runoff and recharge. The runoff results 

Table 3. Process-related model parameters for the Hydrological Simulation Program—FORTRAN

[ET, evapotranspiration; --, none] 

Parameter Description Default

value

Minimum

value

Maximum

value Units

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
INTRODUCTION 9

Table 4. Pervious and impervious land segment areas by geologic and land use categories for the 10 major basins in the s[In acres] 

Category

Basin

San

Geronimo

Creek

Culebra

Creek

Helotes

Creek

Leon

Creek

Olmos/

Huebner

Creeks

Salado

Creek

Panther

Springs

Creek

Mud

Creek

PERLND (pervious land segment)

Outcrop–undeveloped 2,512 8,671 3,158 7,610 2,498 3,447 6,617 7,724 Outcrop–residential 2 0 174 996 1,022 382 1,087 1,715 Outcrop–commercial 0 0 21 503 140 52 188 326 Outcrop–transportation 55 0 8 347 232 86 372 472 Outcrop–quarries 0 0 284 226 120 0 0 108 Catchment area–undeveloped 12,878 2,222 10,962 23,368 482 14,625 4,812 2,108 Catchment area–residential 102 140 2,017 4,332 14 562 135 672 Catchment area–commercial 3 0 124 564 61 12 1 17 Catchment area–transportation 34 32 94 686 41 36 6 16 Catchment area–quarries 0 0 26 87 0 0 0 0 

IMPLND (impervious land segment)

Outcrop–residential 1 0 41 174 340 128 425 469 Outcrop–commercial 0 0 24 504 139 52 191 342 Outcrop–transportation 55 0 5 345 232 86 372 472 Catchment area–residential 11 24 360 764 5 99 35 92 Catchment area–commercial 2 0 139 562 60 2 2 18 Catchment area–transportation 34 32 100 685 40 46 6 17 
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 

were used to compute constituent loads for the entire 

study area. 

Sources of Model Calibration Data 

Sources of data (data-collection stations) for 

model calibration and development of process-related 

parameter sets for the Edwards aquifer outcrop and 

catchment area are shown in figure 5 and listed in 

table 5. Some rainfall and runoff calibration data were 

obtained from a 1970–79 data-collection program in 

several undeveloped basins in the Edwards aquifer outcrop in Bexar County. Most of the stations from that 

program have been discontinued; land use changes, 

including suburban development and construction of 

flood-control structures, have altered the basin characteristics. The 1970s data-collection program provided 

the only available data for characterizing the rainfallrunoff relation in the undeveloped Edwards aquifer outcrop in the study area. The rainfall and streamflow data 

from undeveloped basins are required for the model calibration process, especially for selection of processrelated parameters for PERLNDs associated with the 

Edwards aquifer outcrop. 

Another important factor in the rainfall-runoff 

relation is streamflow losses in the outcrop area. 

Streamflow loss, as a function of streamflow, in Salado 

Creek after a major runoff during October 19981 is 

shown in figure 6. Streamflow losses are not accounted 

for by the HSPF process-related parameters. Instead, 

the channel routing process in HSPF was adjusted to 

incorporate streamflow losses as channel withdrawals 

on the basis of streamflow in a reach. The relation 

shown in figure 6 was used to obtain streamflow losses 

for the major stream reaches crossing the Edwards aquifer outcrop in the study area. The relation was linearly 

extrapolated to simulate streamflow losses for streamflows greater than about 200 cubic feet per second. 

Streamflow losses are assumed to contribute directly to 

ground-water recharge (Puente, 1978). 

The streamflow-loss relation is appropriate for 

computing losses of streamflow as the water moves 

downstream from one RCHRES to the next RCHRES. 

However, water entering a RCHRES as runoff from a 

PERLND or IMPLND does not travel the entire length 

of a model RCHRES; therefore, the streamflow-loss 

relation must be scaled accordingly. The simplifying 

assumption was made that PERLND and IMPLND runoff within a RCHRES occurred at the midpoint of the 

RCHRES. Therefore, streamflow entering a RCHRES 

from an upstream RCHRES incurs streamflow losses 

along the entire RCHRES, and streamflow entering a 

RCHRES as PERLND or IMPLND runoff incurs 

streamflow losses over one-half the RCHRES length. 

Direct infiltration to the Edwards aquifer also 

occurs from storm runoff detained by 10 flood-control 

structures. The infiltration behind these structures was 

estimated on the basis of data collected at Parkers 

Creek Reservoir in Medina County (site no. 14, fig. 5). 

This reservoir in the outcrop collects runoff from a 

10-square-mile basin and is similar in size and design to 

the flood-control structures in the study area in Bexar 

County. The relation between reservoir impoundment 

surface area and infiltration at Parkers Creek was used 

in the model to simulate withdrawals on the basis of reservoir elevation and surface area for the Bexar County 

flood-control reservoirs.

ET is water transpired from vegetation and evaporated from land surfaces. In the HSPF model, rainfall 

infiltrating into the ground is recharge or base flow that 

returns to the stream channel. The base-flow component 

can be calibrated using streamflow records. However, 

to estimate the fraction of infiltration that becomes 

ground-water recharge, ET must be estimated. To calibrate the HSPF parameters affecting ET, actual ET data 

are needed. Actual ET data were not available for the 

study area but were collected (along with rainfall and 

potential ET [PET]) from the Seco Creek Basin in 

Medina and Uvalde Counties (Dugas and others, 1998) 

during the months of February–September2 from 1991 

through 1995. The area of the Seco Creek Basin where 

the ET data were collected is undeveloped rangeland on 

Glen Rose Limestone, similar to that in the catchment 

area in Bexar County. 

To use the Seco Creek ET data for model calibration, an empirical relation between measured ET, PET, 

and rainfall at Seco Creek was developed. A sample of 

the Seco Creek data is shown in figure 7. The empirical 

ET relation then was used with PET from the National 

1 Streamflow-loss data (unpublished) were collected by the 

USGS along Salado Creek after a storm during October 17–19, 

1998. Flood runoff stored by flood-control structures in the upper 

basin (Salado structures 1 and 2) was released over 2 weeks after 

the storm, providing an opportunity to measure streamflow at 

several locations along a 5.2-mile reach of Salado Creek in the 

Edwards aquifer outcrop and to compute streamflow losses per mile 

for a range of discharges.

2 ET data were not collected during November–February 

because the study area was inaccessible.
INTRODUCTION 11

99 ̊30' 99 ̊ 98 ̊30'

29 ̊30' 

30 ̊

29 ̊45' 

Base modified from U.S. Geological Survey 1:24,000 quadrangles Universal Transverse Mercator Projection, Zone 14

Boerne

Huebner Creek

tributary

Salado Creek

at Loop 1604

Salado Creek

at Old Blanco Road

Bulverde

Stone Mountain drainage ch

Olmos Creek

Huebner

Creek

Helotes

Creek

Panther

Springs

Creek

East 

Elm Creek

West Elm

Creek

Seco Creek

Leon Creek

Sea World

Govenment Canyon

Creek

Parkers Creek 

Parkers Creek Reservoir 

KERR

COUNTY

UVALDE COUNTY BEXAR COUNTY

MEDINA COUNTY

BANDERA

COUNTY

KENDALL

COUNTY

20

13 14

11

1

7

9

10

8

3 4

6

2 15

16

12

19

18

17

0 10 5 15 MILES

EXPLANATION

Study area

Gaged basin

Station description—Number refers to table 5

Reservoir

Streamflow Evapotranspiration

Pan evaporation

Water quality

12 Rainfall

20

17

15

7

14

Figure 5. Data-collection stations used for Hydrological Simulation Program—FORTRAN model calibration. 
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 

Weather Service (NWS) station at Sea World of San 

Antonio and with rainfall data from the Helotes Creek 

at Helotes streamflow-gaging station to estimate actual 

ET for the Helotes Creek Basin during 1997–98. This 

estimated actual ET then was used to calibrate the HSPF 

parameters that influence ET.

SIMULATION OF RUNOFF AND 

RECHARGE

Model Calibration and Testing

Calibration and testing of process-related 

parameters involved four gaged basins in the study 

area: Helotes Creek, East Elm Creek, West Elm Creek, 

and Panther Springs Creek. Helotes Creek Basin primarily is in the catchment area. East Elm Creek and West 

Elm Creek Basins are in the Edwards aquifer outcrop. 

Panther Springs Creek Basin is partly in the Edwards 

aquifer outcrop and partly in the catchment area. 

Helotes Creek Basin

About 98 percent of the gaged part (upper 9,500 

acres) of the Helotes Creek Basin is undeveloped. The 

uppermost geologic unit is the Glen Rose Limestone. 

Data from six storms during April 1997–October 1998 

were used for calibration of the parameters related to 

runoff (table 6). 

Measured and simulated streamflow during the 

storm of Oct. 15–31, 1998, are shown in figure 8. The 

overall error in simulated runoff (computed as the sum 

of simulated runoff, minus the sum of measured runoff, 

divided by the sum of measured runoff, quantity times 

100) for the six storms at Helotes Creek was 4.4 percent. 

The mean absolute error in simulated runoff for individual storms (average of the absolute values of all storm 

runoff percent differences) was 60 percent. This large 

error is heavily influenced by the 220-percent difference 

from the April 1997 storm. Generally, large runoffs are 

simulated more accurately than smaller runoffs. Therefore, mean absolute error was recomputed to weight the 

Loss per mile = (0.062)streamflow + 8.3

Measured

Regression

0 25 75 125 50 100 200 150 175

STREAMFLOW, IN CUBIC FEET PER SECOND

0

4

8

12

16

20

24

STREAMFLOW LOSS PER MILE, IN CUBIC FEET PER SECOND

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

October 1998. 
SIMULATION OF RUNOFF AND RECHARGE 13

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

Reference no.

(fig. 5)

Name and USGS no. 

(if applicable)

Type

of data

Period of

operation Comment

1 USGS station 08181400 Helotes 

Creek at Helotes

Rainfall/

streamflow 

1997–2000 15-square-mile basin; data used for 

calibration of process-related parameters 

for catchment area

2 USGS station 08178645 East Elm 

Creek at San Antonio

Rainfall/

streamflow

1976 2.33-square-mile basin; data used for 

calibration of process-related parameters 

for outcrop–undeveloped

3 USGS station 08178640 West Elm 

Creek at San Antonio

Rainfall/

streamflow

1976–78 2.45-square-mile basin; data used for 

testing of process-related parameters for 

outcrop–undeveloped

Measured evapotranspiration (Bowen ratio method)

Potential evapotranspiration

Rainfall

1991

MAY JUNE JULY AUGUST

0.1

0.2

0.3

0.4

DAILY EVAPOTRANSPIRATION, IN INCHES

0

0.5

1.0

1.5

2.0

2.5

3.0

DAILY RAINFALL, IN INCHES

0

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 

Resources Conservation Service]
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 

4 USGS station 08178600 Panther 

Springs Creek at FM 2696 near San 

Antonio

Rainfall/

streamflow

1973 9.54-square-mile basin; data used for 

testing process-related parameters for 

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 

Mountain drainage channel at 

Granite Path, San Antonio

Rainfall/water 

quality

1997–2000 Rainfall data used for 1997–2000 

simulations; water-quality data used to 

characterize residential runoff

7 USGS station 08181420 Huebner 

Creek at De Zavala Rd., San 

Antonio

Water quality 1996 Data used to characterize commercial 

runoff

8 USGS station 08181425 Cedar Elm 

outfall at Huebner Creek tributary, 

San Antonio

Water quality 1996–97 Data used to characterize commercial 

runoff

9 USGS station 08181440 Leon Creek 

outfall at Ingram Rd. near SH 151, 

San Antonio

Water quality 1992–99 Data for 1997–99 used to characterize 

transportation runoff

10 USGS station 08177600 Olmos Creek 

tributary at FM 1535 at Shavano 

Park

Water quality 1997–98 Data used to characterize residential runoff

11 USGS station 08180940 Government 

Canyon Creek near Helotes

Water quality 1997–98 Data used to characterize undeveloped land 

runoff

12 USGS station 08185000 Cibolo Creek 

at Selma

Rainfall 1997–2000 Data used for 1997–2000 simulations

13 USGS station 08202790 Parkers 

Creek Reservoir inflow near 

D’Hanis

Streamflow 1992–97 Measured inflow used to estimate water 

balance of Parkers Creek Reservoir and, 

in turn, to estimate infiltration at floodcontrol reservoirs in Bexar County

14 USGS station 08202800 Parkers 

Creek Reservoir near D’Hanis

Reservoir 

elevation and 

storage

1992–97 Measured infiltration rates used to estimate 

infiltration at flood-control reservoirs in 

Bexar County

15 USGS station 08178592 Salado 

Creek at Loop 1604 near San 

Antonio

Streamflow 1998 Gain-loss measurement data used to 

estimate streamflow losses in outcrop

16 USGS station 08178593 Salado 

Creek at Old Blanco Rd. near San 

Antonio

Streamflow 1998 Gain-loss measurement data used to 

estimate streamflow losses in outcrop

17 NWS station at Sea World, San 

Antonio

Pan evaporation 1997–2000 Data used for 1997–2000 simulations

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, 

Medina County

Evapotranspiration1992–96 Data used for calibration simulations, 

Helotes Creek Basin

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

Reference no.

(fig. 5)

Name and USGS no. 

(if applicable)

Type

of data

Period of

operation Comment
SIMULATION OF RUNOFF AND RECHARGE 15

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

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

times 100.

Date Rainfall amount 

(inches)

Measured runoff 

(inches)

Simulated runoff

(inches)

Percent

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

Simulated

Measured

15

OCTOBER 1998

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

200

400

600

800

1,000

1,200

1,400

STREAMFLOW, IN CUBIC FEET PER SECOND

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

1998. 
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 

0

Estimated

Simulated

JAN

MAR

APR

MAY

JUNE

JULY

SEPT

FEB

AUG

OCT

NOV

DEC

JAN

FEB

MAR

APR

1997–98

MAY

JUNE

JULY

SEPT

AUG

OCT

NOV

DEC

4.0

2.0

1.0

3.0

EVAPOTRANSPIRATION, IN INCHES

error of each storm by measured (gaged) runoff. The 

mean absolute runoff-weighted error of the six Helotes 

Creek storms was 12 percent. 

Along with parameters related to runoff, parameters related to ET also were calibrated. Rainfall from 

the Helotes Creek at Helotes gage and PET (from the 

Sea World NWS station) were used to estimate actual 

ET on undeveloped Glen Rose Limestone using the 

empirical relation between measured ET, PET, and rainfall from the Seco Creek Basin previously described. 

The model parameters (primarily UZSN, LZSN, and 

LZETP) were adjusted so that ET simulated by the 

model approximated ET estimated from the empirical 

relation. Monthly simulated ET compared with ET 

estimated from the empirical relation is shown in 

figure 9. 

The following table shows rainfall for the Helotes 

Creek Basin and simulated ET for the undeveloped 

Glen Rose Limestone and ET expressed as a percentage 

of annual rainfall. During 1997, 1998, and 2000, annual 

rainfall was greater than normal, and much of the rainfall occurred in a few large storms that generated considerable runoff that exited the study area and was not 

subject to ET. The result was low ET as a percentage of 

rainfall in 1997, 1998, and 2000. In contrast, 1999 was 

dryer than normal with no major storms; ET as a percentage of rainfall was high.

Helotes Creek annual rainfall and HSPF simulated 

evapotranspiration (ET), 1997–2000 

Year

Rainfall

(inches)

Glen Rose 

Limestone ET

(inches)

Glen Rose

Limestone ET

(percent of 

annual rainfall)

1997 36.75 18.93 51.5

1998 41.38 18.96 45.8

1999 15.72 15.32 97.4

2000 36.30 18.22 50.2

Figure 9. Hydrograph showing simulated and estimated evapotranspiration, Helotes Creek Basin, 1997–98. 
SIMULATION OF RUNOFF AND RECHARGE 17

0.01 0.10 1.00

MEASURED STORM RUNOFF, IN INCHES

0.01

0.10

1.00

SIMULATED STORM RUNOFF, IN INCHES

East Elm Creek

West Elm Creek

East Elm Creek and West Elm Creek Basins

Calibration and testing of East Elm Creek and 

West Elm Creek Basins were done to obtain processrelated parameters for the undeveloped Edwards aquifer 

outcrop. Rainfall and runoff data during May 1976–

September 1978 (when the basins were undeveloped) 

were used for the model calibration and testing. East 

Elm Creek Basin was calibrated using data from four 

storms during 1976. To test the calibration and also 

to test the transferability of model parameters to similar 

ungaged basins, the calibration model parameters from 

the East Elm Creek Basin were applied to the West 

Elm Creek Basin in simulations of four storms during 

1976–78. Simulated runoff volumes were 4.7-percent 

larger than the gaged volumes for the East Elm Creek 

storms and 3.1-percent smaller for the West Elm Creek 

storms (table 7). Simulated and measured runoff volumes for the East and West Elm Creek Basins are shown 

in figure 10; a hydrograph showing measured and simulated streamflow for the East Elm Creek storm on September 28, 1976, is shown in figure 11.

The overall errors in simulated runoff for the four 

storms at East Elm Creek and the four storms at West 

Elm Creek were 4.7 and -3.1 percent, respectively. The 

mean absolute errors in simulated runoff for individual 

storms were 17 and 20 percent, respectively; and the 

mean absolute runoff-weighted errors were 17 and 12 

percent, respectively.

Data to calibrate ET-related parameters in the 

Edwards aquifer outcrop were not available. Although 

the infiltration capacity and permeability of the 

Edwards aquifer outcrop and Glen Rose Limestone 

likely are different, other factors affecting ET, such 

as soil and vegetation, are similar. The ET-related 

parameters developed for the catchment area (Glen 

Rose Limestone) in the Helotes Creek Basin calibration 

Figure 10. Measured and simulated storm runoff volumes, East Elm Creek and West Elm Creek Basins, 1976–78. 
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 

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

1976–78 

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

times 100.

Date Rainfall amount

(inches)

Measured runoff

(inches)

Simulated runoff

(inches)

Percent

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

0

20

40

60

80

100

120

140

160

0.5

STREAMFLOW, IN CUBIC FEET PER SECOND

1.0

1.5

2.0

CUMULATIVE RAINFALL, IN INCHES

0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TIME, IN HOURS

Measured streamflow

Simulated streamflow

Cumulative rainfall

Total rainfall - 1.67 inches

Measured runoff - 0.23 inch

Simulated runoff - 0.26 inch

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

1976. 
SIMULATION OF RUNOFF AND RECHARGE 19

0.25 0.50 0.75 1.00 1.25 1.50

MEASURED RUNOFF, IN INCHES

0

0.25

0.50

0.75

1.00

1.25

1.50

SIMULATED RUNOFF, IN INCHES

0

Figure 12. Measured and simulated storm runoff volumes, Panther Springs Creek Basin, 1973. 

were used for calibration, testing, and simulations of 

basins in the Edwards aquifer outcrop.

Panther Springs Creek Basin

The calibration parameters from Helotes Creek, 

East Elm Creek, and West Elm Creek Basins were further tested by applying them in simulations of the 

Panther Springs Creek Basin. The gaged area of the 

basin in the 1970s (about 6,100 acres) was about equally 

divided between catchment area and Edwards aquifer 

outcrop. The gaged area was largely undeveloped in 

1973 when rainfall and runoff data were collected during four storms (table 8). Simulated runoff during these 

storms is compared to measured runoff in figure 12.

Table 8. Summary of model testing results for Panther Springs Creek Basin, 1973 

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

times 100.

Date Rainfall amount 

(inches)

Measured runoff 

(inches)

Simulated runoff 

(inches)

Percent

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
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 

The simulations of the four storms for the Panther 

Springs Creek Basin yielded an overall error in runoff 

volumes of 3.3 percent. The mean absolute error for the 

individual storms was 35 percent, and the mean absolute 

runoff-weighted error was 43 percent. Unlike the simulations for the other three basins, weighting the errors by 

runoff volume did not result in a smaller mean error.

The simulation errors for all four basins are summarized in table 9. As a result of model calibration and 

testing with data for the four basins, a set of processrelated model parameters for simulation of all of the 

basins in the study area was obtained; those parameters 

are summarized in table 10. 

Error Analysis 

The types of error from the model calibration and 

testing can be classified as measurement errors or systematic errors (Raines, 1996). Measurement errors are 

introduced as a result of missing data or of inaccurate 

data used in calibration or testing. For example, measured or gaged streamflow is subject to potential error in 

rating tables of stage and discharge; insufficient data 

also contribute to potential error. For example, relatively 

few gaged storms (from only four of the 10 basins) were 

available for calibration, and the results (calibration 

parameters) were transferred to ungaged basins. Possible seasonal hydrologic factors could not be investigated because of the small number of storms. ET is a 

substantial component of the hydrologic cycle, yet ET 

data were not available for the Edwards aquifer outcrop. 

Also, streamflow-loss data from one stream (Salado 

Creek) was used to characterize losses for most of the 

major streams in the study area. Finally, the spatial variability of rainfall in a basin might not be adequately represented by the available network of rain gages.

Systematic errors are associated with limited ability of the simulation model to represent the hydrologic 

processes of the basins in the study area. Limits exist on 

Table 9. Errors, mean absolute errors, and mean absolute runoff-weighted errors for storm runoff volumes used in 

model calibrations 

[In percent] 

Watershed Number of

storm events

Error Mean absolute

error

Mean absolute runoffweighted 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

Table 10. Selected process-related calibration 

parameters used for basin simulations

[Parameter definitions in table 3; --, not applicable] 

1 The users manual for Hydrological Simulation Program—

FORTRAN (Bicknell and others, 1997) provides a more complete 

description of each parameter.

Parameter Value Unit

AGWRC–outcrop 0.50 per day

AGWRC–catchment area .98 per day

AGWETP 0 --

BASETP 0 --

CEPSC .1 inches

DEEPFR–outcrop .95 --

DEEPFR–catchment area .05 --

INFEXP 2.0 --

INFILD 2.0 --

INFILT–outcrop .40 inches per hour

INFILT–catchment area .15 inches per hour

INTFW–outcrop .2 --

INTFW–catchment area 2.0 --

IRC–outcrop .1 per day

IRC–catchment area .4 per day

LZETP .1 --

LZSN–outcrop 4.0 inches

LZSN–catchment area 5.0 inches

NSUR .15 --

UZSN–outcrop .20 inches

UZSN–catchment area .25 inches
SIMULATION OF RUNOFF AND RECHARGE 21

how well the model parameters and equations describe 

the physical properties of runoff. Also, the configuration of the model segments (PERLNDs, IMPLNDs, 

and RCHRESs) and the selection of simulation time 

step (in this case, hourly) can only approximate the 

actual physical configuration and hydrologic response 

of the basins.

Despite incomplete data, the HSPF model provides reasonable simulations of runoff volumes compared with observed data. For each of the calibration 

basins, total simulated runoff was within 5 percent of 

measured runoff. For individual storms, mean absolute 

runoff-weighted errors were within 20 percent for 

Helotes Creek, East Elm Creek, and West Elm Creek 

Basins. 

Figures 8 and 11 show how the model simulations 

represent the peak flow and runoff response of the 

basins. There is a characteristic difference in runoff 

response between basins in the catchment area and 

those in the Edwards aquifer outcrop. Runoff for 

Helotes Creek (catchment area) lasted weeks, whereas 

runoff at East Elm Creek (outcrop) ceased within hours. 

Although the Panther Springs Creek storms exhibited 

the highest mean absolute error in runoff volumes, the 

HSPF model was able to account for differences in rainfall intensity and antecedent conditions in runoff simulation. For example, the June 1973 rainfall (4.96 inches) 

occurred after a relatively dry month (0.16 inch in May). 

The June 1973 measured runoff of 0.45 inch is less than 

the runoff that occurred during the July and September 

storms (table 8), which featured less, but more intense, 

rainfall. 

Sensitivity Analysis

Panther Springs Creek Basin parameters were 

used in a sensitivity analysis of selected HSPF model 

parameters to determine the effects of changes in the 

values of selected parameters on simulated runoff, 

recharge, and streamflow exiting the basin. (Streamflow 

exiting the basin is runoff minus streamflow losses and 

water lost to infiltration behind recharge dams.) Each 

parameter was changed by a hydrologically reasonable 

amount while keeping other parameters unchanged, and 

simulations were run. The resulting changes in surface 

runoff (surface plus interflow runoff), recharge, and 

streamflow exiting the basin are listed in table 11.

The parameters to which simulated model outputs 

were most sensitive were lower-zone ET (LZETP) 

and index to soil infiltration capacity (INFILT). A 50-

percent increase in LZETP resulted in a 9.1-percent 

decrease in recharge, a 1.4-percent decrease in surface 

runoff, and a 6.2-percent decrease in streamflow exiting 

the basin. Also, recharge was somewhat sensitive to 

interception storage capacity (CEPSC). A 50-percent 

increase in CEPSC resulted in a 4.9-percent decrease in 

recharge. 

In addition to the process parameters, the percentage of impervious area in residential and commercial land use categories is subject to some uncertainty. 

The effect of a percentage change in the effective 

impervious cover for residential land use was simulated. 

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

[Parameter definitions in table 3]

Parameter Initial

value

Adjusted

value

Change in

parameter

(percent)

Change in

surface runoff

(percent)

Change in Edwards

aquifer recharge

(percent)

Change in streamflow exiting basin

(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
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 

The overall, effective impervious cover for residential 

land use in the Panther Springs Creek Basin in 1998 was 

estimated to be 29 percent. A model simulation was 

done with a residential impervious cover of 32 percent 

(10-percent increase). The increase in overall basin 

impervious cover was from 7.2 to 7.6 percent. The 

resulting percentage changes in runoff and recharge are 

listed in the following table:

Results of sensitivity analysis of change in percentage 

of residential land use effective impervious cover

[In percent] 

Increasing the residential effective impervious 

cover from 29 percent to 32 percent resulted in a negligible increase in recharge of 0.04 percent. Although 

recharge from direct infiltration decreased because of 

pervious area loss, recharge occurring from streamflow 

losses increased, and the two changes effectively canceled one another.

Runoff From and Recharge to the Major 

Basins

The process-related model parameters calibrated 

and tested in the Helotes Creek, East Elm Creek, West 

Elm Creek, and Panther Springs Creek Basins were 

applied to the 10 major basins in the study area to simulate runoff from (streamflow exiting) the basins and 

recharge to the basins during 1997–2000 (table 12). 

During 1997, 39.23 inches of rain (area-weighted 

average of rain-gage data used for the simulations) fell 

on the study area—about 35 percent above normal. In 

June 1997, a major rainfall occurred in the western part 

of the study area where as much as 10 inches of rain fell 

during a 2-day period. Rainfall in 1998 was 44.06 in, 

about 50 percent above normal. Some areas in the northeastern part of the study area received more than 20 

inches of rainfall in October 1998. In contrast, rainfall 

in 1999 was 15.85 inches, about 45 percent below normal; rainfall in 2000 was 34.66 inches, about 20 percent 

above normal.

In 1997, simulated streamflow exiting the 10 

major basins (and thus runoff from the study area) was 

5.62 inches. Simulated recharge to the 10 major basins 

(essentially in the Edwards aquifer outcrop in the study 

area) was 7.85 inches (20 percent of rainfall). In 1998, 

simulated streamflow exiting was 11.05 inches, and 

simulated recharge was 10.99 inches (25 percent of 

rainfall). In 1999, simulated streamflow exiting was 

0.66 inch, and simulated recharge was 3.03 inches (19 

percent of rainfall). In 2000, simulated streamflow exiting was 5.29 inches; simulated recharge was 7.19 inches 

(21 percent of rainfall).

During 1997–2000, simulated recharge in the 

study area (essentially in the Edwards aquifer outcrop) 

was about 22 percent of rainfall, simulated runoff from 

the study area was about 17 percent of rainfall, and simulated ET in the study area was about 51 percent of rainfall. The remaining 10 percent of rainfall during 1997–

2000 was accounted for by simulated changes in soil 

and unsaturated-zone storages and by a small amount of 

simulated recharge in the catchment area. Figure 13 

shows simulated monthly runoff from and recharge to 

the study area during 1997–2000. About 37 percent of 

the recharge (177,000 acre-ft) and about 66 percent of 

the runoff (248,000 acre-ft) occurred during four 

months—June 1997, October 1998, and October and 

November 2000. Thus the proportions of rainfall 

accounted for by recharge, runoff, and ET during 1997–

2000 might not reflect the proportions characteristic of 

long-term average hydrologic conditions 

Simulated recharge and runoff during 1997–2000 

varied widely among the 10 basins. The Leon Creek 

Basin (mostly Glen Rose Limestone) had 20.60 inches 

of recharge; the Elm Creek Basin (mostly Edwards 

aquifer outcrop; includes four flood-control structures) 

had 49.30 inches of recharge. For the study area as a 

whole, recharge during 1997–2000 was 29.06 inches 

The Helotes Creek Basin had the least amount of runoff 

(6.60 inches compared with 22.62 inches for the entire 

study area) mostly because of relatively large streamflow losses in a stream reach that passes through a 

quarry near the exit of the basin. Cibolo Creek had the 

largest amount of runoff (36.18 inches); it also received 

the largest amount of rainfall (150.86 inches compared 

with the area-weighted average of 133.80 inches for the 

entire study area).

Primary sources of recharge to the Edwards 

aquifer are direct infiltration of rainfall in interstream 

areas, streamflow losses in the outcrop, and infiltration 

of impounded runoff behind flood-control structures. 

Scenario

Change in

residential

surface

runoff

Change in

Edwards

aquifer

recharge

Change in

volume of 

streamflow

exiting basin

10-percent increase in 

residential effective 

impervious cover

5.6 0.04 1.1
ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 23

Simulated recharges, by primary sources, are listed in 

table 13. 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.

ESTIMATION OF CONSTITUENT LOADS IN 

RUNOFF

In this report a constituent load is the mass of a 

constituent moved past a point by water in a specified 

time. Loads for suspended solids, dissolved solids, dissolved nitrite plus nitrate nitrogen, and total lead in runoff were computed using the equation 

L = R x EMC x Cf, (1)

where 

L = constituent load, in pounds;

R = runoff volume, in acre-feet;

EMC = median event-mean concentration during runoff, in milligrams per liter or micrograms per 

liter; and

Cf = conversion factor, 2.718 for concentrations in 

milligrams per liter or 0.00272 for concentrations in micrograms per liter.

Constituent yield, a measure of the loadproducing characteristics of a basin, is computed by the 

equation

Y = L/DA, (2)

where 

Y = constituent yield, in pounds per acre; 

L = constituent load, in pounds; and 

DA = contributing drainage area of the basin, in 

acres.

Recharge

Streamflow

1997 1998 1999 2000

0

20,000

40,000

60,000

80,000

100,000

120,000

STREAMFLOW OR RECHARGE, IN ACRE-FEET

Figure 13. Simulated monthly runoff from (streamflow exiting) and recharge to the study area, 1997–2000. 
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 12. Annual rainfall and estimates of runoff from (streamflow exiting) and recharge to the 10 major basins in the study 

San

Geronimo

Creek

Culebra

Creek

Helotes

Creek

Leon

Creek

Olmos/

Huebner

Creeks

Salado

Creek

Panther

Springs

Creek

Mud

Creek

Elm

Creek

1997

Basin area, acres 15,690 11,121 17,540 41,759 5,426 19,637 14,245 14,546 19,244 Annual rainfall, inches 36.75 36.75 36.75 40.05 34.73 40.00 33.08 33.08 36.31 Exiting streamflow, acre-feet 9,410 2,940 2,560 29,600 1,690 6,750 2,120 2,450 2,350 Exiting streamflow, inches 7.20 3.17 1.75 8.51 3.74 4.12 1.79 2.02 1.47 Edwards recharge, acre-feet 3,110 11,600 9,960 18,800 5,390 7,720 12,200 13,400 21,200 Edwards recharge, inches 2.38 12.52 6.81 5.40 11.93 4.72 10.27 11.08 13.24 1998Basin area, acres 15,690 11,121 17,540 41,759 5,426 19,637 14,245 14,546 19,244 Annual rainfall, inches 40.33 40.33 40.33 42.46 41.28 45.06 41.19 41.19 45.94 Exiting streamflow, acre-feet 16,800 5,600 4,840 50,300 4,440 15,300 7,540 7,410 12,200 Exiting streamflow, inches 12.82 6.04 3.31 14.45 9.82 9.35 6.35 6.11 7.59 Edwards recharge, acre-feet 3,810 14,900 13,000 24,500 7,060 10,000 21,200 22,500 32,700 Edwards recharge, inches 2.91 16.08 8.89 7.04 15.60 6.12 17.86 18.53 20.38 1999Basin area, acres 15,690 11,121 17,540 41,759 5,426 19,637 14,245 14,546 19,244 Annual rainfall, inches 15.72 15.72 15.72 16.24 15.80 15.87 15.87 15.87 14.74 Exiting streamflow, acre-feet 935 26 20 4,690 440 150 510 730 1,260 Exiting streamflow, inches .71 .03 .01 1.33 .97 .09 .43 .60 .78 Edwards recharge, acre-feet 1,210 4,160 4,040 7,900 2,300 2,500 5,440 6,120 9,170 Edwards recharge, inches .93 4.49 2.76 2.27 5.08 1.53 4.58 5.05 5.72 2000Basin area, acres 15,690 11,121 17,540 41,759 5,426 19,637 14,245 14,546 19,244 Annual rainfall, inches 36.30 36.30 36.30 36.30 37.03 37.03 29.00 29.00 29.03 Exiting streamflow, acre-feet 10,900 2,700 2,240 33,800 2,720 8,240 1,190 1,510 1,320 Exiting streamflow, inches 8.33 2.91 1.53 9.71 6.01 5.03 1.00 1.24 .83 Edwards recharge, acre-feet 3,310 12,500 11,200 20,500 6,100 7,630 11,200 12,200 16,000 Edwards recharge, inches 2.53 13.49 7.66 5.89 13.50 4.66 9.44 10.07 9.96 
ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 25

Event-mean concentration (EMC) (Charbeneau 

and Barrett, 1998) represents a discharge-weighted 

average constituent concentration during storm runoff. 

Two sources of EMCs for Bexar County were available 

for this study: (1) Data collected during 1996–98 for a 

water-quality study of five basins with specific land 

uses in the Edwards aquifer outcrop, Bexar County 

(Ockerman and others, 1999); and (2) data obtained 

from a study of the San Antonio National Pollutant Discharge Elimination System (B.L. Petri, U.S. Geological 

Survey, written commun., 2000). Median EMCs from 

the two sources (table 14) were used to compute loads.

The computed constituent loads in surface-water 

runoff are estimates of annual total loads originating 

from pervious and impervious surfaces and transported 

to streams in the study area. The transport and fate of 

these constituent loads are not considered. For example, 

some of the load will be retained behind flood-control 

structures; some of the load infiltrates the aquifer with 

streamflow lost to the aquifer; and some of the load is 

transported from the study area in streamflow exiting 

the study area. Constituents transported to streams by 

interflow or base flow are not included in the load 

estimates. Also, basin protection ordinances in Bexar 

County require detention, sedimentation, and filtration 

structures for commercial land use in the Edwards 

aquifer outcrop designed to capture and isolate the 

first 0.5 inch of runoff during a storm to minimize pollutants entering the aquifer (San Antonio Water System, 

2000). The estimates of constituent loads in runoff 

do not consider the effects of these pollution-control 

structures.

Estimates of annual loads and yields by land use 

for suspended solids, dissolved solids, dissolved nitrite 

plus nitrate nitrogen, and total lead during 1997–2000 

for each of the 10 major basins are listed in tables 15–

18. Annual loads for the four constituents were consistently largest from undeveloped land and smallest from 

commercial land or transportation corridors. Annual 

loads varied with rainfall (table 12), with the maximum 

loads for the study area produced in 1998 (wettest year) 

and the minimum loads produced in 1999 (driest year). 

For example, in 1998 when rainfall was about 50 percent above normal, nitrite plus nitrate nitrogen load 

from undeveloped areas was 134 tons; in 1999, when 

rainfall was about 45 percent below normal, nitrite plus 

nitrate nitrogen load from undeveloped areas was 7.5 

tons (table 17).

Because they are computed from loads, annual 

yields for the four constituents also varied with rainfall. 

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

[In acre-feet] 

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 

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

Source of recharge 1997 1998 1999 2000 Total, 

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

Constituent Undeveloped Residential Commercial Transportation

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 
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 

Table 15. Estimates of annual loads, by land use, and yields for suspended solids in surface-water runoff for the 10 major b1997–2000 

San

Geronimo

Creek

Culebra

Creek

Helotes

Creek

Leon

Creek

Olmos/

Huebner

Creeks

Salado

Creek

Panther

Springs

Creek

Mud

Creek

Elm

Cree

1997

Load (undeveloped), tons 540 280 500 1,000 38 490 180 140 180 Load (residential), tons 6.2 10 170 380 72 69 95 130 71 Load (commercial), tons 1.0 0 79 530 81 29 74 81 78 Load (transportation corridor), tons 25 11 35 340 73 41 98 127 130 Yield, pounds per acre 73 54 90 110 97 64 63 66 48 1998Load (undeveloped), tons 1,000 540 950 2,000 140 1,300 720 570 980 Load (residential), tons 11 16 260 590 130 120 180 280 170 Load (commercial), tons 2 0 100 660 120 41 120 130 130 Load (transportation corridor), tons 36 14 44 430 110 58 150 200 220 Yield, pounds per acre 140 100 150 170 180 160 160 160 160 1999Load (undeveloped), tons 59 11 60 110 4 39 75 53 130 Load (residential), tons 2 3 45 100 28 18 44 61 33 Load (commercial), tons .4 0 29 190 34 10 35 39 32 Load (transportation corridor), tons 11 4 13 130 31 14 47 60 53 Yield, pounds per acre 9.1 3 17 25 36 8 28 29 26 2000Load (undeveloped), tons 620 290 580 1,200 79 680 130 92 130 Load (residential), tons 11 18 180 410 200 150 79 110 57 Load (commercial), tons 2.5 0 80 530 220 70 63 70 64 Load (transportation corridor), tons 68 24 36 350 200 98 84 110 110 Yield, pounds per acre 89 60 100 120 260 100 50 52 37 
ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 27

Table 16. Estimates of annual loads, by land use, and yields for dissolved solids in surface-water runoff for the 10 major ba

1997–2000 

San

Geronimo

Creek

Culebra

Creek

Helotes

Creek

Leon

Creek

Olmos/

Huebner

Creeks

Salado

Creek

Panther

Springs

Creek

Mud

Creek

Elm

Cree

1997

Load (undeveloped), tons 1,300 690 1,200 2,600 94 1,200 450 350 45Load (residential), tons 6.3 11 170 390 73 70 97 130 7Load (commercial), tons .5 0 36 240 37 13 34 37 3Load (transportation corridor), tons 14 6 19 190 40 23 54 70 7Yield, pounds per acre 170 130 170 160 90 130 89 81 61998Load (undeveloped), tons 2,500 1,300 2,400 4,900 360 3,300 1,800 1,400 2,40Load (residential), tons 11 16 260 600 130 120 180 290 17Load (commercial), tons .7 0 46 300 55 19 55 59 5Load (transportation corridor), tons 20 8 24 240 60 32 83 110 12Yield, pounds per acre 330 240 310 290 220 350 300 260 291999Load (undeveloped), tons 150 27 150 270 10 97 180 130 32Load (residential), tons 2.0 3.1 46 100 29 18 45 62 5Load (commercial), tons .2 0 13 87 16 4.6 16 18 2Load (transportation corridor), tons 6 2 7 72 17 8 26 33 2Yield, pounds per acre 20 6 25 26 26 13 38 34 42000Load (undeveloped), tons 1,500 720 1,400 3,000 200 1,700 320 230 32Load (residential), tons 11 18 180 420 200 150 80 110 3Load (commercial), tons 1.1 0 36 240 100 32 29 32 1Load (transportation corridor), tons 38 13 20 190 110 54 46 59 2Yield, pounds per acre 200 140 190 180 220 200 67 59 4
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 

Table 17. Estimates of annual loads, by land use, and yields for dissolved nitrite plus nitrate nitrogen in surface-water runoff in the study area, 1997–2000 

San

Geronimo

Creek

Culebra

Creek

Helotes

Creek

Leon

Creek

Olmos/

Huebner

Creeks

Salado

Creek

Panther

Springs

Creek

Mud

Creek

Elm

Cre

1997

Load (undeveloped), tons 6.3 3.3 5.8 12 0.4 5.7 2.1 1.6 2.Load (residential), tons .03 .06 .90 2.0 .38 .36 .50 .69 .Load (commercial), tons 0 0 .22 1.5 .23 .08 .21 .23 .Load (transportation corridor), tons .10 .04 .14 1.4 .30 .17 .40 .52 .Yield, pounds per acre .82 .61 .81 .81 .50 .64 .45 .42 .1998Load (undeveloped), tons 12 6.3 11 23 1.7 15 8.4 6.7 11 Load (residential), tons .06 .08 1.4 3.1 .68 .63 .95 1.5 .Load (commercial), tons 0 0 .28 1.9 .34 .12 .34 .36 .Load (transportation corridor), tons .15 .06 .18 1.8 .44 .24 .61 .82 .Yield, pounds per acre 1.5 1.2 1.5 1.4 1.2 1.7 1.4 1.3 1.1999Load (undeveloped), tons .69 .13 .70 1.3 .05 .46 .86 .62 1.Load (residential), tons .01 .02 .24 .53 .15 .10 .23 .32 .Load (commercial), tons 0 0 .08 .53 .10 .03 .10 .11 .Load (transportation corridor), tons .04 .02 .05 .53 .13 .06 .19 .24 .Yield, pounds per acre .09 .03 .12 .14 .15 .06 .19 .18 .2000Load (undeveloped), tons 7.2 3.4 6.8 14 .92 7.9 1.5 1.1 1.Load (residential), tons .06 .10 .95 2.2 1.1 .77 .42 .58 .Load (commercial), tons .01 0 .22 1.5 .62 .20 .18 .20 .Load (transportation corridor), tons .28 .10 .15 1.4 .82 .40 .34 .44 .Yield, pounds per acre .97 .64 .92 .91 1.3 .95 .34 .31 .
ESTIMATION OF CONSTITUENT LOADS IN RUNOFF 29

Table 18. Estimates of annual loads, by land use, and yields for total lead in surface-water runoff for the 10 major basins in 

1997–2000

San

Geronimo

Creek

Culebra

Creek

Helotes

Creek

Leon

Creek

Olmos/

Huebner

Creeks

Salado

Creek

Panther

Springs

Creek

Mud

Creek

Elm

Cree

1997

Load (undeveloped), tons 50 26 46 94 3.5 45 17 13 17 Load (residential), tons .96 1.6 26 59 11 11 15 20 11 Load (commercial), tons .16 0 12 83 13 4.5 12 13 12 Load (transportation corridor), tons 2.0 .90 2.9 28 6.0 3.3 8.0 10 11 Yield, pounds per acre .003 .002 .006 .017 .002 .004 .003 .004 .001998Load (undeveloped), tons 94 50 87 180 13 120 66 52 90 Load (residential), tons 1.7 2.5 40 91 20 19 28 43 26 Load (commercial), tons .23 0 16 100 19 6.4 19 20 20 Load (transportation corridor), tons 2.9 1.1 3.6 35 8.9 4.7 12 16 18 Yield, pounds per acre .006 .003 .009 .03 .004 .01 .008 .008 .01999Load (undeveloped), tons 5 1.0 5.5 10 .37 3.4 6.8 4.9 12 Load (residential), tons .31 .46 7.0 15 4.3 2.8 6.8 9.4 8.8 Load (commercial), tons .06 0 4.5 30 5.3 1.6 5.5 6.1 10 Load (transportation corridor), tons .90 .33 1.1 11 2.5 1.1 3.8 4.9 4.3 Yield, pounds per acre .0004 .0001 .001 .004 .0008 .0006 .002 .002 .002000Load (undeveloped), tons 57 27 53 110 7.2 62 12 8.4 12 Load (residential), tons 1.7 2.8 28 63 31 22 12 17 5.1 Load (commercial), tons .39 0 12 83 34 11 10 11 5.0 Load (transportation corridor), tons 5.6 2.0 2.9 29 16 8.0 6.8 8.7 4.4 Yield, pounds per acre .004 .002 .006 .018 .006 .007 .003 .003 .00
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 yields for suspended solids (table 15) ranged 

from 20 pounds per acre 1999 (driest year) to 160 

pounds per acre in 1998 (wettest year). Among basins, 

the average annual yield for suspended solids in 1999 

ranged from 3 pounds per acre in the Culebra Creek 

Basin (mostly Edwards aquifer outcrop and one of the 

least developed basins) to 36 pounds per acre in the 

Olmos/Huebner Creeks Basin (the basin with the highest estimated percentage of impervious area [table 1], 

primarily related to residential development).

Annual yields for dissolved solids (table 16) and 

dissolved nitrite plus nitrate nitrogen (table 17) showed 

variations with rainfall similar to those for suspended 

solids—generally smallest in 1999 and largest in 1998. 

Yields of these constituents among basins, as with suspended solids, varied widely.

The annual yield for total lead (table 18) was 

about eight times greater in 1998, the wettest year 

(0.008 pound per acre) than in 1999, the driest year 

(0.001 pound per acre). The Culebra Creek Basin had 

the smallest annual yield of total lead (0.0001 pound per 

acre in 1999), and the Leon Creek watershed (mostly 

Glen Rose Limestone and relatively greater residential 

and commercial development) had the largest annual 

yield (0.018 pound per acre in 2000, a relatively wet 

year).

SUMMARY

The USGS developed an HSPF watershed model 

to simulate runoff and recharge and to estimate constituent loads in surface-water runoff in the Edwards 

aquifer outcrop and catchment area in Bexar County. 

Rainfall and runoff data collected during 1970–98 from 

four gaged basins in the Edwards aquifer 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 of the study area to obtain runoff and recharge 

volumes for 4 years, 1997–2000. The simulated runoff 

volumes were used with EMC data from basins in the 

study area and from other Bexar County basins to compute constituent loads and yields for various land uses. 

Calibration and testing of process-related model 

parameters (those that represent the physical processes 

of infiltration, ET, interception, interflow, ground-water 

recession, and surface runoff) involved four gaged 

basins in the study area: Helotes Creek, East Elm Creek, 

West Elm Creek, and Panther Springs Creek. Helotes 

Creek Basin primarily is in the catchment area. East 

Elm and West Elm Creek basins are in the Edwards 

aquifer outcrop. Panther Springs Creek Basin is partly 

in the Edwards aquifer outcrop and partly in the catchment area. A computerized expert system was used to 

guide the adjustment of process-related parameters during calibration and testing. 

For the Helotes Creek Basin, data from six storms 

were used for calibration of the parameters related to 

runoff. The mean absolute runoff-weighted error for 

calibration simulations of the six Helotes Creek storms 

was 12 percent. East Elm Creek Basin was calibrated 

using data from four storms. To test the calibration and 

also to test the transferability of model parameters to 

similar ungaged basins, the calibration parameters from 

the East Elm Creek Basin were applied to the West Elm 

Creek Basin in simulations of four storms. The mean 

absolute runoff-weighted errors for the calibration (East 

Elm) and testing (West Elm) simulations were 17 and 12 

percent, respectively. The calibration parameters from 

Helotes Creek, East Elm Creek, and West Elm Creek 

Basins were further tested by applying them in simulations of the Panther Springs Creek Basin. The mean 

absolute runoff-weighted error for the Panther Creek 

testing simulations was 43 percent.

The process-related model parameters calibrated 

and tested in the Helotes Creek, East Elm Creek, West 

Elm Creek, and Panther Springs Creek Basins were 

applied to the 10 major basins in the study area to simulate streamflow exiting and recharge to the basins during 1997–2000. In 1997, simulated streamflow exiting 

the 10 major basins (and thus runoff from the study 

area) was 5.62 inches; simulated recharge to the 10 

major basins (essentially in the recharge zone in the 

study area) was 7.85 inches (20 percent of rainfall). In 

1998, simulated streamflow exiting was 11.05 inches; 

simulated recharge was 10.99 inches (25 percent of 

rainfall). In 1999, simulated streamflow exiting was 

0.66 inch; simulated recharge was 3.03 inches (19 percent of rainfall). In 2000, simulated streamflow exiting 

was 5.29 inches; simulated recharge was 7.19 inches 

(21 percent of rainfall).

Primary sources of recharge to the Edwards 

aquifer are direct infiltration of rainfall in interstream 

areas, streamflow losses in the outcrop, and infiltration 

of impounded runoff behind flood-control structures. 

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.
REFERENCES 31

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 varied with rainfall, with the 

maximum loads produced in the wettest year (1998) and 

the minimum loads produced in the driest year (1999).

Because they are computed from loads, annual 

yields for the four constituents also varied with rainfall. 

Annual yields for suspended solids ranged from 20 

pounds per acre in 1999 (driest year) to 160 pounds per 

acre in 1998 (wettest year). Annual yields for dissolved 

solids and dissolved nitrite plus nitrate nitrogen showed 

variations with rainfall similar to those for suspended 

solids—generally smallest in 1999 and largest in 1998. 

The annual yield for total lead was about eight times 

greater in 1998 (0.008 pound per acre) than in 1999 

(0.001 pound per acre).

REFERENCES

Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Donigian, 

A.S., Jr., and Johanson, R.C., 1997, Hydrological 

Simulation Program—FORTRAN, user’s manual for 

version 11: Athens, Ga., U.S. Environmental Protection 

Agency, National Exposure Research Laboratory, 

EPA/600/R–97–080, 755 p.

Charbeneau, R.J., and Barrett, M.E., 1998, Evaluation 

of methods for estimating stormwater pollutant 

loads: Water Environment Research, v. 70, no. 7, 

p. 1,295–1,302.

Dugas, W.A., Hicks, R.A., and Wright, P.W., 1998, Effect of 

removal of Juniperus ashei on evapotranspiration and 

runoff in the Seco Creek watershed: Water Resources 

Research, v. 34, no. 6, p. 1,499–1,506. 

Land, L.F., Boning, C.W., Harmsen, Lynn, and Reeves, 

R.D., 1983, Streamflow losses along the Balcones fault 

zone, Nueces River Basin, Texas: U.S. Geological Survey Water-Resources Investigations Report 83–4168, 

72 p.

Lumb, A.M., McCammon, R.B., and Kittle, J.L., Jr., 1994, 

Users manual for an expert system (HSPEXP) for 

calibration of the Hydrological Simulation Program—

FORTRAN: U.S. Geological Survey Water-Resources 

Investigations Report 94–4168, 102 p.

Ockerman, D.J., Petri, B.L., and Slattery, R.N., 1999, 

Stormwater runoff for selected watersheds in the 

Edwards aquifer recharge zone, Bexar County, Texas, 

1996–98: U.S. Geological Survey Fact Sheet 

FS–172–98, 6 p.

Puente, Celso, 1978, Method of estimating natural recharge 

to the Edwards aquifer in the San Antonio area, Texas: 

U.S. Geological Survey Water-Resources Investigations 

Report 78–10, 34 p.

Raines, T.H., 1996, Simulation of storm peaks and storm 

volumes for selected subbasins in the West Fork Trinity 

River Basin, Texas, water years 1993–94: U.S. Geological Survey Water-Resources Investigations Report 

96–4110, 41 p.

San Antonio River Authority, 1996, Salado Creek watershed 

protection and flood prevention project: San Antonio 

River Authority Fact Sheet, 2 p. 

San Antonio Water System, 2000, Outline of ordinance 

#81491, aquifer recharge zone and watershed protection, 

San Antonio Texas, approved January 12, 1995: 

Accessed January 10, 2000, at URL 

http:www.saws.org/our_water/Source_Water_

Watershed_Protection/aquifer_protection/

documents/4outline.pdf

Stein, W.G., and Ozuna, G.B., 1995, Geologic framework 

and hydrogeologic characteristics of the Edwards 

aquifer recharge zone, Bexar County, Texas: U.S. Geological Survey Water-Resources Investigations Report 

95–4030, 8 p.

U.S. Department of Agriculture, Soil Conservation Service, 

1966, Soil survey of Bexar County, Texas: 126 p., 

88 sheets.

University of Texas, Bureau of Economic Geology, 1985, The 

Guadalupe-Lavaca-San Antonio-Nueces River Basins 

regional study, San Antonio East-Llano East sheets: 

Austin, Tex., scale 1:250,000, 5 sheets.