Cover:

San Antonio River looking upstream toward downtown San Antonio. 
U.S. Department of the Interior 

U.S. Geological Survey 

Simulation of Streamflow and Estimation 

of Streamflow Constituent Loads in 

the San Antonio River Watershed, 

Bexar County, Texas, 1997–2001 

By Darwin J. Ockerman and Kenna C. McNamara 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 03–4030 

In cooperation with the San Antonio Water System 

Austin, Texas 

2003 
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 

ii 
CONTENTS

Abstract ................................................................................................................................................................................ 1


Introduction .......................................................................................................................................................................... 1


Purpose and Scope .................................................................................................................................................... 4


Description of Study Area ........................................................................................................................................ 4


Description of Simulation Model, Modeling Process, and Loads Estimation Method ............................................ 8


Simulation of Streamflow .................................................................................................................................................... 10


Model Calibration and Testing ................................................................................................................................. 10


Lower Medina River Basin ........................................................................................................................... 10


Leon Creek Basin .......................................................................................................................................... 11


Upper San Antonio River Basin .................................................................................................................... 19


Salado Creek Basin ....................................................................................................................................... 20


Lower San Antonio River Basin ................................................................................................................... 22


Error Analysis ........................................................................................................................................................... 24


Sensitivity Analysis .................................................................................................................................................. 28


Simulation of Streamflow by Basin and Source Category ....................................................................................... 28


Estimation of Streamflow Constituent Loads ...................................................................................................................... 28


Source Concentrations Used for San Antonio River Constituent Loads Estimation ................................................ 30


Estimation of Streamflow Constituent Loads Using Source Concentrations With HSPF-Simulated Flow ............. 36


Summary .............................................................................................................................................................................. 36


References ............................................................................................................................................................................ 37


FIGURES 

1–4. Maps showing: 

1.	 San Antonio River watershed, Bexar County, Texas ................................................................................. 2


2.	 Data-collection sites used for Hydrological Simulation Program—FORTRAN model calibration,


testing, simulations, and estimation of streamflow water-quality constituent loads ................................. 3


3.	 Study area land use, Bexar County, Texas ................................................................................................. 7


4.	 Lower Medina River Basin stream and reservoir (RCHRES) segments ................................................... 11


5.	 Graphs showing measured and simulated 7-day mean streamflow at Medina River at San Antonio, Texas


(08181500), 1997–2001 ....................................................................................................................................... 12


6.	 Map showing Leon Creek Basin stream and reservoir (RCHRES) segments ...................................................... 20


7.	 Graphs showing measured and simulated 7-day mean streamflow at Leon Creek at IH–35 at San Antonio,


Texas (08181480), 1997–2001 ............................................................................................................................. 21


8.	 Map showing upper San Antonio River Basin stream and reservoir (RCHRES) segments ................................. 22


9.	 Graphs showing measured and simulated 7-day mean streamflow at San Antonio River at Loop 410 at


San Antonio, Texas (08178565), 1997–2001 ....................................................................................................... 23


10.	 Map showing Salado Creek Basin stream and reservoir (RCHRES) segments ................................................... 24


11.	 Graphs showing measured and simulated 7-day mean streamflow at Salado Creek at Loop 13 at San 


Antonio, Texas (08178800), 1997–2001 .............................................................................................................. 25


12.	 Map showing lower San Antonio River Basin stream and reservoir (RCHRES) segments ................................. 26


13.	 Graphs showing measured and simulated 7-day mean streamflow at San Antonio River near Elmendorf,


Texas (08181800), 1997–2001 ............................................................................................................................. 27


14.	 Pie diagram showing contribution to streamflow, in percent, at San Antonio River at Elmendorf, Texas 


(08181800), 1997–2001 ....................................................................................................................................... 29


15.	 Pie diagram showing sources of simulated streamflow, in percent, at San Antonio River near Elmendorf, 


Texas (08181800), 1997–2001 ............................................................................................................................. 29


CONTENTS iii 
16.	 Graphs showing comparison of streamflow loads estimated from Hydrological Simulation Program—


FORTRAN-simulated flow and source concentrations with loads computed from actual samples at San 


Antonio River near Elmendorf, Texas (08181800), 1997–2001 ........................................................................... 32


17.	 Pie diagrams showing sources of selected streamflow constituent loads, in percent, San Antonio River 


near Elmendorf, Texas (08181800), 1997–2001 ................................................................................................... 33


TABLES 

1.	 Selected characteristics of basins in the San Antonio River watershed, Bexar County, Texas ........................... 1


2.	 Data-collection sites used for Hydrological Simulation Program—FORTRAN model calibration, testing,


and simulations and estimation of water-quality constituent loads ...................................................................... 5


3.	 Major recycled wastewater discharges, Bexar County, Texas ............................................................................. 8


4.	 Basin-related model parameters for the Hydrological Simulation Program—FORTRAN ................................. 8


5.	 Process-related model parameters for the Hydrological Simulation Program—FORTRAN .............................. 9


6.	 Pervious and impervious land segment areas for basins in the San Antonio River watershed, Bexar 


County, Texas ........................................................................................................................................................ 13


7.	 Selected annual PERLND and IMPLND process-related parameters for flow, San Antonio River 


watershed models, Bexar County, Texas ............................................................................................................... 14


8.	 Annual measured and simulated streamflow for basins in the San Antonio River watershed, Bexar 


County, Texas, 1997–2001 .................................................................................................................................... 19


9.	 Sensitivity of selected annual process-related parameters, Salado Creek Basin, 1997–2001 ............................... 28


10.	 Sources of simulated streamflow at San Antonio River near Elmendorf, Texas (08181800), 1997–2001 .......... 30


11.	 Water-quality concentrations used for estimation of streamflow constituent loads ............................................. 31


12.	 Estimates of selected annual streamflow constituent loads, by source, San Antonio River watershed,


Bexar County, Texas, 1997–2000 ......................................................................................................................... 34


13.	 Sources of selected streamflow constituent loads, San Antonio River near Elmendorf, Texas


(08181800), 1997–2001 ........................................................................................................................................ 35


iv 
Simulation of Streamflow and Estimation 

of Streamflow Constituent Loads in 

the San Antonio River Watershed, 

Bexar County, Texas, 1997–2001 

By Darwin J. Ockerman and Kenna C. McNamara 

Abstract 

The U.S. Geological Survey developed 

watershed models (Hydrological Simulation Program—FORTRAN) to simulate streamflow and 

estimate streamflow constituent loads from five 

basins that compose the San Antonio River watershed in Bexar County, Texas. Rainfall and streamflow data collected during 1997–2001 were used to 

calibrate and test the model. The model was configured so that runoff from various land uses and 

discharges from other sources (such as wastewater 

recycling facilities) could be accounted for to 

indicate sources of streamflow. Simulated streamflow volumes were used with land-use-specific, 

water-quality data to compute streamflow loads of 

selected constituents from the various streamflow 

sources. 

Model simulations for 1997–2001 indicate 

that inflow from the upper Medina River (originating outside Bexar County) represents about 22 percent of total streamflow. Recycled wastewater 

discharges account for about 20 percent and base 

flow (ground-water inflow to streams) about 18 

percent. Storm runoff from various land uses represents about 33 percent. 

Estimates of sources of streamflow constituent loads indicate recycled wastewater as the largest source of dissolved solids and nitrate plus nitrite 

nitrogen (about 38 and 66 percent, respectively, of 

the total loads) during 1997–2001. Stormwater runoff from urban land produced about 49 percent of 

the 1997–2001 total suspended solids load. Stormwater runoff from residential and commercial land 

(about 23 percent of the land area) produced about 

70 percent of the total lead streamflow load during 

1997–2001. 

INTRODUCTION 

The San Antonio River watershed drains most 

of the San Antonio, Tex., metropolitan area in Bexar 

County and comprises five basins—lower Medina 

River, Leon Creek, upper San Antonio River, Salado 

Creek, and lower San Antonio River (fig. 1). The lower 

San Antonio River is the receiving water body for the 

other basins. Together, the drainage area of these 

streams is about 934 square miles (mi2) (table 1). 

Since August 1992 the U.S. Geological Survey 

(USGS), in cooperation with the San Antonio Water 

System (SAWS), has operated a stormwater monitoring 

network consisting of seven stations located in the San 

Antonio area (fig. 2, sites 13–19). This network is part 

of the National Pollutant Discharge Elimination System 

(NPDES) program designed to monitor stormwater 

quality of small (less than 2 mi

2) urban watersheds 

(U.S. Environmental Protection Agency, 2002). Each 

of the monitored watersheds consists of a single, 

Table 1. Selected characteristics of basins in the San 

Antonio River watershed, Bexar County, Texas 

Drainage Edwards aquifer 

Basin 

area 

(square 

catchment area 

and outcrop 

miles) (square miles) 

Lower Medina River 306 0 

Leon Creek 237 114 

Upper San Antonio River 125 4 

Salado Creek 223 106 

Lower San Antonio River 43 0

 Total 934 224 

Abstract 1 
WILSON COUNTY 

COMAL COUNTY

GUADALUPE

 COUNTY 

BEXAR COUNTY

ATASCOSA COUNTY 

BANDERA

COUNTY 

Base modified from U.S. Geological Survey 

Universal Transverse Mercator Projection, Zone14 

KENDALL 

Figure 1. San Antonio River watershed, Bexar County, Texas. 

98o

41' 98o

27' 

COUNTY 

predominant land-use type such as residential, commercial, or industrial. In addition, since May 1996 the 

USGS, in cooperation with SAWS, has operated a network of seven gages and sampling stations (fig. 2, sites 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 MEDINA COUNTY

EXPLANATION 

Edwards aquifer catchment area 

Edwards aquifer recharge zone (outcrop) 

Edwards aquifer upper confining unit—Blackland Prairie 

South Texas Plains 

Major basin boundary 

29o35' 

98o

13' 

SALADO CREEK 

LEON CREEK 

UPPER


 SAN ANTONIO


 RIVER


29o21' 

LOWER

 MEDINA Study area RIVER 

LOWER

 SAN ANTONIO

 RIVER 

San

 Antonio 

BEXAR

1:24,000 quadrangles COUNTY

0 2 4 6 8 MILES 

TEXAS


BEXAR MEDINA COUNTY COUNTY 

ATASCOSA COUNTY 

5–9, 11–12) in the Edwards aquifer recharge zone in 

Bexar County to monitor runoff quality from specific 

land-use watersheds on the recharge zone (Ockerman 

and others, 1999). (The Edwards aquifer recharge zone 

2 
MEDINA COUNTY

1 

98o

41' 98o

27' 

KENDALL 

29o21' 

1:24,000 quadrangles 

EXPLANATION


Study area 0 2 4 6 8 MILES


Major basin boundary


LOWER

 SAN ANTONIO

 RIVER 

UPPER

 SAN ANTONIO

 RIVER 

LEON CREEK 

SALADO CREEK 

LOWER

 MEDINA

 RIVER 

Base modified from U.S. Geological Survey 

Universal Transverse Mercator Projection, Zone14 

BEXAR COUNTY

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

COUNTY 

COUNTY 

BANDERA

COUNTY 

29o

35' 

98o

13' 

SAN 

RIVER 

ANTONIO 

Cibolo 

Creek 

Leon 

Creek 

MEDINA 

Salado 

Creek

 RIVER 

BRAUNIG

LAKE 

I–10 

I–35 

US 281 

FM 1604 

I 35 

7 11 

15 

I-37 

US 281 

LOOP 410 

–

US 90 

FM 1604 

I–10 

9 8

5 12 10 

6 

3 

2 

4 

30 

29 

28 

19 

14 

16 17 21 

18 

13 

25 

24 

26 27 

31 

34 

35 

32 

33 

22 

23 

20 

Site description–Number refers to table 2


7
 Streamflow


4
 Pan evaporation


5
 Rainfall


17
 Water quality 

Figure 2. Data-collection sites used for Hydrological Simulation Program—FORTRAN model calibration, testing, 

simulations, and estimation of streamflow water-quality constituent loads. 

is approximately coincident with the Edwards aquifer In another USGS and SAWS cooperative project, 

outcrop and is hereinafter referred to as the Edwards water quality was monitored at five USGS gaging staaquifer outcrop.) tions (fig. 2, sites 24–26, 30–31) located near the outlets 

INTRODUCTION 3 
of the basins that compose the San Antonio River watershed in Bexar County. The objective of this collection of 

water-quality data was to characterize baseline waterquality conditions of urban streams and to assess the 

effects of stormwater runoff on the quality of receiving 

streams (R.N. Slattery, U.S. Geological Survey, written 

commun., 2002). In this study, water-quality samples of 

base flow and stormflow were collected during 1992– 

98. Since 1998, water-quality sampling has continued at 

the USGS streamflow-gaging station San Antonio River 

near Elmendorf (site 31) as part of the USGS National 

Water-Quality Assessment program (Bush and others, 

2000). 

In 2002, the USGS published results of a modeling project to simulate runoff and aquifer recharge and 

to estimate constituent loads in runoff on the Edwards 

aquifer outcrop and catchment area in northern Bexar 

County during 1997–2000 (Ockerman, 2002). 

Data and results from these projects were compiled and incorporated into a watershed model to 

develop an improved understanding of the hydrology 

(sources of runoff and streamflow) and water quality 

(sources of constituent loads in streams) of the urban 

San Antonio area. 

Purpose and Scope 

This report describes the calibration of a Hydrological Simulation Program—FORTRAN (HSPF) 

watershed model used to simulate streamflow during 

1997–2001 for the basins that compose the San Antonio 

River watershed in Bexar County. Also described is the 

use of HSPF-simulated streamflow with water-quality 

data collected in the area to estimate streamflow constituent loads at the outlet of the study area. Constituent 

loads were estimated for dissolved solids, nitrate plus 

nitrite nitrogen, suspended solids, and total lead. Data 

collected during 1992–2002 at 35 sampling sites in 

Bexar County and adjacent areas and data from other 

sources were compiled and used to calibrate and test the 

model and to estimate constituent loads. The types of 

data-collection sites used in this study include rainfall, 

streamflow, pan evaporation, and water quality (fig. 2, 

table 2). 

Description of Study Area 

The study area includes most of Bexar County 

and small areas of adjacent Atascosa and Medina Counties that drain to the San Antonio River (fig. 1). The 

San Antonio River watershed is subdivided into five 

basins—lower Medina River, Leon Creek, upper San 

Antonio River, Salado Creek, and lower San Antonio 

River. Parts of Calaveras Creek and Cibolo Creek 

Basins also are located in Bexar County. Cibolo Creek 

forms the boundary of Bexar County with Comal 

County on the north and with Guadalupe County on 

the east and flows into the San Antonio River downstream of the study area. Calaveras Creek, in southeastern Bexar County adjacent to Braunig Lake and 

outside the lower San Antonio River Basin, also flows 

to the San Antonio River downstream of the study area. 

These basins are not included in the study area and 

simulations. 

The direction of drainage and streamflow in the 

study area generally is from northwest to southeast 

(fig. 1). The study area can be divided into four general 

hydrologic response areas on the basis of geology, soils, 

and runoff response to rainfall. The northern part of the 

study area includes 224 mi2

 of (1) Edwards aquifer 

catchment area and (2) Edwards aquifer outcrop (fig. 1). 

Both areas are characterized by thin, rocky soils and 

fairly steep slopes. The Edwards aquifer outcrop has 

greater permeability and infiltration of rainfall than the 

catchment area. Also, stream channels crossing the 

Edwards aquifer outcrop lose substantial flow to karst 

features such as fractures, sinkholes, and caves. 

Through central Bexar County, an area that comprises 

(3) the Edwards aquifer upper confining unit and Blackland Prairie consists of gently sloping, deep clayey soils 

characterized by slow to moderate rainfall infiltration 

rates and moderate to high water-holding capacity (U.S. 

Department of Agriculture, Soil Conservation Service, 

1966). This area of Bexar County is highly developed 

and includes the urban San Antonio area. The southern 

part of the county is classified as (4) South Texas Plains 

(Texas State Historical Association, 2002). This part of 

the study area is level to rolling, has dense brush, and 

features a transition from clayey soils to more permeable loam and sandy soils. 

Streamflow in the northern part of Bexar County 

is relatively infrequent because of high infiltration rates 

in the stream channels; even after heavy rains, streamflow might last only a few hours. In the central part of 

the county, base flow is evident, especially in wet 

weather. Two Edwards aquifer springs (San Pedro and 

San Antonio) also contribute flow to the San Antonio 

River (Brune, 1975). Flow is perennial in the lower San 

Antonio River in the southern part of Bexar County, 

largely because of discharge from wastewater treatment 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

4 
Table 2. Data-collection sites used for Hydrological Simulation Program—FORTRAN model calibration, testing, 

and simulations and estimation of water-quality constituent loads—Continued 

Table 2. Data-collection sites used for Hydrological Simulation Program—FORTRAN model calibration, testing, 

and simulations and estimation of water-quality constituent loads 

[USGS, U.S. Geological Survey; NWS, National Weather Service; FM, Farm Road; IH, Interstate Highway] 

Site 

no. 

(fig. 2) 

Name and 

USGS station no. 

(if applicable) 

Type of data 

(no. of waterquality samples, 

if applicable) 

Datacollection 

period 

Comment 

1 NWS station near Boerne, Tex. Rainfall 1997–2000 Data used for calibration and simulation of Leon 

Creek Basin 

2 NWS station near Bulverde, Tex. Rainfall 1997–2000 Data used for calibration and simulation of Salado 

Creek Basin 

3 NWS station at San Antonio International Rainfall 1997–2001 Data used for calibration and simulation of Salado 

Airport, San Antonio, Tex. Creek, Leon Creek, and upper and lower San 

Antonio River Basins 

4 NWS station at Sea World, San Antonio, Pan evaporation 1997–2001 Data used for calibration and simulation of all San 

Tex. Antonio River watershed models 

5 08181400 Helotes Creek at Helotes, Tex. Rainfall, 1997–2001 Rainfall and streamflow data used for calibration 

streamflow, and and simulation of Leon Creek Basin; waterwater quality (6) quality data used to characterize runoff– 

undeveloped land use 

6 08180945 Leon Creek at Scenic Loop Rd. Rainfall and 2001–02 Rainfall data used for calibration and simulation of 

near Leon Springs, Tex. water quality (1) Leon Creek Basin; water-quality data used to 

characterize runoff–undeveloped land use 

7 08180941 Government Canyon Creek site 2 Rainfall and 1997–2001 Rainfall data used for calibration and simulation of 

near Helotes, Tex. water quality (5) Leon Creek and lower Medina River Basins; 

water-quality data used to characterize runoff– 

undeveloped land use 

8 08178627 Elm Waterhole Creek Tributary Rainfall and 2001–02 Rainfall data used for calibration and simulation of 

at San Antonio, Tex. water quality (3) Salado Creek Basin; water-quality data used to 

characterize runoff–undeveloped land use 

9 08178595 Stone Mountain drainage Rainfall and 1996–2001 Rainfall data used for calibration and simulation of 

channel at Granite Path, San Antonio, water quality (12) Salado Creek Basin; water-quality data used to 

Tex. characterize runoff–residential land use 

10 08185000 Cibolo Creek at Selma, Tex. Rainfall 1997–2000 Data used for calibration and simulation of Salado 

Creek Basin 

11 08181425 Cedar Elm Outfall at Huebner Rainfall and 1996–2001 Rainfall data used for calibration and simulation of 

Creek Tributary, San Antonio, Tex. water quality (13) Leon Creek Basin; water-quality data used to 

characterize runoff–commercial land use 

12 08177600 Olmos Creek Tributary at FM Rainfall and 1997–98 Rainfall data used for calibration and simulation of 

1535 at Shavano Park, Tex. water quality (5) Leon Creek Basin; water-quality data used to 

characterize runoff– residential land use 

13 08178690 Salado Creek Tributary at Bitters Water quality (30) 1992–2001 Data used to characterize runoff–residential land 

Rd. at San Antonio, Tex. use 

14 08178420 Bandera Rd. Outfall at Water quality (23) 1992–2000 Data used to characterize runoff–commercial land 

Zarzamora Creek, San Antonio, Tex. use 

15 08177720 Olmos Creek Outfall at San Water quality (21) 1992–2001 Data used to characterize runoff–commercial land 

Pedro Ave., San Antonio, Tex. use 

16 08181440 Ingram Rd. Outfall at Leon Water quality (23) 1993–2000 Data used to characterize runoff–transportation 

Creek Tributary at San Antonio, Tex. corridor 

17 08178430 Zarzamora Creek Outfall at Water quality (13) 1993–2000 Data used to characterize runoff–residential land 

Alderette Park, San Antonio, Tex. use 

18 08178820 Rosillo Creek Tributary near Water quality (29) 1992–2001 Data used to characterize runoff–industrial land 

Rittiman Rd., San Antonio, Tex. use 

19 08178520 Harlandale Creek Outfall at Water quality (24) 1992–2001 Data used to characterize runoff–residential land 

South Flores St., San Antonio, Tex. use 

INTRODUCTION 5 
Table 2. Data-collection sites used for Hydrological Simulation Program—FORTRAN model calibration, testing, 

and simulations and estimation of water-quality constituent loads—Continued 

Site 

no. 

(fig. 2) 

Name and 

USGS station no. 

(if applicable) 

Type of data 

(no. of waterquality samples, 

if applicable) 

Datacollection 

period 

Comment 

20 08177818 San Antonio Springs at San Streamflow 1997-2001 Data used for calibration and simulation of 

Antonio, Tex. springflow from San Antonio Springs in upper 

San Antonio River Basin model 

21 08178090 San Pedro Springs at San Streamflow 1997-2001 Data used for calibration and simulation of 

Antonio, Tex. springflow from San Pedro Springs in upper 

San Antonio River Basin model 

22 08178700 Salado Creek at Loop 410 at San Streamflow 1997–2001 Data used for calibration of Salado Creek Basin 

Antonio, Tex. model 

23 08178050 San Antonio River at Mitchell Streamflow 1997–2001 Data used for calibration of upper San Antonio 

St., San Antonio, Tex. River Basin model 

24 08178800 Salado Creek at Loop 13 at San Streamflow and 1992–2001 Streamflow data used for calibration of Salado 

Antonio, Tex. water quality (80) Creek Basin model; water-quality data used to 

characterize base-flow quality 

25 08178565 San Antonio River at Loop 410 Streamflow and 1992–2001 Streamflow data used for calibration of upper San 

at San Antonio, Tex. water quality (26) Antonio River Basin model; water-quality data 

used to characterize base-flow quality 

26 08181480 Leon Creek at IH–35 at San Streamflow and 1992–2001 Streamflow data used for calibration of Leon 

Antonio, Tex. water quality (24) Creek Basin model; water-quality data used to 

characterize base-flow quality 

27 08180640 Medina River at La Coste, Tex. Streamflow and 1997–2000 Streamflow data used for calibration and 

water quality (34) simulation of lower Medina River Basin model; 

water-quality data used to characterize water 

quality of upper Medina River inflow 

28 08180700 Medina River near Macdona, Streamflow and 1997–2001 Streamflow data used for calibration of lower 

Tex. water quality (34) Medina River Basin model; water-quality data 

used to characterize water quality of upper 

Medina River inflow 

29 08180800 Medina River near Somerset, Streamflow 1997–2001 Data used for calibration of lower Medina River 

Tex. Basin model 

30 08181500 Medina River at San Antonio, Streamflow and 1997–2001 Streamflow data used for calibration of Medina 

Tex. water quality (7) River Basin model; water-quality data used to 

estimate streamflow constituent loads for 1997– 

2001 simulations 

31 08181800 San Antonio River near Streamflow and 1992–2001 Streamflow data used for calibration of lower San 

Elmendorf, Tex. water quality (106) Antonio River Basin model; water-quality data 

used to estimate streamflow constituent loads 

for 1997–2001 simulations 

32 USGS sampling site 291407098243701 Water quality (35) 1992–95 Data used to characterize recycled wastewater 

Medio Creek WWRC Outfall at San quality 

Antonio, Tex. 

33 USGS sampling site 291632098305001 Water quality (34) 1992–95 Data used to characterize recycled wastewater 

Leon Creek WWRC Outfall at San quality 

Antonio, Tex. 

34 USGS sampling site 291655098253601 Water quality (35) 1992–95 Data used to characterize recycled wastewater 

Salado Creek WWRC Outfall at San quality 

Antonio, Tex. 

35 USGS sampling site 292354098400701 Water quality (27) 1992–95 Data used to characterize recycled wastewater 

Dos Rios WWRC Outfall at San quality 

Antonio, Tex. 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

6 
BEXAR COUNTY 

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

 COUNTY 

BANDERA

 COUNTY 

plants and inflow originating from the upper Medina 

Land use in the study area for 1998 is shown in

figure 3 (David Kruse, Alamo Area Council of Governments, written commun., 1999). The estimated 2001 

Undeveloped 

Residential 

Commercial 

Transportation corridor 

Extractions 

Major basin boundary 

EXPLANATION 

Land use from Alamo Area Council of Governments, 1998 

KENDALL COUNTY 

Figure 3. Study area land use, Bexar County, Texas.

98o

41' 98o

27' 

29o

35' 

98o

13' 

29o21' 

Land use 

0 24 6 8 MILES 

Industrial 

Military 

Services 

Lakes 

population in Bexar County is 1,418,000 (U.S. Census

River. Bureau, 2002). Development in Bexar County has 

occurred rapidly over the past decade, with a population 

MEDINA COUNTY

increase of 17.5 percent during 1990–2000 (U.S.

Census Bureau, 2002). 

INTRODUCTION 7 
Four major municipal wastewater-recycling 

centers (WWRCs) discharge about 120 million gallons 

per day to Leon Creek and the lower Medina and San 

Antonio Rivers (table 3). There also are several smaller 

treatment facilities, including military and industrial 

dischargers. These smaller discharges, combined, are 

less than 2 percent of the total wastewater flow and are 

not included in the model simulations. Data for recycled 

water discharges were obtained from the Texas Natural 

Resource Conservation Commission (Michael Veazey, 

written commun., 2001). 

Description of Simulation Model, Modeling 

Process, and Loads Estimation Method 

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 

channel), 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 important 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 channel 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 

Table 3. Major recycled wastewater discharges, Bexar 

County, Texas 

Average daily 

Recycling 

facility 

Discharge 

site 

discharge 

(million 

gallons 

per day) 

Medio Creek Medio Creek (Medina River) 4.89 

Leon Creek Leon Creek 34.4 

Dos Rios Lower Medina River 53.8 

Salado Creek Lower San Antonio River 31.0 

over a land segment. Potential evaporation data can be 

estimated from measured pan evaporation or computed 

using minimum and maximum daily temperatures. 

Time series of measured streamflow are used for model 

calibration and testing. 

The six basin-related model parameters (table 4) 

define the areal extent of each land segment, the reach 

length, and a table of values (FTABLE) of surface area, 

volume, and discharge, as a function of depth for each 

reach or reservoir of the watershed. These parameters 

represent the physical characteristics of each land segment or reach of a basin and generally remain 

unchanged during calibration of the model. 

The 21 process-related model parameters 

(table 5) represent the physical processes of soil infiltration, soil moisture, evapotranspiration (ET), interception storage of plants, interflow recession, groundwater recession, and surface runoff for each land 

segment. The process-related model parameters for 

each land segment are adjusted to calibrate the model. 

Some process-related parameters can be automatically 

adjusted by month to account for seasonal variations: 

interception storage capacity (CEPSC), interflow 

inflow (INTFW), interflow recession rate (IRC), lowerzone ET (LZETP), Manning's n for assumed overland 

flow plane (NSUR), and upper-zone nominal storage 

(UZSN). For this study, monthly variation of these 

parameters were not investigated or implemented primarily because of insufficient data to characterize 

monthly variations in the parameters. The HSPF users 

manual (Bicknell and others, 1997) provides a more 

complete description of each parameter. 

Table 4. 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] 

Description Parameter (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) 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

8 
Table 5. Process-related model parameters for the Hydrological Simulation Program—FORTRAN 

[--, none; ET, evapotranspiration] 

Parameter 

AGWS 

AGWETP 

AGWRC 

Description1 

Initial active ground-water storage 

Available ET satisfied by active ground water 

Active ground-water recession rate 

Default 

0 

--

--

Minimum 

0 

0 

.001 

Maximum 

1.0 

1.0 

--

Units 

inches 

per day 

--

BASETP 

CEPSC 

DEEPFR 

Available ET satisfied by base flow 

Interception storage capacity 

Fraction of inflow that enters inactive ground water 

0 

0 

0 

0 

0 

0 

1.0 

1.0 

1.0 

--

inches 

--

INFEXP 

INFILD 

INFILT 

Infiltration equation exponent 

Ratio of maximum to mean infiltration capacities 

Index to infiltration capacity of soil 

2.0 

2.0 

--

0 

1.0 

.0001 

10.0 

2.0 

100.0 

--

--

inches per hour 

INTFW 

IRC 

KVARY 

Interflow inflow 

Interflow recession rate 

Nonexponential ground-water recession rate 

--

--

0 

0 

0 

0 

--

1.0 

--

--

per day 

per inch 

LSUR 

LZETP 

LZS 

Length of assumed overland flow plane 

Lower-zone ET 

Initial lower-zone storage 

--

0 

--

1.0 

0 

0 

--

1.0 

--

feet 

inches 

--

LZSN 

NSUR 

RETSC 

Lower-zone nominal storage 

Manning’s n for assumed overland flow plane 

Impervious retention storage capacity 

--

.1 

0 

.01 

.001 

0 

100.0 

1.0 

10.0 

inches 

inches 

--

SLSUR 

UZS 

UZSN 

Slope of assumed overland flow plane 

Initial upper-zone storage 

Upper-zone nominal storage 

--

--

--

.000001 

0 

.01 

10.0 

10.0 

--

feet per foot 

inches 

inches 

1

 The users manual for Hydrological Simulation Program—FORTRAN (Bicknell and others, 1997) provides a more complete description of each 

parameter. 

The HSPEXP (Lumb and others, 1994) 

computerized expert system was used to help adjust 

process-related parameters to achieve improved model 

calibration. 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(s) through a systematic evaluation of 

the model parameters. 

Loads of selected constituents (dissolved 

solids, nitrate plus nitrite nitrogen, suspended solids, 

and total lead) were estimated by applying source 

concentrations to HSPF-derived flow volumes to 

compute constituent loads by source and land-use 

category. HSPF model land segments (PERLNDs and 

IMPLNDs) usually are configured according to land 

use, geology, soil type, slope, and other characteristics 

that affect the hydrologic response to rainfall. For the 

San Antonio River watershed, model segments also 

were configured according to land-use categories or 

other sources of flow that might have different runoffquality characteristics. In addition, HSPF includes the 

capability to assign water originating from various 

sources to unique categories as it moves through the 

simulation process. For example, runoff from all commercial land-use areas can be assigned an exclusive category and can be accounted for separately from other 

categories (such as residential, transportation, or even 

recycled water discharged from treatment plants) as it 

INTRODUCTION 9 
moves through the watershed. Therefore, simulated 

streamflow at any point in the model can be separated 

into components of flow that originate from various 

sources. To estimate loads, water-quality concentrations 

characteristic of each source of flow were applied to the 

appropriate flow volumes to compute streamflow loads. 

SIMULATION OF STREAMFLOW 

Model Calibration and Testing

 The HSPF model for each of the five basins of the 

San Antonio River watershed was calibrated and tested 

separately. All five models were calibrated using 1997– 

2001 data (table 2). Data from 2001 were used independently to test the calibration of each model. Sensitivity 

analyses were done for selected parameters for each 

model to assess the effects of parameter uncertainty on 

simulated flow. 

The first step in developing an HSPF model for 

each basin was to divide it into RCHRES segments, taking into consideration the stream and reservoir (floodcontrol impoundment) configuration of each basin. 

After subdivision, each basin contained from 4 to 36 

RCHRES segments for a total of 103. Stream channel 

characteristics (surface area, volume, and discharge as a 

function of depth) were determined for each of the 

model stream reaches and entered in HSPF FTABLES 

(tables of stream channel parameters). For gaged stream 

reaches, FTABLE parameters were based on discharge 

measurements made at USGS gaging stations. FTABLE 

information for ungaged reaches were estimated from 

similar gaged reaches. FTABLES for flood-impoundment reservoirs were developed from the dam and reservoir design specifications. 

PERLND (pervious) and IMPLND (impervious) 

land segments were configured according to geologysoil characteristics and land use. Four primary 

physiographic categories (fig. 1) are: (1) Edwards 

aquifer catchment area (relatively low permeability); 

(2) Edwards aquifer outcrop (relatively high permeability); (3) Edwards aquifer upper confining unitBlackland Prairie (deeper clay soils and moderate permeability); and (4) South Texas Plains (loam and sandy 

soils and high permeability). Land-use categories comprise undeveloped, residential, commercial, transportation, industrial, services (mostly public parks), quarries, 

and military. Table 6 lists areas of PERLNDs and 

IMPLNDs for each basin, by land-use category. For 

model simulation, various military land segments were 

re-categorized as the most similar land cover (for example, undeveloped rangeland or industrial), resulting in 

28 PERLNDS (table 7). 

Models were calibrated by adjusting various 

process-related parameters so that simulated streamflow corresponded to gaged streamflow in the basin, 

considering such criteria as error in total streamflow 

volume for the calibration period, low-flow and highflow distribution, and error in flow volumes for selected 

storms. A summary of process-related parameters for 

the calibrated models is shown in table 7. 

Lower Medina River Basin 

The lower Medina River Basin (306 mi2) and 

its subdivision into RCHRES segments are shown in 

figure 4. The upper Medina River Basin, including 

Medina Lake, is not included in the model. Medina 

River streamflow entering the lower watershed was 

input (as a boundary condition) to the model according 

to streamflow records from USGS streamflow-gaging 

station Medina River at La Coste (08180640). Small 

parts of the basin also extend into Medina and Atascosa 

Counties (fig. 4). 

Besides the station at La Coste (discontinued in 

2001), USGS streamflow-gaging stations are on the 

Medina River near Macdona (08180700), near Somerset (08180800), and at San Antonio (08181500). Data 

collected from the Macdona and Somerset stations were 

used to help calibrate the parts of the model representing drainage upstream of the respective stations. The 

San Antonio station is the primary station used for calibration of the model. The Medina River Basin model 

also includes inflow from the Leon Creek Basin that 

occurs upstream of the San Antonio station. For calibration, gaged inflow from Leon Creek was used as input 

to the model; for subsequent simulations and analysis, 

simulated Leon Creek streamflow data were used as 

input. 

An annual comparison of measured and simulated streamflow is shown in table 8. Overall error in 

simulated streamflow for the calibration period (1997– 

2000) was -4.3 percent; 1999 and 2000 were undersimulated (-16.8 and -21.7 percent error, respectively). The 

2001 testing results oversimulated volume by 6.8 percent. Overall error for 1997–2001 was -2.9 percent. A 

comparison of measured and simulated (7-day mean) 

streamflow at the Medina River at San Antonio is shown 

in figure 5. 

10 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
98o

41' 

The Leon Creek Basin and its subdivision into 

RCHRES segments are shown in figure 6. The upper 

219 mi

2 of the 237-mi2 basin is gaged by USGS streamflow-gaging station Leon Creek at IH–35 at San Antonio (08181480). The Helotes Creek Basin (15 mi

2

) in 

the upper part of the Leon Creek Basin also is gaged 

(Helotes Creek at Helotes [08181400]). Data collected 

from the Leon Creek station are the primary source of 

UPPER

 SAN ANTONIO

 RIVER 

LEON CREEK 

SALADO CREEK 

LOWER

 SAN ANTONIO

 RIVER 

EXPLANATION 

Lower Medina River Basin 

Reservoir 

Major basin boundary 

RCHRES segment boundary 

0 2 4 6 8 MILES 

29o

35' 

98o

27' 

98o

13' 

Base modified from U.S. Geological Survey 

Universal Transverse Mercator Projection, Zone14 

BEXAR COUNTY

MEDINA COUNTY

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

COUNTY 

KENDALL COUNTY 

BANDERA

COUNTY 

Figure 4. Lower Medina River Basin stream and reservoir (RCHRES) segments. 

29o21' 

1:24,000 quadrangles 

Leon Creek Basin data used to calibrate the Leon Creek Basin model. 

HSPF parameters obtained from the gaged part of the 

basin were applied to the ungaged part to estimate 

streamflow for the entire basin. 

Overall error in simulated streamflow for the calibration period (1997–2000) was -9.2 percent (table 8); 

1997 and 1998 results (-8.7 and -14.2 percent error, 

respectively) were influenced by rather extreme events 

(June 1997 and October 1998) that were undersimulated. Simulated streamflow for 1999 (a dry year) and 

SIMULATION OF STREAMFLOW 11 
1997 1998 1999 2000 2001 

YEAR 

0.5	 2 5 10 20 30 50 70 80 90 95 98 99.5 

EXCEEDANCE PROBABILITY, IN PERCENT 

Measured 

Simulated 

10 

100 

1,000 

10,000 

FLOW, IN CUBIC FEET PER SECOND 

Measured 

Simulated 

10 

10,000 

FLOW, IN CUBIC FEET PER SECOND

100 

1000 

SIMULATED FLOW, IN CUBIC FEET PER SECOND

10,000 

1,000 

100 

10 

10 100 1,000 10,000 

MEASURED FLOW, IN CUBIC FEET PER SECOND 

Figure 5. Measured and simulated 7-day mean streamflow at Medina River at San Antonio, Texas (08181500), 

1997–2001. 

12 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
Table 6. Pervious and impervious land segment areas for basins in the San Antonio River watershed, Bexar 

County, Texas 

[In acres; A, Edwards aquifer catchment area; --, not applicable; B, Edwards aquifer outcrop; C, Edwards aquifer upper 

confining unit-Blackland Prairie; D, South Texas Plains] 

Land-use 

category Lower 

Medina River 

PERLND (pervious land segment) 

Undeveloped, A 

Undeveloped, B 

Undeveloped, C 94,142 

Undeveloped, D 73,569 

Residential, A 

Residential, B 

Residential, C 6,256 

Residential, D 6,691 

--

--

--

--

Leon Creek 

36,756 

18,392 

46,803 

7,470 

1,211 

8,632 

--

--

Basin 

Upper San 

Antonio River 

Salado Creek Lower San 

Antonio River 

21,315 

2,897 32,882 

9,398 26,211 10,913 

10,090 

1,440 

1,150 4,587 

23,110 16,197 1,103 

902 

-- --

--

-- --

-- --

--

-- --

Commercial, A 

Commercial, B 

Commercial, C 2,269 

Commercial, D 539 

--

--

708 

496 

3,859 

--

-- 5 

82 800 

9,151 5,071 299 

95 

--

--

-- --

Transportation, A 

Transportation, B 

Transportation, C 792 

Transportation, D 481 

Industrial, A 

Industrial, B 

Industrial, C 8 

Industrial, D 1 

--

--

--

--

1,163 

526 

2,812 

0 

0 

3,071 

--

--

-- 63 

251 1,414 

7,556 796 118 

177 

0 

155 0 

374 4,276 6 

515 

--

--

-- --

-- --

--

-- --

Services, A 

Services, B 

Services, C 143 

Services, D 0 

--

--

0 

528 

590 

--

-- 0 

79 488 

1,432 2,160 894 

230 

--

--

-- --

Quarries, A 

Quarries, B 

Quarries, C 339 

Quarries, D 0 

--

--

300 

421 

0 

--

-- 0 

215 659 

0 1,206 0 

--

--

-- -- --

Military, A 

Military, B 

Military, C 2,948 

Military, D 0 

--

--

849 

0 

841 

--

-- 0 

0 0 

1,018 2,323 0 

--

--

-- -- --

IMPLND (impervious land segment) 

Residential 2,283 

Commercial 2,809 

Transportation 1,274 

Industrial 6 

Services 36 

Quarries 18 

Military 983 

5,121 

4,709 

2,982 

1,074

589

0 

561 

6,065 4,235 214 

9,233 63 132 

7,807 2,378 127 

 353 4,276 108 

 168 485 203 

11 63 0 

1,244 1,117 0 

SIMULATION OF STREAMFLOW 13 
14 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

Ta

ble 7. Selecte

d annual P

E

R

LND and I

M

P

LND process-related p

ara

m

eters f

or flo

w, San Antonio Riv

er w

atershed model

[Parameter definitions and units in table 5; PE

R

L

N

D, pervious land segment; A, E

d

wards aquifer catchment area; --, not applicable; B, 

C, Edwards aquifer upper confining unit-Blacklan

d Prairie; D, South Texas Plains; IMP

L

N

D, i

mper

vious land segment] 

Land-use category AGWETP A

G

WRC BASETP CEPSC DEEPFR INFILT INTFW IRC KVARY LSUR LZETP LZSN NSUR R

Lower Medina River Basin 

PERLND 

Undeveloped, A -- -- -- -- -- -- -- -- -- -- -- -- -- Undeveloped, B -- -- -- -- -- -- -- -- -- -- -- -- -- Undeveloped, C 0 0.90 0 0.30 0.40 0.60 0.20 0.50 0 800 0.40 2.0 0.25 Undeveloped, D 0 .98 0 .30 .30 .70 .50 .60 0 800 .40 2.0 .25 Residential, A -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, B -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, C 0 .90 0 .35 .40 .60 .20 .50 0 800 .40 2.0 .25 Residential, D 0 .98 0 .30 .30 .70 .50 .60 0 800 .40 2.0 .25 Com

mercial, A -- -- -- -- -- -- -- -- -- -- -- -- -- 

Com

mercial, B -- -- -- -- -- -- -- -- -- -- -- -- -- 

Com

mercial, C 0 .90 0 .30 .40 .60 .20 .50 0 800 .40 2.0 .25 

Com

mercial, D 0 .98 0 .30 .30 .70 .50 .60 0 800 .40 2.0 .25 

Transportation, A -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, B -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Transportation, D 0 .98 0 .25 .30 .70 .50 .60 0 800 .40 2.0 .25 Industrial, A -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, B -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Industrial, D 0 .98 0 .25 .30 .70 .50 .60 0 800 .40 2.0 .25 Services, A -- -- -- -- -- -- -- -- -- -- -- -- -- Services, B -- -- -- -- -- -- -- -- -- -- -- -- -- Services, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Services, D 0 .98 0 .25 .30 .70 .50 .60 0 800 .40 2.0 .25 Quarries, A -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, B -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Quarries, D -- -- -- -- -- -- -- -- -- -- -- -- -- 

IMPLND 

All categories -- -- -- -- -- -- -- -- -- 800 -- -- .10 
Table 7. Selected annual PERLND and IMPLND process-related parameters for flow, San Antonio River watershed models

Continued 

Land-use category AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC KVARY LSUR LZETP LZSN NSUR R

Leon Creek Basin 

PERLND 

Undeveloped, A 0 0.90 0 0.50 0.05 0.50 0.30 0.30 0 800 0.40 2.0 0.25 Undeveloped, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Undeveloped, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Undeveloped, D -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, A 0 .90 0 .50 .05 .50 .30 .30 0 800 .40 2.0 .25 Residential, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Residential, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Residential, D -- -- -- -- -- -- -- -- -- -- -- -- -- Commercial, A 0 .90 0 .50 .05 .50 .30 .30 0 800 .40 2.0 .25 Commercial, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Commercial, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Commercial, D -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, A 0 .90 0 .50 .05 .50 .30 .30 0 800 .40 2.0 .25 Transportation, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Transportation, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Transportation, D -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, A 0 .90 0 .50 .05 .50 .30 .30 0 800 .40 2.0 .25 Industrial, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Industrial, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Industrial, D -- -- -- -- -- -- -- -- -- -- -- -- -- Services, A 0 .90 0 .50 .05 .50 .30 .30 0 800 .40 2.0 .25 Services, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Services, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Services, D -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, A 0 .90 0 .50 .05 .50 .30 .30 0 800 .40 2.0 .25 Quarries, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.0 .25 Quarries, C 0 .90 0 .25 .70 .60 .20 .50 0 800 .40 2.5 .25 Quarries, D -- -- -- -- -- -- -- -- -- -- -- -- -- 

IMPLND 

All categories -- -- -- -- -- -- -- -- -- 800 -- -- .10 

SIMULATION OF STREAMFLOW 15 
16 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

Ta

ble 7. Selecte

d annual PERLND and IMPLN

D process-related p

arameters for fl

o

w, San Antonio Ri

v

er w

atershed models

Continued 

Land-use category AGWETP A

G

WRC BASETP CEPSC DEEPFR INFILT INTFW IRC KVARY LSUR LZETP LZSN NSUR R

Upper San Antonio River Basin 

PERLND 

Undeveloped, A -- -- -- -- -- -- -- -- -- -- -- -- -- Undeveloped, B 0 0.40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 Undeveloped, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 Undeveloped, D -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, A -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, B 0 .40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 Residential, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 Residential, D -- -- -- -- -- -- -- -- -- -- -- -- -- Com

mercial, A -- -- -- -- -- -- -- -- -- -- -- -- -- 

Com

mercial, B 0 .40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 

Com

mercial, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 

Com

mercial, D -- -- -- -- -- -- -- -- -- -- -- -- -- 

Transportation, A -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, B 0 .40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 Transportation, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 Transportation, D -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, A -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, B 0 .40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 Industrial, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 Industrial, D -- -- -- -- -- -- -- -- -- -- -- -- -- Services, A -- -- -- -- -- -- -- -- -- -- -- -- -- Services, B 0 .40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 Services, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 Services, D -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, A -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, B 0 .40 0 .30 .90 .60 .10 .10 0 800 .30 2.5 .25 Quarries, C 0 .90 0 .30 .70 .60 .20 .50 0 800 .40 3.5 .25 Quarries, D -- -- -- -- -- -- -- -- -- -- -- -- -- 

IMPLND 

All categories -- -- -- -- -- -- -- -- -- 800 -- -- .10 
Table 7. Selected annual PERLND and IMPLND process-related parameters for flow, San Antonio River watershed models

Continued 

Land-use category AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC KVARY LSUR LZETP LZSN NSUR R

Salado Creek Basin 

PERLND 

Undeveloped, A 0 0.90 0 0.50 0.10 0.50 0.30 0.30 0 800 0.40 2.5 0.25 Undeveloped, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Undeveloped, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Undeveloped, D -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, A 0 .90 0 .50 .10 .50 .30 .30 0 800 .40 2.5 .25 Residential, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Residential, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Residential, D -- -- -- -- -- -- -- -- -- -- -- -- -- Commercial, A 0 .90 0 .50 .10 .50 .30 .30 0 800 .40 2.5 .25 Commercial, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Commercial, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Commercial, D -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, A 0 .90 0 .50 .10 .50 .30 .30 0 800 .40 2.5 .25 Transportation, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Transportation, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Transportation, D -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, A 0 .90 0 .50 .10 .50 .30 .30 0 800 .40 2.5 .25 Industrial, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Industrial, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Industrial, D -- -- -- -- -- -- -- -- -- -- -- -- -- Services, A 0 .90 0 .50 .10 .50 .30 .30 0 800 .40 2.5 .25 Services, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Services, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Services, D -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, A 0 .90 0 .50 .10 .50 .30 .30 0 800 .40 2.5 .25 Quarries, B 0 .40 0 .40 .90 .60 .10 .10 0 800 .30 2.5 .25 Quarries, C 0 .90 0 .30 .70 .75 .20 .50 0 800 .40 3.0 .25 Quarries, D -- -- -- -- -- -- -- -- -- -- -- -- -- 

IMPLND 

All categories -- -- -- -- -- -- -- -- -- 800 -- -- .10 

SIMULATION OF STREAMFLOW 17 
18 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

Ta

ble 7. Selecte

d annual PERLND and IMPLN

D process-related p

arameters for fl

o

w, San Antonio Ri

v

er w

atershed models

Continued 

Land-use category AGWETP A

G

WRC BASETP CEPSC DEEPFR INFILT INTFW IRC KVARY LSUR LZETP LZSN NSUR R

Lower San Antonio River Basin 

PERLND 

Undeveloped, A -- -- -- -- -- -- -- -- -- -- -- -- -- Undeveloped, B -- -- -- -- -- -- -- -- -- -- -- -- -- Undeveloped, C 0 0.90 0 0.30 0.40 .60 .20 .50 0 800 .40 2.0 .25 Undeveloped, D 0 .98 0 .30 .30 .70 .50 .60 0 800 .40 2.0 .25 Residential, A -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, B -- -- -- -- -- -- -- -- -- -- -- -- -- Residential, C 0 .90 0 .35 .40 .60 .20 .50 0 800 .40 2.0 .25 Residential, D 0 .98 0 .30 .30 .70 .50 .60 0 800 .40 2.0 .25 Com

mercial, A -- -- -- -- -- -- -- -- -- -- -- -- -- 

Com

mercial, B -- -- -- -- -- -- -- -- -- -- -- -- -- 

Com

mercial, C 0 .90 0 .30 .40 .60 .20 .50 0 800 .40 2.0 .25 

Com

mercial, D 0 .98 0 .30 .30 .70 .50 .60 0 800 .40 2.0 .25 

Transportation, A -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, B -- -- -- -- -- -- -- -- -- -- -- -- -- Transportation, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Transportation, D 0 .98 0 .25 .30 .70 .50 .60 0 800 .40 2.0 .25 Industrial, A -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, B -- -- -- -- -- -- -- -- -- -- -- -- -- Industrial, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Industrial, D 0 .98 0 .25 .30 .70 .50 .60 0 800 .40 2.0 .25 Services, A -- -- -- -- -- -- -- -- -- -- -- -- -- Services, B -- -- -- -- -- -- -- -- -- -- -- -- -- Services, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Services, D 0 .98 0 .25 .30 .70 .50 .60 0 800 .40 2.0 .25 Quarries, A -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, B -- -- -- -- -- -- -- -- -- -- -- -- -- Quarries, C 0 .90 0 .25 .40 .60 .20 .50 0 800 .40 2.0 .25 Quarries, D -- -- -- -- -- -- -- -- -- -- -- -- -- 

IMPLND 

All categories -- -- -- -- -- -- -- -- -- 800 -- -- .10 
Table 8. Annual measured and simulated streamflow for basins in the San Antonio River watershed, Bexar County, 

Texas, 1997–2001 

Lower Medina River Basin Leon Creek Basin Upper San Antonio River Basin 

Period Measured 

streamflow 

(acre-feet) 

Simulated 

streamflow 

(acre-feet) 

Error 

(percent) 

Measured 

streamflow 

(acre-feet) 

Simulated 

streamflow 

(acre-feet) 

Error 

(percent) 

Measured 

streamflow 

(acre-feet) 

Simulated 

streamflow 

(acre-feet) 

Error 

(percent) 

Calibration 

1997 266,000 270,000 1.5 33,400 30,500 -8.7 52,200 64,800 24.1 

1998 311,000 326,000 4.8 73,400 62,900 -14.2 75,300 90,000 19.5 

1999 125,000 104,000 -16.8 10,200 10,100 -.9 52,000 35,000 -32.7 

2000 161,000 126,000 -21.7 30,700 30,800 .6 64,000 58,600 -8.4 

1997–2000 863,000 826,000 -4.3 148,000 134,000 -9.2 244,000 248,000 1.6 

Testing 

2001 176,000 188,000 6.8 31,200 33,800 8.3 91,700 81,400 -11.2 

1997–2001 

Total 1,040,000 1,010,000 -2.9 179,000 168,000 -6.1 335,000 330,000 -1.5 

Salado Creek Basin Lower San Antonio River Basin 

Period Measured 

streamflow 

(acre-feet) 

Simulated 

streamflow 

(acre-feet) 

Error 

(percent) 

Measured 

streamflow 

(acre-feet) 

Simulated 

streamflow 

(acre-feet) 

Error 

(percent) 

Calibration 

1997 32,800 28,900 -11.9 396,000 447,000 12.9 

1998 93,900 96,800 3.1 619,000 616,000 .5 

1999 13,000 14,400 10.8 235,000 226,000 -3.8 

2000 22,200 31,000 39.6 345,000 298,600 -13.6 

1997–2000 162,000 171,000 5.5 1,595,000 1,587,000 -.5 

Testing 

2001 39,600 42,400 7.1 402,000 414,400 3.0 

1997–2001 

Total 202,000 214,000 5.9 1,997,000 2,000,000 .2 

2000 (a normal year) was within 1 percent of observed 

streamflow. The 2001 testing results oversimulated volume by 8.3 percent. Overall error for 1997–2001 was 

-6.1 percent. A comparison of measured and simulated 

(7-day mean) streamflow at the Leon Creek station is 

shown in figure 7. 

Upper San Antonio River Basin 

The upper San Antonio River Basin and its subdivision into RCHRES segments are shown in figure 8. 

Two long-term USGS streamflow-gaging stations are 

used for model calibration. The upper 42.4 mi2 of the 

basin, including most of downtown San Antonio, is 

gaged by the San Antonio River at Mitchell Street station (08178050). The lower station, San Antonio River 

at Loop 410 at San Antonio (08178565), gages the 

entire 125 mi

2 of the basin. 

Overall error in simulated streamflow for the 

calibration period (1997–2000) was 1.6 percent 

(table 8); 1999 was substantially undersimulated 

(-32.7 percent error). The 2001 testing results undersimulated volume by 11.2 percent. Overall error 

SIMULATION OF STREAMFLOW 19 
98o

41' 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

for 1997–2001 was -1.5 percent. A comparison of 

measured and simulated (7-day mean) streamflow at the 

San Antonio River at Loop 410 station is shown in 

The Salado Creek Basin and its subdivision 

into RCHRES segments are shown in figure 10. Two 

long-term USGS streamflow-gaging stations were 

UPPER

 SAN ANTONIO

 RIVER 

SALADO CREEK 

LOWER

 SAN ANTONIO

 RIVER 

LOWER

 MEDINA

 RIVER 

EXPLANATION 

Leon Creek Basin 

Reservoir 

Major basin boundary 

RCHRES segment boundary 

29o21' 

29o

35' 

98o

27' 

98o

13' 

Base modified from U.S. Geological Survey 

1:24,000 quadrangles 

Universal Transverse Mercator Projection, Zone14 

BEXAR COUNTY

MEDINA COUNTY

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

COUNTY 

KENDALL COUNTY 

BANDERA

COUNTY 

Figure 6. Leon Creek Basin stream and reservoir (RCHRES) segments. 

0 2 4 6 8 MILES 

used for model calibration. The upper 125 mi2

 of the 

basin is gaged by the San Antonio River at Loop 410 

(08178565). The lower station, Salado Creek at Loop 13 

figure 9. (08178800), gages 189 mi2 of the entire 223-mi2

 basin. 

HSPF parameters obtained from the gaged part of the 

Salado Creek Basin basin were applied to the ungaged part to estimate 

streamflow and constituent loads for the entire basin.

Overall error in simulated streamflow for the calibration period (1997–2000) was 5.5 percent (table 8); 

2000 was substantially oversimulated (39.6 percent 

20 
SIMULATED FLOW, IN CUBIC FEET PER SECOND FLOW, IN CUBIC FEET PER SECOND FLOW, IN CUBIC FEET PER SECOND 

10,000 

1,000 

MEASURED


SIMULATED


100


10


1


1997 1998 1999 2000 2001


YEAR


10,000 

MEASURED 

SIMULATED 

1,000 

100


10


1


0.5 2 5 10 20 30 50 70 80 90 95 98 99.5


EXCEEDANCE PROBABILITY, IN PERCENT 

10,000 

1,000 

100


10


1


1 10 100 1,000 10,000


MEASURED FLOW, CUBIC FEET PER SECOND


Figure 7. Measured and simulated 7-day mean streamflow at Leon Creek at IH–35 at San Antonio, Texas 

(08181480), 1997–2001. 

SIMULATION OF STREAMFLOW 21 
98o

41' 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

error). The 2001 testing results oversimulated volume 

by 7.1 percent. Overall error for 1997–2001 was 

5.9 percent. A comparison of measured and simulated 

(7-day mean) streamflow at the Salado Creek at Loop 

13 station is shown in figure 11. 

Lower San Antonio River Basin 

The Lower San Antonio River Basin and its subdivision into RCHRES segments are shown in figure 12. 

The USGS streamflow-gaging station San Antonio 

EXPLANATION 

Upper San Antonio River Basin 

Major basin boundary 

RCHRES segment boundary 

0 2 4 6 8 MILES 

Base modified from U.S. Geological Survey 

Universal Transverse Mercator Projection, Zone14 

29o

35' 

98o

27' 

98o

13' 

LEON CREEK 

SALADO CREEK 

LOWER

 MEDINA

 RIVER 

LOWER

 SAN ANTONIO

 RIVER 

BEXAR COUNTY

MEDINA COUNTY

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

COUNTY 

KENDALL COUNTY 

BANDERA

COUNTY 

OLMOS

 RESERVOIR 

Figure 8. Upper San Antonio River Basin stream and reservoir (RCHRES) segments. 

29o21' 

1:24,000 quadrangles 

River near Elmendorf (08181800) was used for model 

calibration. Besides runoff from the basin, inflows from 

the four other basins, including recycled water discharges, enter this basin. Withdrawals from the San 

Antonio River are made to maintain normal levels in 

two power-generating plant reservoirs. These withdrawals mostly occur during dry summer months. 

Accuracy of simulated flow at the Elmendorf station 

depends largely on the accuracy of simulated flows 

from the contributing basins. 

22 
10,000 

SIMULATED FLOW, IN CUBIC FEET PER SECOND FLOW, IN CUBIC FEET PER SECOND FLO

W, IN CUBIC FEET PER SECOND

Measured


Simulated


1,000 

100


10


1


0.1


1997 1998 1999 2000 2001


YEAR 

10,000 

Measured 

Simulated 

1,000 

100


10


1


0.5	 2 5 10 20 30 50 70 80 90 95 98 99.5


EXCEEDANCE PROBABILITY, IN PERCENT


10,000 

1,000 

100


10


1


0.1 

1 10 100 1,000 10,000


MEASURED FLOW, IN CUBIC FEET PER SECOND


0.1 

Figure 9. Measured and simulated 7-day mean streamflow at San Antonio River at Loop 410 at San Antonio, 

Texas (08178565), 1997–2001. 

SIMULATION OF STREAMFLOW 23 
98o

41' 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

The model calibration results of the lower San 

Antonio River Basin represent the integrated results of 

all study area models (table 8). Overall error in simulated streamflow for the calibration period (1997–2000) 

was -0.5 percent. The 2001 testing results oversimulated 

volume by 3.0 percent. Overall error for 1997–2001 was 

0.2 percent. A comparison of measured and simulated 

(7-day mean) streamflow at the San Antonio River near 

Elmendorf station is shown in figure 13. 

UPPER

 SAN ANTONIO

 RIVER 

LEON CREEK 

LOWER

 MEDINA

 RIVER 

LOWER

 SAN ANTONIO

 RIVER 

Salado Creek Basin 

Flood-control reservoirs 

Major basin boundary 

RCHRES segment boundary 

BEXAR COUNTY

MEDINA COUNTY

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

COUNTY 

KENDALL COUNTY 

BANDERA

COUNTY 

29o21' 

29o35' 

98o

27' 

98o

13' 

Base modified from U.S. Geological Survey 

1:24,000 quadrangles 

Universal Transverse Mercator Projection, Zone14 

Figure 10. Salado Creek Basin stream and reservoir (RCHRES) segments. 

EXPLANATION 0 2 4 6 8 MILES 

Error Analysis 

The HSPF model provides reasonable simulations of runoff volumes compared with observed data. 

For each of the basins, total simulated volume for 

1997–2001 was within 10 percent of measured runoff 

(table 8). At the basin outlet (San Antonio River near 

Elmendorf station), simulated volume was within 1 percent of measured volume. Annual errors in streamflow 

24 
0.1 

1 

10 

100 

1,000 

10,000 

FLOW, IN CUBIC FEET PER SECOND 

Measured 

Simulated 

1997 1998 1999 2000 2001 

YEAR 

0.5 2 5 10 20 30 50 70 80 90 95 98 99.5

0.1 

1 

10 

100 

1,000 

10,000 

FLOW, IN CUBIC FEET PER SECOND 

Measured 

Simulated 

EXCEEDANCE PROBABILITY, IN PERCENT 

0.1	 1 10 100 1,000 10,000 

MEASURED FLOW, IN CUBIC FEET PER SECOND 

Figure 11. Measured and simulated 7-day mean streamflow at Salado Creek at Loop 13 at San Antonio, Texas 

(08178800), 1997–2001. 

1 

10 

100 

1,000 

10,000

SIMULATED FLOW, IN CUBIC FEET PER SECOND

0.1 

SIMULATION OF STREAMFLOW 25 
98o

41' 

 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 

among the five basins ranged from -32.7 percent to 39.6 

The types of error from 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 inaccurate data 

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

rating tables of stage and discharge. The spatial variability of rainfall in a watershed might not be adequately 

represented by the available network of rain gages. 

UPPER

 SAN ANTONIO

 RIVER 

LEON CREEK 

SALADO CREEK 

LOWER

 MEDINA

 RIVER 

Base modified from U.S. Geological Survey 

1:24,000 quadrangles 

Universal Transverse Mercator Projection, Zone14 

29o21' 

29o

35' 

98o

27' 

98o

13' 

BEXAR COUNTY

MEDINA COUNTY

ATASCOSA COUNTY 

WILSON COUNTY 

COMAL COUNTY

GUADALUPE

COUNTY 

KENDALL COUNTY 

BANDERA

COUNTY 

BRAUNIG

 LAKE 

Lower San Antonio River Basin 

Major basin boundary 

RCHRES segment boundary 

Figure 12. Lower San Antonio River Basin stream and reservoir (RCHRES) segments. 

0 2 4 6 8 MILES EXPLANATION 

Systematic errors are associated with limited ability of 

percent. the simulation model to represent the hydrologic processes of the basins in the study area. Limits on how 

accurately the model parameters and equations describe 

the physical properties of runoff are possible. Also, the 

configuration of the model segments (PERLNDS, 

IMPLNDs, and RCHRESs) and selection of simulation 

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

actual physical configuration and hydrologic responses 

of the basins. 

26 
1997 1998 1999 2000 2001 

10 

100 

1,000 

10,000 

100,000 

FLOW, IN CUBIC FEET PER SECOND 

Measured 

Simulated 

YEAR 

100,000 

10,000 

1,000 

100 

10 

0.5 2 5 10 20 30 50 70 80 90 95 98 99.5

EXCEEDANCE PROBABILITY, IN PERCENT 

Measured 

Simulated 

FLOW, IN CUBIC FEET PER SECOND

10 100 1,000 10,000 100,000 

10 

100 

1,000 

10,000 

100,000 

SIMULATED FLOW, IN CUBIC FEET PER SECOND 

MEASURED FLOW, IN CUBIC FEET PER SECOND 

Figure 13. Measured and simulated 7-day mean streamflow at San Antonio River near Elmendorf, Texas 

(08181800), 1997–2001. 

SIMULATION OF STREAMFLOW 27 
Sensitivity Analysis 

Because there is still some uncertainty in model 

parameter values after calibration and testing, sensitivity of selected HSPF process-related parameters was 

analyzed by altering values of selected parameters 

and evaluating the resulting changes in simulated 

streamflow. 

The Salado Creek Basin was used to do a sensitivity analysis of selected HSPF model parameters to indicate what effect a change in a parameter value has on 

streamflow volumes. Each parameter was modified to 

represent a reasonable change. The resulting changes in 

streamflow at the lower gaging station in the basin 

(Salado Creek at Loop 13 at San Antonio) are shown in 

table 9. 

The most sensitive parameters are lower-zone ET 

(LZETP) and lower-zone nominal storage (LZSN). A 

reduction in LZSN from 3.0 to 2.5 for all land uses in 

the C soils and geology grouping (Edwards aquifer confining unit-Blackland Prairie) resulted in a 2.4-percent 

increase in streamflow. Individually, none of the parameter changes caused relatively large changes to simulation results. 

Simulation of Streamflow by Basin and Source 

Category 

The 1997–2001 average annual contribution to 

streamflow at the San Antonio River near Elmendorf 

station, by basin, is shown in figure 14. The lower 

Medina River Basin, which includes inflow from the 

upper Medina River and discharge from the Medio 

Creek and Dos Rios WWRCs, is the largest contributor 

(31 percent). The lower San Antonio River Basin and 

Leon Creek Basin were the next largest contributors (23 

and 19 percent, respectively). Both basins also include 

discharges from WWRCs. Sources of simulated streamflow for 1997–2001 at the outlet of the study area (San 

Antonio River near Elmendorf station) are shown in figure 15 and listed in table 10. 

The average annual streamflow at the San 

Antonio River near Elmendorf station during 1997– 

2001 was 399,000 acre-feet (acre-ft). About 22 percent 

(88,200 acre-feet per year [acre-ft/yr]) entered the study 

area as inflow from the upper Medina River, and about 

20 percent (79,800 acre-ft/yr) originated from recycled 

wastewater. Base flow accounted for about 18 percent 

(72,600 acre-ft/yr) of the total streamflow. Surface runoff from various land uses provided about 33 percent 

(132,000 acre-ft/yr) of the total streamflow. Edwards 

aquifer springflow accounted for about 5 percent 

(21,100 acre-ft/yr) of streamflow. Other sources of 

flow (primarily runoff from minor land uses) contributed about 1.4 percent (5,600 acre-ft/yr) of the total 

streamflow. 

ESTIMATION OF STREAMFLOW 

CONSTITUENT LOADS 

In this report a constituent load is the mass of a 

constituent moved past a point by water in a specified 

Table 9. Sensitivity of selected annual process-related parameters, Salado Creek Basin, 1997–2001 

[Parameter definitions and units in table 5; B, Edwards aquifer outcrop; C, Edwards aquifer upper confining unit-Blackland 

Prairie] 

Change = 100 * [(A – I)/I] where A = adjusted value; I = initial value] 

Land-use Initial Adjusted Change in parameter Change in streamflow Parameter 

category value value (percent) (percent) 

CEPSC Undeveloped, B 0.40 0.30 -25 0.30 

DEEPFR All categories, B .90 .80 -11 1.1 

INFILT All categories, B .60 .70 17 -.70 

INFILT Residential, C .75 .85 13 -.10 

INTFW Undeveloped, C .20 .40 100 -.10 

IRC Undeveloped, C .50 .40 -20 0 

LZETP All categories, B .30 .25 -17 1.2 

LZETP All categories, C .40 .50 25 -2.1 

LZSN All categories, C 3.0 2.5 -17 2.4 

UZSN All categories, C .80 .75 -6.2 .70 

28 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
Lower San Antonio River Basin Leon Creek Basin 

Salado Creek Basin


12


Upper San Antonio River Basin 

15 

Lower Medina River Basin 

31 

23 19

Average annual streamflow, 1997–2001, 

San Antonio River near Elmendorf, Texas: 

399,000 acre-feet per year 

Figure 14. Contribution to streamflow, in percent, at San Antonio River at Elmendorf, Texas (08181800), 1997– 

2001. 

Other Runoff–undeveloped 

Edwards aquifer 12 5.6 Runoff–residential 

springflow 7.4 

5.3 

Runoff–commercial 

Recycled wastewater 

20 

9.9 

Inflow–upper 

Medina River 

22 Base flow 

18 

Figure 15. Sources of simulated streamflow, in percent, at San Antonio River near Elmendorf, Texas (08181800), 

1997–2001. 

time. Streamflow loads were estimated using average or flow could be accounted for during the simulations. 

median concentrations of dissolved solids, dissolved These sources include surface runoff from various landnitrate plus nitrite nitrogen, suspended solids, and total use categories (undeveloped, residential, commercial, 

lead. transportation, and industrial), stream base flow (origiThe HSPF models were configured by land-use nating as ground-water inflow to streams), recycled 

and flow-source category so that the sources of stream- wastewater discharges, upper Medina River inflow from 

ESTIMATION OF STREAMFLOW CONSTITUENT LOADS 29 
Table 10. Sources of simulated streamflow at San Antonio River near Elmendorf, Texas (08181800), 1997–2001 

Simulated streamflow 

Source (percent of total flow1

) 

1997 1998 1999 2000 2001 1997–2001 

Runoff–undeveloped 3.1 14 0.10 1.4 3.3 5.6 

Runoff–residential 6.5 8.3 4.2 9.1 7.6 7.4 

Runoff–commercial 9.2 9.3 6.3 13 11 9.9 

Runoff–transportation corridor 5.7 5.6 3.6 7.7 6.6 25.9 

Runoff–industrial 2.2 2.3 1.6 2.9 2.4 22.3 

Runoff–services 1.6 2.1 1.1 2.2 1.9 21.9 

Other (land use) .80 1.9 1.7 .90 1.3 21.4 

Base flow 19 17 17 17 20 18 

Inflow–upper Medina River 32 22 21 16 18 22 

Recycled wastewater 18 13 34 26 20 20 

Edwards aquifer springflow 2.0 4.7 10 3.8 8.0 5.3 

1 Might not total 100 percent because of rounding.
 2 Grouped as other in figure 15.


outside the study area, and springflow originating from 

Edwards aquifer springs. Average or median constituent 

concentrations were determined (using available waterquality data) to characterize water quality from each of 

these various flow sources. Then constituent loads originating from each source were computed using the 

equation 

Ls = Rs x Cs x cf, (1) 

where 

Ls = constituent load from a particular source of flow 

(for example, residential runoff or recycled 

wastewater), in pounds; 

Rs = HSPF-simulated runoff volume from a particular 

source of flow, in acre-feet; 

Cs = source concentration—average or median concentration characteristic of flow from a particular source, 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 loads from each source were summed to 

estimate overall streamflow constituent loads at the 

study area outlet. 

Source Concentrations Used for San Antonio 

River Constituent Loads Estimation 

Available water-quality data include stormwater 

runoff samples, base-flow samples, streamflow samples 

representative of the upper Medina River, recycled 

wastewater effluent samples, and ground-water samples 

from Edwards aquifer wells. Data-collection sites 

(excluding wells and sites outside the study area) are 

shown in figure 2 and listed in table 2. 

An event-mean concentration (EMC) represents a 

discharge-weighted average constituent concentration 

during storm runoff (Charbeneau and Barrett, 1998). 

Two sources of Bexar County EMC data were available 

to characterize stormwater runoff quality for this study. 

EMC data collected during 1997–2000 for undeveloped, residential, and commercial land uses are available from a water-quality study of basins in the Edwards 

aquifer outcrop and catchment area in Bexar County 

(Ockerman, 2002). EMC data for residential, commercial, transportation, and industrial land use were 

obtained from a study done by the USGS and SAWS 

for the San Antonio NPDES program (B.L. Petri, U.S. 

Geological Survey, written commun., 2002). Only one 

industrial basin and one transportation corridor basin 

were sampled in the study. Because of the small number 

of sites to characterize water quality for these land uses, 

data from an NPDES characterization study done in the 

30 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
Table 11. Water-quality concentrations used for estimation of streamflow constituent loads 

[mg/L, milligrams per liter; μg/L, micrograms per liter] 

Average concentration, stormwater runoff 

Constituent 

Undeveloped Residential Commercial Transportation Industrial Other 

corridor (land use) 

Dissolved solids (mg/L) 124 71 65 152 76 98 

Nitrate plus nitrite nitrogen (mg/L) .51 .54 .55 .55 .60 .55 

Suspended solids (mg/L) 172 140 204 159 192 173 

Total lead (μg/L) 8 37 65 21 39 34 

Constituent 

Median 

concentration, 

base flow 

Average 

concentration, 

inflow–upper 

Medina River 

Median 

concentration, 

recycled 

wastewater 

Average 

concentration, 

Edwards aquifer 

springflow 

Dissolved solids (mg/L) 400 350 609 327 

Nitrate plus nitrite nitrogen (mg/L) 1.5 4.22 15 1.84 

Suspended solids (mg/L) 10 129 3 1 

Total lead (μg/L) .5 .5 .5 0 

Dallas-Fort Worth metropolitan area also were used 

(Baldys and others, 1997). The Dallas-Fort Worth study 

included eight industrial sites and four transportation 

corridor sites. Combined, the data from the three studies 

represent about 275 storm samples collected among five 

land-use categories and 26 sites. The data were grouped 

by land use, and average runoff concentrations were 

computed (table 11). 

Base flow is defined as shallow ground-water 

contributions to streamflow. Water-quality concentrations were obtained from 133 base-flow (low flow, 

assumed to consist primarily of base flow) samples collected at USGS streamflow-gaging stations Leon Creek 

at IH–35 at San Antonio (08181480), San Antonio 

River at Loop 410 at San Antonio (08178565), and 

Salado Creek at Loop 13 at San Antonio (08178800) 

(table 2). Base-flow samples at these stations were collected upstream of wastewater discharges. Median concentrations used for estimation of constituent loads are 

listed in table 11. 

Quality of flow entering the study area from the 

upper Medina River was characterized from 31 samples 

collected at USGS streamflow-gaging stations Medina 

River at La Coste (08180640) and Medina River near 

Macdona (08180700) during 1997–2000. Average constituent concentrations are listed in table 11. 

Recycled wastewater quality was obtained from 

131 effluent samples collected from the four major 

WWRCs and analyzed by the USGS during 1992–95. 

Median constituent concentrations are listed in table 11. 

Water quality of Edwards aquifer springflow was 

characterized from 50 ground-water samples collected 

from 30 Bexar County Edwards aquifer wells during 

1998–2002 (C.L. Otero, U.S. Geological Survey, written commun., 2002). Sample analyses included dissolved solids and nitrate plus nitrite nitrogen but did not 

include suspended solids and total lead. However, unfiltered turbidity and dissolved lead were measured and 

used to estimate suspended solids and total lead, respectively. Average constituent concentrations are listed in 

table 11. 

Some runoff occurs from minor land-use categories for which concentration data are not available. Services, quarries, and some military land use are primary 

examples of this type of land use. Constituent concentrations used to estimate runoff loads originating from 

these land uses were computed as the average of the 

other five land-use concentrations and are listed in table 

11 under “other.” 

ESTIMATION OF STREAMFLOW CONSTITUENT LOADS 31 
100 

1,000 

10,000 

SIMULATED DISSOLVED SOLIDS LOAD,

IN TONS PER DAY 

Comparison of simulated daily loads with measured daily loads 

computed for 58 samples collected during 1997–2001 

100 1,000 10,000 

MEASURED DISSOLVED SOLIDS LOAD, IN TONS PER DAY 

0 10 20 30 40 50 

MEASURED NITRATE PLUS NITRITE NITROGEN LOAD, IN TONS PER DAY 

0 

10 

20 

30 

40 

50 

SIMULATED NITRATE PLUS NITRITE

NITROGEN LOAD, IN TONS PER DAY

Comparison of simulated daily loads and measured daily loads 

computed for 58 samples collected during 1997–2001 

100 1,000 10 10,000 

10,000 

1,000 

100 

10

SIMULATED DISSOLVED SOLIDS LOAD,

IN TONS PER DAY 

Comparison of simulated daily loads and measured daily loads 

computed for 31 samples collected during 1997–2001 

MEASURED SUSPENDED SOLIDS LOAD, IN TONS PER DAY 

MEASURED TOTAL LEAD LOAD, IN POUNDS PER DAY 

Figure 16. Comparison of streamflow loads estimated from Hydrological Simulation Program—FORTRANsimulated flow and source concentrations with loads computed from actual samples at San Antonio River near 

Elmendorf, Texas (08181800), 1997–2001. 

0 250 500 750 1,000 

1,000 

750 

500 

250 

0 

SIMULATED TOTAL LEAD LOAD,

IN POUNDS PER DAY 

Comparison of simulated daily loads and measured daily loads 

computed for 3 samples collected during 1997–2001 

32 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
E

STIMATION OF STREAMFLOW CONSTITUENT LOADS 33 

Dissolved solids Nitrate plus nitrite nitrogen 

Edwards aquifer 


springflow Runoff– 
 all land uses 5.5 9.7 Recycled wastewater 

Recycled 

Inflow–upper Medina River 

22 

Base flow 

24 

66 

Base f

6.6 Inflow–upper Medina River 

38 

wastewater 

21 

Suspended solids Total lead Runoff–ind

6.9 

Runoff–transportation 

Inflow–upper Medina River corridor 

37 9.6 

Base flow Runoff–undeveloped 

9.8 

Runoff– 

2.0 

other land uses 

Runoff–residential 

8.6 

11


Runoff–transportation 
 Runoff–commercial corridor Runoff–commercial 49 Runoff–residentia

9.9 21 21 

Figure 17. Sources of selected streamflow constituent loads, in percent, San Antonio River near Elmendorf, Texas (08181
1999 

Table 12. Estimates of selected annual streamflow constituent loads, by source, San Antonio River watershed, 

Bexar County, Texas, 1997–2001 

Source Dissolved solids 

(tons) 

Nitrate plus 

nitrite nitrogen 

(tons) 

Suspended solids 

(tons) 

Total lead 

(pounds) 

1997 

Runoff–undeveloped 

Runoff–residential 

Runoff–commercial 

 2,400

 2,880

 3,700

 10.0 

 22.0 

 31.0 

3,330 

5,670 

11,600 

310 

2,990 

7,410 

Runoff–transportation corridor

Runoff–industrial 

Runoff–services 

Other (land use) 

Base flow

Inflow–upper Medina River

Recycled wastewater 

Edwards aquifer springflow

Total 

 5,370

1,050 

700 

470 

 46,400

 62,200

67,600 

 4,030

197,000 

 19.0 

8.00 

5.00 

3.00 

230 

840 

1,690 

 23.0 

2,880 

5,610 

2,660 

1,390 

80.0 

1,160 

56,200 

340 

12.0 

88,100 

1,490 

1,080 

420 

320 

120 

400 

110 

0 

14,600 

Source Dissolved solids 

(tons) 

Nitrate plus 

nitrite nitrogen 

(tons) 

Suspended solids 

(tons) 

Total lead 

(pounds) 

1998 

Runoff–undeveloped 

Runoff–residential 

Runoff–commercial 

Runoff–transportation corridor 

Runoff–industrial 

Runoff–services 

Other (land use) 

Base flow 

Inflow–upper Medina River

Recycled wastewater 

Edwards aquifer springflow 

Total 

15,140 

5,080 

5,220 

7,310 

1,480 

 1,280 

6,690 

57,900 

 58,700 

68,600 

13,300 

241,000 

62.0 

39.0 

44.0 

26.0 

12.0 

10.0 

10.0 

220 

790 

1,720 

74.0 

3,010 

21,000 

10,000 

16,400 

7,640 

3,730 

2,520 

2,890 

1,450 

23,800 

340 

41.0 

89,800 

1,950 

5,300 

10,400 

2,020 

1,520 

760 

1,120 

150 

370 

110 

0 

23,700 

Nitrate plus Dissolved solids Suspended solids Total lead Source nitrite nitrogen (tons) (tons) (pounds) (tons) 

Runoff–undeveloped 24.0 0.100 33.0 3.00 

Runoff–residential 960 7.00 1,890 1,000 

Runoff–commercial 1,320 11.0 4,130 2,630 

Runoff–transportation corridor 1,770 6.00 1,850 490 

Runoff–industrial 390 3.00 970 400 

Runoff–services 240 2.00 480 140 

Other (land use) 560 4.00 970 200 

Base flow 21,600 81.0 540 27.0 

Inflow–upper Medina River 20,900 280 3,400 130 

Recycled wastewater 65,800 1,650 330 110 

Edwards aquifer springflow 10,800 60.0 33.0 0 

Total 124,000 2,100 14,600 5,130 

34 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
Table 12. Estimates of selected annual streamflow constituent loads, by source, San Antonio River watershed, 

Bexar County, Texas, 1997–2001—Continued 

Source Dissolved solids 

(tons) 

Nitrate plus 

nitrite nitrogen 

(tons) 

Suspended solids 

(tons) 

Total lead 

(pounds) 

2000 

Runoff–undeveloped 540 2.00 750 70.0 

Runoff–residential 2,730 21.0 5,390 2,850 

Runoff–commercial 3,610 31.0 11,300 7,220 

Runoff–transportation corridor 4,950 18.0 5,180 1,370 

Runoff–industrial 940 7.00 2,370 960 

Runoff–services 670 5.00 1,310 390 

Other (land use) 370 3.00 170 240 

Base flow 29,100 110 730 72.0 

Inflow–upper Medina River 21,200 290 9,900 135 

Recycled wastewater 65,600 1,640 330 110 

Edwards aquifer springflow 5,190 29.0 16.0 0 

Total 135,000 2,160 37,400 13,400 

Source Dissolved solids 

(tons) 

Nitrate plus 

nitrite nitrogen 

(tons) 

Suspended solids 

(tons) 

Total lead 

(pounds) 

2001 

Runoff–undeveloped 590 2.00 820 77.0 

Runoff–residential 3,070 23.0 6,060 3,200 

Runoff–commercial 4,110 35.0 12,900 8,220 

Runoff–transportation corridor 5,740 21.0 6,000 1,590 

Runoff–industrial 1,040 8.00 2,640 1,070 

Runoff–services 790 6.00 1,570 470 

Other (land use) 740 4.00 1,290 510 

Base flow 52,900 198 1,320 130 

Inflow–upper Medina River 31,800 430 5,160 200 

Recycled wastewater 68,300 1,710 340 115 

Edwards aquifer springflow 15,100 84.0 46.0 0 

Total 184,000 2,520 38,100 15,600 

Table 13. Sources of selected streamflow constituent loads, San Antonio River near Elmendorf, Texas 

(08181800), 1997–2001 

Dissolved solids Nitrate plus nitrite nitrogen Suspended solids Total lead Source (percent of total1

) (percent of total

1) (percent of total1

) (percent of total1) 

Runoff–undeveloped 2.1 0.60 9.8 3.3 

Runoff–residential 1.7 .90 11 21 

Runoff–commercial 2.0 1.2 21 49 

Runoff–transportation corridor 2.9 .70 9.9 9.6 

Runoff–industrial .60 .30 4.7 6.9 

Runoff–services .40 .20 2.7 3.0 

Other (land use) 1.0 .30 1.2 3.3 

Base flow 24 6.6 2.0 .70 

Inflow–upper Medina River 22 21 37 1.7 

Recycled wastewater 38 66 .60 .80 

Edwards aquifer springflow 5.5 2.1 .10 0 

1 Might not total 100 percent because of rounding. 

ESTIMATION OF STREAMFLOW CONSTITUENT LOADS 35 
Estimation of Streamflow Constituent Loads 

Using Source Concentrations With HSPFSimulated Flow 

Source concentrations from table 11 were used 

with HSPF-simulated streamflow to estimate selected 

constituent loads at the San Antonio River near Elmendorf (08181800). To assess the reasonableness of the 

estimated loads and whether data used to characterize 

water quality from various sources are representative of 

overall streamflow loads in the study area, selected 

daily loads at the Elmendorf station were compared 

with loads computed by a different method. On the days 

during 1997–2001 when water-quality samples were 

collected at the Elmendorf station, selected daily constituent loads were computed using the “measured” 

(from water-quality sample) instream concentration and 

mean daily streamflow. This comparison of estimated 

and measured loads for dissolved solids, nitrate plus 

nitrite nitrogen, suspended solids, and total lead is 

shown in figure 16. 

Loads estimated by the two methods are in relative agreement for smaller daily loads (which correspond to lower daily flows), especially for dissolved 

solids and nitrate plus nitrite nitrogen (both dissolved 

constituents). The HSPF source-concentration method 

underestimated loads for higher flows (compared with 

loads computed from actual samples and gaged mean 

daily flows), especially suspended solids and total lead. 

Also, only three samples were analyzed for total lead, 

and only one of those samples was at higher flow 

(fig. 16). Because loads were underestimated mostly 

for higher flows, water quality might not be adequately 

characterized for sources that contribute to higher flows 

such as stormwater runoff. Moreover, because suspended solids appear to be underestimated, other constituents that tend to be associated with sediment 

(metals, for example) might also be underestimated by 

the HSPF source-concentration method. Despite limitations at higher flows, the source-concentration method 

of estimating loads does provide a method to quantify 

loads from various land uses and sources. 

Annual estimates of loads (HSPF sourceconcentration method) are listed by source in table 12; 

1997–2001 constituent loads for each source as a percentage of total streamflow loads are listed in table 13. 

Sources of loads for dissolved solids, nitrate plus nitrite 

nitrogen, suspended solids, and total lead for 1997– 

2001 at the San Antonio River near Elmendorf station 

are shown in figure 17. 

Sources of streamflow loads varied according to 

constituent. Recycled wastewater is the largest source of 

dissolved solids and nitrate plus nitrite nitrogen (about 

38 and 66 percent, respectively). Inflow from the upper 

Medina River and base flow are the other major contributors of these constituents. Runoff from urban sources 

(all runoff except “runoff-undeveloped” in fig. 17) is the 

largest source of suspended solids loads (about 49 percent) and total lead loads (about 90 percent). Runoff 

from residential and commercial land, which represents 

about 23 percent of the land area, produced about 70 

percent of the total lead streamflow load. 

SUMMARY 

The USGS developed HSPF watershed models to 

simulate runoff and streamflow constituent loads for the 

major basins of the San Antonio River watershed in 

Bexar County. A model parameter set for use with the 

HSPF model was developed from available data and 

used for model simulations to estimate streamflow and 

selected constituent loads. The San Antonio River Basin 

in Bexar County was divided into five major basins for 

the simulations. Each basin was further subdivided, 

resulting in a total of 103 stream or reservoir segments. 

Twenty-eight pervious land segments were defined for 

the study on the basis of four physiographic categories 

and seven land-use categories. To adequately represent 

the relatively rapid rainfall-runoff response of the study 

area basins, the model simulations were done using a 1­

hour time step. Streamflow and constituent loads were 

estimated for the period 1997–2001. 

Model parameter calibration and simulations 

were done using data collected during 1997–2001 from 

six rain gages, 10 streamflow gages, and one evaporation station. The error in simulated runoff (difference 

between simulated and measured streamflow) at the 

study area outlet (USGS streamflow gaging station 

08181800 San Antonio River near Elmendorf) for 

1997–2001 was less than 1 percent. Annual error 

among the basins was greater, ranging from -32.7 to 

39.6 percent. 

The HSPF models were configured to assign runoff to various categories on the basis of land use or 

source of flow so that the sources, or components, of 

total streamflow at the study area outlet could be determined. During 1997–2001, about 22 percent of streamflow originated as inflow from the upper Medina River. 

Recycled wastewater accounted for about 20 percent of 

streamflow. Base flow (shallow ground water returning 

36 Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the San Antonio River Watershed, Bexar County, 

Texas, 1997–2001 
to streams) provided about 18 percent of total streamflow. Springflow, originating from Edwards aquifer 

springs, produced 5.3 percent of total streamflow. Storm 

runoff from various land uses represented about 33 percent of streamflow, mostly from commercial (9.9 percent) and residential (7.6 percent). 

Streamflow constituent loads were computed by 

obtaining representative water-quality concentrations 

for each flow category or source, then applying the concentrations to the appropriate source category of HSPFsimulated flow volume. Data from more than 600 waterquality samples were analyzed to determine concentrations used to characterize the quality of streamflow 

originating from various sources. In addition, waterquality samples were collected at the study outlet (San 

Antonio River near Elmendorf station) so selected daily 

loads could be computed using the measured (sampled) 

concentration and the gaged streamflow. These measured loads were compared with loads estimated using 

HSPF-simulated streamflow and source concentrations. 

Overall, the HSPF source-concentration method underestimated loads (compared to loads computed from 

samples collected at the San Antonio River near Elmendorf station). Loads estimated using the two methods 

tended to agree more closely for the dissolved constituents (dissolved solids and nitrate plus nitrite nitrogen) 

than for constituents associated with suspended sediment (suspended solids and total lead). Also, loads 

estimated by both methods were in relatively good 

agreement for lower flows (smaller daily loads). 

On the basis of the source-concentration estimates for 1997–2001, recycled wastewater is the largest 

source of dissolved solids and nitrate plus nitrite nitrogen streamflow loads (about 38 and 66 percent, respectively). Inflow from the upper Medina River and base 

flow also were major contributors to dissolved solids 

and nitrate plus nitrite nitrogen. In contrast, storm runoff was the primary source for suspended solids and 

total lead streamflow loads. Storm runoff from urban 

land accounted for about 49 percent of all suspended 

solids. Runoff from residential and commercial land 

(which represents about 23 percent of the land area) 

produced most of the total lead in streamflow (about 70 

percent). 

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REFERENCES 37 
Ockerman and McNamara— Simulation of Streamflow and Estimation of Streamflow Constituent Loads in the 

San Antonio River Watershed, Bexar County, Texas, 1997–2001