U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5191

In cooperation with the U.S. Army Corps of Engineers and the San Antonio River Authority

Simulation of Streamflow and Water Quality in the Leon 

Creek Watershed, Bexar County, Texas, 1997–2004
Front and back cover:

Leon Creek near confluence of the Medina River, looking upstream, August 26, 2009. 
Simulation of Streamflow and Water 

Quality in the Leon Creek Watershed, 

Bexar County, Texas, 1997–2004

By Darwin J. Ockerman and Meghan C. Roussel

In cooperation with the U.S. Army Corps of Engineers and the 

San Antonio River Authority

Scientific Investigations Report 2009–5191

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

This and other USGS information products are available at http://store.usgs.gov/

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Denver, CO 80225

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ockerman, D.J., and Roussel, M.C., 2009, Simulation of streamflow and water quality in the Leon Creek watershed, 

Bexar County, Texas, 1997–2004: U.S. Geological Survey Scientific Investigations Report 2009–5191, 50 p.
iii

Contents

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

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

Purpose and Scope .............................................................................................................................3

Description of Leon Creek Watershed .............................................................................................3

Simulation of Streamflow and Water Quality ...........................................................................................4

Functional Description of Hydrological Simulation Program—FORTRAN .................................4

Input Data for Hydrological Simulation Program—FORTRAN .....................................................7

Model Configuration ..........................................................................................................................10

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

Streamflow .................................................................................................................................10

Water Quality .............................................................................................................................21

Water Temperature .........................................................................................................21

Dissolved Oxygen ............................................................................................................21

Suspended Sediment ......................................................................................................25

Dissolved Ammonia Nitrogen and Dissolved Nitrate Nitrogen ................................26

Dissolved and Total Lead and Dissolved and Total Zinc ............................................30

Simulation of Constituent Loads and Yields ..................................................................................33

Model Limitations ...............................................................................................................................37

Summary .......................................................................................................................................................38

References ...................................................................................................................................................39

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) Process-Related 

Parameters Used to Simulate the Leon Creek Watershed, Bexar County, 

Texas .....................................................................................................................................43

Figures

1. Map showing location of Leon Creek watershed, Bexar County, Texas ............................2

2. Map showing land use in the Leon Creek watershed, Bexar County, Texas, 2004 ...........5

3. Diagram showing Hydrological Simulation Program—FORTRAN (HSPF) flowchart 

for (A) impervious and (B) pervious land segments ...............................................................6

4. Map showing data-collection sites that provided data for Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas ...........8

5. Map showing delineated subwatersheds for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas .........11

6–12. Graphs showing: 

6. Measured and simulated daily mean streamflow at station 08180945 Leon 

Creek at Scenic Loop Road near Leon Springs, Texas, 2001–04 ..............................17

7. Measured and simulated daily mean streamflow at station 08181050 Leon 

Creek at Prue Road at San Antonio, Texas, 2001–03 ..................................................18

8. Measured and simulated daily mean streamflow at station 08181480 Leon 

Creek at Interstate Highway 35 at San Antonio, Texas, 2001–04 ..............................19

9. Measured and simulated daily mean water temperature at station 08181480 

Leon Creek at Interstate Highway 35 at San Antonio, Texas, 1997–2000 ................22
iv

10. Measured and simulated daily mean dissolved oxygen at station 08181480 

Leon Creek at Interstate Highway 35 at San Antonio, Texas, 1997–2000 ................23

11. Relation between daily mean streamflow and daily suspended-sediment 

load based on 20 streamflow-concentration data pairs for station 08181480 

Leon Creek at Interstate Highway 35 at San Antonio, Texas, 1992–2007 ................26

12. Relation between estimated monthly suspended-sediment loads and 

Hydrological Simulation Program—FORTRAN-simulated suspendedsediment loads for station 08181480 Leon Creek at Interstate Highway 35 at 

San Antonio, Texas, 2001–04 ...........................................................................................27

Tables

1. Data-collection sites that provided data for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas ...........9

2. Streamflow calibration (five gaging stations) and testing (one gaging station) 

results for the Hydrological Simulation Program—FORTRAN model of the Leon 

Creek watershed, Bexar County, Texas .................................................................................13

3. Summary of calibrated values for selected hydrology-related parameters for the 

Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, 

Bexar County, Texas ..................................................................................................................20

4. Water temperature calibration results for the Hydrological Simulation Program—

FORTRAN model of the Leon Creek watershed, Bexar County, Texas .............................24

5. Dissolved oxygen calibration results for the Hydrological Simulation Program—

FORTRAN model of the Leon Creek watershed, Bexar County, Texas .............................24

6. Simulated 1992–2004 suspended-sediment runoff concentrations from four 

pervious and impervious land-use types and 1992–2004 medians of measured 

suspended-sediment runoff concentrations from watersheds that represent the 

respective land-use types (sites 8–14) for the Hydrological Simulation Program—

FORTRAN model of the Leon Creek watershed, Bexar County, Texas .............................28

7. Measured and simulated suspended-sediment concentrations and loads at 

upper-watershed sites for selected stormflows for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas .........28

8. Measured and simulated dissolved ammonia nitrogen concentrations and loads 

at upper-watershed sites for selected stormflows for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas .........29

9. Measured and simulated dissolved nitrate nitrogen concentrations and loads at 

upper-watershed sites for selected stormflows for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas .........29

10. Measured and simulated concentrations of dissolved ammonia nitrogen and 

dissolved nitrate nitrogen for selected stormflows at station 08181480 Leon Creek 

at Interstate Highway 35 at San Antonio, Texas ...................................................................29

11. Measured and simulated concentrations of dissolved and total lead at upperwatershed sites for selected stormflows for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas .........31

12. Measured and simulated concentrations of dissolved and total zinc at upperwatershed sites for selected stormflows for the Hydrological Simulation 

Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas .........32
v

13. Measured and simulated concentrations of total lead and total zinc for selected 

stormflows at station 08181480 Leon Creek at Interstate Highway 35 at San 

Antonio, Texas ............................................................................................................................33

14. Summary statistics for measured streamflow and concentrations of total lead and 

total zinc for nine base-flow samples collected during 1992–94 at station 08181480 

Leon Creek at Interstate Highway 35 at San Antonio, Texas .............................................33

15. Summary statistics for simulated daily mean total lead and total zinc 

concentrations for base-flow periods at station 08181480 Leon Creek at 

Interstate Highway 35 at San Antonio, Texas, 1997–2004 ...................................................33

16. Summary of calibrated values for selected water-quality parameters for the 

Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, 

Bexar County, Texas, 1997–2004 ..............................................................................................34

17. Simulated mean annual streamflow and annual streamflow loads and yields of 

suspended sediment, dissolved nitrate nitrogen, and total lead at the outlet of 

Leon Creek watershed, Bexar County, Texas, 1997–2004 ...................................................37

18. Simulated mean annual streamflow and mean annual streamflow yields of 

suspended sediment, dissolved nitrate nitrogen, and total lead at selected stream 

reach (RCHRES) outlets, Leon Creek watershed, Bexar County, Texas, 1997–2004 .......37

Conversion Factors, Datums, and Water-Quality Units

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Slope

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 megagram (Mg) 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8
vi

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Water-Quality Units

mg/L, milligram per liter

μg/L, microgram per liter
Abstract 

The U.S. Geological Survey, in cooperation with the U.S. 

Army Corps of Engineers and the San Antonio River Authority, configured, calibrated, and tested a Hydrological Simulation Program—FORTRAN watershed model for the approximately 238-square-mile Leon Creek watershed in Bexar 

County, Texas, and used the model to simulate streamflow and 

water quality (focusing on loads and yields of selected constituents). Streamflow in the model was calibrated and tested 

with available data from five U.S. Geological Survey streamflow-gaging stations for 1997–2004. Simulated streamflow 

volumes closely matched measured streamflow volumes at all 

streamflow-gaging stations. Total simulated streamflow volumes were within 10 percent of measured values. Streamflow 

volumes are greatly influenced by large storms. Two months 

that included major floods accounted for about 50 percent of 

all the streamflow measured at the most downstream gaging 

station during 1997–2004. 

Water-quality properties and constituents (water temperature, dissolved oxygen, suspended sediment, dissolved 

ammonia nitrogen, dissolved nitrate nitrogen, and dissolved 

and total lead and zinc) in the model were calibrated using 

available data from 13 sites in and near the Leon Creek watershed for varying periods of record during 1992–2005. Average 

simulated daily mean water temperature and dissolved oxygen 

at the most downstream gaging station during 1997–2000 were 

within 1 percent of average measured daily mean water temperature and dissolved oxygen. Simulated suspended-sediment 

load at the most downstream gaging station during 2001–04 

(excluding July 2002 because of major storms) was 77,700 

tons compared with 74,600 tons estimated from a streamflowload regression relation (coefficient of determination = .869). 

Simulated concentrations of dissolved ammonia nitrogen and 

dissolved nitrate nitrogen closely matched measured concentrations after calibration. At the most downstream gaging 

station, average simulated monthly mean concentrations of 

dissolved ammonia and nitrate concentrations during 1997–

2004 were 0.03 and 0.37 milligram per liter, respectively. For 

the most downstream station, the measured and simulated 

concentrations of dissolved and total lead and zinc for stormflows during 1993–97 after calibration do not match particularly closely. For base-flow conditions during 1997–2004 at 

the most downstream station, the simulated/measured match 

is better. For example, median simulated concentration of 

total lead (for 2,041 days) was 0.96 microgram per liter, and 

median measured concentration (for nine samples) of total 

lead was 1.0 microgram per liter. 

To demonstrate an application of the Leon Creek watershed model, streamflow constituent loads and yields for 

suspended sediment, dissolved nitrate nitrogen, and total lead 

were simulated at the mouth of Leon Creek (outlet of the 

watershed) for 1997–2004. The average suspended-sediment 

load was 51,800 tons per year. The average suspendedsediment yield was 0.34 ton per acre per year. The average 

load of dissolved nitrate at the outlet of the watershed was 802 

tons per year. The corresponding yield was 10.5 pounds per 

acre per year. The average load of lead at the outlet was 3,900 

pounds per year. The average lead yield was 0.026 pound per 

acre per year.

The degree to which available rainfall data represent 

actual rainfall is potentially the most serious source of measurement error associated with the Leon Creek model. Major 

storms contribute most of the streamflow loads for certain constituents. For example, the three largest stormflows contributed 

about 64 percent of the entire suspended-sediment load at the 

most downstream station during 1997–2004. 

Introduction

South-central Texas, including the Leon Creek watershed 

in Bexar County (fig. 1), is one of the most flash-flood-prone 

areas of the United States (Bomar, 1995). A large flash flood 

occurred during October 1998 when as much as 30 inches of 

rain occurred in a 2-day period in south-central Texas. Thirtytwo lives were lost, and property damage was estimated to be 

$500 million (Slade and Patton, 2003). During the October 

1998 flash flood, more than 12 inches of rain fell in parts of 

the Leon Creek watershed in less than 24 hours. The peak discharge at U.S. Geological Survey (USGS) streamflow-gaging 

station 08181480 Leon Creek at Interstate Highway (IH) 35 at 

San Antonio, Tex., exceeded the 100-year recurrence interval 

(Slade and Persky, 1999). Since the October 1998 flash flood, 

ongoing development in the Leon Creek watershed and subsequent increase of impervious cover have increased the risk of 

flood damage.

In the “Texas Water Quality Inventory 2000” (Texas 

Commission on Environmental Quality, 2002), part of lower 

Leon Creek (32 river miles of Leon Creek upstream from its 

confluence with the Medina River) (fig. 1) was identified as 

impaired on the basis of the criteria of suitability for contact 

Simulation of Streamflow and Water Quality in the Leon 

Creek Watershed, Bexar County, Texas, 1997–2004 

By Darwin J. Ockerman and Meghan C. Roussel
2 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Figure 1. Location of Leon Creek watershed, Bexar County, Texas. 

Leon Creek

Watershed

WILSON COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

MEDINA COUNTY

BANDERA

COUNTY

COMAL COUNTY

GUADALUPE

 COUNTY

KENDALL 

COUNTY

98°20'

98°34'

98°48'

29°42'

29°28'

29°14'

Salado Creek

San Antonio River

Cibolo Creek

Leon Creek

Elm Creek

Leon Creek

Cibolo Creek

Medina River

Medio Creek

Medina River

Culebra Creek

San Antonio River

Balcones Creek

Salado Creek

Calaveras

 Lake

Braunig

 Lake

Mitchell

 Lake

San

 Antonio

BEXAR

 COUNTY

LOCATION MAP

TEXAS

Bexar County

Beeville

Sonora

Base modified from U.S. Geological Survey

digital data 1:24,000 quadrangles

Universal Transverse Mercator Projection, Zone 14

North American Datum of 1983

0 2 4 6 8 MILES

EXPLANATION

Edwards aquifer recharge zone (outcrop)

Leon Creek watershed boundary
Introduction 3

recreation and sustainability of high aquatic life. Contact 

recreation use was not supported because of elevated fecal 

coliform counts. Aquatic life use was only partially supported 

because of depressed dissolved oxygen concentrations. In 

2004, the Texas Commission on Environmental Quality (2005) 

also determined that sections of lower Leon Creek did not 

support fish consumption because of high levels of polychlorinated biphenyls.

In 2004, the U.S. Army Corps of Engineers (USACE), 

Fort Worth District, began a study of water-resource issues 

in the Leon Creek watershed. The purpose of that study (U.S. 

Army Corps of Engineers, 2004) is to (1) establish and 

document the existing hydrologic, engineering, economic, 

and environmental conditions for the watershed (phase 1), 

and (2) evaluate best-management practices for flood-risk 

management, improvement in water quality, and ecosystem 

restoration (phase 2). As part of phase 1, the USGS, in cooperation with the USACE and the San Antonio River Authority (SARA), configured, calibrated, and tested a watershed 

model for the Leon Creek watershed and used the model to 

simulate streamflow and water quality (focusing on loads and 

yields [load per acre of contributing drainage area] of selected 

constituents). 

The Leon Creek watershed model is configured so that 

it might be used in potential simulations as part of phase 2. 

Potential simulations would involve detention/retention structures, sediment-control structures, and vegetative management 

practices. Another use of the model is to predict streamflow 

and water-quality conditions for scenarios involving population increases and associated land development in the watershed. Finally, the modular nature of the watershed model 

will allow for the configuration, calibration, and simulation 

of water-quality constituents not included in the simulations 

reported here.

Purpose and Scope

The purpose of this report is to describe simulation 

of streamflow and water quality in the Leon Creek watershed 

using a watershed model, the Hydrological Simulation Program—FORTRAN (HSPF model) (Bicknell and others, 

2001), with input data collected during 1997–2004. The 

HSPF model for this application was configured to simulate 

streamflow, water temperature, dissolved oxygen, suspended 

sediment, dissolved ammonia nitrogen, dissolved nitrate 

nitrogen, and dissolved and total lead and zinc. The HSPF 

model and the input data are described first and then the 

process of configuration, calibration, and testing of the 

model. Following that is description of an application of 

the model involving simulation of loads and yields of suspended sediment, dissolved nitrate nitrogen, and total lead; 

and finally, discussion of the limitations on the application 

of the model to the Leon Creek watershed. The report documents the contributions of the USGS as part of phase 1 of 

the USACE study of water-resource issues in the Leon Creek 

watershed. 

Description of Leon Creek Watershed

The Leon Creek watershed is in western Bexar County 

in the greater San Antonio area (fig. 1). Leon Creek originates in northwestern Bexar County and runs south-southeast 

for about 57 river miles to its confluence with the Medina 

River. The drainage area of the Leon Creek watershed (at the 

confluence with the Medina River) is about 238 square miles. 

The watershed is in a region described as having a subtropical, sub-humid climate characterized by hot summers and 

mild, dry winters (Larkin and Bomar, 1983). Average monthly 

low temperatures range from 38.6 degrees Fahrenheit (°F) in 

January to 74.0 °F in July; average monthly high temperatures 

range from 62.1 °F in January to 94.7 °F in August (National 

Climatic Data Center, 2009). Average annual rainfall in the 

watershed is about 33 inches per year based on 1971–2000 

rainfall data from the National Weather Service station at the 

San Antonio International Airport. During 1942–2008, daily 

rainfall greater than 0.01 inch occurred, on average, 76 days 

per year (National Climatic Data Center, 2009). Daily rainfall 

1.0 inch or greater occurs at the San Antonio International 

Airport about every 43 days according to Asquith and Roussel 

(2003, table 5, sequence number 1047). Most rainfall tends to 

occur in spring, early summer, and fall, but abundant rainfall 

can occur anytime during the year (Larkin and Bomar, 1983). 

Four predominant geologic formations or groups of 

formations crop out in the watershed: From north to south 

according to the San Antonio sheet of the “Geologic Atlas of 

Texas” (Brown and others, 1983), the surficial rocks primarily 

are (1) Glen Rose Limestone; (2) Edwards Group undivided; 

(3) Navarro Group and Marlbrook Marl, Pecan Gap Chalk, 

and Austin Chalk; and (4) Leona Formation and fluviatile 

terrace deposits. The outcropping Glen Rose Limestone is 

characterized by shallow, rocky, and clayey soils with relatively low to moderate infiltration capacity based on the Bexar 

County Soil Survey (U.S. Department of Agriculture, Soil 

Conservation Service, 1966). The outcropping Edwards Group 

undivided is characterized by shallow- to moderate-depth 

clayey soils with relatively high infiltration largely because 

of faults, sinkholes, and other karst features. The outcropping 

Navarro Group and Marlbrook Marl, Pecan Gap Chalk, and 

Austin Chalk are characterized by deep clayey soils with moderate infiltration capacity. The outcropping Leona Formation 

and fluviatile terrace deposits are characterized by deep clayey 

and sandy loam soils with relatively high infiltration.

The recharge zone (outcrop) of the Edwards aquifer 

crosses the northern part of the watershed (fig. 1). Numerous 

wells tap the Edwards aquifer, most of which are concentrated 

in the confined freshwater zone of the aquifer (the watershed 

south of the outcrop). Many wells in the confined freshwater 

zone can yield more than 1,000 gallons per minute (Maclay, 

1995, p. 43). Although the Edwards aquifer supplies water for 

several major springs in the region, no major springs are in the 

Leon Creek watershed. Also, Leon Creek streamflow in the 

area upstream from and on the Edwards aquifer recharge zone 

outcrop is intermittent, flowing only for a short period (days 
4 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

or weeks) during and after abundant rainfall. Streamflow on 

the recharge zone is lost to faults, fractures, and other geologic 

features, and streamflow losses on the recharge zone contribute directly to Edwards aquifer recharge (Puente, 1978). Most 

recharge to the Edwards aquifer occurs as streamflow losses. 

Direct infiltration of rainfall through soil and rock layers (diffuse recharge) is a relatively small part of the total recharge 

to the Edwards aquifer (Maclay, 1995). Other than relatively 

small stock ponds, no lakes or reservoirs are in the study 

watershed. There is a relatively large municipal wastewater 

treatment facility (annual discharge of about 39,000 acre-feet 

during 1997–2004 [M. Veazy, Texas Commission on Environmental Quality, written commun., 2001]) near the mouth of 

Leon Creek.

Land use in the northwestern part of the Leon Creek 

watershed is largely undeveloped (rangeland and juniper and 

oak forest) (fig. 2). Undeveloped land in the southern part of 

the watershed is largely agricultural. The central area of the 

watershed includes relatively intense residential and commercial development. Much undeveloped land in the watershed is 

undergoing conversion to suburban residential and commercial 

land use. The 2004 population of the watershed was about 

278,000 (City of San Antonio, Department of Public Works, 

2006). Demographic statistics for all of Bexar County indicate 

a population increase of about 11 percent during the period 

represented by the data of this report (1997–2004) (Texas 

Department of State Health Services, 2008).

Elevation in the Leon Creek watershed ranges from about 

460 to 1,930 feet above sea level. Land slopes generally are 

steeper in the northern (upstream) part of the watershed than 

in the southern (downstream) part. Overall, the Leon Creek 

stream channel slope is about 18 feet per mile. Some stream 

slopes in the northern part of the watershed (upper Culebra 

Creek and upper Helotes Creek) are greater than 60 feet per 

mile. 

Simulation of Streamflow and Water 

Quality 

Streamflow and water quality in the Leon Creek watershed were simulated with the Hydrological Simulation Program—FORTRAN (HSPF) (Bicknell and others, 2001). 

HSPF was selected for the study watershed because it is one 

of the more comprehensive watershed models available, can 

simulate a variety of stream and watershed conditions with 

reasonable accuracy (including water-quality processes), and 

enables flexibility in adjusting the model to simulate alternative conditions or scenarios (Donigian and others, 1995). To 

simulate the hydrologic processes that occur in a watershed, 

HSPF uses data (referred to as parameters in the HSPF documentation) such as the time history of rainfall, air temperature, 

and other meteorological parameters; parameters related to 

land cover and land-use practices; and soil characteristics. 

The result of an HSPF simulation is an output of streamflow 

and concentrations (or loads) of water-quality constituents at 

a user-specified interval, or time step. A 1-hour time step was 

used for this report.

Continuous simulation models facilitate simulation of 

important watershed processes for a full range in streamflow 

regime for the simulation period. The relative importance of 

various processes and factors influencing water quality can 

vary considerably with the magnitude of streamflow. For 

example, processes that appreciably affect water-quality conditions during low flows might have relatively minor effects 

on water-quality conditions during high flows. For assessment 

of peak-streamflow characteristics, continuous simulation 

models can provide a more realistic evaluation of antecedent 

soil-moisture conditions than is generally possible with eventbased models (Martin and others, 2001, p. 66).

Functional Description of Hydrological 

Simulation Program—FORTRAN

In HSPF, a watershed is represented by a group of 

hydrologically similar areas referred to as hydrologic response 

units (HRUs) that drain to a stream segment, lake, or reservoir 

referred to as a RCHRES (ReaCH REServoir). Each RCHRES 

(composed of open or closed channels, or completely mixed 

lakes) is contained in and corresponds to a subwatershed. 

HRUs reflect areas in a subwatershed of similar land use, 

surficial geology, and other factors deemed important to 

produce a similar hydrologic response to rainfall and potential 

(pan) evaporation. HRUs are categorized as either pervious 

or impervious land segments, termed PERLND (PERvious 

LaND) or IMPLND (IMPervious LaND), respectively. A 

PERLND is represented conceptually within HSPF by three 

interconnected water storage zones—an upper zone, a lower 

zone, and a groundwater zone. An IMPLND is represented 

by surface storage, evaporation, and runoff processes. The 

hydraulics of stream reaches is simulated using storage routing 

(Donigian and others, 1995). 

HSPF, a continuous, semi-lumped-parameter model 

(Singh, 1995), provides continuous water and mass balance 

by tracking rainfall and water-quality constituents through 

the conceptual pathways of the hydrologic cycle. HSPF is 

composed of a series of computational routines that separately 

simulate processes of the hydrologic cycle. Specifically, HSPF 

simulates the hydrologic cycle as an interconnected series of 

storage (and processing) segments with water fluxes (volume per unit area per unit time) and constituent fluxes (mass 

[weight] per unit area per unit time) moving between the various storages (fig. 3). Storages and fluxes are controlled by system inputs and user-supplied parameter values. For each HRU 

and RCHRES, the model computes a water budget (inflows, 

outflows, and changes in storage) for each time step. 

HSPF is an empirical model; although some parameters 

are directly measurable, most are estimated during model 

calibration (Martin and others, 2001). The HSPF model of 

the Leon Creek watershed was developed by (1) compiling 
Simulation of Streamflow and Water Quality 5

WILSON COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

MEDINA COUNTY

BANDERA

COUNTY

COMAL COUNTY

GUADALUPE

 COUNTY

KENDALL 

COUNTY

98°20'

98°34'

98°48'

29°42'

29°28'

29°14'

Land use from S. Gonzales, San Antonio

 River Authority, written commun. (2005)

0 2 4 6 8 MILES

EXPLANATION

Land use

Undeveloped

Open space

Residential

Commercial

Industrial

Transportation corridor

Mining

Parks, services

Leon Creek watershed boundary

Base modified from U.S. Geological Survey

digital data 1:24,000 quadrangles

Universal Transverse Mercator Projection, Zone 14

North American Datum of 1983

Figure 2. Land use in the Leon Creek watershed, Bexar County, Texas, 2004. 
6 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Figure 3. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for (A) impervious and (B) pervious land segments (modified from Wicklein and Schiffer, 2002, fig. 3). 

INFILT*

(B) Pervious land segments

(A) Impervious land segments

Rainfall

potential ET

Interception

storage Surface

detention

storage

Interflow

storage

Active groundwater

storage

Groundwater

flow

Shallow

subsurface

flow

Overland

flow

To

stream

Upper-zone

storage

Directinfiltration

DelayedInfiltration

Lowerzone

storage

Actual

evaporation

Actual

ET

ETETETETET

Inactive groundwater

To

stream

Potentialrunoff

RETSC*

CEPSC*

LZETP*

LZSN*

INFILD*

INFEXP*

INTFW*

AGWETP*

BASETP*

DEEPFR*

UZSN*

LSUR*

NSUR*

SLSUR*

LSUR*

NSUR*

SLSUR*

IRC*

KVARY*

AGWRC*

Surface detention

storage

Overland

flow

Surface retention

Rainfall

storage

potential ET Model input

StorageOutflowDecision pointEvapotranspirationDesignates controllingHSPF parameter—Refer to table 3

EXPLANATION

*

ET
Simulation of Streamflow and Water Quality 7

and processing required input data, (2) configuring the model 

to represent the watershed, and (3) calibrating the model to 

improve simulation accuracy. A complete description of the 

computational processes and required input model parameters 

is provided in the HSPF user’s manual (Bicknell and others, 

2001). The definitions of the HSPF model process parameters 

used in the Leon Creek model are listed in appendix 1. 

Input Data for Hydrological Simulation 

Program—FORTRAN

Input data for the HSPF model include spatial data (land 

use, topography, geology, and soils), hydraulic characteristics 

of stream segments (RCHRESs), meteorological data, and 

streamflow and water-quality data. Spatial data were used to 

develop model HRUs (PERLNDs, IMPLNDs) and RCHRESs. 

Watershed land-use data for 2004 are from an engineering 

consultant report to SARA on the development of land-use 

information for hydrologic modeling (S. Gonzalez, San Antonio River Authority, written commun., 2005). The consultant 

report also provided estimates of impervious cover for each 

land-use category. Topography (slope) was obtained from 

USGS 7.5-minute digital elevation models (U.S. Geological 

Survey, 2001). The digital elevation models also were used 

to delineate subwatersheds as part of the model development. 

Surface geology was obtained from the San Antonio sheet of 

the “Geologic Atlas of Texas” (Brown and others, 1983). Soils 

were obtained from the “Soil Survey of Bexar County, Texas” 

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

1966). Hydraulic characteristics for each stream segment 

(surface area, volume, and discharge as a function of depth) 

were estimated from a HEC-RAS model of the Leon Creek 

watershed developed by the USACE (D. Prochaska, U.S. 

Army Corps of Engineers, written commun., 2006). As part of 

its Next Generation Weather Radar (NEXRAD) program, the 

National Weather Service (NWS) produces gridded rainfall 

estimates for the region; these estimates are known as stage 

III data (National Oceanic and Atmospheric Administration, 

2006). This spatial rainfall information can be incorporated 

into watershed models as an improvement to using somewhat 

sparse rain-gage networks to obtain rainfall data. For the rainfall input to the HSPF model of the Leon Creek watershed, a 

procedure for combining a gridded NEXRAD rainfall surface 

with subwatersheds of the Leon Creek watershed was used. A 

Geographic Information System (GIS) provided standardized 

functions to accomplish this task. 

The NEXRAD stage III product offers rainfall estimates 

spatially averaged over grid cells of about 16 square kilometers (about 10 square miles) and temporally averaged over 1 

hour. In the stage III data product, estimates from one or more 

radars are combined into a common grid system so that basinwide streamflow forecasts can be made. The radar that provides data for the Leon Creek watershed is the NWS Austin/

San Antonio NEXRAD station (site 1, table 1), located about 

40 miles northwest of the Leon Creek watershed. Stage III 

products provide hourly estimates in the Hydrologic Rainfall 

Analysis Project (HRAP) grid system, a 4-kilometer (about 

2.5-mile) grid in a Polar Stereographic map projection (Shedd 

and Fulton, 1993). The HRAP grid, as defined by Greene and 

Hudlow (1982), is used to define the location of each average 

rainfall value in stage III data. Flat map coordinate systems 

like HRAP are defined using a datum and a map projection. 

Stage III data for 1999–2004 were obtained for the 

HRAP grid cells that cover the Leon Creek watershed. These 

hourly rainfall time series were then used to determine Leon 

Creek watershed average hourly rainfall time series for each 

subwatershed: The HRAP grid cells were intersected with 

the predetermined subwatersheds of the Leon Creek model. 

Hourly values of rainfall associated with each HRAP cell 

were then averaged on the basis of the area of each HRAP 

grid cell within each subwatershed. This was done for each 

subwatershed by weighting the HRAP grid cell hourly rainfall 

values on the basis of the percentage of subwatershed area 

covered by each HRAP grid cell using a custom-built, operation-specific PERL script (William H. Asquith, U.S. Geological Survey, written commun., 2006). For example, if 100 

percent of an HRAP cell is within a subwatershed, the rainfall 

values are multiplied by 1; if 25 percent of an HRAP cell is 

within a subwatershed then the rainfall values are multiplied 

by 0.25. After the area-weighted hourly rainfall values were 

determined for each subwatershed, the values were averaged 

and the averages were compiled into an hourly time series for 

the subwatershed.

Other meteorological data required for the Leon Creek 

model simulations comprise air temperature, dew point 

temperature, wind speed, solar radiation, cloud cover, and 

evaporation. These data (except evaporation) were obtained 

from the NWS station at the San Antonio airport (site 2) 

(fig. 4; table 1) and were generally available at 3-hour 

intervals. Daily pan evaporation data were obtained from the 

NWS station at San Antonio Sea World (site 3). The 3-hour 

data were disaggregated to hourly rates for each day (on the 

basis of site latitude and time of year) using a computer program that is part of the BASINS (Better Assessment Science 

Integrating Point and Nonpoint Sources) watershed modeling 

software package (U.S. Environmental Protection Agency, 

2008). 

Continuous streamflow and instream water-quality data 

(temperature and dissolved oxygen) and discrete water-quality 

data from analysis of samples were used to calibrate model 

parameters pertaining to constituent simulations. Streamflow 

and water-quality data were available for five USGS streamflow-gaging stations in the watershed (sites 4–7, 15) (fig. 4; 

table 1). Water-quality data also were obtained from other 

USGS gaging stations in Bexar County (sites 8–14, 16). These 

data typically were used to help characterize water quality 

from specific land-use categories.

Discharge data (monthly averages) from the wastewater 

treatment facility located near the mouth of Leon Creek (site 

16) (fig. 4; table 1) were obtained from the Texas Commission on Environmental Quality (M. Veazy, Texas Commission 
8 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

WILSON COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

MEDINA COUNTY

BANDERA

COUNTY

COMAL COUNTY

GUADALUPE

 COUNTY

KENDALL 

COUNTY

98°20'

98°34'

98°48'

29°42'

29°28'

29°14'

FM 1604

US Hwy 281

IH-10

IH-10

US Hwy 90

I-410

IH-37

IH-35

IH-35

>

>

>

> >

>

>

>

>

>

>

>

>

>

>

14

13

10

9

8

11

12

16

7

5

4 6

15

3

2

Base modified from U.S. Geological Survey

digital data 1:24,000 quadrangles

Universal Transverse Mercator Projection, Zone 14

North American Datum of 1983

0 2 4 6 8 MILES EXPLANATION

Leon Creek watershed

Culebra Creek drainage area

Upper Leon Creek drainage area

Lower Leon Creek drainage area

Leon Creek watershed boundary

Site type and number—See table 1

Streamflow

Pan evaporation

Meteorological

Water quality

>

>

>

>

Leon Creek

Leon Creek

Culebra Creek

Figure 4. Data-collection sites that provided data for Hydrological Simulation Program—FORTRAN model of the Leon Creek 

watershed, Bexar County, Texas. 
Simulation of Streamflow and Water Quality 9

Table 1. Data-collection sites that provided data for the Hydrological Simulation Program—FORTRAN model of the Leon Creek 

watershed, Bexar County, Texas.

[ddmmss, degrees minutes seconds; NWS, National Weather Service; --, not available; NEXRAD, Next Generation Weather Radar; USGS, U.S. Geological 

Survey]

Site 

number Site name Latitude(ddmmss)

Longitude

(ddmmss) Type of data Period of record used

1 NWS station 12971, KEWX, Austin/San 

Antonio, Comal County, Tex.1

29°42'--" 98°02'--" NEXRAD rainfall 1997–2004

2 NWS station 12921, KSAT, at San Antonio 

International Airport, San Antonio, Tex.

29°32'--" 98°28'--" Meterological data 1997–2004

3 NWS station 418169 at Sea World, San 

Antonio, Tex.

29°27'--" 98°42'--" Pan evaporation 1997–2004

4 USGS station 08180941 Government 

Canyon Creek site 2 near Helotes, Tex.

29°32'21" 98°45'05" Streamflow and water 

quality 

1997–2004

5 USGS station 08180945 Leon Creek at 

Scenic Loop Road near Leon Springs, Tex.

29°40'32" 98°40'33" Streamflow and water 

quality 

2001–04

6 USGS station 08181050 Leon Creek at Prue 

Road at San Antonio, Tex.

29°32'29" 98°37'54" Streamflow and water 

quality 

2001–04

7 USGS station 08181400 Helotes Creek at 

Helotes, Tex.

29°34'42" 98°41'29" Streamflow and water 

quality 

1997–2003

8 USGS station 08177720 Olmos Creek outfall 

at San Pedro Avenue, San Antonio, Tex.

29°29'27" 98°29'55" Water quality 1992–2005 

9 USGS station 08178430 Zarzamora Creek 

outfall at Alderette Park, San Antonio, 

Tex.

29°26'33" 98°34'20" Water quality 1992–2005 

10 USGS station 08178520 Harlandale Creek 

outfall at South Flores Street, San 

Antonio, Tex.

29°20'44" 98°29'08" Water quality 1992–2005 

11 USGS station 08178690 Salado Creek 

tributary at Bitters Road, San Antonio, 

Tex.

29°31'36" 98°26'25" Water quality 1992–2005 

12 USGS station 08178820 Rosillo Creek 

tributary near Rittiman Road, San 

Antonio, Tex.

29°28'29" 98°23'37" Water quality 1992–2005 

13 USGS station 08188420 Bandera Road 

outfall at Zarzamora Creek, San Antonio, 

Tex.

29°28'52" 98°35'49" Water quality 1992–2005 

14 USGS station 08181440 Ingram Road outfall 

at Leon Creek tributary, San Antonio, 

Tex.

29°26'25" 98°39'26" Water quality 1993–2005 

15 USGS station 08181480 Leon Creek at 

Interstate Highway 35 at San Antonio, 

Tex.

29°19'47" 98°35'01" Streamflow and water 

quality 

1992–2004

16 USGS sampling site 291632098305001 Leon 

Creek WWRC outfall at San Antonio, Tex.

29°16'32" 98°30'50" Water quality 1992–95 

1

 About 40 miles northwest of Leon Creek watershed.
10 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

on Environmental Quality, written commun., 2001). Water 

quality of wastewater discharge was obtained from analyses 

of 131 effluent samples collected from four major wastewater 

recycling centers that discharge to Leon Creek and the lower 

Medina and San Antonio Rivers by the USGS during 1992–95 

(Ockerman and McNamara, 2003, p. 8 and 31).

Model Configuration

The HSPF model of the Leon Creek watershed was 

configured by (1) defining subwatersheds for the Leon 

Creek watershed; (2) identifying unique HRUs on the basis 

of combinations of land use, geology and soils, and slope; 

and (3) determining initial (uncalibrated) values of HSPF 

process-related parameters. Initial estimates of parameters 

were obtained primarily from previous studies (Brown 

and Raines, 2002; Ockerman and McNamara, 2003; and 

Ockerman, 2005). 

Corresponding to each subwatershed is an HSPF stream 

segment (RCHRES). The stream-segment configuration 

(fig. 5) was developed with regard to defining segments with 

(1) similar streamflow travel times that approximate the model 

time step; (2) homogeneous channel properties such as slope 

and conveyance; and (3) outlets of subwatersheds at important 

points such as streamflow-gaging stations, tributary confluence, aquifer recharge-zone boundaries, and withdrawal or 

discharge sites. 

In each subwatershed, PERLND and IMPLND HRUs 

were categorized on the basis of combinations of three 

factors: geology-soils, land use, and land slope. The geologysoils factor involves the four predominant formation outcrops 

or groups of outcrops and the soil types of each as described 

in the “Description of Leon Creek Watershed” section. The 

Glen Rose Limestone, characterized by shallow, rocky, and 

clayey soils, occurs in all or part of subwatersheds 10, 12, 

14, 16, 18, 22, 26, 40, 42, 44, 46, 48, and 60 (fig. 5) . The 

Edwards Group, characterized by shallow- to moderate-depth 

clayey soils, occurs in all or parts of subwatersheds 20, 22, 26, 

28, 30, 32, 50, 60, and 62. The Navarro Group and Marlbrook 

Marl, Pecan Gap Chalk, and Austin Chalk, characterized by 

deep clayey soils, occur in all or parts of subwatersheds 20, 

22, 26, 28, 30, 32, 34, 50, 60, 62, 64, 66, 68, 70, and 82. The 

Leona Formation and fluviatile terrace deposits, characterized 

by deep clayey and sandy loam soils, encompass most of subwatersheds 80, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 

106, 108, and 109.

PERLND and IMPLND land-use categories were 

adapted from the 2004 land-use map shown in figure 2. 

Land slope was determined from the digital elevation models 

and input to the model as the average slope for each subwatershed. Average subwatershed slopes range from about 1 

percent in the lower, downstream areas of the watershed to 

about 7 percent in the upstream, headwater areas. Combinations of geology-soils, land use, and land slope in each of 

the 40 subwatersheds result in 520 possible unique HRU 

types. 

Streamflow losses in the Edwards aquifer recharge zone 

in the Leon Creek watershed have not been quantified to the 

level required for HSPF simulation. However, streamflow 

gain and loss measurements in the recharge zone have been 

made in nearby Salado Creek (fig. 1) (Ockerman, 2002). The 

Salado Creek data were used to develop initial estimates of 

streamflow losses in the Leon Creek watershed. HSPF does 

not include explicit process parameters or specific features 

to account for streamflow losses. For the Leon model, these 

losses were simulated as water withdrawals from the stream 

reaches. The withdrawals were then routed to groundwater 

recharge. The withdrawals are established in HSPF hydraulic routing (FTABLEs) as a function of stream discharge or 

stream volume. 

The HSPF software used for this report (version 12) 

limits a single simulation to no more than 500 model “operations.” Each unique PERLND, IMPLND, and RCHRES model 

element (as well as certain model utility functions) is considered an operation. Because the Leon Creek model comprises 

more than 500 model elements, the model was actually configured as three separate models, one for upper Leon Creek, 

one for Culebra Creek, and one for lower Leon Creek (fig. 5). 

To simulate the entire watershed, the separate models were 

run sequentially. The outputs from the upper Leon Creek and 

Culebra Creek models were used as input, or boundary conditions, for the lower Leon Creek model. However, for simplicity, reference in the remainder of the report will continue to be 

to “the Leon Creek model.” 

Model Calibration and Testing

Model calibration is the process by which initial estimates of model process-related parameters are adjusted until 

simulated streamflows and constituent concentrations and 

loads acceptably match measured (observed) data. Various 

acceptance criteria can be used, such as matching of peak 

streamflows, high streamflows, base flows, and low flows or 

combinations. The match between measured and simulated 

values can be assessed by statistics such as means, medians, 

variances, and by graphical comparisons. For the HSPF model, 

calibration is an inherently iterative procedure of parameter 

evaluation and adjustment based on comparison of measured 

and simulated values. 

Model testing involves comparing calibrated-model 

simulations with measured data that were not used in the 

calibration. For this report, model testing was done for 

hydrologic simulations at the most downstream streamflowgaging station (08181480 Leon Creek at IH 35 at San Antonio). Because of few water-quality data, model testing was not 

done for water-quality simulations.

Streamflow

A primary goal of model calibration is to adjust 

simulated streamflow to match streamflow measured at a 
Simulation of Streamflow and Water Quality 11

14

20

28

62

66

32

64 50

68 34

70

82

88

94

92

96

98

102

106

104

100

86

26

18

48

40

42

60

46

22

44

84

90

30

10

16

12

109

108

80

WILSON COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

MEDINA COUNTY

BANDERA

COUNTY

COMAL COUNTY

GUADALUPE

 COUNTY

KENDALL 

COUNTY

98°20'

98°34'

98°48'

29°42'

29°28'

29°14'

Base modified from U.S. Geological Survey

digital data 1:24,000 quadrangles

Universal Transverse Mercator Projection, Zone 14

North American Datum of 1983

0 2 4 6 8 MILES

EXPLANATION

Leon Creek watershed

Culebra Creek drainage area

Upper Leon Creek drainage area

Lower Leon Creek drainage area

Leon Creek watershed boundary

Leon Creek subwatershed boundary and associated

stream reach (RCHRES), identified by number 82

Leon Creek

Leon Creek

Culebra Creek

Figure 5. Delineated subwatersheds for the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar 

County, Texas. 
12 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

streamflow-gaging station. The Leon Creek model was 

calibrated in accordance with the guidelines of Donigian and 

others (1984) and Lumb and others (1994). The calibration 

generally entailed adjusting the process-related parameter 

values to improve the model fit. Criteria such as error in total 

streamflow volume for the calibration period, low-flow and 

high-flow distribution, and error in peak flows were used to 

evaluate how well simulated streamflow represented measured 

streamflow. For this report, simulation errors were evaluated 

by comparing total streamflow volume, 50-percent lowest 

daily flows, 10-percent highest daily flows, and peak flows for 

selected storms.

Model-fit statistics generated by the software program 

GenScn (Generation and Analysis of Model Simulation 

Scenarios) (Aqua Terra Consultants, 2007) were used to examine the quality of the model fit on annual, monthly, daily, and 

hourly bases for the (1) coefficient of determination (R2) of the 

linear regression between measured and simulated streamflow; 

(2) Nash-Sutcliff coefficient of model-fit efficiency (NSE), 

which measures the proportion of variance in the measured 

streamflow explained by the simulated streamflow (Nash and 

Sutcliff, 1970); (3) mean absolute error (MAE); and (4) root 

mean square error (RMSE). R2

 and NSE are similar because 

each provides a measure of the variation in the simulated value 

explained by the measured value. NSE, however, provides 

a generally preferable evaluation of the fit quality than does 

R2 because NSE measures the magnitude of the differences 

between measured and simulated values, whereas the R2 measures the difference between mean values (Zarriello and Ries, 

2000, p. 44). MAE and RMSE express the difference between 

measured and simulated streamflow in original units (cubic 

feet per second).

Data were obtained for model calibration, testing, and 

simulation from five USGS streamflow-gaging stations in 

the watershed. Station 08181400 Helotes Creek at Helotes, 

Tex. (site 7) (fig. 4; table 1) monitors streamflow from 15.0 

square miles in the upper part of the watershed and has been 

in operation since 1969. Station 08181480 Leon Creek at IH 

35 at San Antonio (site 15), which is on lower Leon Creek, 

monitors streamflow from 219 square miles and has been in 

operation since 1984. Three partial-record gaging stations1

also provided stormflow data used in the calibration process. 

Station 08180945 Leon Creek at Scenic Loop Road near Leon 

Springs, Tex. (site 5) monitors 10.0 square miles in the upper 

part of the watershed. Station 08180941 Government Canyon 

Creek site 2 near Helotes, Tex. (site 4) monitors 12.4 square 

miles in the upper part of the watershed. Station 08181050 

Leon Creek at Prue Road at San Antonio, Tex. (site 6), which 

is in the central part of the watershed, monitors the upper 

58.2 square miles of the Leon Creek watershed. From this 

network of five stations, selected streamflow data from the 

period 1997–2004 were used for model streamflow calibration 

1

 When few streamflow data are collected on a systematic basis over a 

period of years, the site at which the data are collected is called a partialrecord station.

and testing. All streamflow data used are stored by site in the 

USGS National Water Information System (U.S. Geological 

Survey, 2009).

The streamflow calibration process was accomplished 

beginning with the most upstream subwatersheds, using 

available gaging-station data to adjust model process-related 

parameters. Data from station 08181400 Helotes Creek at 

Helotes (site 7) (fig. 4; table 1) were used to calibrate model 

streamflow for the model subwatersheds upstream from the 

gage (subwatersheds 40, 42, 44, and 46) (fig. 5). Data from 

the station 08180945 Leon Creek at Scenic Loop Road (site 

5) were used to calibrate model subwatershed 10. Data from 

station 08180941 Government Canyon Creek near Helotes 

(site 4) were used to calibrate model subwatershed 60. The 

process-related parameter values resulting from the calibration using data from upstream gages were then applied to 

subwatersheds upstream from the next downstream gage, 

station 08181050 Leon Creek at Prue Road (site 6). Additional streamflow calibration was done using data from station 

08181050 to adjust process-related parameters for subwatersheds between stations 08180945 and 08181050. Next, the 

calibration of streamflow at station 08181480 Leon Creek at 

IH 35 (site 15) was refined by further adjusting parameters 

for subwatersheds downstream from the previously calibrated 

gaging stations. Finally, the same process-related parameters 

for subwatershed 98 associated with station 08181480 were 

applied to ungaged subwatersheds 100, 102, 104, 106, 108, 

and 109 downstream from this station. 

The Leon Creek model output at station 08181480 Leon 

Creek at IH 35 (site 15) was calibrated using available streamflow data from the period January 1, 2001, to December 31, 

2004. The response of the calibrated model was tested using 

available data from the period January 1, 1997, to December 31, 2000. Because of intermittent streamflow at gaging 

stations upstream from station 08181480 and because some 

stations were not in operation during the entire calibration and 

testing time periods, the post-calibration testing was done only 

for the model output at station 08181480. 

Streamflow calibration results for stations 08180941 

(site 4), 08180945 (site 5), 08181050 (site 6), 08181400 (site 

7), and 08181480 (site 15), and testing results for station 

08181480 (site 15), are listed in table 2. For each station, 

simulation errors and other statistics related to the quality of 

the model calibration are included. 

Simulation errors for other criteria generally were within 

acceptable limits, on the basis of applicable criteria guidelines suggested by Lumb and others (1994). Although runoff 

volumes for most storms were adequately simulated, simulated 

peak streamflows, especially those for very large floods (June 

1997, October 1998, and July 2002), were less than reported 

values. Simulated runoff volumes and simulated peak streamflows for the October 1998 flood were substantially lower than 

reported values. Measured runoff volumes at station 08181480 

Leon Creek at IH 35 for October 1998 (a short period of 

unusually high flood flows) accounted for about 49 percent 

of the total volume during the 1997–2000 testing period. 
Simulation of Streamflow and Water Quality 13

Table 2. Streamflow calibration (five gaging stations) and testing (one gaging station) results for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas.

[acre-ft, acre-feet; ft3/s, cubic feet per second; --, not applicable]

08180941 Government Canyon Creek site 2 near Helotes, Texas (site 4)

Calibration period 2003–04

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 1,750 1,860 5.9 10

Average daily mean flow rate (ft3

/s) 1.21 1.28 5.9 10

Total of highest 10 percent of daily 

flows (acre-ft)

 1,750 1,860 5.9 10

Total of lowest 50 percent of daily 

flows (acre-ft)

 0 0 0 10

Average of selected storm peaks 

(ft3

/s) (two storms)

 1,890 1,600 -15.3 --

Model-fit statistics 2003–04 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 2 24 731 17,544

Coefficient of determination (R2

) 1.00 1.00 .94 .36

Nash-Sutcliff coefficient of modelfit efficiency (NSE).99 1.00 .94 .14

Mean absolute error (ft3

/s) .07 .10 .43 .95

Root mean square error (ft3

/s) .10 .32 4.6 28

08180945 Leon Creek at Scenic Loop Road near Leon Springs, Texas (site 5)

Calibration period 2001–04

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 20,000 19,200 -4.0 10

Average daily mean flow rate (ft3

/s) 6.92 6.64 -4.0 10

Total of highest 10 percent of daily 

flows (acre-ft)

 19,800 18,300 -7.6 10

Total of lowest 50 percent of daily 

flows (acre-ft)

 0 0 0 10

Average of selected storm peaks 

(ft3

/s) (four storms)

 4,520 4,080 -9.7 --

Model-fit statistics 2001–04 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 4 48 1,461 35,064

Coefficient of determination (R2

) .98 .97 .94 .45

Nash-Sutcliff coefficient of modelfit efficiency (NSE).97 .97 .97 .20

Mean absolute error (ft3

/s) 1.2 2.4 3.4 5.0

Root mean square error (ft3

/s) 1.3 5.4 17 79
14 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Table 2. Streamflow calibration (five gaging stations) and testing (one gaging station) results for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas—Continued.

08181050 Leon Creek at Prue Road at San Antonio, Texas (site 6)

Calibration period 2001–03

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 57,100 60,900 6.5 10

Average daily mean flow rate (ft3

/s) 26.3 28.0 6.5 10

Total of highest 10 percent of daily 

flows (acre-ft)

 56,900 60,900 7.0 10

Total of lowest 50 percent of daily 

flows (acre-ft)

 0 0 0 10

Average of selected storm peaks 

(ft3

/s) (three storms)

 10,900 6,440 -40.9 --

Model-fit statistics 2001–03 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 3 36 1,095 26,280

Coefficient of determination (R2

) 1.00 1.00 .91 .71

Nash-Sutcliff coefficient of modelfit efficiency (NSE).97 .99 .91 .65

Mean absolute error (ft3

/s) 3.8 4.8 10 14

Root mean square error (ft3

/s) 5.0 11 84 200

08181400 Helotes Creek at Helotes, Texas (site 7)

Calibration period 2000–2001

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 6,270 5,930 -5.7 10

Average daily mean flow rate (ft3

/s) 4.33 4.08 -5.7 10

Total of highest 10 percent of daily 

flows (acre-ft)

 4,740 5,050 6.5 10

Total of lowest 50 percent of daily 

flows (acre-ft)

 0 0 0 10

Average of selected storm peaks 

(ft3

/s) (three storms)

 673 1,000 48.6 --

Model-fit statistics 2000–2001 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 2 24 731 17,544

Coefficient of determination (R2

) 1.00 .57 .82 .50

Nash-Sutcliff coefficient of modelfit efficiency (NSE).43 .53 .80 .50

Mean absolute error (ft3

/s) .81 3.4 3.6 3.9

Root mean square error (ft3

/s) .85 5.6 10 25
Simulation of Streamflow and Water Quality 15

Table 2. Streamflow calibration (five gaging stations) and testing (one gaging station) results for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas—Continued.

08181480 Leon Creek at Interstate Highway 35 at San Antonio, Texas (site 15)

Calibration period 2001–04

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 343,000 366,000 6.8 10

Average daily mean flow rate (ft3

/s) 118 126 6.8 10

Total of highest 10 percent of daily 

flows (acre-ft)

159,000 168,000 5.7 10

Total of lowest 50 percent of daily 

flows (acre-ft)

3,890 4,070 4.6 10

Average of selected storm peaks 

(ft3

/s) (five storms)

21,100 16,200 -23.2 --

Model-fit statistics 2001–04 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 4 48 1,461 35,064

Coefficient of determination (R2

) .99 .99 .97 .81

Nash-Sutcliff coefficient of modelfit efficiency (NSE).98 .99 .97 .79

Mean absolute error (ft3

/s) 15.5 21.6 38.0 64.6

Root mean square error (ft3

/s) 19.1 51.8 187 512

08181480 Leon Creek at Interstate Highway 35 at San Antonio, Texas (site 15)

Testing period 1997–2000

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 226,000 212,000 -6.0 10

Average daily mean flow rate (ft3

/s) 78 73 -6.0 10

Total of highest 10 percent of daily 

flows (acre-ft)

62,500 62,400 -.2 10

Total of lowest 50 percent of daily 

flows (acre-ft)

3,680 3,440 -6.5 10

Average of selected storm peaks 

(ft3

/s) (three storms)

14,100 11,600 -17.7 --

Model-fit statistics 1997–20003 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 4 47 1,430 34,320

Coefficient of determination (R2

) .94 .87 .77 .33

Nash-Sutcliff coefficient of modelfit efficiency (NSE).83 .82 .76 .01

Mean absolute error (ft3

/s) 18.8 23.5 33.6 114

Root mean square error (ft3

/s) 19.5 38.0 160 407
16 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Consequently, simulation errors during the 1997–2000 testing 

period are substantially influenced by the October 1998 flood. 

For this reason, model-fit statistics listed in table 2 excluded 

October 1998 data. Possible reasons for poor simulation of the 

October 1998 streamflow include general uncertainty in rainfall and peak streamflow values. NEXRAD rainfall data used 

as input for the Leon Creek model might have been underestimated for the October 1998 flood. October 1998 NEXRAD 

rainfall totals used for model simulation ranged from about 

11.4 to 14.8 inches. In contrast, rainfall measured at the San 

Antonio International airport (site 2) (fig. 4; table 1) during 

October 1998 was 18.1 inches. Also, the reported peak streamflow at station 08181480 for October 1998 was not directly 

measured but was estimated by indirect methods (Gandara and 

others, 2000, p. 184).

Simulated flows at the five USGS stations also were 

evaluated graphically by comparing measured and simulated 

daily time series and exceedance-probability (flow-duration) 

curves. General agreement between the measured and simulated exceedance-probability curves indicate adequate calibration over the range of flow conditions. Figures 6–8 show daily 

time series, exceedance probability curves, and scatter plots 

of measured daily and simulated daily streamflow at stations 

08180945 Leon Creek at Scenic Loop Road, 08181050 Leon 

Creek at Prue Road, and 08181480 Leon Creek at IH 35, 

respectively. The authors judge that plots for all three sites 

show reasonable agreement for the range of flow conditions. 

Also, comparison of the exceedance-probability plots between 

the three gaging stations show the differences in actual (measured) streamflow duration between the upstream and downstream parts of the Leon Creek watershed. 

Donigian and others (1984) presents general, qualitative guidelines for characterizing HSPF calibrations. For 

annual and monthly streamflow volumes, model calibration 

is considered very good when the error is less than 10 percent, good when error is 10–15 percent, and fair when error 

is 15–25 percent. By these guidelines, calibration results 

for annual streamflow volumes at all stations are very good. 

Annual, monthly, and daily R2

 and NSE values are considered 

acceptable for all stations. The NSE for daily streamflow 

volumes ranged from .80 to .97 for the calibration periods at 

all stations (table 2). The NSE for daily streamflow volumes 

Table 2. Streamflow calibration (five gaging stations) and testing (one gaging station) results for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas—Continued.

Leon Creek at Interstate Highway 35 at San Antonio, Texas – 08181480 (site 15)

Simulation period 1997–2004

Streamflow volumes and peaks Measured Simulated Error1

(percent)

Criterion2

(percent)

Total flow volume (acre-ft) 569,000 579,000 1.8 10

Average daily mean flow rate (ft3

/s) 98 100 1.8 10

Total of highest 10 percent of daily 

flows (acre-ft)

264,000 261,000 -1.1 10

Total of lowest 50 percent of daily 

flows (acre-ft)

6,040 6,640 9.9 10

Average of selected storm peaks 

(ft3

/s) (8 storms)

18,500 14,500 -21.6 --

Model-fit statistics 1997–20043 Annual Monthly Daily Hourly

Number of years, months, days, or 

hours

 7.9 95 2,891 69,384

Coefficient of determination (R2

) .99 .99 .95 .74

Nash-Sutcliff coefficient of modelfit efficiency (NSE).97 .98 .95 .70

Mean absolute error (ft3

/s) 15.7 22.5 35.8 54.9

Root mean square error (ft3

/s) 18.5 45.5 174 463

1

 Error = [(simulated – measured)/measured] x100. 

2

 Default error criteria from HSPEXP (Lumb and others, 1994). 

3

 Excludes October 1998. 
Simulation of Streamflow and Water Quality 17

Figure 6. Measured and simulated daily mean streamflow at station 08180945 Leon Creek at Scenic Loop Road near Leon Springs, 

Texas, 2001–04. 

1 10 100 1,000 10,000

MEASURED DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

SIMULATED DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

Line of equal value

0.2 0.5 2 5 1 10 20 30 50 70 80 90 95 98 99 99.5 99.8

EXCEEDANCE PROBABILITY, IN PERCENT

DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

MEASURED

SIMULATED

2001 2002 2003 2004

YEAR

DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

MEASURED

SIMULATED

10

1

100

1,000

10,000

10

1

100

1,000

10,000

10

1

100

1,000

10,000
18 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Figure 7. Measured and simulated daily mean streamflow at station 08181050 Leon Creek at Prue Road at San Antonio, Texas, 2001–03. 

1 10 100 1,000

10

1

100

1,000

10,000

10,000

MEASURED DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

SIMULATED DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

Line of equal value

1 10

10

1

100

1,000

10,000

0.2 0.5 2 5 20 30 50 70 80 90 95 98 99 99.5 99.8

EXCEEDANCE PROBABILITY, IN PERCENT

DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

MEASURED

SIMULATED

YEAR

DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

MEASURED

SIMULATED

2001 2002 2003

10

1

100

1,000

10,000
Simulation of Streamflow and Water Quality 19

Figure 8. Measured and simulated daily mean streamflow at station 08181480 Leon Creek at Interstate Highway 35 at San Antonio, 

Texas, 2001–04. 

1 10 100 1,000

10

1

100

1,000

10,000

100,000

MEASURED DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

SIMULATED DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

Line of equal value

10,000

100,000

0.2 0.5 2 5 1 10 20 30 50 70 80 90 95 98 99 99.5 99.8

EXCEEDANCE PROBABILITY, IN PERCENT

DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

10

1

100

10,000

100,000

1,000

MEASURED

SIMULATED

2001 2002 2003 2004

10

1

100

10,000

100,000

YEAR

DAILY STREAMFLOW, IN CUBIC FEET PER SECOND

MEASURED

SIMULATED

1,000
20 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

for the testing period at the Leon Creek at IH 35 station was 

.76 as opposed to .97 for the calibration period. Generally, R2

and NSE were lower for hourly simulations, especially the 

1997–2000 testing period at Leon Creek at IH 35. The NSE 

for hourly simulations ranged from .14 to .79 for the calibration periods at all stations. The NSE for hourly simulations for 

the testing period at Leon Creek at IH 35 station was .01 and is 

considered poor compared with .79 for the calibration period. 

The NSE for hourly values for the entire 1997–2004 simulation period at the Leon Creek at IH 35 station was .70, which 

is considered good.

Although streamflow is perennial at station 08181480 

Leon Creek at IH 35, streamflows in the upstream areas of 

the watershed (stations 08180941, 08180945, 08181050, and 

08181400) are intermittent. Model simulations at the outlets 

of upper Leon Creek (RCHRES 34) (fig. 5) and Culebra Creek 

(RCHRES 70) indicate that during 1997–2004, streamflow at 

these sites occurred about 20 percent of the time. The drainage 

area of the Leon Creek watershed upstream from these sites 

accounts for about two-thirds of the watershed drainage area. 

Streamflow volumes are greatly influenced by large 

storms. Two months that included major floods (October 

1998 and July 2002) accounted for about 50 percent of all the 

streamflow measured at station 08181480 Leon Creek at IH 35 

(site 15) (fig. 4) during 1997–2004. 

Wastewater discharged from the large municipal treatment facility near the mouth of Leon Creek accounted for 

a substantial part of the total streamflow exiting the Leon 

Creek watershed during 1997–2004. The average recycled 

wastewater discharge was about 39,000 acre-feet per year (M. 

Veazy, Texas Commission on Environmental Quality, written 

commun., 2001) compared with the average simulated total 

streamflow of about 127,000 acre-feet per year at the outlet of 

Leon Creek, or about 31 percent of the average simulated total 

streamflow. The median simulated streamflow at the mouth of 

Leon Creek was 66 cubic feet per second compared with 8.3 

cubic feet per second at Leon Creek at IH 35 for 1997–2004. 

Most of this difference was because of the wastewater inflow 

near the mouth of Leon Creek.

As a result of the streamflow calibration and testing process, a set of hydrology-related parameters were developed for 

the Leon Creek model. A summary of PERLND and IMPLND 

hydrology parameter values for the Leon Creek model are 

listed in table 3.

Table 3. Summary of calibrated values for selected hydrology-related parameters for the Hydrological Simulation Program—FORTRAN model of the 

Leon Creek watershed, Bexar County, Texas.

[PERLND, pervious land surface; IMPLND, impervious land surface]

Parameter Hydrologic response 

unit category Description Value

AGWETP PERLND Fraction of remaining evapotranspiration from active groundwater 0

AGWRC PERLND Basic groundwater recession rate .90–.985

BASETP PERLND Fraction of available evapotranspiration to be met from base flow 0 

CEPSC PERLND Interception storage capacity .10–.40

DEEPFR PERLND Fraction of groundwater inflow to deep recharge .15–.80

INFEXP PERLND Infiltration equation exponent 2

INFILD PERLND Ratio of maximum and mean infiltration rates 2

INFILT PERLND Index to infiltration capacity of soil .16–.25

INTFW PERLND Index to amount of water that infiltrates and flows as interflow 1.0–1.5

IRC PERLND Interflow recession coefficient .50–.60

KVARY PERLND Groundwater outflow modifier 0

LSUR PERLND or IMPLND Average length of overland-flow plane 250–300

LZETP PERLND Lower-zone evapotranspiration 0–.70

LZSN PERLND Lower-zone nominal storage 3.1–6.0

NSUR PERLND or IMPLND Average roughness coefficient for overland-flow plane .1–.35

RETSC IMPLND Impervious retention storage capacity .25

SLSUR PERLND or IMPLND Average slope of overland-flow plane .021–.072

UZSN PERLND Upper-zone nominal storage .42–.80
Simulation of Streamflow and Water Quality 21

Water Quality

The HSPF model was calibrated for selected water-quality properties and constituents using data obtained at the sites 

listed in table 1 and shown in figure 4. The model was used to 

simulate water temperature, dissolved oxygen, suspended sediment, dissolved ammonia nitrogen, dissolved nitrate nitrogen, 

and dissolved and total lead and zinc. 

PERLNDs and IMPLNDs are based on land use/cover. 

Therefore, water-quality data were needed to characterize 

runoff quality from several land-use types. Water-quality sites 

4–7 (fig. 4; table 1) are at outlets of relatively large subbasins 

in the upstream areas of the Leon Creek watershed. Data from 

these sites characterize runoff quality from rural and undeveloped land. Also, data from these sites were used to calibrate 

water-quality parameters for specific subwatersheds upstream 

from each site. Water-quality sites 8–14 are data-collection 

sites for seven relatively small, urban land-use watersheds 

sampled for a previous study during 1992–2005 to characterize runoff quality (B.L. Petri, U.S. Geological Survey, written commun., 2006). Each of the seven watersheds reflects 

a dominant land use—residential, industrial, commercial, or 

transportation corridor. Site 15 monitored most of the entire 

drainage area of the Leon Creek watershed (219 of the 238 

square miles), so water-quality data from this site integrate 

overall water quality from the entire watershed. Data from 

this site are the most important source of water-quality data 

for model calibration. Site 16 monitored water quality of 

discharge from the large wastewater treatment facility near the 

mouth of the watershed. These data characterize water quality 

for a substantial source of inflow and associated constituent 

loads in the watershed. All water-quality data used are stored 

by site in the USGS National Water Information System (U.S. 

Geological Survey, 2009).

Water Temperature

Water-temperature simulations are accomplished in HSPF 

by the HTRCH section of the RCHRES module. Changes in 

RCHRES water temperature are simulated by three major processes: (1) heat transfer through movement of water into and 

out of each RCHRES; (2) heat transfer across the air-water 

interface; and (3) heat transfer across the water-streambed 

boundary. 

Continuous (15-minute interval) water-temperature data 

are available for RCHRES 98 (site 15, Leon Creek at IH 35) 

(figs. 4, 5) for the period 1997–2000. Comparison of these 

data with simulated water temperature for this site was done to 

calibrate process-related parameters related to water temperature. Compared to available data for streamflow calibration, 

data available for water-temperature calibration were few. 

Therefore, all available data for 1997–2000 were used for 

the calibration; a post-calibration evaluation of model performance during a separate testing period was not done. Graphs 

of measured and simulated daily mean water temperatures are 

shown in figure 9. Measured and simulated mean water temperature and model-fit statistics for monthly, daily, and hourly 

simulations are listed in table 4. The average daily mean measured water temperature during 1997–2000 was 71.7 °F. The 

average simulated daily mean temperature during 1997–2000 

was 71.6 °F. R2

 values for simulated water temperature were 

.89 for daily values and .84 for hourly values. NSE values for 

simulated water temperature were .89 for daily values and .83 

for hourly values. 

RCHRES 98 (site 15, Leon Creek at IH 35) (figs. 4, 5) is 

the only reach with continuous water-temperature data available for model calibration. The calibrated values of parameters 

related to RCHRES water temperature at RCHRES 98 were 

applied to all other model RCHRES. One factor that accounts 

for differences in water temperature among RCHRESs is 

elevation. HSPF simulations include a lapse-rate calculation 

to simulate differences in air temperature because of elevation 

differences (Bicknell and others, 2001, p. 39). Air temperature, in turn, affects RCHRES water temperature. For example, 

during July 2002 the simulated average water temperature in 

RCHRES 98 (elevation about 595 feet above sea level) was 

84.3 °F. During the same period, the simulated average water 

temperature in RCHRES 10 (elevation about 1,260 feet above 

sea level) was 83.3 °F. If a RCHRES is dry, the water temperature is the same as the simulated air temperature.

Dissolved Oxygen

Dissolved oxygen simulations are accomplished in HSPF 

by the OXRX and NUTRX subroutines within the RQUAL 

RCHRES module. OXRX processes considered in the dissolved oxygen simulations include benthal oxygen release, 

reaeration, and oxygen depletion from decay of biochemical 

oxygen demand. NUTRX processes considered in the dissolved oxygen simulations include nitrification of ammonia 

and nitrite and denitrification of nitrate.

Continuous (15-minute interval) dissolved oxygen data 

are available for RCHRES 98 (site 15, Leon Creek at IH 35) 

(figs. 4, 5) for the period 1997–2000. These data, as well as 

simulated dissolved oxygen data for this site, were used to 

calibrate process-related parameters related to dissolved oxygen. Graphs of measured and simulated daily mean dissolved 

oxygen are shown in figure 10. Measured and simulated average dissolved oxygen concentrations and model-fit statistics 

are listed in table 5. The average daily mean measured dissolved oxygen during this period was 6.64 milligrams per liter 

(mg/L). The average simulated daily mean dissolved oxygen 

was 6.57 mg/L. Average simulated dissolved oxygen compares 

satisfactorily with the average measured value. However, the 

daily and hourly R2

 values and NSE values were relatively 

low. R2

 values of simulated dissolved oxygen were .39 and .30 

for daily and hourly values, respectively. NSE values of simulated dissolved oxygen were .35 and .18 for daily and hourly 

values, respectively. Similar to calibration and simulation of 

water temperature, calibrated values for dissolved oxygen 

model parameters for RCHRES 98 were applied to all other 

RCHRESs.
22 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Figure 9. Measured and simulated daily mean water temperature at station 08181480 Leon Creek at Interstate Highway 35 at San 

Antonio, Texas, 1997–2000. WATER TEMPERATURE, IN DEGREES FAHRENHEIT

MEASURED

SIMULATED

40

50

60

70

80

90

100

 JAN 

1997

JULY

1997

JAN

1998

JULY

1998

JAN

1999

JULY

1999

JAN

2000 

JULY

2000

DATE

40 50 60 70 80 90 100

40

50

60

70

80

90

100

MEASURED WATER TEMPERATURE, IN DEGREES FAHRENHEIT

SIMULATED WATER TEMPERATURE, IN DEGREES FAHRENHEIT
Simulation of Streamflow and Water Quality 23

Figure 10. Measured and simulated daily mean dissolved oxygen at station 08181480 Leon Creek at Interstate Highway 35 at San 

Antonio, Texas, 1997–2000. 

JAN

1997

JULY

1997

JAN

1998

JULY

1998

JAN

1999

JULY

1999

JAN

2000

JULY

2000

DATE

0

2

4

6

8

10

12

14

 DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

MEASURED

SIMULATED

MEASURED DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

SIMULATED DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

10 12 14

0

2

4

6

8

10

12

14

0 2 4 6 8
24 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Table 4. Water temperature calibration results for the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar 

County, Texas.

[°F, degrees Fahrenheit]

08181480 Leon Creek at Interstate Highway 35, San Antonio, Texas (site 15)

Calibration period January 1997–September 2000 

Measured and simulated values Measured Simulated Error1

(percent)

Average daily mean water temperature (°F) 71.7 71.6 -0.14

Model-fit statistics Monthly Daily Hourly

Number of months, days, or hours 45 1,368 32,856

Coefficient of determination (R2

) .97 .89 .84

Nash-Sutcliff coefficient of model-fit 

efficiency (NSE)

.97 .89 .83

Mean absolute error (°F) 1.3 2.5 4.4

Root mean square error (°F) 1.7 3.2 4.0

1

 Error = [(simulated – measured)/measured] x100. 

Table 5. Dissolved oxygen calibration results for the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar 

County, Texas.

[mg/L, milligrams per liter]

08181480 Leon Creek at Interstate Highway 35, San Antonio, Texas (site 15)

Calibration period January 1997–September 2000

Measured and simulated values Measured Simulated Error1

(percent)

Average daily mean dissolved oxygen (mg/L) 6.64 6.57 -0.11

Model-fit statistics Monthly Daily Hourly

Number of months, days, or hours 45 1,368 32,856

Coefficient of determination (R2

) .51 .39 .30

Nash-Sutcliff coefficient of model-fit 

efficiency (NSE)

.50 .35 .18

Mean absolute error (mg/L) 1.2 1.5 1.7

Root mean square error (mg/L) 1.6 1.9 2.2

1

 Error = [(simulated – measured)/measured] x100. 
Simulation of Streamflow and Water Quality 25

Suspended Sediment

Suspended sediment was simulated using the HSPF 

secondary modules SEDMNT for PERLND simulation, 

SOLIDS for IMPLND simulation, and SEDTRN for RCHRES 

simulation. For each PERLND, the processes of detachment 

of sediment from the soil matrix and washoff of this sediment 

are simulated in SEDMNT on the basis of rainfall intensity, 

surface runoff, and the model parameters that control the 

accumulation, detachment, and transport of soils. For each 

IMPLND, the processes of accumulation and washoff of sediment are simulated in the SOLIDS module. In each RCHRES, 

the sediment-transport processes in the stream channel are 

accomplished by the SEDTRN module. Transport processes 

in RCHRES include deposition and scour and are functions 

of the sediment size, settling velocity, density, erodibility, bed 

depth, and critical shear stress. RCHRES sediment transport is 

computed separately for each fraction of sediment size—sand, 

silt, and clay. 

Selection of initial values and calibration of sedimentrelated process parameters of the SEDMNT, SOLIDS, and 

SEDTRN modules (appendix 1) were based on guidelines of 

Donigian and Love (2003) and U.S. Environmental Protection Agency (2006). Calibration of sediment-related process 

parameters involved three steps: (1) Estimating soil-erosion 

parameters that affect generation of sediment flux from 

PERLNDs and sediment-washoff parameters that affect 

generation of sediment flux from IMPLNDs; (2) calibrating 

sediment-transport (RCHRES) parameters by comparison 

of simulation results with available monitoring data from 

upstream stations; and (3) further adjusting selected sedimentrelated process parameters on the basis of comparison of measured and simulated data at the most downstream streamflowgaging station. 

To develop estimates of soil-erosion and sedimentwashoff parameters for the various PERLNDs and IMPLNDs, 

respectively, simulated suspended-sediment runoff concentrations from representative PERLNDs and IMPLNDs were 

compared with measured concentrations from the seven relatively small, urban land-use watersheds (sites 8–14) (fig. 4; 

table 1) sampled for a previous study, as noted at the beginning of the “Water Quality” section. First, median suspendedsediment runoff concentrations were computed for each of 

four land-use types (residential, industrial, commercial, transportation corridor) represented in the measured data from sites 

8–14. Then, iteratively, PERLND and IMPLND sedimentrelated parameters were adjusted until simulated concentrations of suspended sediment in PERLND and IMPLND 

runoff matched the target runoff concentrations from the 

measured data. Table 6 shows simulated suspended-sediment 

concentrations for four PERLND and IMPLND land-use 

types compared with median concentrations from the measured data. 

After calibrating estimates for PERLND and IMPLND 

parameters related to soil erosion and sediment washoff, the 

parameters related to sediment transport were calibrated by 

comparing simulated suspended-sediment concentrations and 

loads to computed concentrations and loads from measured 

data collected at USGS gaging stations 08180941 Government 

Canyon Creek near Helotes (site 4) (fig. 4; table 1), 08180945 

Leon Creek at Scenic Loop Road (site 5), 08181050 Leon 

Creek at Prue Road (site 6), and 08181400 Helotes Creek at 

Helotes (site 7) (hereinafter, referred to as the upper-watershed 

sites). Because of intermittent flow at the upper-watershed 

sites and relative scarcity of suspended-sediment data, calibrations were based on selected stormflows. Stormflows were 

selected for calibration of sediment transport on the basis of 

adequate agreement (a judgment call) between measured and 

simulated streamflow. Unreliable suspended-sediment calibration could result if the measured and simulated streamflow 

hydrographs were not reasonably congruent. As a result, only 

three stormflows were considered for sediment calibration 

at the upper-watershed sites (none were available for site 7). 

Generally, for each upper-watershed site, each PERLND and 

IMPLND land-use type in all subwatersheds upstream from 

a particular site were assigned the same parameter values. 

For example, all PERLNDS upstream from site 5 that represent residential land use were assigned the same sedimenttransport parameter values. Because no suspended-sediment 

data for site 7 were available for calibration, parameters for 

adjacent upper-watershed sites (site 4 or 5) were used. Table 7 

shows calibration results at the upper-watershed sites, including a comparison of measured and simulated flow volumes, 

suspended-sediment concentrations, and suspended-sediment 

loads. 

Further adjusting of selected sediment-related process 

parameters was done on the basis of comparison of measured 

and simulated data for streamflow-gaging station 08181480 

Leon Creek at IH 35 (site 15). Calibration of sedimentrelated process parameters for site 15 involved adjustment of 

PERLND, IMPLND, and RCHRES parameters (appendix 1) 

for subwatersheds downstream from the upper-watershed sites. 

Twenty suspended-sediment samples (and associated 

streamflows) were collected during 1992–97 and 2007. 

Most of these samples were collected outside the 1997–2004 

simulation period; thus a direct comparison of measured and 

simulated concentrations, or loads, was not possible. Instead, 

loads computed from streamflows and suspended-sediment 

concentrations for the 20 samples were used to develop a 

regression relation between daily mean streamflow and daily 

suspended-sediment load at station 08181480 Leon Creek at 

IH 35 (site 15) (fig. 11). From this relation, estimated daily 

sediment loads at site 15 were computed for 2001–04. These 

estimated daily sediment loads were used as targets for HSPF 

suspended-sediment calibration and simulations at site 15. 

The relation between estimated monthly suspendedsediment loads (computed from regression relation in fig. 

11) and HSPF-simulated monthly suspended-sediment loads 

at Leon Creek at IH 35 (site 15) for the period 2001–04 is 

shown in figure 12. The HSPF-simulated suspended-sediment 

load during the period was 77,700 tons compared with an 

estimate of 74,600 tons from the relation in figure 11 (less 

than 5-percent difference; coefficient of determination, .869). 
26 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

July 2002 data were excluded from the relation in figure 12 

and in the comparison of simulated and estimated loads for 

2001–04 because of major storms and resulting floods then. 

The HSPF-simulated sediment load for July 2002 was 117,000 

tons, greatly exceeding the load for the entire 2001–04 period 

excluding July 2002. However, that value is unreliable because 

of the inability of the model to adequately simulate extremely 

high streamflows, as noted in the “Streamflow” section. The 

match between HSPF-simulated loads and loads estimated 

from regression is closer (smaller percentage error) for months 

of relatively high streamflow and sediment load than for 

months of relatively low streamflow and sediment load.

Dissolved Ammonia Nitrogen and Dissolved Nitrate 

Nitrogen

Nitrogen in outflow of constituents from pervious and 

impervious land segments is simulated using the PQUAL and 

IQUAL subroutines (within modules PERLND and IMPLND, 

respectively). PQUAL and IQUAL simulation routines used 

for the Leon Creek model include rainfall deposition; accumulation, storage, and washoff of constituents; and simulation of 

constant interflow and groundwater constituent concentrations. 

Instream simulation of nitrogen is accomplished through 

the RQUAL subroutine of the RCHRES module. RQUAL 

processes used in the Leon Creek model include ammonia 

volatilization, denitrification, and ammonia adsorption.

Wet deposition (rainfall) data in the form of weekly 

ammonia and nitrate concentrations for the period 1992–2004 

were obtained from the National Atmospheric Deposition Program stations at Beeville and Sonora, Tex. (fig. 1) (National 

Atmospheric Deposition Program, 2006). Median concentrations were computed from the sample data (0.24 mg/L 

dissolved ammonia nitrogen and 0.23 mg/L dissolved nitrate 

nitrogen) and used as input rainfall concentrations for wet 

deposition onto PERLND, IMPLND, and RCHRES segments 

for the HSPF simulations. Dry deposition was not simulated 

explicitly because local information on dry atmospheric 

loadings of ammonia and nitrate was not available. However, 

through calibration of parameters related to accumulation, 

DAILY MEAN STREAMFLOW (Q), IN CUBIC FEET PER SECOND

0.5 1 10 100 1,000 10,000

SUSPENDED-SEDIMENT LOAD (L), IN TONS PER DAY

0.01

0.1

1

10

100

1,000

10,000

L=0.012 x Q1.626

R2=.869

Load estimated by least-squares

regression

Load computed from measured

streamflow and sample

suspended-sediment concentration

Figure 11. Relation between daily mean streamflow and daily suspended-sediment load based on 20 streamflow-concentration data 

pairs for station 08181480 Leon Creek at Interstate Highway 35 at San Antonio, Texas, 1992–2007. 
Simulation of Streamflow and Water Quality 27

storage, and washoff, deposition for nutrients is represented in 

the general constituent simulations. 

The method used to develop initial estimates of dissolved 

ammonia and nitrate nitrogen accumulation, storage, and 

washoff parameters for the various PERLNDs and IMPLNDs 

was similar to the method used to develop soil-erosion and 

sediment-washoff parameter estimates. Median dissolved 

ammonia and dissolved nitrate concentrations were computed 

for each of the four land-use types (residential, industrial, 

commercial, transportation corridor) from measured runoff 

concentration data from sites 8–14 that reflect small, urban 

land-use watersheds. Simulated concentrations from selected 

PERLNDs and IMPLNDs were compared with median 

measured runoff concentration values in an iterative series of 

simulations. Values for parameters related to accumulation, 

storage, and washoff of ammonia and nitrate were adjusted 

until simulated concentrations adequately matched median 

measured concentrations. 

These initial parameter values for PERLND and 

IMPLND processes, as well as parameters related to instream 

(RCHRES) processes, were further adjusted in a manner 

similar to that for calibration of the parameters related to 

sediment transport—that is, by comparing simulated ammonia 

and nitrate concentrations and loads with measured concentrations and computed streamflow loads for selected stormflows 

at the upper-watershed sites (4–7) (fig. 4; table 1). Calibration 

results for dissolved ammonia nitrogen and dissolved nitrate 

nitrogen for the upper-watershed sites are listed in tables 8 and 

9, respectively.

At station 08181480 Leon Creek at IH 35 (site 15), only 

two water-quality samples were collected and analyzed by the 

USGS for dissolved ammonia nitrogen and dissolved nitrate 

nitrogen during the 1997–2004 period; thus calibration of 

HSPF parameters related to nutrient simulations in the lower 

part of the watershed is based on few measured data. Measured and simulated nutrient concentrations are compared in 

1 10 100 1,000 10,000 100,000

ESTIMATED MONTHLY SUSPENDED-SEDIMENT LOAD, IN TONS

1

10

100

1,000

10,000

100,000

HYDROLOGICAL SIMULATION PROGRAM—FORTRAN-SIMULATED

MONTHLY SUSPENDED-SEDMENT LOAD, IN TONS

Line of equal value

Figure 12. Relation between estimated monthly suspended-sediment loads and Hydrological Simulation Program—FORTRANsimulated suspended-sediment loads for station 08181480 Leon Creek at Interstate Highway 35 at San Antonio, Texas, 2001–04. 
28 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

table 10. Although the number of samples is small, simulated 

concentrations of dissolved ammonia nitrogen and dissolved 

nitrate nitrogen closely matched measured concentrations 

(within 0.1 mg/L).

Average simulated monthly mean dissolved ammonia 

nitrogen and dissolved nitrate nitrogen concentrations in 

streamflow at station 08180945 Leon Creek at Scenic Loop 

Road (site 5) during 1997–2004 were 0.05 and 0.25 mg/L, 

respectively. The concentration of nitrate is similar to median 

rainfall concentrations of nitrate (0.23 mg/L). Streamflow 

concentrations of ammonia are much lower than rainfall 

concentrations because ammonia in rainfall and runoff quickly 

undergoes volatilization to nitrite and nitrate (Bowie and 

others, 1985). At station 08181480 Leon Creek at IH 35 (site 

15), average simulated monthly mean concentrations of dissolved ammonia and nitrate concentrations during 1997–2004 

were 0.03 and 0.37 mg/L, respectively. Simulated concentrations of dissolved ammonia and nitrate in streamflow were 

relatively low at most stream reach outlets. However, dissolved 

nitrate concentrations were much greater at the most downstream reach (RCHRES 109) because of the inflow of wastewater. The average nitrate concentration associated with the 

wastewater was 15 mg/L. This inflow greatly increases nitrate 

concentrations at the mouth of Leon Creek. The average 

simulated monthly mean concentration of nitrate at RCHRES 

109 during 1997–2004 was 7.4 mg/L. For comparison, just 

Table 6. Simulated 1992–2004 suspended-sediment runoff concentrations from four pervious and impervious land-use types and 

1992–2004 medians of measured suspended-sediment runoff concentrations from watersheds that represent the respective land-use 

types (sites 8–14) for the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas. 

[in/acre, inches per acre; mg/L, milligrams per liter]

Land use Percent 

pervious

Percent 

impervious

Simulated pervious Simulated impervious Simulated total Median 

measured 

concentration 

(mg/L)

Runoff

(in/acre)

Concentration

(mg/L)

Runoff

(in/acre)

Concentration

(mg/L)

Runoff

(in/acre)

Concentration

(mg/L)

Residential 48 52 148 214 351 173 258 184 184

Commercial 27 73 148 214 351 196 294 198 198

Industrial 20 80 148 181 351 156 310 157 157

Transportation corridor 16 84 148 219 351 156 328 160 160

Table 7. Measured and simulated suspended-sediment concentrations and loads at upper-watershed sites for selected stormflows for 

the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas.

[ft3/s, cubic feet per second; mg/L, milligrams per liter]

Stormflow 

date

Daily mean streamflow

(ft3/s)

Suspended-sediment concentration

(mg/L)

Suspended-sediment load

(tons)

Error1

 in simulated 

suspended-sediment load

Measured Simulated Measured Simulated Measured Simulated (percent)

08180941 Government Canyon Creek site 2 near Helotes, Texas (site 4)

August 23, 

2004

34 30 185 163 17.0 16.5 -2.7

08180945 Leon Creek at Scenic Loop Road near Leon Springs, Texas (site 5)

October 24, 

2002

87 101 154 119 36.1 46.8 29.5

08181050 Leon Creek at Prue Road at San Antonio, Texas (site 6)

October 24, 

2002

253 360 171 169 117 163 39.7

1

 Error = [(simulated – measured)/measured] x100. 
Simulation of Streamflow and Water Quality 29

Table 8. Measured and simulated dissolved ammonia nitrogen concentrations and loads at upper-watershed sites for selected 

stormflows for the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas.

[ft3/s, cubic feet per second; mg/L, milligrams per liter; lb, pounds; <, less than]

Stormflow date

Daily mean streamflow

(ft3

/s)

Dissolved ammonia 

nitrogen concentration

(mg/L)

Dissolved ammonia 

nitrogen load

(lb)

Error1

 in simulated dissolved 

ammonia nitrogen load

(percent) Measured Simulated Measured Simulated Measured Simulated 

08180941 Government Canyon Creek site 2 near Helotes, Texas (site 4)

August 23, 2004 34 30 <0.04 0.03 7.3 5.3 -28

08180945 Leon Creek at Scenic Loop Road near Leon Springs, Texas (site 5)

October 24, 2002 87 101 <.04 .04 19 27 42

08181050 Leon Creek at Prue Road at San Antonio, Texas (site 6)

October 24, 2002 253 360 <.04 .05 55 96 75

1 Error = [(simulated – measured)/measured] x100. 

Table 9. Measured and simulated dissolved nitrate nitrogen concentrations and loads at upper-watershed sites for selected 

stormflows for the Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas.

[ft3/s, cubic feet per second; mg/L, milligrams per liter; lb, pounds]

Stormflow date

Daily mean streamflow

(ft3

/s)

Dissolved nitrate 

nitrogen concentration

(mg/L)

Dissolved nitrate 

nitrogen load

(lb)

Error1

 in simulated dissolved 

nitrate nitrogen load

(percent) Measured Simulated Measured Simulated Measured Simulated

08180941 Government Canyon Creek site 2 near Helotes, Texas (site 4)

August 23, 2004 34 30 0.38 0.10 70 24 -66

08180934 Leon Creek at Scenic Loop Road near Leon Springs, Texas (site 5)

October 24, 2002 87 101 .10 .11 47 78 66

08181050 Leon Creek at Prue Road at San Antonio, Texas (site 6)

October 24, 2002 253 360 .34 .22 464 420 -9.5

1

 Error = [(simulated – measured)/measured] x100.

Table 10. Measured and simulated concentrations of dissolved ammonia nitrogen and dissolved nitrate nitrogen for selected 

stormflows at station 08181480 Leon Creek at Interstate Highway 35 at San Antonio, Texas.

[ft3/s, cubic feet per second; mg/L, milligrams per liter]

Stormflow date

Daily mean streamflow

(ft3

/s)

Dissolved ammonia nitrogen 

concentration

(mg/L)

Dissolved nitrate nitrogen 

concentration

(mg/L)

Measured Simulated Measured Simulated Measured Simulated

April 2, 1997 113 442 0.04 0.04 0.41 0.51

April 26, 1997 160 123 .11 .12 .79 .63
30 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

upstream from the recycled wastewater inflow at the outlet of 

RCHRES 108, the simulated average monthly mean concentration of nitrate was 0.36 mg/L. 

Dissolved and Total Lead and Dissolved and Total Zinc

Lead and zinc transport from land surfaces is simulated 

by accumulation and washoff of the dissolved form and the 

constituent associated with sediment. These processes are 

accomplished by the PQUAL and IQUAL modules (within 

PERLND and IMPLND modules, respectively). Interflow and 

groundwater inflow of lead and zinc are simulated by input of 

constant concentration values assigned to simulated interflow 

and groundwater inflow. Instream changes in metals concentrations are simulated by the GQUAL module within the 

RCHRES module. GQUAL simulates dissolved constituent 

concentrations and concentrations associated with sand, silt, 

and clay. Process-related parameters affecting distribution of 

metals between the dissolved phase and sediment adsorption 

include partitioning coefficients and adsorption/desorption 

rate parameters (Allison and Allison, 2005). Because of the 

availability of some sample data that included analyses of dissolved and total metal concentrations, initial estimates of these 

parameters could be adjusted to better calibrate simulated 

dissolved and total lead and zinc concentrations to measured 

values.

Initial estimates for daily accumulation of lead and zinc 

onto land surfaces were values from Paode and others (1998). 

Calibration was accomplished by comparing simulated concentrations and loads to measured concentrations and loads at 

the upper-watershed sites (tables 11, 12). During the simulation period (1997–2004), 31 samples for lead and 35 samples 

for zinc were collected and analyzed from the four upperwatershed sites. Most of the samples were analyzed for the 

total constituent (dissolved plus adsorbed to sediment). About 

one-half of the samples also included analyses of the dissolved 

phase of the constituent. 

Generally, simulated lead concentrations compared 

adequately with measured concentrations in 1997–2004 stormflows at the upper-watershed sites (table 11). Process-related 

parameters controlling accumulation and washoff of lead were 

relatively consistent for all four sites. Accumulation rates for 

the subwatershed monitored at station 08180941 Government Canyon Creek near Helotes (site 4) (fig. 4; table 1) were 

slightly lower than the rates for subwatersheds monitored at 

the other three stations because that subwatershed includes 

relatively more undeveloped land (including a State natural 

area and park). Most measured dissolved lead concentrations 

for the four upper-watershed sites were less than or near the 

laboratory reporting level of 0.08 microgram per liter (μg/L); 

and the median simulated dissolved lead concentration for 

those sites was 0.05 μg/L. The median measured total lead 

concentration for the four sites was 2.9 μg/L, and the median 

simulated concentration was 4.2 μg/L. 

Overall, simulated total zinc concentrations for the 

upper-watershed sites compared adequately with measured 

data (table 12). However, dissolved zinc was under-simulated. 

Process-related parameters for accumulation and washoff of 

zinc were consistent for most of the upstream subwatersheds. 

The exception is subwatershed 10 (fig. 5), which is monitored by station 08180945 Leon Creek at Scenic Loop Road 

(site 5) (fig. 4) at the outlet of RCHRES 10. Analyses from six 

stormflow samples (collected during 2002–07) yielded total 

and dissolved zinc concentrations that were roughly an order 

of magnitude greater than measured concentrations from the 

other upper-watershed sites (collected during 1997–2004). 

The reason for greater zinc concentrations in subwatershed 

10 is not known. Monitoring station 08181050 Leon Creek at 

Prue Road (site 6) is at the outlet of RCHRES 28 downstream 

from RCHRES 10. Samples collected at site 6 had lower zinc 

concentrations than samples collected at site 5; and measured 

concentrations at site 6 were more similar to measured concentrations at sites 4 and 7 than to those at site 5. Therefore 

zinc accumulation and washoff parameters were adjusted to 

simulate higher zinc runoff concentrations from RCHRES 10 

and associated PERLNDs and IMPLNDs than from downstream RCHRES and associated PERLNDs and IMPLNDs. 

The median measured dissolved zinc concentration for the 

four sites was 2.2 μg/L, and the median simulated concentration was 0.38 μg/L. The median measured total zinc concentration for the four sites was less than 25 μg/L, and the median 

simulated concentration was 9.9 μg/L.

At the most downstream streamflow station (08181480 

Leon Creek at IH 35) (site 15), only three samples were 

collected and analyzed for lead and zinc during 1997–2002 

(all in 1997). The samples were analyzed only for the total 

form of the constituents. Simulated total lead and total zinc 

concentrations were much less than measured concentrations 

for two samples collected in April 1997 (table 13). For the 

October 1997 sample, the simulated total lead concentration 

closely matched the measured concentration, but the simulated 

total zinc concentration was much greater than the measured 

concentration. 

Because of the relatively large differences between 

most of the measured and simulated values and the few data 

available for 1997–2002, additional sample data for site 15 

outside the simulation period were analyzed. Seven additional 

stormflow samples collected and analyzed for total lead and 

total zinc during 1993–96 were available. Because simulated 

total lead and total zinc concentrations were not available for 

comparison with measured concentrations from these seven 

samples, median concentrations of all 10 available measured 

sample concentrations for 1993–97 are compared with median 

simulated concentrations for the three 1997 stormflows 

(table 13). The matches between measured and simulated still 

are not particularly close: The median measured total lead 

concentration for the 1993–97 stormflow data is 6.5 μg/L, and 

the median simulated total lead concentration for the three 

1997 stormflows is 14 μg/L. The median measured total zinc 

concentration for the 1993–97 stormflow data is 25 μg/L, and 

the median simulated total zinc concentration for the three 

1997 stormflows is 47 μg/L.
Simulation of Streamflow and Water Quality 31

Table 11. Measured and simulated concentrations of dissolved and total lead at upper-watershed sites for selected stormflows for the 

Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas.

[ft3/s, cubic feet per second; μg/L, micrograms per liter; --, no data or not applicable; <, less than; E, estimated1]

Stormflow date

Daily mean streamflow

(ft3

/s)

Total lead concentration

(μg/L)

Dissolved lead concentration

(μg/L)

Measured Simulated Measured Simulated Measured Simulated

08180941 Government Canyon Creek site 2 near Helotes, Texas (site 4)

June 23, 1997 -- 152 <1.0 2.7 -- 0.08

June 24, 1997 -- 57 <1.0 2.0 -- .10

March 16, 1998 -- 26 3.3 3.9 -- .08

August 22, 1998 -- 2.8 1.3 5.1 -- .06

October 17, 1998 -- 645 9.7 17 -- .01

October 23, 2000 557 659 16.7 14 -- .01

July 2, 2002 -- 2,020 2.9 5.8 0.13 .01

June 28, 2004 3.8 37 1.4 6.7 <.08 .04

August 23, 2004 43 30 3.4 4.5 <.08 .05

 Median concentration 2.2 5.1 <.08 .05

08180945 Leon Creek at Scenic Loop Road near Leon Springs, Texas (site 5)

July 1, 2002 462 367 2.4 8.6 <.08 .02

October 24, 2002 87 101 3.4 3.8 E.05 .11

 Median concentration 2.9 6.2 <.08 .06

08181050 Leon Creek at Prue Road at San Antonio, Texas (site 6)

March 14, 2001 1.5 1.7 4.4 1.1 .09 .05

October 13 2001 253 351 4.2 3.4 <.08 .02

November 15, 2001 1,430 983 10 11 E.04 .03

April 7, 2002 3.7 14 2.4 2.2 .09 .03

June 30, 2002 129 478 2.3 5.2 <.08 .02

October 24, 2002 253 360 2.0 3.4 <.08 .03

February 20, 2003 6.8 29 2.2 1.9 .12 .04

May 1, 2004 -- 6.3 1.8 1.8 .06 .04

 Median concentration 2.4 2.8 <.08 .03

08181400 Helotes Creek at Helotes, Texas (site 7)

April 4, 1997 14 23 6.6 1.4 -- .09

June 22, 1997 1,040 918 6.1 13 -- .07

June 25, 1997 149 134 <1.0 1.8 -- .09

January 6, 1998 .1 7.6 1.6 2.2 -- .12

August 22, 1998 2.4 46 4.2 4.4 -- .07

October 17, 1998 1,340 688 9.8 14 -- .07

June 21, 1999 12.6 12 8.9 3.3 -- .10

June 10, 2000 .5 15 6.1 2.7 -- .07

July 1, 2002 580 919 1.9 13 <.08 .29

July 2, 2002 641 2,150 28 9.3 E.07 .10

October 24, 2002 61 178 <1.0 5.1 <.08 .05

July 5, 2003 6.2 34 2.1 4.2 .24 .01

 Median concentration 5.2 4.3 <.08 .08

1

 Estimated concentrations are concentrations measured between the laboratory reporting level and the long-term method detection level (Childress and others, 

1999).
32 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Table 12. Measured and simulated concentrations of dissolved and total zinc at upper-watershed sites for selected stormflows for the 

Hydrological Simulation Program—FORTRAN model of the Leon Creek watershed, Bexar County, Texas.

[ft3/s, cubic feet per second; μg/L, micrograms per liter; --, no data or not applicable; <, less than]

Stormflow date

Daily mean streamflow

(ft3

/s)

Total zinc concentration

(μg/L)

Dissolved zinc concentration

(μg/L)

Measured Simulated Measured Simulated Measured Simulated 

08180941 Government Canyon Creek site 2 near Helotes, Texas (site 4)

June 23, 1997 -- 152 <10 5.0 -- 0.29

June 24, 1997 -- 57 <10 2.9 -- .29

March 16, 1998 -- 26 10 8.0 -- .49

August 22, 1998 -- 2.8 <10 7.0 -- .29

October 17, 1998 -- 645 30 36 -- .19

October 23, 2000 557 659 25 33 -- .20

July 2, 2002 -- 2,020 <25 13 1.6 .10

June 28, 2004 3.8 37 5 24 .70 .43

August 23, 2004 42 30 9 9.9 .70 .38

 Median concentration <10 9.9 .70 .30

08180945 Leon Creek at Scenic Loop Road near Leon Springs, Texas (site 5)

July 1, 2002 462 367 250 240 30 3.1

October 24, 2002 87 101 496 220 46 16

November 16–17, 2004 -- 649 36

May 6, 2006 -- -- 834 -- 100 --

March 26, 2007 -- -- -- -- 155 --

March 30, 2007 -- -- 502 -- 130 --

 Median concentration 375 230 38 9.4

08181050 Leon Creek at Prue Road at San Antonio, Texas (site 6)

March 14, 2001 1.5 1.7 -- 1.3 4.3 .48

October 13 2001 253 351 -- 22 1.5 .57

November 15, 2001 1,430 983 -- 46 1.0 .78

April 7, 2002 3.7 14 -- 36 3.2 1.8

June 30, 2002 129 478 <25 16 1.6 .29

October 24, 2002 253 360 <25 24 1.0 .61

February 20, 2003 6.8 28.7 <25 3.4 2.3 .41

May 1, 2004 -- 6.3 11 2.6 2.2 .45

 Median concentration <25 16 2.2 .48

08181400 Helotes Creek at Helotes, Texas (site 7)

April 4, 1997 14 23 20 1.7 -- .27

June 22, 1997 1,040 918 20 26 -- 1.0

June 25, 1997 149 134 <10 2.6 -- .28

January 6, 1998 .1 7.6 20 1.9 -- .29

August 22, 1998 2.4 46 <10 4.6 -- .28

October 17, 1998 1,340 688 30 29 -- 1.17

June 21, 1999 13 12 21 3.1 -- .28

June 10, 2000 .5 15 6.1 2.5 -- .25

July 1, 2002 580 919 17 24 1.2 1.69

July 2, 2002 641 2,150 58 11 <1.0 .14

October 24, 2002 60 178 <25 5.3 2.2 .87

July 5, 2003 6.2 34 8 3.8 2.7 .22

 Median concentration 5.2 4.2 1.7 .28
Simulation of Streamflow and Water Quality 33

Besides the 10 stormflow samples for total lead and 

total zinc collected at station 08181480 Leon Creek at IH 35 

(site 15) during 1993–97, nine base-flow samples also were 

collected and analyzed for total lead and total zinc during 

1992–94. Summary statistics for these samples are listed in 

table 14. The median measured total lead concentration was 

1.0 μg/L, and the median measured total zinc concentration 

was 10 μg/L. These median concentrations were used as target 

values for calibration of simulated total lead and total zinc during base-flow periods. 

Summary statistics for simulated daily mean total lead 

and total zinc concentrations for base-flow periods at site 

15 during 1997–2004 are listed in table 15. The summary 

statistics were compiled for 2,041 days when the daily mean 

streamflow at site 15 was less than 20 cubic feet per second. 

For these base-flow days, average and median concentrations 

of simulated total lead (1.3 and 0.96 mg/L, respectively) and 

total zinc (13 and 9.8 mg/L, respectively) match the corresponding measured concentrations (table 14) closely, although 

maximums not as well. 

As a result of the water-quality calibration process, a 

set of water-quality-related parameters was developed for the 

Leon Creek model. A summary of the calibrated parameter 

values for the Leon Creek model is in table 16.

Simulation of Constituent Loads and Yields

To demonstrate an application of the Leon Creek HSPF 

model, streamflow constituent loads and yields for three 

selected constituents were simulated: suspended sediment, dissolved nitrate nitrogen, and total lead. Mean annual (1997–

2004) streamflow and annual constituent loads and yields were 

simulated at the mouth of Leon Creek (outlet of the watershed). These results are listed in table 17.

The annual suspended-sediment load ranged from 7,900 

tons in 2003 (year of lowest annual streamflow) to 163,000 

tons in 2002 (year of highest annual streamflow). The average load was 51,800 tons per year. The corresponding annual 

suspended-sediment yields ranged from 0.05 ton per acre in 

Table 13. Measured and simulated concentrations of total lead and total zinc for selected stormflows at station 08181480 Leon Creek 

at Interstate Highway 35 at San Antonio, Texas.

[ft3/s, cubic feet per second; μg/L, micrograms per liter; <, less than. Median simulated concentrations for three 1997 stormflows and median measured concentrations for 1993–97 stormflows shown in bold for comparison.]

Stormflow date

Daily mean streamflow

(ft3

/s)

Total lead concentration

(μg/L)

Total zinc concentration

(μg/L)

Measured Simulated Measured Simulated Measured Simulated

April 2, 1997 113 442 37 7.5 120 27

April 26, 1997 160 123 64 28 190 56

October 10, 1997 108 493 12 14 <10 47

 Median measured stormflow-sample 

 concentration (and number of 

 samples) for 1993–97 

6.5 (10) -- 25 (10) --

Table 14. Summary statistics for measured streamflow and 

concentrations of total lead and total zinc for nine base-flow 

samples collected during 1992–94 at station 08181480 Leon Creek 

at Interstate Highway 35 at San Antonio, Texas.

[ft3/s, cubic feet per second; μg/L, micrograms per liter; <, less than]

Streamflow 

or 

constituent

Number of 

samples

Average Median Maximum Minimum

Streamflow 

(ft3

/s)

9 13 10 27 3.3

Total lead 

(μg/L)

9 <1.0 1.0 1.0 <1.0

Total zinc 

(μg/L)

9 <14 10 30 <10

Table 15. Summary statistics for simulated daily mean total lead 

and total zinc concentrations for base-flow periods1 at station 

08181480 Leon Creek at Interstate Highway 35 at San Antonio, 

Texas, 1997–2004.

[μg/L, micrograms per liter; ft3/s, cubic feet per second]

Constituent Number 

of days Average Median Maximum Minimum

Total lead 

(μg/L)

2,041 1.3 0.96 20 0.54

Total zinc 

(μg/L)

2,041 13 9.8 167 .58

1

Base-flow periods are days when simulated daily mean streamflow is less 

than 20 ft3/s; during 1997–2004, 2,041 of 2,922 days met this criterion.
34 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Table 16. Summary of calibrated values for selected water-quality parameters for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas, 1997–2004—Continued.

Parameter Model module Description Value Unit

Water temperature

CFSAEX RCHRES Correction factor for solar radiation, the fraction of RCHRES 

surface exposed to solar radiation

0.45 --

KATRAD RCHRES Longwave radiation coefficient 9.0 --

KCOND RCHRES Conduction-convection heat transport coefficient 3.0 --

KEVAP RCHRES Evaporation coefficient 2.0 --

Dissolved oxygen

IDOXP PERLND Concentration of dissolved oxygen in interflow 5.0–8.0 milligram/liter

ADOXP PERLND Concentration of dissolved oxygen in base flow 4.0–8.0 milligram/liter

KBOD20 RCHRES Unit biochemical oxygen demand decay rate at 20 °C .004 /hour

KODSET RCHRES Rate of biochemical oxygen demand settling .027 foot/hour

SUPSAT RCHRES Maximum allowable dissolved oxygen supersaturation factor 1.15 --

BENOD RCHRES Benthal oxygen demand at 20 °C 75 milligram/square meterhour

BRBOD(1) RCHRES Benthal release rate of biochemical oxygen demand under 

anaerobic conditions

.001 milligram/square meterhour

BRBOD(2) RCHRES Increment to benthal release rate of biochemical oxygen 

demand under anaerobic conditions

.001 milligram/square meterhour

EXPREL RCHRES Exponent in dissolved oxygen term of benthal biochemical 

oxygen demand release equation

2.82 --

Suspended sediment

KRER PERLND Coefficient of soil-detachment equation .01–.30 complex

JRER PERLND Exponent of soil-detachment equation 2.0–2.5 complex

KSER PERLND Coefficient of detached-sediment washoff equation .1–.8 complex

JSER PERLND Exponent of detached-sediment washoff equation 2.0 complex

AFFX PERLND Fraction by which detached sediment decreases daily 

through soil compaction

.01–.02 1/day

COVER PERLND Fraction of land surface shielded by vegetation or mulch 

from erosion by direct rainfall impact

.10–.85 --

NVSI PERLND Rate at which sediment enters detached-sediment storage 

from atmosphere

.4–.5 pound/acre-day

KEIM IMPLND Coefficient of solids washoff equation .03–.04 complex

JEIM IMPLND Exponent of solids washoff equation 1.8–2.0 complex

ACCSDM IMPLND Solids accumulation rate .01–.03 ton/acre-day

RHO RCHRES Density of sediment particle 2.0–2.6 gram/cubic centimeter

M (silt) RCHRES Erodibility coefficient of sediment .005 pound/square foot-day

M (clay) RCHRES Erodibility coefficient of sediment .01 pound/square foot-day

W (silt and 

clay)

RCHRES Settling velocity of sediment particle in still water .00005–.0005 inch/second

TAUCD (silt) RCHRES Critical bed shear stress for sediment deposition .08–.12 pound/square foot

TAUCS (silt) RCHRES Critical bed shear stress for sediment scour .24–.28 pound/square foot

Table 16. Summary of calibrated values for selected water-quality parameters for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas, 1997–2004.

[RCHRES, stream reach; --, none; PERLND, pervious land surface; °C, degrees Celsius; IMPLND, impervious land surface]
Simulation of Streamflow and Water Quality 35

Table 16. Summary of calibrated values for selected water-quality parameters for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas, 1997–2004—Continued.

Parameter Model module Description Value Unit

Suspended sediment—Continued

TAUCD 

(clay)

RCHRES Critical bed shear stress for sediment deposition 0.07–0.10 pound/square foot

TAUCS 

(clay)

RCHRES Critical bed shear stress for sediment scour .20–.24 pound/square foot

Dissolved ammonia nitrogen

ACQOP PERLND or 

IMPLND

Accumulation rate of constituent on surface .0025–.012 pound/acre-day

SQOLIM PERLND or 

IMPLND

Maximum storage of constituent on surface .01–.12 pound/acre

WSQOP PERLND Rate of surface runoff to remove 90 percent of stored constituent in 1 hour.5–.7 inch/acre

KTAM20 RCHRES Nitrification rate of ammonia at 20 °C .24–.32 /hour

KNO220 RCHRES Nitrification rate of nitrite at 20 °C .003 /hour

Dissolved nitrate nitrogen

ACQOP PERLND or 

IMPLND

Accumulation rate of constituent on surface .003–.012 pound/acre-day

SQOLIM PERLND or 

IMPLND

Maximum storage of constituent on surface .004–.12 pound/acre

WSQOP PERLND Rate of surface runoff to remove 90 percent of stored constituent in 1 hour.5–.8 inch/acre

KNO320 RCHRES Denitrification rate of nitrate at 20 °C .003 /hour

Dissolved and total lead

ACQOP PERLND or 

IMPLND

Accumulation rate of constituent on surface .02–.03 gram/acre-day

SQOLIM PERLND or 

IMPLND

Maximum storage of constituent on surface .01–.80 gram/acre

WSQOP PERLND Rate of surface runoff to remove 90 percent of stored constituent in 1 hour.6–1.0 inch/acre

POTFW PERLND or 

IMPLND

Potency factor of sediment in washoff 6–12 gram/ton

ADPM(1,1) RCHRES Distribution coefficient for lead with suspended sand .20–.33 /milligram

ADPM(2,1) RCHRES Distribution coefficient for lead with suspended silt .22–.75 /milligram

ADPM(3,1) RCHRES Distribution coefficient for lead with suspended clay .24–.80 /milligram

ADPM(4,1) RCHRES Distribution coefficient for lead with bed sand .10–.20 /milligram

ADPM(5,1) RCHRES Distribution coefficient for lead with bed silt .15–.40 /milligram

ADPM(6,1) RCHRES Distribution coefficient for lead with bed clay .15–.40 /milligram

ADPM(1,2) RCHRES Transfer rate between adsorbed and desorbed states of lead 

with suspended sand

1.8–2.5 /day

ADPM(2,2) RCHRES Transfer rate between adsorbed and desorbed states of lead 

with suspended silt

1.8–2.5 /day

ADPM(3,2) RCHRES Transfer rate between adsorbed and desorbed states of lead 

with suspended clay

1.8–2.5 /day

ADPM(4,2) RCHRES Transfer rate between adsorbed and desorbed states of lead 

with bed sand

.0001 /day

ADPM(5,2) RCHRES Transfer rate between adsorbed and desorbed states of lead 

with bed silt

.0001 /day

ADPM(6,2) RCHRES Transfer rate between adsorbed and desorbed states of lead 

with bed clay

.0001 /day
36 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

2003 to 1.07 tons per acre in 2002, with an average yield of 

0.34 ton per acre per year.

Annual streamflow loads of dissolved nitrate at the 

watershed outlet were relatively constant because the source of 

most of the dissolved nitrate load was discharge of wastewater, 

which is relatively constant compared to stormflow-dominated 

streamflows in the Leon Creek watershed. The wastewater discharge accounts for about 95 percent of the entire nitrate load 

in streamflow exiting the Leon Creek watershed. The average 

annual (1997–2004) streamflow load of dissolved nitrate at the 

outlet of the watershed was 802 tons per year. The corresponding yield was 10.5 pounds per acre per year.

Annual total lead load showed a correlation with streamflow, ranging from 1,260 pounds in 2003 to 11,000 pounds 

in 2002. The average annual load was 3,900 pounds per 

year. The corresponding annual total lead yields ranged from 

0.008 pound per acre in 2003 to 0.072 pound per acre in 

2002, with an average annual yield of 0.026 pound per acre 

per year.

Mean annual yields of suspended sediment, dissolved 

nitrate, and total lead during 1997–2004 were simulated for 

six selected reach (RCHRES) outlets (table 18). The simulated 

reaches in table 18 are listed in order of increasing drainage 

area. The amount and effect of impervious area increase in 

the downstream direction. Suspended-sediment yields tend 

to decrease with distance downstream because the upstream 

part of the watershed includes more undeveloped land, which 

contributes larger sediment yields than more developed land. 

In the upstream part of the watershed (subwatershed 10 

[RCHRES 10]), suspended-sediment yield averaged 0.56 ton 

per acre per year compared with an average of 0.34 ton per 

acre per year for the entire Leon Creek watershed (outlet of 

RCHRES 109). 

Dissolved nitrate yield increases in the downstream direction. At the outlet of RCHRES 10, the yield was 0.20 pound 

per acre per year. Through the more developed midsection of 

the watershed, runoff yield gradually increases and was 0.46 

pound per acre per year at the outlet of RCHRES 102. At the 

Table 16. Summary of calibrated values for selected water-quality parameters for the Hydrological Simulation Program—FORTRAN 

model of the Leon Creek watershed, Bexar County, Texas, 1997–2004—Continued.

Parameter Model module Description Value Unit

Dissolved and total zinc

ACQOP PERLND or 

IMPLND

Accumulation rate of constituent on surface 0.24–30 gram/acre-day

SQOLIM PERLND or 

IMPLND

Maximum storage of constituent on surface 16–180 gram/acre

WSQOP PERLND Rate of surface runoff to remove 90 percent of stored constituent in 1 hour.6 inch/acre

POTFW PERLND or 

IMPLND

Potency factor of sediment in washoff 1.5–2.0 gram/ton

ADPM(1,1) RCHRES Distribution coefficient for zinc with suspended sand .15–.20 /milligram

ADPM(2,1) RCHRES Distribution coefficient for zinc with suspended silt .15–.22 /milligram

ADPM(3,1) RCHRES Distribution coefficient for zinc with suspended clay .15–.24 /milligram

ADPM(4,1) RCHRES Distribution coefficient for zinc with bed sand .15–.21 /milligram

ADPM(5,1) RCHRES Distribution coefficient for zinc with bed silt .15–.21 /milligram

ADPM(6,1) RCHRES Distribution coefficient for zinc with bed clay .15–.21 /milligram

ADPM(1,2) RCHRES Transfer rate between adsorbed and desorbed states of zinc 

with suspended sand

1.8 /day

ADPM(2,2) RCHRES Transfer rate between adsorbed and desorbed states of zinc 

with suspended silt

1.8 /day

ADPM(3,2) RCHRES Transfer rate between adsorbed and desorbed states of zinc 

with suspended clay

1.8 /day

ADPM(4,2) RCHRES Transfer rate between adsorbed and desorbed states of zinc 

with bed sand

.0001 /day

ADPM(5,2) RCHRES Transfer rate between adsorbed and desorbed states of zinc 

with bed silt

.0001 /day

ADPM(6,2) RCHRES Transfer rate between adsorbed and desorbed states of zinc 

with bed clay

.0001 /day
Simulation of Streamflow and Water Quality 37

outlet of RCHRES 109, the yield increases appreciably to 10.5 

pounds per acre per year because of the effect of the wastewater inflow.

Total lead yield also increased in the downstream direction, which is attributed to greater runoff yields for developed 

and impervious land, percentages of which increase in the 

downstream direction of the Leon Creek watershed. Total lead 

yield at the outlet of RCHRES 10 was 0.008 pound per acre 

per year compared with 0.026 pound per acre per year for the 

entire watershed (outlet of RCHRES 109).

Model Limitations 

Errors in model calibration can be classified as either 

systematic or measurement errors (Raines, 1996). Systematic errors are associated with limitations of the model to 

Table 17. Simulated mean annual streamflow and annual streamflow loads and yields of suspended sediment, dissolved nitrate 

nitrogen, and total lead at the outlet of Leon Creek watershed, Bexar County, Texas, 1997–2004.

[ft3/s, cubic feet per second]

Year or period

Simulated 

streamflow

(ft3

/s)

Simulated suspended sediment Simulated dissolved 

nitrate nitrogen

Simulated 

total lead

Load

(tons per 

year)

Yield

(tons per acre 

per year)1

Load

(tons per 

year)

Yield

(pounds per acre 

per year)1

Load

(pounds per 

year)

Yield

(pounds per

acre per year)1

1997 90.4 33,600 0.22 790 10.4 3,050 0.020

1998 145 111,000 .73 825 10.8 5,690 .037

1999 50.9 13,100 .09 765 10.0 1,880 .012

2000 76.2 19,500 .13 770 10.1 2,590 .017

2001 77.3 29,700 .20 810 10.6 2,640 .017

2002 393 163,000 1.07 860 11.3 11,000 .072

2003 40.2 7,900 .05 790 10.4 1,260 .008

2004 93.8 36,200 .24 810 10.6 3,120 .020

Average 

(1997–2004)

121 51,800 .34 802 10.5 3,900 .026

1

 Leon Creek drainage area upstream from outlet of watershed is 152,239 acres. 

Table 18. Simulated mean annual streamflow and mean annual streamflow yields of suspended sediment, dissolved nitrate nitrogen, 

and total lead at selected stream reach (RCHRES) outlets, Leon Creek watershed, Bexar County, Texas, 1997–2004.

[ft3/s, cubic feet per second]

Site

(fig. 5)

Average simulated 

streamflow

(ft3

/s)

Drainage area

(acres)

Simulated suspendedsediment yield

(tons per acre 

per year)

Simulated dissolved 

nitrate nitrogen 

yield

(pounds per acre 

per year)

Simulated 

total lead yield

(pounds per 

acre per year)

Outlet of RCHRES 10 5.2 6,875 0.56 0.20 0.008

Outlet of RCHRES 34 26 48,618 .42 .31 .008

Outlet of RCHRES 70 31 52,525 .43 .33 .008

Outlet of RCHRES 98 100 133,424 .29 .43 .017

Outlet of RCHRES 102 106 137,902 .32 .46 .020

Outlet of RCHRES 109 175 152,239 .34 10.5 .026
38 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

represent the hydrologic processes of the watershed. As a 

result, there are limits to how well model parameters and equations represent the complex physical properties of streamflow 

and water-quality processes. 

Measurement errors are introduced as a result of 

inaccurate or missing data. The degree to which available 

rainfall data represent actual rainfall is potentially the 

most serious source of measurement error. Although the 

NEXRAD rainfall data used for the study provide adequate 

spatial representation of rainfall, rainfall totals and intensity 

might not always be accurately represented. For example, 

NEXRAD rainfall used as input for the simulation model 

possibly underestimated actual rainfall during the storm 

of October 17–18, 1998. Streamflow volume at station 

08181480 Leon Creek at IH 35 (site 15) during October 

1998 was under-simulated by nearly 50 percent. Also, 

runoff from this storm contributed about 20 percent of the 

entire streamflow at station 08181480 during the 1997–2004 

simulation period. Thus, errors in rainfall input for major 

events can have substantial effects on calibration and simulation results.

Overall, streamflow is simulated with reasonable 

accuracy at streamflow-gaging stations. At other sites, accuracy of streamflow simulation is uncertain. Streamflow 

characteristics, especially flow duration, are much different between the upstream and downstream gaging stations 

(figs. 6–8). No measured streamflow data are available in 

the stream reaches between the upper-watershed sites and 

the downstream station 08181480 Leon Creek at IH 35. 

The midsection of the watershed is an area of physical 

changes: changes from steeper to flatter slopes, changes in 

geology and soils, increase in development and impervious 

cover, and the merging of upper Leon Creek and Culebra 

Creek (fig. 1). Also, stream reaches immediately downstream 

from the confluence of upper Leon Creek and Culebra Creek 

(reaches 80, 84, 88, 92, and 96) (fig. 5) are susceptible to 

flash flooding. Continuous streamflow data in the area of 

reach 80 or 84 could improve model streamflow calibration. 

Water-quality simulations also are affected by uncertainties in rainfall and streamflow. For example, overestimation 

of rainfall might result in simulated runoff and streamflow 

greater than actual. In turn, simulated suspended-sediment 

concentrations likely would be greater than actual. Simulated metals concentrations, because of close association 

of metals with sediment, also likely would be greater than 

actual. 

Major storms contribute most of the streamflow and most 

of the streamflow loads for certain water-quality constituents. 

For example, the three largest stormflows (June 1997, October 1998, and July 2002) contributed about 64 percent of the 

entire suspended-sediment load at station 08181480 Leon 

Creek at IH 35 during 1997–2004; however, no water-quality 

samples were collected during these stormflows. Water-quality 

constituent calibration and simulation might be improved with 

sample data that better characterize water-quality conditions 

during major storms.

Summary

The U.S. Geological Survey (USGS), in cooperation 

with the U.S. Army Corps of Engineers (USACE) and the San 

Antonio River Authority, configured, calibrated, and tested 

a Hydrological Simulation Program—FORTRAN (HSPF) 

watershed model for the approximately 238-square-mile Leon 

Creek watershed in Bexar County, Texas, and used the model 

to simulate streamflow and water quality (focusing on loads 

and yields of selected constituents). The USGS cooperative study was a part of phase 1 of a larger USACE study to 

establish and document the existing hydrologic, engineering, 

economic, and environmental conditions for the watershed.

 Streamflow in the model was calibrated and tested with 

available data from five USGS streamflow-gaging stations 

for the period 1997–2004. Streamflow data from four upperwatershed sites, where streamflows are intermittent, were 

used to calibrate model parameters for the subwatersheds 

upstream from the respective upper-watershed sites. The most 

downstream station, station 08181480 Leon Creek at Interstate Highway (IH) 35, was the primary source of data for 

streamflow calibration of subwatersheds downstream from the 

upper-watershed sites. Data from this perennial-flow station 

for 2001–04 were used for calibration. Data for 1997–2000 

were used to test the calibrated model.

Simulated streamflow volumes closely matched measured streamflow volumes at all streamflow-gaging stations. 

Total simulated streamflow volumes were within 10 percent of 

measured values. Annual, monthly, and daily coefficients of 

determination of the linear regression between measured and 

simulated streamflow and Nash-Sutcliff coefficients of modelfit efficiency (NSE) are considered acceptable for all stations. 

The NSE for daily streamflow volumes ranged from .80 to .97 

for the calibration periods at all stations. The NSE for hourly 

values for the entire 1997–2004 simulation period at the Leon 

Creek at IH 35 station was .70, which is considered good.

Streamflow volumes are greatly influenced by large 

storms. Two months that included major floods (October 

1998 and July 2002) accounted for about 50 percent of all the 

streamflow measured at station 08181480 Leon Creek at IH 35 

during 1997–2004. 

Wastewater discharged from the large municipal treatment facility near the mouth of Leon Creek accounted for a 

substantial part of the total streamflow exiting the Leon Creek 

watershed during 1997–2004. The average recycled wastewater discharge was about 39,000 acre-feet per year compared 

with the average simulated total streamflow of about 127,000 

acre-feet per year at the outlet of Leon Creek, or about 31 

percent of the average simulated total streamflow.

The HSPF model was calibrated for selected waterquality properties and constituents using available data from 

13 sites in and near the Leon Creek watershed for varying 

periods of record during 1992–2005. The model was used to 

simulate water temperature, dissolved oxygen, suspended sediment, dissolved ammonia nitrogen, dissolved nitrate nitrogen, 

and dissolved and total lead and zinc. Average simulated daily 
References 39

mean water temperature at station 08181480 Leon Creek at 

IH 35 during 1997–2000 was within 1 percent of average 

measured daily mean water temperature. NSE values for simulated water temperature were .89 for daily values and .83 for 

hourly values. Average simulated daily mean dissolved oxygen 

during 1997–2000 was within 1 percent of average measured 

daily mean dissolved oxygen. NSE values for simulated dissolved oxygen were .35 for daily values and .18 for hourly 

values.

Suspended-sediment data were available for seven sites 

in and near the watershed for 1992–2005. Soil-erosion and 

sediment-washoff parameters were calibrated by comparing 

simulated suspended-sediment concentrations to measured 

concentrations in runoff from four urban land-use types represented in the measured data. Parameters related to sediment 

transport were calibrated primarily by comparing simulated 

suspended-sediment concentrations and loads to computed 

concentrations and loads from measured data collected at three 

of the four upper-watershed sites during three stormflows 

in 2002 and 2004. From a regression relation between daily 

mean streamflow and daily suspended-sediment load at station 08181480 Leon Creek at IH 35 developed from 1992–97 

measured data (coefficient of determination = .869), estimated 

daily sediment loads at that site were computed for 2001–04. 

These estimated daily sediment loads were used as targets 

for HSPF suspended-sediment calibration and simulations at 

station 08181480 Leon Creek at IH 35. The HSPF-simulated 

suspended-sediment load during 2001–04 (excluding July 

2002 because of major storms) was 77,700 tons compared 

with an estimate of 74,600 tons from the streamflow-load 

relation (less than 5 percent difference). 

Median concentrations of dissolved ammonia nitrogen 

and dissolved nitrate nitrogen from wet deposition (rainfall) 

data at Beeville and Sonora, Tex., were used as input rainfall 

concentrations for wet deposition onto the watershed for the 

HSPF simulations. Calibration to obtain related parameters 

was similar to that for soil-erosion and sediment-washoff 

parameters using measured nutrient data collected at three 

of the four upper-watershed sites during three stormflows in 

2002 and 2004. Calibration of parameters related to nutrient 

simulations in the lower part of the watershed is based on very 

few measured data at station 08181480 Leon Creek at IH 35; 

however, simulated concentrations of dissolved ammonia 

nitrogen and dissolved nitrate nitrogen for two stormflows 

closely matched (within 0.1 mg/L) measured concentrations. 

At station 08181480 Leon Creek at IH 35, average simulated monthly mean concentrations of dissolved ammonia 

and nitrate concentrations during 1997–2004 were 0.03 and 

0.37 mg/L, respectively. Because of the inflow of wastewater, the average simulated monthly mean concentration of 

nitrate at the most downstream reach during 1997–2004 was 

7.4 mg/L.

Analyses of stormflow samples collected at all five 

streamflow-gaging stations and base-flow samples collected 

at station 08181480 Leon Creek at IH 35 were available for 

calibrating with regard to dissolved and total lead and zinc. 

Generally, simulated lead and zinc concentrations for stormflows compared adequately with measured concentrations at 

the upper-watershed sites. For example, the median measured 

total lead concentration in selected stormflows for 1997–2004 

for the four upper-watershed sites was 2.9 μg/L, and the 

median simulated concentration was 4.2 μg/L. For station 

08181480 Leon Creek at IH 35, the measured and simulated 

concentrations for stormflows match less closely. For example, 

the median measured total lead concentration for 1993–97 

stormflow data is 6.5 μg/L, and the median simulated total 

lead concentration for three 1997 stormflows is 14 μg/L. For 

base-flow conditions during 1997–2004 at station 08181480 

Leon Creek at IH 35, the simulated/measured concentrations 

match closely. For example, average and median simulated 

concentrations of total lead (for 2,041 days) were 1.3 and 

0.96 μg/L, respectively, and average and median measured 

concentrations (for nine samples) of total lead were less than 

1.0 and 1.0 μg/L, respectively. 

To demonstrate an application of the Leon Creek HSPF 

model, streamflow constituent loads and yields for suspended 

sediment, dissolved nitrate nitrogen, and total lead were 

simulated at the mouth of Leon Creek (outlet of the watershed) 

for 1997–2004. The average suspended-sediment load was 

51,800 tons per year. The average suspended-sediment yield 

was 0.34 ton per acre per year. The average load of dissolved 

nitrate at the outlet of the watershed was 802 tons per year. 

The corresponding yield was 10.5 pounds per acre per year. 

The average load of lead at the outlet was 3,900 pounds per 

year. The average lead yield was 0.026 pound per acre per 

year.

The degree to which available rainfall data represent 

actual rainfall is potentially the most serious source of 

measurement error associated with the Leon Creek model. 

Overall, streamflow is simulated with reasonable accuracy at 

streamflow-gaging stations. At other sites, accuracy of streamflow simulation is uncertain. Water-quality simulations also 

are affected by uncertainties in rainfall and streamflow. Major 

storms contribute most of the streamflow loads for certain constituents. For example, the three largest stormflows contributed about 64 percent of the entire suspended-sediment load 

at station 08181480 Leon Creek at IH 35 during 1997–2004. 

Water-quality constituent calibration and simulation might 

be improved with sample data that better characterize waterquality conditions during major storms.

References

Allison, J.D., and Allison, T.L., 2005, Partition coefficients for 

metals in surface water, soil, and waste: Washington, DC, 

U.S. Environmental Protection Agency, Office of Research 

and Development, 93 p.

Aqua Terra Consultants, 2007, Scenario generator for 

enhanced use of watershed models: accessed May 30, 2007, 

at http://www.aquaterra.com/genscn.html
40 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Asquith, W.H., and Roussel, M.C., 2003, Atlas of interoccurrence intervals of selected thresholds of daily precipitation 

in Texas: U.S. Geological Survey Water-Resources Investigations Report 03–4281, 106 p.

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

and Johanson, R.C., 2001, Hydrological Simulation Program—FORTRAN, user’s manual for version 12: Research 

Triangle Park, N.C., U.S. Environmental Protection Agency, 

National Exposure Research Laboratory, Office of Research 

and Development, 843 p.

Bomar, G.W., 1995, Texas weather (2d ed.): Austin, University 

of Texas Press, 275 p.

Bowie, G.L., Mills, W.B., Porcella, D.B., Campbell, C.L., 

Pagenkopf, J.R., Rupp, G.L., Johnson, K.M., Chan, P., 

Gherini, S.A., and Chamberlin, C.E., 1985, Rates, constants, 

and kinetic formulations in surface water quality modeling (2d ed.): Athens, Ga., U.S. Environmental Protection 

Agency, Environmental Research Laboratory, Office of 

Research and Development, EPA/600/3–85/040, 455 p.

Brown, D.S., and Raines, T.H., 2002, Simulation of flow and 

effects of best-management practices in the upper Seco 

Creek Basin, south-central Texas, 1991–98: U.S. Geological Survey Water-Resources Investigations Report 02–4249, 

22 p.

Brown, T.E., Waechter, N.B., Rose, P.R., and Barnes, V.E., 

1983, Geologic atlas of Texas, GA0029 San Antonio sheet: 

Austin, The University of Texas, Bureau of Economic Geology, 8 p., 1 sheet.

Childress, C.J.O., Foreman, W.T., Connor, B.F., and Maloney, T.J., 1999, New reporting procedures based on longterm method detection levels and some considerations for 

interpretations of water-quality data provided by the U.S. 

Geological Survey National Water Quality Laboratory: U.S. 

Geological Survey Open-File Report 99–103, 19 p.

City of San Antonio, Department of Public Works, 2006, 

Storm water utility: accessed November 14, 2007, at 

http://www.sanantonio.gov/publicworks/stormwater/.

Donigian, A.S., Jr., Bicknell, B.R., and Imhoff, J.C., 1995, 

Hydrological Simulation Program—FORTRAN (HSPF), in

Singh, V.P., ed., Computer models of watershed hydrology: 

Highlands Ranch, Colo., Water Resources Publications, 

p. 395–442.

Donigian, A.S., Jr., Imhoff, J.C., Bicknell, B.R., and Kittle, 

J.L., Jr., 1984, Application guide for Hydrological Simulation Program—FORTRAN (HSPF): Athens, Ga., U.S. 

Environmental Protection Agency, Environmental Research 

Laboratory, EPA–600/3–84–065, 177 p.

Donigian, A.S., Jr., and Love, J.T., 2003, Sediment calibration procedures and guidelines for watershed modeling: 

accessed August 14, 2009, at http://www.hspf.com/pdf/

TMDL2003PaperDonigian.pdf

Gandara, S.C., Gibbons, W.J., Barbie, D.L., and Jones, R.E., 

2000, Water resources data, Texas, water year 1999—Volume 5. Guadalupe River Basin, Nueces River Basin, Rio 

Grande Basin, and intervening coastal basins: U.S. Geological Survey Water-Data Report TX–99–05, 400 p.

Greene, D.G., and Hudlow, M.D., 1982, Hydrometeorologic 

grid mapping procedures, in Johnson, A.I., and Clark, 

R.A., American Water Resources Association International 

Symposium on Hydrometeorology, Denver, Colorado, June 

13–17, 1982: Proceedings, 20 p.

Larkin, T.J., and Bomar, G.W., 1983, Climatic atlas of Texas: 

Texas Department of Water Resources Report LP–192, 

151 p.

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

Users manual for an expert system (HSPEXP) for calibration of the Hydrological Simulation Program—FORTRAN: 

U.S. Geological Survey Water-Resources Investigations 

Report 94–4168, 102 p.

Maclay, R.W., 1995, Geology and hydrology of the Edwards 

aquifer in the San Antonio area, Texas: U.S. Geological 

Survey Water-Resources Investigations Report 95–4186, 

64 p., 12 pl.

Martin G.R., Zarriello, P.J., and Shipp, A.A., 2001, Hydrologic and water-quality characterization and modeling of the 

Chenoweth Run Basin, Jefferson County, Kentucky: U.S. 

Geological Survey Water-Resources Investigations Report 

00–4239, 197 p. 

Nash, J.E., and Sutcliffe, J.V., 1970, River flow forecasting 

through conceptual models, part 1—A discussion of principles: Journal of Hydrology, v. 10, p. 282–290. 

National Atmospheric Deposition Program, 2006, National 

Research Support Project–3: Champaign, Ill., National 

Atmospheric Deposition Program Office, Illinois State 

Water Survey, accessed August 12, 2007, at http://nadp.sws.

uiuc.edu/sites/ntnmap.asp?state=tx

National Climatic Data Center, 2009, Comparative climatic 

data for the United States through 2008: Asheville, N.C., 

National Oceanic and Atmospheric Administration, 

National Environmental Satellite, Data and Information Service, accessed August 14, 2009, at http://www1.ncdc.noaa.

gov/pub/data/ccd-data/CCD-2008.pdf

National Oceanic and Atmospheric Administration, 2006, 

NOAA hydrologic data systems group—ABRFC operational NEXRAD stage III data: accessed December 13, 

2008, at http://dipper.nws.noaa.gov/hdsb/data/nexrad/

abrfc_stageiii.php

Ockerman, D.J., 2002, Simulation of runoff and recharge 

and estimation of constituent loads in runoff, Edwards 

aquifer recharge zone (outcrop) and catchment area, Bexar 

County, Texas, 1997–2000: U.S. Geological Survey WaterResources Investigations Report 02–4241, 31 p.
References 41

Ockerman, D.J., 2005, Simulation of streamflow and estimation of recharge to the Edwards aquifer in the Hondo Creek, 

Verde Creek, and San Geronimo Creek watersheds, southcentral Texas, 1951–2003: U.S. Geological Survey Scientific Investigations Report 2005–5252, 37 p.

Ockerman, D.J., and McNamara, K.C., 2003, Simulation of 

streamflow and estimation of streamflow constituent loads 

in the San Antonio River watershed, Bexar County, Texas, 

1997–2001: U.S. Geological Survey Water-Resources 

Investigations Report 03–4030, 37 p.

Paode, R.D., Sofuoglu, S.C., Sivadechathep, J., Noll, K.E., 

Holsen, T.M., and Keeler, G.J., 1998, Dry deposition fluxes 

and mass size distributions of Pb, Cu, and Zn measured in 

southern Lake Michigan during AEOLOS: Environmental 

Science & Technology, v. 32, no. 11, p. 1,629–1,635.

Puente, Celso, 1978, Method of estimating natural recharge to 

the Edwards aquifer in the San Antonio area, Texas: U.S. 

Geological Survey Water-Resources Investigations Report 

78–10, 34 p.

Raines, T.H., 1996, Simulation of storm peaks and storm volumes for selected subbasins in the West Fork Trinity River 

Basin, Texas, water years 1993–94: U.S. Geological Survey 

Water-Resources Investigations Report 96–4110, 41 p.

Shedd, R.C., and Fulton, R.A., 1993, WSR–88D Precipitation 

processing and its use in National Weather Service hydrologic forecasting, in International Symposium on Engineering Hydrology, San Francisco, California, July 25–29, 

1993, Proceedings: American Society of Civil Engineers, 

p. 16–21.

Singh, V.P., 1995, Watershed modeling, in Singh, V.P., ed., 

Computer models of watershed hydrology: Highlands 

Ranch, Colo., Water Resources Publications, p. 1–22.

Slade, R.M., Jr., and Patton, John, 2003, Major and catastrophic storms and floods in Texas: U.S. Geological Survey 

Open-File Report 03–193, 1 CD-ROM.

Slade, R.M., Jr., and Persky, Kristie, 1999, Floods in the 

Guadalupe and San Antonio River Basins in Texas: U.S. 

Geological Survey Fact Sheet 147–99, 4 p. 

Texas Commission on Environmental Quality, 2002, Texas 

water quality inventory 2000 (SFR–050/00): accessed 

November 15, 2007, at http://www.tceq.state.tx.us/assets/

public/comm_exec/pubs/sfr/050_00/index.html

Texas Commission on Environmental Quality, 2005, 2004 

Texas water quality inventory and 303(d) list: accessed 

November 15, 2007, at http://www.tceq.state.tx.us/

compliance/monitoring/water/quality/data/04twqi/

twqi04.html

Texas Department of State Health Services, 2008, Bexar 

County health facts for 2004 and 1997: accessed December 

12, 2008, at http://www.dshs.state.tx.us/chs/cfs/.

U.S. Army Corps of Engineers, 2004, Project management 

plan, Leon Creek watershed, Texas—Interim feasibillity 

study, Guadalupe-San Antonio River Basin, Texas: U.S. 

Army Corps of Engineers, Fort Worth District, 28 p.

U.S. Department of Agriculture, Soil Conservation Service 

[Natural Resources Conservation Service], 1966, Soil 

survey of Bexar County, Texas: Soil Conservation Service, 

126 p., 88 map sheets.

U.S. Environmental Protection Agency, 2006, Sediment 

parameter and calibration guidance for HSPF: U.S. Environmental Protection Agency, EPA BASINS Technical 

Note 8, accessed April 28, 2008, at http://www.epa.gov/

waterscience/BASINS/docs/tecnot8.pdf

U.S. Environmental Protection Agency, 2008, Better assessment science integrating point and nonpoint sources 

(BASINS): accessed December 13, 2008, at http://www.epa.

gov/OST/BASINS/.

U.S. Geological Survey, 2001, Rocky Mountain Mapping 

Center—Elevation program: accessed December 15, 2004, 

at http://rockyweb.cr.usgs.gov/elevation/.

U.S. Geological Survey, 2009, National Water Information 

System (NWISWeb) data available on the World Wide Web: 

at http://waterdata.usgs.gov/tx/nwis/nwis

Wicklein, S.M., and Schiffer, D.M., 2002, Simulation of runoff and water quality for 1990 and 2008 land-use conditions 

in the Reedy Creek watershed, east-central Florida: U.S. 

Geological Survey Water-Resources Investigations Report 

02–4018, 62 p.

Zarriello, P.J., and Ries, K.G., III, 2000, A precipitation-runoff 

model for analysis of the effects of water withdrawals on 

streamflow, Ipswich River Basin, Massachusetts: U.S. 

Geological Survey Water-Resources Investigations Report 

00–4029, 99 p.
Blank Page
Appendix 1—Hydrological Simulation Program—FORTRAN 

(HSPF) process-related parameters used to simulate the Leon 

Creek watershed, Bexar County, Texas

Appendix 1
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Appendix 1 45

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Pervious Land (PERLND)—Continued

Secondary 

module Parameter Unit Description

Water balance

Interception storage

PWATER CEPSC inch Interception storage capacity 

CEPS inch Initial interception storage

Surface and subsurface storages

UZSN inch Upper-zone nominal storage; an index to amount of depression and surface-layer 

storage of a pervious area

LZSN inch Lower-zone nominal storage; an index to soil-moisture-holding capacity

SURS inch Initial surface storage

IFWS inch Initial interflow storage

UZS inch Initial upper-zone storage

LZS inch Initial lower-zone storage

AGWS inch Initial active-groundwater storage

Evapotranspiration

LZETP -- Lower-zone evapotranspiration; an index to the density of deep-rooted vegetation 

on a pervious area

AGWETP -- Fraction of available potential evapotranspiration demand that can be met with active groundwater

BASETP -- Fraction of available potential evapotranspiration demand that can be met with 

baseflow (groundwater outflow)

Recession rates

KVARY 1/inch Groundwater outflow modifier; an index of how much effect recent recharge has on 

groundwater outflow

AGWRC 1/day Basic groundwater recession rate if KVARY is zero and there is no inflow to 

groundwater 

IRC 1/day Interflow recession coefficient

GWVS inch Index to groundwater slope

Infiltration

INFILT inch/hour Index to infiltration capacity of soil

INFILD -- Ratio of maximum to mean infiltration rate of a pervious area

INFEXP -- Infiltration equation exponent

INTFW -- Index to amount of water that infiltrates and flows as interflow (shallow subsurface 

runoff)

DEEPFR -- Fraction of groundwater inflow to deep recharge

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas.

[--, none; °C, degrees Celsius; BOD, biochemical oxygen demand]

Pervious Land (PERLND)
46 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Pervious Land (PERLND)—Continued

Secondary 

module Parameter Unit Description

Water balance—Continued

Overland flow

LSUR foot Average length of overland-flow plane

SLSUR -- Average slope of overland-flow plane

NSUR -- Average roughness coefficient of overland-flow plane

Soil erosion

SEDMNT SMPF -- Management factor to account for use of erosion-control factors

KRER complex Coefficient of soil-detachment equation

JRER complex Exponent of soil-detachment equation

AFFX 1/day Fraction by which detached sediment decreases daily through soil compaction

COVER -- Fraction of land surface shielded by vegetation or mulch from erosion by direct 

rainfall impact

NVSI pound/acre-day Rate at which sediment enters detached-sediment storage from atmosphere

KSER complex Coefficient of detached-sediment washoff equation

JSER complex Exponent of detached-sediment washoff equation

Water-quality constituent flux

PQUAL SQO pound/acre 

(or gram/acre)

Initial constituent storage on surface

POTFW pound/ton 

(or gram/ton)

Potency factor of sediment in washoff

POTFS pound/ton

(or gram/ton)

Potency factor of scoured sediment 

ACQOP pound/acre-day 

(or gram/acre-day)

Accumulation rate of constituent on surface

SQOLIM pound/acre

(or gram/acre)

Maximum storage of constituent on surface

WSQOP inch/hour Rate of surface runoff to remove 90 percent of stored constituent in 1 hour

IDOXP milligram/liter Concentration of dissolved oxygen in interflow

ADOXP milligram/liter Concentration of dissolved oxygen in base flow
Appendix 1 47

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Impervious Land (IMPLND)

Secondary 

module Parameter Unit Description

Water balance

IWATER LSUR foot Average length of overland-flow plane

SLSUR -- Average slope of overland-flow plane

NSUR -- Average roughness coefficient of overland-flow plane

RETSC inch Retention storage capacity of impervious areas

RETS inch Initial retention storage

SURS inch Initial overland-flow storage

Sediment washoff

SOLIDS KEIM complex Coefficient of solids washoff equation

JEIM complex Exponent of solids washoff equation

REMSDP 1/day Fraction of solids removed on each day without runoff

ACCSDM ton/acre-day Solids accumulation rate

SLDS ton/acre Initial storage of solids

Water-quality constituent flux

IQUAL SQO pound/acre

(or gram/acre)

Initial constituent storage on surface

POTFW pound/ton

(or gram/ton)

Potency factor of sediment in washoff

ACQOP pound/acre-day

(or gram/acre-day)

Accumulation rate of constituent on surface

SQOLIM pound/acre

(or gram/acre)

Maximum storage of constituent on surface

WSQOP inch/hour Rate of surface runoff to remove 90 percent of stored constituent in 1 hour
48 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Stream Reaches (RCHRES)—Continued

Secondary 

module Parameter Unit Description

Water balance

HYDR FTABNO -- Number of FTABLE that contains RCHRES geometric and hydraulic 

properties

LEN mile Length of stream reach

DELTH foot Drop in water elevation within stream reach

STCOR foot Correction in reach depth to calculate stage

KS -- Weighting factor for flow routing

DB50 millimeter Median diameter of bed sediment

ADCALC CRRAT -- Ratio of maximum velocity to mean velocity in reach cross section under 

typical flow conditions

Heat balance

HTRCH ELEV feet Mean RCHRES elevation

CFSAEX -- Correction factor for solar radiation; the fraction of RCHRES exposed to 

radiation

KATRAD -- Longwave radiation coefficient

KCON -- Conduction-convection heat transport coefficient

KEVAP -- Evaporation coefficient

Dissolved oxygen and nutrient balance

OXRX KBOD20 /hour Unit BOD decay rate at 20 °C

KODSET foot/hour Rate of BOD settling

SUPSAT -- Maximum allowable dissolved oxygen supersaturation factor

BENOD milligram/square meter-hour Benthal oxygen demand at 20 °C

BRBOD(1) milligram/square meter-hour Benthal release rate of BOD demand under anaerobic conditions

BRBOD(2) milligram/square meter-hour Increment to benthal release rate of BOD under anaerobic conditions

EXPREL -- Exponent in dissolved oxygen term of benthal BOD release equation

NUTRX KTAM20 /hour Nitrification rate of ammonia at 20 °C 

KNO220 /hour Nitrification rate of nitrite at 20 °C

KNO320 /hour Denitrification rate of nitrate at 20 °C

Sediment transport

SEDTRN BEDWID foot Width of streambed

BEDWRN foot Depth of streambed

POR -- Porosity of streambed

D inch Effective diameter of sediment particle

W inch/second Settling velocity of sediment particle in still water

RHO gram/cubic centimeter Density of sediment particle

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Stream Reaches (RCHRES)
Appendix 1 49

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Stream Reaches (RCHRES)—Continued

Secondary 

module Parameter Unit Description

Sediment transport—Continued

KSAND complex Coefficient of HSPF sand-load equation

EXPSAND complex Exponent of HSPF sand-load equation

TAUCD pound/square foot Critical bed shear stress for sediment deposition

TAUCS pound/square foot Critical bed shear stress for sediment scour

M pound/square foot-day Erodibility coefficient of sediment

BEDDEP foot Initial thickness of bed material

SSAND milligram/liter Initial concentration of sand in suspension

SSILT milligram/liter Initial concentration of silt in suspension

SCLAY milligram/liter Initial concentration of clay in suspension

FRACSAND -- Initial fraction by weight of sand in bed material

FRACSILT -- Initial fraction by weight of silt in bed material

FRACCLAY -- Initial fraction by weight of clay in bed material

Heat and temperature

HTRCH ELEV foot Mean reach elevation

CFSAEX -- Correction factor for solar radiation; the fraction of RCHRES surface 

exposed to radiation

KATRAD -- Longwave radiation coefficient.

KCOND -- Conduction-convection heat transport coefficient

KEVAP -- Evaporation coefficient

TGRNDM degrees Fahrenheit Monthly ground temperature

KMUD kilocalorie/square meter/

degree Celsius/hour

Heat conduction coefficient between water and mud/ground

TW degrees Fahrenheit Initial water temperature of RCHRES

AIRTMP degrees Fahrenheit Initial air temperature at RCHRES

Oxygen balance

RQUAL KBOD20 1/hour Unit BOD decay rate at 20 °C

TCBOD -- Temperature correction coefficient for BOD decay

KODSET foot/hour Rate of BOD settling

SUPSAT -- Allowable dissolved oxygen supersaturation multiplier

ELEV foot RCHRES elevation above sea level

BENOD milligram/square meter-hour Benthal oxygen demand at 20 °C

TCBEN -- Temperature-correction coefficient for benthal oxygen demand

EXPOD -- Exponential factor in dissolved-oxygen term of benthal-oxygen-demand 

equation
50 Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) process-related parameters used to simulate the Leon Creek 

watershed, Bexar County, Texas—Continued.

Stream Reaches (RCHRES)—Continued

Secondary 

module Parameter Unit Description

Oxygen balance—Continued

BRBOD(1) milligram/square meter-hour Benthal release of BOD at high oxygen concentration

BRBOD(2) milligram/square meter-hour Increment to benthal release of BOD under anaerobic conditions

EXPREL -- Exponential factor in dissolved oxygen term of benthal oxygen release 

equation

TCGINV -- Temperature correction coefficient for surface-gas invasion

DOX milligram/liter Initial dissolved oxygen concentration

BOD milligram/liter Initial BOD concentration

SATDO milligram/liter Initial dissolved oxygen-saturation concentration

Water-quality constituent balance—Nutrients

RQUAL KTAM20 1/hour Nitrification rate of ammonia at 20 °C

KNO220 1/hour Nitrification rate of nitrite at 20 °C

KNO320 1/hour Denitrification rate of nitrate at 20 °C

TCNIT -- Temperature correction coefficient for nitrification

TCDEN -- Temperature correction coefficient for denitrification

DENOXT milligram/liter Dissolved oxygen concentration threshold for denitrification

NO3, TAM, 

NO2

milligram/liter Initial concentrations of nitrate, total ammonia, and nitrite

Water-quality constituent balance—Metals

GQUAL DQAL microgram/liter Initial dissolved concentration of constituent

ADPM(1,1) /milligram Distribution coefficient for constituent with suspended sand

ADPM(2,1) /milligram Distribution coefficient for constituent with suspended silt

ADPM(3,1) /milligram Distribution coefficient for constituent with suspended clay

ADPM(4,1) /milligram Distribution coefficient for constituent with bed sand

ADPM(5,1) /milligram Distribution coefficient for constituent with bed silt

ADPM(6,1) /milligram Distribution coefficient for constituent with bed clay

ADPM(1,2) /day Transfer rate between adsorbed and desorbed states of constituent with 

suspended sand

ADPM(2,2) /day Transfer rate between adsorbed and desorbed states of constituent with 

suspended silt

ADPM(3,2) /day Transfer rate between adsorbed and desorbed states of constituent with 

suspended clay

ADPM(4,2) /day Transfer rate between adsorbed and desorbed states of constituent with 

bed sand

ADPM(5,2) /day Transfer rate between adsorbed and desorbed states of constituent with 

bed silt

ADPM(6,2) /day Transfer rate between adsorbed and desorbed states of constituent with 

bed clay
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Ockerman and Roussel—Simulation of Streamflow and Water Quality in the Leon Creek Watershed, Bexar County, Texas, 1997–2004—SIR 2009–5191

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