Cover:

Top: Empty Parkers Creek Reservoir (photograph by Jon R. Gilhousen, U.S. Geological Survey, 

December 4, 1991). 

Bottom: Full Parkers Creek Reservoir (photograph by David S. Brown, U.S. Geological Survey, 

June 25, 1997). 
U.S. Department of the Interior 

U.S. Geological Survey 

Simulation of Flow and Effects of BestManagement Practices in the Upper Seco 

Creek Basin, South-Central Texas, 1991–98 

By David S. Brown and Timothy H. Raines 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 02–4249 

In cooperation with the Texas State Soil and Water Conservation Board and 

U.S. Department of Agriculture, Natural Resources Conservation Service 

Austin, Texas 

2002 
U.S. DEPARTMENT OF THE INTERIOR 

Gale A. Norton, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

Any use of trade, product, or firm names is for 

descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

For additional information write to 

District Chief 

U.S. Geological Survey 

8027 Exchange Dr. 

Austin, TX 78754–4733 

E-mail: dc_tx@usgs.gov 

Copies of this report can be purchased from 

U.S. Geological Survey 

Information Services 

Box 25286 

Denver, CO 80225–0286 

E-mail: infoservices@usgs.gov 

ii 
CONTENTS

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


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


Purpose and Scope .................................................................................................................................................... 1


Description of Study Area ........................................................................................................................................ 2


Description of Simulation Model ............................................................................................................................. 2


Acknowledgments .................................................................................................................................................... 9


Simulation of Flow ............................................................................................................................................................... 9


Model Setup .............................................................................................................................................................. 9


Model Calibration and Testing ................................................................................................................................. 11


Error Analysis ........................................................................................................................................................... 18


Sensitivity Analysis .................................................................................................................................................. 18


Simulation of Best-Management Practices .......................................................................................................................... 20


Brush Management ................................................................................................................................................... 20


Weather Modification ............................................................................................................................................... 21


Summary .............................................................................................................................................................................. 22


References ............................................................................................................................................................................ 22


FIGURES 

1–4. Maps showing: 

1. Location of the upper Seco Creek Basin, south-central Texas .................................................................. 3


2. Data-collection network in the upper Seco Creek Basin ........................................................................... 4


3. Land use/cover in the upper Seco Creek Basin ......................................................................................... 5


4. Surface and shallow subsurface geology in the upper Seco Creek Basin ................................................. 6


5–10. Graphs showing simulated and observed flow for:


5. 08201500 Seco Creek at Miller Ranch near Utopia ......................................................................................... 12


6. 08202450 Seco Creek Reservoir Inflow near Utopia ....................................................................................... 12


7. 08202490 Seco Creek Reservoir Outflow near Utopia .................................................................................... 13


8. 08202700 Seco Creek at Rowe Ranch near D’Hanis ....................................................................................... 13


9. 08202790 Parkers Creek Reservoir Inflow near D’Hanis ................................................................................. 14


10.	 08202810 Parkers Creek Reservoir Outflow near D’Hanis .............................................................................. 14


11.	 Graphs showing difference between simulated and observed monthly discharge for upper Seco Creek 


subbasins ............................................................................................................................................................... 19


TABLES 

1. Daily rainfall, streamflow, and reservoir-content stations in the upper Seco Creek Basin .................................. 7


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


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


4. Basin-related parameters for each gaged subbasin of the upper Seco Creek Basin ............................................. 10


5. Calibration annual parameters .............................................................................................................................. 15


6. Calibration monthly parameter ............................................................................................................................. 16


7. Initial-condition values for model calibration ...................................................................................................... 16


8. Summary of calibration and testing results for upper Seco Creek subbasins ....................................................... 17


9. Summary of calibration and testing errors for upper Seco Creek subbasins ........................................................ 17


10. 	 Simulation results from a 5- to 6-percent reduction in evapotranspiration associated with brush 


management .......................................................................................................................................................... 20


11.	 Simulation results from a 10-percent increase in rainfall totals and intensities associated with weather


modification .......................................................................................................................................................... 21


CONTENTS iii 
Simulation of Flow and Effects of BestManagement Practices in the Upper Seco 

Creek Basin, South-Central Texas, 1991–98 

By David S. Brown and Timothy H. Raines 

Abstract 

The Hydrological Simulation Program— 

FORTRAN model was used to assess the effects of 

two best-management practices—brush management (removal of woody species locally known as 

cedar) and weather modification (rainfall enhancement)—on selected hydrologic processes in six 

subbasins that compose the upper Seco Creek 

Basin in south-central Texas. A parameter set for 

use with the model was developed to simulate 

surface-water-budget components for the six gaged 

subbasins. 

Simulation of brush management, represented by decreases in simulated evapotranspiration of 5 to 6 percent, resulted in increases of 1 to 

47 percent in annual runoff and increases of 14 to 

48 percent in surface runoff for the six subbasins. 

Simulation of weather modification, represented 

by a 10-percent increase in rainfall totals and intensities, resulted in increases of 5 to 6 percent in 

evapotranspiration, increases of 2 to 92 percent in 

annual runoff, and increases of 36 to 101 percent in 

surface runoff. 

Rainfall and runoff data for the study were 

collected during January 1, 1991–September 30, 

1998. Data from 60 storms were used for the simulations. The model was calibrated with data from 

33 storms (in two subbasins) and tested with data 

from 27 storms (in four subbasins). Twenty-one 

pervious land segments were defined for the study 

on the basis of geology and land cover. An error 

analysis and a sensitivity analysis were done on 

each subbasin, and the results were used to develop 

the final parameter set. 

INTRODUCTION 

In April 1990, the Seco Creek Water-Quality 

Demonstration Project was established as a State of 

Texas and U.S. Department of Agriculture cooperative 

project involving several State and Federal agencies, 

groups, and universities. The Seco Creek Water-Quality 

Demonstration Project is intended to demonstrate and 

transfer technology to farmers and ranchers and, 

thereby, encourage the implementation of agricultural 

best-management practices (BMPs) that will protect 

surface- and ground-water quality and potentially 

increase surface-water availability in the Seco Creek 

Basin. At least 60 different BMPs were implemented at 

56 sites within the study area (Steffens and Wright, 

1995). The BMPs included prescribed burning, installation of grass filter strips, and various agricultural management strategies for application and control of brush, 

crops, grazing, herbicides, nutrients, and pesticides. In 

1991, the U.S. Geological Survey (USGS), in cooperation with the Texas State Soil and Water Conservation 

Board and the U.S. Department of Agriculture, Natural 

Resources Conservation Service, began a study to simulate flow for six selected subbasins in the upper Seco 

Creek Basin and to evaluate the effects of two BMPs on 

surface-water quantity. 

Purpose and Scope 

This report describes the use of a model to 

simulate selected hydrologic processes for six gaged 

subbasins in the upper Seco Creek Basin and presents 

an assessment of the effects on surface-water quantity 

in the basin of two BMPs, brush management and 

weather modification. Rainfall and runoff data collected 

from 60 storms during January 1, 1991–September 30, 

1998, were used to calibrate and test the continuoussimulation model. Each subbasin was characterized 

using a minimum of 2 to a maximum of 19 unique pervious land segments that were defined on the basis of 

geology and land-cover types, and each subbasin was 

Abstract 1 
Simulation of Flow

and Effects of BestManagement

Practices in the 

Upper Seco Creek 

Basin, SouthCentral Texas,

1991–98

subdivided into a minimum of 1 to a maximum of 

10 reaches for input to the simulation model. Twenty 

process-related parameters were defined for each land 

segment, and six basin-related parameters were defined 

for each subbasin. The calibrated model was used to 

evaluate the changes in surface-water quantity that are 

likely to result from brush management and weather 

modification. 

Description of Study Area 

The study area, upper Seco Creek Basin, is in 

south-central Texas (fig. 1). The Seco Creek Basin 

upstream from Seco Creek at Rowe Ranch (fig. 2) 

drains about 165 square miles (mi2

). The basin is 

divided into six major subbasins and contains two major 

reservoirs—Seco Creek Reservoir and Parkers Creek 

Reservoir (fig. 2). Both reservoirs are Edwards aquifer 

recharge structures. Recharge occurs when captured 

stormflows pond behind the dam and infiltrate the 

Edwards aquifer outcrop. Parkers Creek Reservoir also 

functions as a flood-control structure. The dam at 

Parkers Creek Reservoir is about 40 feet (ft) high, constructed of earthen material, and captures nearly all 

streamflow. A drop inlet structure and an earthen emergency spillway release streamflow only when the reservoir is filled to capacity during large runoffs. A 12-fthigh, uncontrolled concrete ogee-crested dam at Seco 

Creek Reservoir diverts all flows less than about 350 to 

400 cubic feet per second (ft

3

/s) into a sinkhole in the 

Edwards aquifer outcrop. Flows in excess of about 

350 to 400 ft3/s overtop the dam and pass downstream. 

Thirteen rainfall stations, six streamflow-gaging 

stations, and one reservoir-content station are in the 

upper basin (table 1). 

The study area is characterized by a moderate 

climate with hot, dry summers; warm, wet autumns; 

cool, dry winters; and warm, wet springs. Mean 

monthly temperature for the study area is 68 degrees 

Fahrenheit (°F), with mean monthly temperatures ranging from 51 °F in January to 84 °F in July and August 

(Hydrosphere, 2000). Mean annual rainfall at Utopia 

(location shown in fig. 1) was about 32.6 inches (in.) 

(Hydrosphere, 2000). Rainfall is generated from frontal 

systems and convective heating. Frontal systems in the 

spring and fall produce moderate- to high-intensity, 

long-duration storms that generally result in peak 

streamflows for the year. Convective thunderstorms, 

that occur mostly in the summer, produce widely 

scattered, high-intensity, short-duration storms. 

Basin slopes vary from steep (typically 0.08 to 

0.12 foot/foot [ft/ft]) in the upper part of the basin to 

moderate (typically 0.01 to 0.08 ft/ft) in the lower part 

of the upper basin. 

Major land uses are ranching and farming (rangeland 88 percent; cropland 9.3 percent) (Steffens and 

Wright, 1995) (fig. 3). The predominant land cover, 

rangeland, is used for grazing cattle, goats, deer, and 

exotic game animals; whereas, cropland is used for 

growing corn, cotton, milo, and wheat. 

On the surface and in the shallow subsurface, the 

northern part of the study area (essentially the upper 

three subbasins) comprises rocks of the Trinity aquifer, 

primarily Glen Rose Limestone. The southern part of 

the study area (essentially the lower three subbasins) 

comprises rocks of the Edwards aquifer, primarily 

Devils River Formation, and rocks of several formations 

that are considered local shallow aquifers or confining 

units of the Edwards aquifer (fig. 4). The area where the 

Edwards aquifer (Devils River Formation) crops out 

delineates the Edwards aquifer recharge zone in the 

study area. 

Description of Simulation Model 

The Hydrological Simulation Program— 

FORTRAN (HSPF) (Bicknell and others, 1997) is a 

continuous-simulation model that uses a conceptual 

framework to represent hydrologic processes including 

infiltration, evaporation, interception storage, surface 

runoff, interflow, and base flow on a pervious land 

segment (PERLND) and to represent retention storage 

and surface runoff on an impervious land segment 

(IMPLND). Each user-defined land segment represents 

its own unique hydrologic response system on the basis 

of soils, geology and land cover, basin slope, or other 

basin characteristics. These land segments do not need 

to be contiguous. The runoff from each land segment is 

moved through a system of reaches or reservoirs using 

storage routing. 

The HSPF model uses input from three types of 

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

basin-related model parameters. Continuous time series 

of precipitation and potential evaporation are needed for 

model simulations. Point-precipitation data, measured 

by rain gages, are assumed to be uniform over a land 

segment. Potential evaporation data can be estimated 

from measured pan evaporation or can be computed 

using minimum and maximum temperatures. Time 

series of measured runoff are used for model calibration 

and testing. 

2 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
99o

30' 99o

15' 

0 5 10 15 20 MILES 

99o 

29o45' 

99o

45' 

29o

30' 

29o

15' 

100o 

BANDERA COUNTY 

UVALDE COUNTY 

Upper Seco MEDINA COUNTY 

 Creek Basin 

Utopia 

D'Hanis 

Yancy 

Tarpley 

BANDERA

 COUNTY 

KINNEY

 COUNTY UVALDE

 COUNTY 

MEDINA

 COUNTY 

HAYS

 COUNTY

 COMAL 

COUNTY

 BEXAR

 COUNTY SAN

 ANTONIO 

Seco Creek

 Basin 

Sea World

 of Texas 

TEXAS 

CANYON

 LAKE 

LOCATION MAP


Figure 1. Location of the upper Seco Creek Basin, south-central Texas.


INTRODUCTION 3 
EXPLANATION 

BANDERA

 COUNTY 

UVALDE

 COUNTY 

MEDINA

 COUNTY 

Seco 

Creek 

Creek

Parkers 

Little Seco Creek 

Seco 

Creek 

13 

15 

14 

1 

2 

3 

4 

6 

8 

9 

11 

10 

Base modified from U.S. Geological 

Survey 1:24,000 quadrangles 

Universal Transverse Mercator Projection, 

7 

7 

Data-collection site—Number referenced in table 

below and in table 1 

Subbasin boundary 

Rainfall 

Streamflow 

Reservoir content 14 

5 

12 

5

 1 Seco Creek rain gage no. 1

 2 Seco Creek rain gage no. 2

 3 Freeman rain gage near Utopia

 4 Seco Creek at Miller Ranch near Utopia

 5 Coffee rain gage near Utopia

 6 Seco Creek Reservoir Inflow near Utopia

 7 Seco Creek Reservoir Outflow near Utopia

 8 Brown rain gage near D'Hanis

 9 Valdina Farms rain gage near D'Hanis 

10 Chaney rain gage near D'Hanis 

11 Seco Creek at Rowe Ranch near D'Hanis 

12 Parkers Creek rain gage no. 1 

13 Parkers Creek Reservoir Inflow near D'Hanis 

14 Parkers Creek Reservoir near D'Hanis 

15 Parkers Creek Reservoir Outflow near D'Hanis 

99o25' 

29o42'30" 

29o

37'30" 

99o

20' 

99o

15' 

29o

37'30" 

29o

27'30" 

29o

22'30" 

Parkers Creek Reservoir 

Seco Creek Reservoir 

Zone 14


0 5 10 15 MILES


Figure 2. Data-collection network in the upper Seco Creek Basin. 

4 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
BANDERA

 COUNTY 

UVALDE

 COUNTY 

MEDINA

 COUNTY 

Source: Philip Wright, U.S. Department of Agriculture,

 Natural Resources Conservation Service,

 written commun., 1995 

99o25' 

29o

42'30" 

29o

37'30" 

99o

20' 

99o

15' 

29o

37'30" 

29o

27'30" 

29o

22'30" 

Parkers Creek Reservoir 

Seco Creek Reservoir 

Cropland 

Headquarters, house, or farmstead 

Irrigated cropland 

Irrigated pasture and hayland 

Pasture and hayland 

Rangeland 

Trap or animal containment facility 

Land use/cover 

EXPLANATION 

Subbasin boundary 

Seco 

Creek 

Creek

Parkers 

Little Seco Creek 

Seco 

Creek 

0 5 10 15 MILES 

Figure 3. Land use/cover in the upper Seco Creek Basin. 

INTRODUCTION 5 
Seco 

Creek 

Parkers

Creek 

Little Seco Creek 

Seco 

Creek 

EXPLANATION 

Edwards aquifer recharge zone 99o

25' 

Geology 

Alluvium 

Leona Formation 29o

42'30" 

Uvalde Gravel 

Escondido Formation 

Anacacho Limestone 

Austin Group 

Eagle Ford Group 

Buda Limestone 

Del Rio Clay 

BANDERA Segovia Formation and Devils River Formation COUNTY 

Fort Terrett Formation and Devils River Formation 29o

37'30" 

UVALDE MEDINA 99o

20' Glen Rose Limestone (upper confining unit) 

COUNTY COUNTY 

Subbasin boundary 

Primary fault 

Inferred fault 

29o

37'30" 

99o

15' 

Seco Creek Reservoir 

Sources: Medina County—Small and

 Clark (2000); outside Medina 29o

27'30" County—modified from Brown

 and others (1999). 

Parkers Creek Reservoir 

29o22'30" 

0 5 10 15 MILES 

Figure 4. Surface and shallow subsurface geology in the upper Seco Creek Basin. 

6 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
Table 1. Daily rainfall, streamflow, and reservoir-content stations in the upper Seco Creek Basin 

[mi

2, square miles; --, not applicable] 

Site 

no. 

(fig. 2) 

Station 

no. Station name Latitude Longitude 

Drainage 

area 

(mi2) 

Period of 

record 

(water 

years1) 

Rainfall 

1 2941250992554 Seco Creek rain gage no. 1 29°41'25" 99°25'54" -- 1991–98 

2 2937170992513 Seco Creek rain gage no. 2 29°37'17" 99°25'13" -- 1991–98 

3 2935500992723 Freeman rain gage near Utopia 29°35'50" 99°27'23" -- 1994–98 

4 08201500 Seco Creek at Miller Ranch near Utopia 29°34'23" 99°24'10" -- 1988–2000 

5 2937550992019 Coffee rain gage near Utopia 29°33'55" 99°20'19" -- 1994–98 

6 08202450 Seco Creek Reservoir Inflow near Utopia 29°31'34" 99°23'42" -- 1991–98 

7 08202490 Seco Creek Reservoir Outflow near Utopia 29°30'58" 99°23'51" -- 1991–97 

8 2931150991651 Brown rain gage near D’Hanis 29°31'15" 99°16'51" -- 1995–98 

9 2928080992030 Valdina Farms rain gage near D’Hanis 29°28'08" 99°20'30" -- 1994–98 

10 2925270991837 Chaney rain gage near D’Hanis 29°25'27" 99°18'37" -- 1994–98 

11 08202700 Seco Creek at Rowe Ranch near D’Hanis 29°22'14" 99°17'15" -- 1994–98 

12 2928480991442 Parkers Creek rain gage no. 1 29°28'48" 99°14'42" -- 1991–98 

13 08202790 Parkers Creek Reservoir Inflow near D’Hanis 29°27'27" 99°15'16" -- 1991–97 

Streamflow 

4 08201500 Seco Creek at Miller Ranch near Utopia 29°34'23" 99°24'10" 45.0 1961–2001 

6 08202450 Seco Creek Reservoir Inflow near Utopia 29°31'34" 99°23'42" 59.4 1991–98 

7 08202490 Seco Creek Reservoir Outflow near Utopia 29°30'58" 99°23'51" 61.4 1991–97 

11 08202700 Seco Creek at Rowe Ranch near D’Hanis 29°22'14" 99°17'15" 165 1961–2001 

13 08202790 Parkers Creek Reservoir Inflow near D’Hanis 29°27'27" 99°15'16" 9.40 1991–97 

15 08202810 Parkers Creek Reservoir Outflow near D’Hanis 29°--'--" 99°--'--" 10.1 1991–97 

Reservoir content 

14 08202800 Parkers Creek Reservoir near D’Hanis 29°26'42" 99°15'09" 10.1 1991–97 

1 A water year is the 12-month period October 1–September 30, designated by the year in which it ends. 

The 20 process-related model parameters listed in 

table 2 represent the physical processes of soil infiltration, soil moisture storage, evapotranspiration (ET), 

interception storage of plants, interflow recession, 

ground-water recession, and surface runoff for each 

land segment. The process-related model parameters for 

each land segment are adjusted to calibrate the model. 

The following parameters can be varied by month to 

account for seasonal variations: interception storage 

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

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

and upper-zone nominal storage (UZSN). The HSPF 

user’s manual (Bicknell and others, 1997) provides a 

more complete description of each parameter. 

The six basin-related model parameters listed in 

table 3 define the areal extent of each land segment, the 

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

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

each reach of the subbasin. These parameters represent 

the physical characteristics of each reach of a subbasin 

and generally remain unchanged during calibration and 

testing of the model. 

One set of process-related parameters was 

developed using uniform parameters for each land 

segment. Annual, monthly, and initial model conditions 

were standardized for each of the six gaged subbasins. 

INTRODUCTION 7 
Table 2. Process-related model parameters for the Hydrological Simulation Program—FORTRAN 

[--, none; ET, evapotranspiration] 

Parameter Description1 Default Minimum Maximum Units 

AGWS Initial active ground-water storage -- 0 100 inches 

AGWETP Available ET satisfied by active ground water 0 0 1.0 -­

AGWRC Active ground-water recession rate -- .001 .999 per day 

BASETP Available ET satisfied by base flow 0 0 1.0 -­

CEPSC Interception storage capacity 0 0 10.0 inches 

DEEPFR Fraction of inflow that enters inactive ground water 0 0 1.0 -­

INFEXP Infiltration equation exponent 2.0 0 10.0 -­

INFILD Ratio of maximum to mean infiltration capacities 2.0 1.0 2.0 -­

INFILT Index to infiltration capacity of soil -- .0001 100.0 inches per hour 

INTFW Interflow inflow -- 0 -- -­

IRC Interflow recession rate -- 1.0–30 .999 per day 

KVARY Nonlinear modifier of ground-water recession rate 0 0 -- per inch 

LSUR Length of assumed overland flow plane -- 1.0 -- feet 

LZETP Available ET satisfied by lower-zone ground water 0 0 .999 -­

LZS Initial lower-zone storage .001 .001 -- inches 

LZSN Lower-zone nominal storage -- .01 100.0 inches 

NSUR Manning’s n for assumed overland flow plane .1 .001 1.0 -­

SLSUR Slope of assumed overland flow plane -- .000001 10.0 feet per foot 

UZS Initial upper-zone storage .001 .001 100.0 inches 

UZSN Upper-zone nominal storage -- .01 10.0 inches 

1 The user’s manual for Hydrological Simulation Program—FORTRAN (Bicknell and others, 1997) provides a more 

complete description of each parameter. 

Table 3. Basin-related model parameters for the Hydrological Simulation 

Program—FORTRAN 

[PERLND, pervious land segment; IMPLND, impervious land segment; FTABLE, 

table of depth, surface area, volume, and discharge for each reach] 

Parameter 

Description1 

(unit) 

AREA Drainage area of each PERLND or IMPLND (acres) 

LEN Reach length (miles) 

DEPTH FTABLE depth (feet) 

SAREA FTABLE surface area (acres) 

VOL FTABLE volume (acre-feet) 

DISCH FTABLE discharge (cubic feet per second) 

1 The user’s manual for Hydrological Simulation Program—FORTRAN (Bicknell and 

others, 1997) provides a more complete description of each parameter. 

8 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
Geology and land-cover data were merged to create 

the land segments. Regionalization (a process of iterative simulations that are conducted within and between 

subbasins during the calibration process to optimize 

parameter values) of the parameter set was done, which 

decreased model accuracy somewhat for individual 

subbasins but increased overall model accuracy. The 

parameter set was assumed to represent the average subbasin conditions for the simulation periods. 

Error and sensitivity analyses were done to qualify the accuracy of the model and the effect that BMPs 

might have had on the model results. The calibration of 

the model was facilitated by a computer program developed by Lumb and others (1994) that provided graphics, 

error statistics, and guidance on which parameters to 

adjust to reduce the differences between simulated and 

measured data. 

Acknowledgments 

Special thanks are extended to Phillip Wright and 

Tom Fillinger, U.S. Department of Agriculture, Natural 

Resources Conservation Service, for assisting with 

data-collection activities and providing invaluable 

insight regarding the Seco Creek Basin. The authors 

also thank the landowners for graciously allowing 

access to their property. 

SIMULATION OF FLOW 

Six selected subbasins in the upper Seco Creek 

Basin were modeled to simulate surface-water-budget 

components. A set of process-related model parameters 

was developed from calibration of data collected from 

the Seco Creek at Miller Ranch and Parkers Creek 

Reservoir Inflow subbasins. The parameter set was 

tested spatially on the Seco Creek Reservoir Inflow 

and Outflow, Seco Creek at Rowe Ranch, and Parkers 

Creek Reservoir Outflow subbasins. Data used in the 

calibration process were collected during July 1, 1992– 

September 30, 1998, and January 1, 1991–June 19, 

1997, for Seco Creek at Miller Ranch and Parkers Creek 

Reservoir Inflow subbasins, respectively. Testing of the 

model parameter set was done using data collected during July 1, 1992–September 30, 1998, and July 1, 1992– 

June 21, 1997, for Seco Reservoir Inflow and Outflow 

subbasins; July 1, 1992–June 19, 1997, for Seco Creek 

at Rowe Ranch subbasin; and January 1, 1991–June 19, 

1997, for Parkers Creek Reservoir Outflow subbasin. 

An error analysis was done to identify sources of error 

that were not explained by the simulation model. A sensitivity analysis was done to identify which parameters 

had the greatest effect on simulation results. 

Model Setup 

Rainfall and streamflow-gaging stations (fig. 2; 

table 1) were installed to collect data needed for calibration and testing of the continuous-simulation model. 

Rainfall data were distributed over a land segment 

using the Theisen-weighting method (Maidment, 1993). 

Rainfall was measured with a network of six float 

rain-gage stations (1991–95) and 12 tipping-bucket 

rain-gage stations (1994–98) in the six gaged subbasins. 

Some rainfall data were lost because of instrumentation 

failure during the study period and were estimated 

using the rainfall measured at nearby stations and best 

professional judgment. Streamflow data used in this 

report were collected at six stations during 1991–98. 

Daily pan-evaporation data measured at Canyon Lake 

(located in Comal County; fig. 1) were used as representative of the study area. Missing pan-evaporation data 

were estimated on the basis of regression analysis of 

data from Canyon Lake and Sea World of Texas located 

in San Antonio (fig. 1). 

Geographic information system (GIS) coverages 

of land cover were provided by the U.S. Department of 

Agriculture (fig. 3). Geology coverages (fig. 4) were 

developed by the USGS from field mapping of geologic 

outcrops, previous studies, and a well inventory. A total 

of 21 PERLNDs were developed for this model on the 

basis of the intersection of geology and land cover 

(table 4). 

The main stream channel for each of the six 

gaged subbasins was subdivided into 1 to 10 reaches. 

Reach lengths were measured from USGS 1:100,000 

hydrography coverages. A channel cross section was 

surveyed for each of four reaches (Seco Creek at Miller 

Ranch, Seco Reservoir Inflow, Seco Reservoir Outflow, 

and Parkers Creek Reservoir Inflow) to compute the 

FTABLE values. A channel cross section was estimated 

for each reach of the Seco Creek at Rowe Ranch and 

Parkers Creek Reservoir Outflow subbasins because 

access was limited. The surface area as a function of 

depth was computed by multiplying the average channel 

width by the reach length. The volume as a function 

of depth was computed by multiplying the average 

cross-sectional area by the reach length. Discharge 

as a function of depth was determined from (1) the 

SIMULATION OF FLOW 9 
Table 4. Basin-related parameters for each gaged subbasin of the upper Seco Creek Basin 

[Land segments characterized and designated by 2-digit geology/land-cover descriptor: 01, Devils River Formation rangeland; 02, Buda 

Limestone rangeland; 03, Devils River Formation pasture/hay; 04, Eagle Ford Group pasture/hay; 05, Eagle Ford Group rangeland; 

06, Austin Group rangeland; 07, Del Rio Clay rangeland; 08, Glen Rose Limestone rangeland; 09, Glen Rose Limestone pasture/hay; 

10, alluvium pasture/hay; 11, alluvium rangeland; 12, Fort Terrett Formation rangeland; 13, Segovia Formation rangeland; 17, Leona 

Formation cropland; 18, Leona Formation rangeland; 20, Leona Formation irrigated cropland; 24 Leona Formation irrigated pasture/hay; 

25, Leona Formation pasture/hay; 33, Anacacho Limestone rangeland; 34, Austin Group cropland; 37, Uvalde Gravel rangeland 

PERLND, pervious land segment; IMPLND, impervious land segment; NRECH, number of reaches; LEN, reach lengths; mi, miles; 

FTABLE, table of depths (DEPTH), surface areas (SAREA), volumes (VOL), discharges (DISCH), and channel losses (CHNLOSS) for 

reaches; ft, feet; acre-ft, acre-feet; ft3

/s, cubic feet per second] 

Subbasin 

Parameter

1 Miller 

Ranch 

Seco Creek 

Reservoir 

Inflow 

Seco Creek 

Reservoir 

Outflow 

Rowe 

Ranch 

Parkers Creek 

Reservoir 

Inflow 

Parkers Creek 

Reservoir 

Outflow 

PERLND AREA (acres) 

01 0 0 351 24,198 4,902 5,310 

02 0 0 0 2,227 142 142 

03 0 0 0 52 70 70 

04 0 0 0 16 49 49 

05 0 0 0 489 44 44 

06 0 0 0 5,107 47 47 

07 0 0 0 162 750 805 

08 20,080 27,383 27,928 42,855 0 0 

09 0 0 82 194 0 0 

10 0 0 52 52 0 0 

11 1,428 2,921 3,148 4,504 0 0 

12 6,933 7,325 7,325 9,662 0 0 

13 393 393 393 393 0 0 

17 0 0 0 2,105 0 0 

18 0 0 0 8,591 0 0 

20 0 0 0 1,020 0 0 

24 0 0 0 352 0 0 

25 0 0 0 586 0 0 

33 0 0 0 1,302 0 0 

34 0 0 0 698 0 0 

37 0 0 0 867 0 0 

IMPLND AREA (acres) 14 14.3 14.4 14.5 .1 .1 

NRECH 5 1 1 10 1 1 

LEN (mi) 1.88–4.35 4.03 .89 4.97–21.7 7.95 1.31 

FTABLE 

DEPTH (ft) 0–24.0 0–19.8 0–20.7 0–19.8 0–6.48 0–48.1 

SAREA (acres) 0–259 0–208 0–106 0–815 0–101 0–306 

VOL (acre-ft) 0–3,270 0–2,140 0–125 0–7,900 0–308 0–3,246 

DISCH (ft3/s) 0–64,900 0–32,000 0–43,400 0–41,400 0–2,200 0–4,000 

CHNLOSS (ft3/s) 0 0–50 0–26 0–167 0–37 0–169 

1 The user’s manual for Hydrological Simulation Program—FORTRAN (Bicknell and others, 1997) provides a more complete 

description of each parameter. 

10 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
stage-discharge relation defined at the streamflowgaging station; (2) water-surface-profile model simulations using WSPRO (Shearman, 1990); or (3) subbasins 

with similar geologic and slope characteristics. 

Model Calibration and Testing 

The HSPF model was calibrated using data from 

the Seco Creek at Miller Ranch and Parkers Creek 

Reservoir Inflow streamflow-gaging stations and spatially tested using data from the four remaining streamflow-gaging stations. Simulation periods for each 

subbasin are as follows: July 1, 1992–September 30, 

1998, for Seco Creek at Miller Ranch; July 1, 1992– 

September 30, 1998, for Seco Creek Reservoir Inflow; 

July 1, 1992–June 21, 1997, for Seco Creek Reservoir 

Outflow; July 1, 1992–June 19, 1997, for Seco Creek 

at Rowe Ranch; January 1, 1991–June 19, 1997, for 

Parkers Creek Reservoir Inflow; and January 1, 1991– 

June 19, 1997, for Parkers Creek Reservoir Outflow. 

These time periods were selected on the basis of the 

results of initial simulations and available data. Initial 

conditions during January 1, 1991–June 30, 1992, were 

very wet. At Seco Creek at Miller Ranch, initial model 

simulations that included the 1991 time period consistently oversimulated runoff in the subsequent years. 

Because it was not possible to determine a reasonable 

set of initial conditions prior to July 1992, the start of 

the simulation period was changed. 

Initial estimates for the 20 process-related 

parameters were (1) based on the physical properties of 

geology, land cover, soils, and slopes in the subbasins 

(Chow and others, 1988); or (2) assigned the default 

values listed in table 2. The 16 calibration annual 

parameter values are listed in table 5, the one calibration 

monthly parameter value is listed in table 6, and the 

three initial-condition values are listed in table 7. 

Parameter values varied by geology and land 

cover corresponding to the physical process that the 

parameter represents. For example, the parameter 

values for AGWETP, AGWRC, BASETP, DEEPFR, 

INFILT, INTFW, IRC, KVARY, and LZSN varied by 

geology and represented the different storage and infiltration capacities of the soil and bedrock, whereas the 

parameter value for UZSN varied by land cover and represented the different surface roughness, interception 

storages, and ET potentials of the land-cover groups 

(table 5). The parameter values for SLSUR varied by 

individual subbasin slopes. The parameter values for 

CEPSC were assumed to be uniform for all land segments. The parameter values for INTFW, IRC, and 

NSUR were not varied monthly because data were not 

available to support use of seasonal variations. Monthly 

values for LZETP were used to account for seasonal differences in ET potential (table 6). ET potential is greatest during late spring and summer, and it is least during 

winter. 

Values of the annual parameters (AGWETP, 

AGWRC, BASETP, CEPSC, DEEPFR, INFEXP, 

INFILD, INFILT, INTFW, IRC, KVARY, LSUR, 

LZSN, NSUR, SLSUR, and UZSN) and values of the 

monthly parameter (LZETP) were adjusted during the 

calibration process using the software program 

HSPEXP (Lumb and others, 1994). The values for the 

initial-condition parameters (AGWS, LZS, and UZS) 

were initially estimated from default values and were 

revised during calibration. These values were varied by 

land segment; the values for all subbasins are presented 

in table 7. The predominant land cover in each subbasin 

had the greatest impact on selecting initial-condition 

values. An iterative process was used to determine 

initial-condition values, which produced the best (best 

match of simulated and beginning observed runoff) 

model simulation results. 

The iteratively calibrated parameter set (tables 5– 

6) was developed from 33 storms—17 in the Seco 

Creek at Miller Ranch subbasin and 16 in the Parkers 

Creek Reservoir Inflow subbasin. Another 27 storms 

were used to test the parameters spatially to assess the 

transferability of the parameter set to the other four 

subbasins. Parameter testing was done in the following 

subbasins: Seco Creek Reservoir Inflow (13 storms), 

Seco Creek Reservoir Outflow (4 storms), Seco Creek 

at Rowe Ranch (4 storms), and Parkers Creek Reservoir 

Outflow (6 storms). 

Simulated and measured (observed) monthly 

flows are presented in figures 5–10 for the six Seco 

Creek subbasins. Simulated flow for the six subbasins 

generally compares favorably with observed flow. 

Flow occurs more than 95 percent of the time in the 

Seco Creek at Miller Ranch subbasin, about 11 percent 

of the time in the Seco Creek Reservoir Inflow subbasin, 

about 1 percent of the time in the Parkers Creek Reservoir Inflow subbasin, and less than 1 percent of the time 

in the Seco Creek Reservoir Outflow, Seco Creek at 

Rowe Ranch, and Parkers Creek Reservoir Outflow 

subbasins. Model simulations indicate surface runoff 

(overland flow) is the major component of the flow for 

the Seco Creek Reservoir Outflow (fig. 7) and Seco 

Creek at Rowe Ranch (fig. 8) subbasins; whereas, base 

SIMULATION OF FLOW 11 
105 

10-1 

1 

10 

102 

103 

104 

10-2 

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

PERCENT CHANCE FLOW EXCEEDED 

FLOW-DURATION CURVE 

FLO

W, IN CUBIC FEET PER SECOND 

Simulated 

Observed 

1992 1993 1994 1995 1996 1997 1998 

7.5 

0 

400 

360 

320 

280 

240 

200 

160 

120 

80 

40 

0 

RAINFALL,

IN INCHES

FLO

W, IN CUBIC FEET PER SECOND 

Surface runoff 

Interflow 

Base flow 

Observed 

15.0 

Simulated 

Figure 5. Simulated and observed flow for 08201500 Seco Creek at Miller Ranch near Utopia. 

15.0 

7.5 

0 

400 

360 

320 

280 

240 

200 

160 

120 

80 

40 

0 

RAINFALL,

IN INCHES

FLO

W, IN CUBIC FEET PER SECOND 

1992 1993 1994 1995 1996 1997 1998 

Surface runoff 

Interflow 

Base flow 

Observed 

105 

10-1 

1 

10 

102 

103 

104 

10-2 

FLO

W, IN CUBIC FEET PER SECOND 

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

PERCENT CHANCE FLOW EXCEEDED 

FLOW-DURATION CURVE 

Simulated 

Observed 

Simulated 

Figure 6. Simulated and observed flow for 08202450 Seco Creek Reservoir Inflow near Utopia. 

flow is the major component of the flow for the Parkers annual runoff, highest 10 percent of flows, lowest 50 

Creek Reservoir Outflow (fig. 10) subbasin. Model sim- percent of flows, storm volumes, simulated storm interulations indicate that surface runoff and base flow both flow, simulated storm surface runoff, summer flow 

contribute to flow for the Seco Creek at Miller Ranch volume, winter flow volume, and summer storm vol­

(fig. 5), Seco Creek Reservoir Inflow (fig. 6), and ume. Table 9 presents calibration and testing errors for 

Parkers Creek Reservoir Inflow (fig. 9) subbasins. the entire simulation period by subbasin. 

A summary of calibration and testing results by Simulated ET ranged from 119 in. for the Seco 

subbasin are presented in table 8. Results comprise ET, Creek at Rowe Ranch subbasin to 164 in. for the Seco 

12 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
10 

5 

0 

40 

36 

32 

28 

24 

20 

16 

12 

8 

4 

0 

RAINFALL,

IN INCHES

FLO

W, IN CUBIC FEET PER SECOND 

1992 1993 1994 1995 1996 1997 

Simulated (surface runoff) 

Observed 

10 

Less than 1 percent chance 

flow exceeded 1 cubic foot per 

second 

Figure 7. Simulated and observed flow for 08202490 Seco Creek Reservoir Outflow near Utopia. 

8 

4 

0 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

RAINFALL,

IN INCHES

FLO

W, IN CUBIC FEET PER SECOND 

1992 1993 1994 1995 1996 1997 

Simulated (surface runoff) 

Observed 

105 

1 

10 

102 

103 

104 

FLO

W, IN CUBIC FEET PER SECOND 

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

PERCENT CHANCE FLOW EXCEEDED 

FLOW-DURATION CURVE 

Simulated 

Observed 

Figure 8. Simulated and observed flow for 08202700 Seco Creek at Rowe Ranch near D’Hanis. 

Creek at Miller Ranch subbasin. Intuitively, this appears 

reasonable because the least amount of rainfall occurs in 

the Seco Creek at Rowe Ranch subbasin (less water is 

available for ET), and the greatest amount of rainfall 

occurs in the Seco Creek at Miller Ranch subbasin 

(more water is available for ET). 

The simulated annual runoff volumes for all subbasins match well with observed annual runoff volumes 

for the calibration and testing periods. Maximum simulated annual runoff volumes occurred at Seco Creek at 

Miller Ranch and Seco Creek Reservoir Inflow (above 

Edwards aquifer recharge zone), and minimum annual 

runoff volumes occurred at Seco Creek at Rowe Ranch 

and Parkers Creek Reservoir Outflow (on and below the 

Edwards aquifer recharge zone). More annual runoff 

occurs in subbasins that are predominantly Glen Rose 

SIMULATION OF FLOW 13 
8 

4 

0 

8.0 

7.2 

6.4 

4.8 

4.0 

3.2 

2.4 

1.6 

0.8 

0 

5.6 

RAINFALL,

IN INCHES

FLO

W, IN CUBIC FEET PER SECOND 

1992 1993 1994 1995 1996 1997 

105 

1 

10 

102 

103 

104 

FLO

W, IN CUBIC FEET PER SECOND 

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

PERCENT CHANCE FLOW EXCEEDED 

FLOW-DURATION CURVE 

Simulated 

Observed 

Surface runoff 

Interflow 

Base flow 

Observed 

Simulated 

Figure 9. Simulated and observed flow for 08202790 Parkers Creek Reservoir Inflow near D’Hanis. 

8 

4 

0 

0.20 

0.18 

0.16 

0.14 

0.12 

0.10 

0.08 

0.06 

0.04 

0.02 

0 

RAINFALL,

IN INCHES

FLO

W, IN CUBIC FEET PER SECOND 

1992 1993 1994 1995 1996 1997 

Interflow 

Base flow 

Observed 

Surface runoff Simulated 

Less than 1 percent chance 

flow exceeded 1 cubic foot per 

second 

Figure 10. Simulated and observed flow for 08202810 Parkers Creek Reservoir Outflow near D’Hanis. 

Limestone than subbasins that are predominantly Devils during 10 percent of the year. In other words, at least 

River Formation, which is reasonable because the Glen 90 percent of the time there is no flow at Seco Creek 

Rose generally is less permeable than the Devils River. Reservoir Inflow, Seco Creek Reservoir Outflow, Seco 

Simulated highest 10 percent of flows for all sub- Creek at Rowe Ranch, Parkers Creek Reservoir Inflow, 

basins match well with observed data. Analysis of the and Parkers Creek Reservoir Outflow. Channel losses to 

lowest 50 percent of flows indicates that Seco Creek at the Glen Rose Limestone and Devils River Formation 

Miller Ranch is the only subbasin with consistent base and high infiltration rates capture low flows in these five 

flow. In the other five subbasins all of the flow occurs subbasins. 

14 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
Table 5. Calibration annual parameters 

[Land segments characterized and designated by 2-digit geology/land-cover descriptor: 01, Devils River Formation rangeland; 02, Buda 

Limestone rangeland; 03, Devils River Formation pasture/hay; 04, Eagle Ford Group pasture/hay; 05, Eagle Ford Group rangeland; 

06, Austin Group rangeland; 07, Del Rio Clay rangeland; 08, Glen Rose Limestone rangeland; 09, Glen Rose Limestone pasture/hay; 

10, alluvium pasture/hay; 11, alluvium rangeland; 12, Fort Terrett Formation rangeland; 13, Segovia Formation rangeland; 17, Leona 

Formation cropland; 18, Leona Formation rangeland; 20, Leona Formation irrigated cropland; 24 Leona Formation irrigated pasture/hay; 

25, Leona Formation pasture/hay; 33, Anacacho Limestone rangeland; 34, Austin Group cropland; 37, Uvalde Gravel rangeland 

Parameter definitions in table 2; units below parameter except where no units. /d, per day; in., inches; in/hr, inches per hour; /in., per inch; 

ft, feet; ft/ft, foot per foot] 

Land AGWRC2,3 CEPSC3 INFILT3 

AGWETP1,3 BASETP3 DEEPFR3 INFEXP1 INFILD1 

segment (/d) (in.) (in/hr) 

01 0 0.01 0.01 0.04 0.96 2.0 2.0 0.32 

02 0 .01 .01 .04 .06 2.0 2.0 .08 

03 0 .01 .01 .04 .96 2.0 2.0 .32 

04 0 .01 .01 .04 .06 2.0 2.0 .08 

05 0 .01 .01 .04 .06 2.0 2.0 .08 

06 0 .01 .01 .04 .06 2.0 2.0 .08 

07 0 .10 .01 .04 .02 2.0 2.0 .08 

08 0 .96 0 .04 .02 2.0 2.0 .32 

09 0 .96 0 .04 .02 2.0 2.0 .32 

10 0 .85 .05 .04 .02 2.0 2.0 .82 

11 0 .85 .05 .04 .02 2.0 2.0 .82 

12 0 .50 0 .04 .50 2.0 2.0 .62 

13 0 .50 0 .04 .50 2.0 2.0 .62 

17 .20 .01 .20 .04 .99 2.0 2.0 1.00 

18 .20 .01 .20 .04 .99 2.0 2.0 1.00 

20 .20 .01 .20 .04 .99 2.0 2.0 1.00 

24 .20 .01 .20 .04 .99 2.0 2.0 1.00 

25 .20 .01 .20 .04 .99 2.0 2.0 1.00 

33 0 .01 .05 .04 .04 2.0 2.0 .15 

34 0 .01 .05 .04 .06 2.0 2.0 .11 

37 0 .01 .05 .04 .02 2.0 2.0 .75 

Land IRC2,3 KVARY1,3 LSUR3 LZSN3 SLSUR3 UZSN3 

INTFW3 NSUR3,4 

segment (/d) (/in.) (ft) (in.) (ft/ft) (in.) 

01 0.001 0.01 0 800 4.00 0.15 0.090 0.22 

02 .100 .10 0 800 4.00 .15 .090 .22 

03 .001 .01 0 800 4.00 .15 .090 .22 

04 .100 .10 0 800 4.00 .15 .090 .22 

05 .100 .10 0 800 4.00 .15 .090 .22 

06 .100 .10 0 800 4.00 .15 .090 .22 

07 .001 .10 0 800 4.00 .15 .090 .22 

08 .80 .60 .04 800 5.50 .15 .080 .40 

09 .80 .60 .04 800 5.50 .15 .080 .40 

10 .10 .30 0 800 5.50 .15 .015 .35 

11 .10 .30 0 800 5.50 .15 .015 .35 

12 .10 .10 0 800 5.50 .15 .097 .35 

13 .10 .10 0 800 5.50 .15 .119 .30 

17 .01 .01 0 800 9.00 .15 .016 .55 

18 .01 .01 0 800 9.00 .15 .016 .55 

20 .01 .01 0 800 9.00 .15 .016 .55 

24 .01 .01 0 800 9.00 .15 .016 .55 

25 .01 .01 0 800 9.00 .15 .010 .55 

33 .01 .01 0 800 4.00 .15 .010 .22 

34 .01 .01 0 800 4.00 .15 .010 .22 

37 .01 .01 0 800 5.00 .15 .010 .35 

1 Default value used. 3 Parameter revised during calibration.
 2 Initial estimate was default value. 4 Initial estimate from Chow and others (1988).


SIMULATION OF FLOW 15 
Table 6. Calibration monthly parameter 

[Land segments characterized and designated by 2-digit geology/land-cover descriptor: 01, Devils River Formation rangeland; 02, Buda 

Limestone rangeland; 03, Devils River Formation pasture/hay; 04, Eagle Ford Group pasture/hay; 05, Eagle Ford Group rangeland; 

06, Austin Group rangeland; 07, Del Rio Clay rangeland; 08, Glen Rose Limestone rangeland; 09, Glen Rose Limestone pasture/hay; 

10, alluvium pasture/hay; 11, alluvium rangeland; 12, Fort Terrett Formation rangeland; 13, Segovia Formation rangeland; 17, Leona 

Formation cropland; 18, Leona Formation rangeland; 20, Leona Formation irrigated cropland; 24 Leona Formation irrigated pasture/hay; 

25, Leona Formation pasture/hay; 33, Anacacho Limestone rangeland; 34, Austin Group cropland; 37, Uvalde Gravel rangeland 

Parameter definition in table 2; LZETP has no units] 

Land 

segment 

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

LZETP1,2

01–13 0.27 0.27 0.32 0.37 0.42 0.45 0.58 0.58 0.60 0.47 0.35 0.35 

17–18 .27 .27 .32 .37 .42 .45 .58 .58 .60 .47 .35 .35 

20 .27 .27 .32 .37 .42 .45 .58 .58 .60 .47 .35 .35 

24–25 .27 .27 .32 .37 .42 .45 .58 .58 .60 .47 .35 .35 

33–34 .27 .27 .32 .37 .42 .45 .58 .58 .60 .47 .35 .35 

37 .27 .27 .32 .37 .42 .45 .58 .58 .60 .47 .35 .35 

1 Initial estimate was default value. 2 Parameter revised during calibration. 

Table 7. Initial-condition values for model calibration 

[Land segments characterized and designated by 2-digit geology/land-cover descriptor: 01, Devils River Formation rangeland; 02, Buda 

Limestone rangeland; 03, Devils River Formation pasture/hay; 04, Eagle Ford Group pasture/hay; 05, Eagle Ford Group rangeland; 

06, Austin Group rangeland; 07, Del Rio Clay rangeland; 08, Glen Rose Limestone rangeland; 09, Glen Rose Limestone pasture/hay; 

10, alluvium pasture/hay; 11, alluvium rangeland; 12, Fort Terrett Formation rangeland; 13, Segovia Formation rangeland; 17, Leona 

Formation cropland; 18, Leona Formation rangeland; 20, Leona Formation irrigated cropland; 24 Leona Formation irrigated pasture/hay; 

25, Leona Formation pasture/hay; 33, Anacacho Limestone rangeland; 34, Austin Group cropland; 37, Uvalde Gravel rangeland 

Parameter definitions in table 2; units below parameter. in., inches] 

Land AGWS1,2 LZS1,2 UZS1,2 

Station segments (in.) (in.) (in.) 

Miller Ranch 08–13 0.9 4.0 0 

Seco Creek Reservoir Inflow 08–12 .9 4.0 0 

Seco Creek Reservoir Outflow 01 0 4.4 0 

08–11 .9 4.0 0 

Rowe Ranch	 01–07 0 4.4 0 

08–13 .9 4.0 0 

17–18 0 2.0 .2 

20 0 2.0 .2 

24–25 0 2.0 .2 

33–34 0 2.0 .2 

37 0 2.0 .2 

Parkers Creek Reservoir Inflow 01–07 0 4.4 0 

Parkers Creek Reservoir Outflow 01–07 0 4.4 0 

16 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
Table 8. Summary of calibration and testing results for upper Seco Creek subbasins 

[Volumes, in inches, are cumulative for entire simulation period for each subbasin and are computed on the basis of individual subbasin 

drainage areas. Sim., simulated; Obs., observed; in., inches; --, not measured] 

Miller 

Ranch 

Seco Creek 

Reservoir 

Inflow 

Seco Creek 

Reservoir 

Outflow 

Evapotranspiration 

Annual runoff 

Highest 10 percent of flows 

Lowest 50 percent of flows 

Volume of selected storms 

Interflow 

Surface runoff 

Summer flow volume 

Winter flow volume 

Summer storm volume 

Sim. 

(in.) 

164 

26.1 

16.0 

1.75 

6.05 

1.16 

5.96 

11.6 

3.03 

2.03 

Obs. 

(in.) 

27.3 

16.9 

2.02 

5.79 

12.5 

3.06 

1.95 

--

--

--

Sim. 

(in.) 

157 

45.2 

45.2 

0 

14.8 

1.31 

5.37 

30.4 

.40 

5.80 

Obs. 

(in.) 

44.5 

44.5 

0 

15.7 

30.4 

.59 

7.17 

--

--

--

Sim. 

(in.) 

127 

23.9 

23.9 

0 

23.9 

.18 

1.17 

.03 

0 

0 

Obs. 

(in.) 

24.0 

24.0 

0 

23.9 

.01 

0 

0 

--

--

--

Rowe 

Ranch 

Parkers Creek 

Reservoir 

Inflow 

Parkers Creek 

Reservoir 

Outflow 

Evapotranspiration 

Annual runoff 

Highest 10 percent of flows 

Lowest 50 percent of flows 

Volume of selected storms 

Interflow 

Surface runoff 

Summer flow volume 

Winter flow volume 

Summer storm volume 

Sim. 

(in.) 

119 

.57 

.56 

0 

.09 

.07 

.57 

.03 

0 

0 

Obs. 

(in.) 

0.58 

.57 

0 

.12 

.01 

0 

.01 

--

--

--

Sim. 

(in.) 

136 

2.48 

2.48 

0 

1.88 

0 

3.11 

.61 

.82 

.09 

Obs. 

(in.) 

2.57 

2.56 

0 

2.01 

.66 

.80 

.13 

--

--

--

Sim. 

(in.) 

131 

.44 

.44 

0 

.38 

0 

1.75 

.08 

.23 

.08 

Obs. 

(in.) 

0.44 

.44 

0 

.35 

.12 

.17 

.12 

--

--

--

Table 9. Summary of calibration and testing errors for upper Seco Creek subbasins 

Miller Seco Creek Seco Creek Parkers Creek Parkers Creek 

Rowe Ranch Reservoir Reservoir Reservoir Reservoir Ranch error error Inflow error Outflow error Inflow error Outflow error (percent) (percent) (percent) (percent) (percent) (percent) 

Annual runoff -4.2 1.5 -0.2 -1.4 -3.4 0 

Highest 10 percent of flows -5.2 1.5 -.2 -1.8 -3.4 0 

Lowest 50 percent of flows -13.4 0 0 0 0 0 

SIMULATION OF FLOW 17 
The simulated storm volumes matched fairly well 

(within 10 percent) with observed data for all subbasins 

except Seco Creek at Rowe Ranch. However, the 

observed storm volume at Seco Creek at Rowe Ranch 

was small in comparison to the other five subbasins. 

Simulated storm interflow was greatest (1.31 in.) 

for the Seco Creek Reservoir Inflow subbasin and zero 

for the Parkers Creek Reservoir Inflow and Outflow 

subbasins. Total simulated storm surface runoff was 

greatest (more than 5.0 in.) for the Seco Creek at Miller 

Ranch and Seco Creek Reservoir Inflow subbasins 

(both located primarily on the Glen Rose Limestone) 

and substantially less (0.57 in.) for the Seco Creek at 

Rowe Ranch subbasin (located on and below the 

Edwards aquifer recharge zone). 

Results of simulated and observed summer and 

winter flow volumes for subbasins varied. Simulated 

summer flow volumes matched fairly well with 

observed data for all subbasins, and simulated winter 

flow volumes matched fairly well with observed data 

for all subbasins. Seco Creek Reservoir Inflow and 

Parkers Creek Reservoir Outflow had the greatest errors 

associated with simulated winter flow volumes. 

The summer storm volumes were the most 

difficult to simulate. Summer storms typically are of 

short duration, intense, and limited in geographical 

extent, which makes modeling of individual summer 

storms difficult. Seco Creek at Miller Ranch, Seco 

Creek Reservoir Outflow, Seco Creek at Rowe Ranch, 

Parkers Creek Reservoir Inflow, and Parkers Creek 

Reservoir Outflow simulations matched well with 

observed data. Simulations for the Seco Creek Reservoir Inflow subbasin substantially undersimulated 

summer storm volumes. 

Error Analysis 

The types of errors from the model calibration 

and testing can be classified as measurement errors or 

systematic errors. Measurement errors are introduced 

as a result of missing data, inaccurate stage-discharge 

relations, and unknown channel losses. Data were 

missing for several rainfall stations and had to be estimated from adjacent rain gages, particularly in the Seco 

Creek at Rowe Ranch subbasin. The streamflow-gaging 

stations at Seco Creek Reservoir Inflow and Seco Creek 

at Rowe Ranch had the least accurate stage-discharge 

relations (for depths greater than wading), which likely 

resulted in inaccurate discharge records. Peak flows at 

Seco Creek Reservoir Inflow approached 17,000 ft3

/s 

(on the basis of an indirect measurement of discharge), 

whereas the maximum wading discharges were 600 to 

900 ft3

/s. Hence, the upper end (greater than 900 ft

3

/s) 

of the Seco Creek Reservoir Inflow rating is less accurate. Likewise, flows at Seco Creek at Rowe Ranch 

typically were measured only at wading stages. Channel 

losses to the Devils River Formation were estimated 

on the basis of channel losses measured in the reach 

between Seco Creek Reservoir Inflow and Seco Creek 

Reservoir Outflow and also in the reach between 

Parkers Creek Reservoir Inflow and Parkers Creek 

Reservoir Outflow. ET is a major component of the 

water budget. Errors between actual ET and simulated 

ET on the basis of measured pan evaporation are 

unknown. 

Systematic errors are associated with the inability 

of the simulation model to represent the physical processes of runoff. These errors are represented in the 

model equations and selected model parameters. The 

PERLNDs used in this model might not have represented adequately all the different hydrologic response 

units of the study area. Also, some uncertainty existed 

in the values of each reach volume and the corresponding discharge for the FTABLES, particularly for the 

Seco Creek at Rowe Ranch subbasin. 

Seasonal systematic errors might be detected by 

looking at differences between simulated and observed 

monthly discharge (fig. 11). Ideally, simulations for a 

given month would exhibit no errors or only those errors 

that are evenly distributed above and below zero. ET 

potential was adjusted to account for seasonal variation 

in runoff. ET rates change with seasons; these changes 

were simulated by adjusting the parameter values for 

LZETP (table 6). Differences in simulated monthly discharge were reasonably well distributed above and 

below zero for all subbasins with no apparent bias. The 

greatest and least differences between simulated and 

observed monthly discharges occurred in the Seco 

Creek at Rowe Ranch and Parkers Creek Reservoir Outflow subbasins, respectively. 

Sensitivity Analysis 

Sensitivity analyses were iteratively done during 

the calibration and testing process. The values of nearly 

all parameters in tables 5–7 were changed, and the 

results were evaluated. Each parameter was modified 

to represent a reasonable change and even an unreasonable change. In order of importance, the parameters 

INFILT, LZSN, LZETP, DEEPFR, UZSN, AGWRC, 

18 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
4.0 

3.2 

2.4 

1.6 

0.8 

0 

-0.8 

-1.6 

-2.4 

-3.2 

-4.0 

J F M A MJ J A S O N D 

MONTH 

Parkers Creek 

Reservoir Inflow 

4.0 

3.2 

2.4 

1.6 

0.8 

0 

-0.8 

-1.6 

-2.4 

-3.2 

-4.0 

Seco Creek

 Reservoir Outflow

SIMULATED MINUS OBSERVED MONTHLY DISCHARGE, IN CUBIC FEET PER SECOND

J F M A MJ J A S O N D 

MONTH 

0.040 

0.032 

0.024 

0.016 

0.008 

0 

-0.008 

-0.016 

-0.024 

-0.032 

-0.040 

Parkers Creek

 Reservoir Outflow 

J F M A MJ J A S O N D 

MONTH 

20 

16 

12 

8 

4 

0 

-4 

-8 

-12 

-16 

-20 

Seco Creek

 Reservoir Inflow 

J F M A MJ J A S O N D 

MONTH 

40 

32 

24 

16 

8 

0 

-8 

-16 

-24 

-32 

-40 

Seco Creek at

 Rowe Ranch

SIMULATED MINUS OBSERVED MONTHLY DISCHARGE, IN CUBIC FEET PER SECOND

J F M A MJ J A S O N D 

MONTH 

20 

16 

12 

8 

4 

0 

-4 

-8 

-12 

-16 

-20 

Seco Creek at

 Miller Ranch 

J F M A MJ J A S O N D 

MONTH 

Figure 11. Difference between simulated and observed monthly discharge for upper Seco Creek subbasins. 

SIMULATION OF FLOW 19 
Table 10. Simulation results from a 5- to 6-percent reduction in evapotranspiration associated with brush 

management 

[Reduction in evapotranspiration produced in the model by 20-percent reduction in value of LZET] 

Miller 

Ranch 

(percent 

change) 

Seco Creek 

Reservoir 

Inflow 

(percent 

change) 

Seco Creek 

Reservoir 

Outflow 

(percent 

change) 

Rowe 

Ranch 

(percent 

change) 

Parkers 

Creek 

Reservoir 

Inflow 

(percent 

change) 

Parkers 

Creek 

Reservoir 

Outflow 

(percent 

change) 

Evapotranspiration -5 -5 -5 -5 -6 -5 

Annual runoff 27 6 1 18 47 2 

Highest 10 percent of flows 19 6 1 16 47 2 

Lowest 50 percent of flows 50 0 0 0 0 0 

Volume of selected storms 24 8 1 8 41 3 

Interflow 27 18 50 17 0 0 

Surface runoff 14 21 33 16 41 48 

Summer flow volume 18 4 0 33 85 13 

Winter flow volume 40 13 0 3 9 0 

Summer storm volume 10 3 0 24 200 13 

and channel losses had the most effect on storm and 

base-flow simulations (tables 5, 6). The values for the 

parameters AGWS, LZS, and UZS represented the initial soil-moisture storage conditions at the start of the 

simulations (table 7). Changes in these values did have 

an effect on the model results early in a simulation 

period, but changes did not have an effect on model 

results more than 8 months into the simulation period. 

SIMULATION OF BEST-MANAGEMENT 

PRACTICES 

Alternative scenarios were developed to evaluate 

the potential effects of the primary BMPs of interest, 

which include brush management (removal of woody 

species) and weather modification (increased rainfall 

totals and rainfall intensities) on surface-water quantity 

in the upper Seco Creek Basin. Evaluation of a single 

BMP on a field scale was not possible because numerous BMPs at multiple sites were being implemented 

during the simulation period. Rather, the effect of all the 

BMPs were reflected in the final model parameter values. The following results of the scenarios provided an 

estimate of BMP effects. The alternative scenarios indicated a direction (increase or decrease) but not necessarily an accurate magnitude of the change. 

Brush Management 

The brush management BMP is the removal 

(treatment) of woody species (Juniperus ashei, locally 

known as cedar) and continued maintenance to eliminate the regrowth of the woody species. The rationale 

for the BMP is that by removing deep-rooted woody 

species, the ET potential in the basin will be reduced. 

Another study done in the upper Seco Creek Basin 

(Dugas and others, 1998) measured springflows downgradient of treatment areas and ET. After treatment, 

results indicated that discharge increased and that ET 

was reduced an average of about 0.003 in. per day. 

In the Seco Creek at Miller Ranch subbasin, 

about 3,700 acres (12.8 percent of the subbasin) was 

treated by brush management (Phillip Wright, U.S. 

Department of Agriculture, Natural Resources Conservation Service, written commun., 2000). Other subbasins in the upper Seco Creek Basin had a minimal 

amount of woody species removed. 

Simulation of woody-species treatment was 

approximated by reducing the values for the parameter 

LZETP by 20 percent. Simulation results are presented 

for all subbasins in table 10. Apparent large-percentage 

increases might not translate into substantial amounts of 

water if simulated volumes of water (table 8) were relatively small (in relation to other subbasins) prior to the 

simulation of LZETP reductions. Simulated results for 

the six subbasins indicated that reducing the values for 

20 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
Table 11. Simulation results from a 10-percent increase in rainfall totals and intensities associated with weather 

modification 

Miller 

Ranch 

(percent 

change) 

Seco Creek 

Reservoir 

Inflow 

(percent 

change) 

Seco Creek 

Reservoir 

Outflow 

(percent 

change) 

Rowe 

Ranch 

(percent 

change) 

Parkers 

Creek 

Reservoir 

Inflow 

(percent 

change) 

Parkers 

Creek 

Reservoir 

Outflow 

(percent 

change) 

Evapotranspiration 6 6 6 6 5 6 

Annual runoff 32 10 2 48 92 9 

Highest 10 percent of flows 30 10 2 48 92 9 

Lowest 50 percent of flows 37 0 0 0 0 0 

Volume of selected storms 42 15 2 38 87 11 

Interflow 45 31 78 40 0 0 

Surface runoff 36 47 62 48 77 101 

Summer flow volume 23 6 0 62 115 13 

Winter flow volume 42 13 0 36 45 4 

Summer storm volume 33 7 0 55 189 13 

LZETP by 20 percent decreases simulated total ET by 

5 to 6 percent. 

The simulated increases in annual runoff and 

highest 10 percent of flows for the six subbasins ranged 

from 1 to 47 percent. The simulated lowest 50 percent 

of flows increased only at the Seco Creek at Miller 

Ranch subbasin because low flows do not occur frequently in the five other subbasins (table 8). Simulated 

volume of selected storms increased from 1 to 41 percent for the six subbasins; interflow ranged from no 

change to an increase of 50 percent; surface runoff 

increased from 14 to 48 percent. Simulated summer 

flow volume ranged from no change to an increase of 

85 percent; winter flow volume ranged from no change 

to an increase of 40 percent; and summer storm volume 

ranged from no change to an increase of 200 percent. 

The simulated annual runoff increased for all 

subbasins, but only the Seco Creek at Miller Ranch subbasin indicated increased base flow. Low flows in the 

Seco Creek at Miller Ranch, Seco Creek Reservoir 

Inflow, Seco Creek at Rowe Ranch, and Parkers Creek 

Reservoir Inflow subbasins are lost as channel losses to 

the Glen Rose Limestone of the Trinity aquifer, Leona 

Formation (a local shallow aquifer), and Devils River 

Formation of the Edwards aquifer. Very small changes 

in runoff were indicated in the Seco Creek Reservoir 

Outflow and Parkers Creek Reservoir Outflow subbasins because most of the flows in these subbasins 

were retained in Seco Creek Reservoir and Parkers 

Creek Reservoir, except those flows from large storms. 

Weather Modification 

Weather modification currently (2001) is being 

conducted by the Edwards Aquifer Authority (EAA) in 

the San Antonio region to enhance rainfall by seeding 

clouds in an effort to increase recharge to the Edwards 

aquifer (Edwards Aquifer Authority, 2001). Among the 

counties treated for rainfall enhancement are Bandera, 

Medina, and Uvalde, which encompass the Seco Creek 

Basin (fig. 1). According to the EAA, rainfall could be 

increased by about 10 to 20 percent.

 A scenario to simulate increased rainfall was 

developed for the Seco Creek subbasins. Rainfall data 

(totals and intensities) were increased 10 percent for 

all six subbasins for every storm and for the entire simulation period. Simulations results are presented in 

table 11. 

A 10-percent increase in rainfall resulted in a 5- to 

6-percent increase in ET for the six subbasins. The simulated annual runoff and the highest 10 percent of flows 

for the six subbasins increased from 2 to 92 percent. 

The simulated lowest 50 percent of flows increased only 

at the Seco Creek at Miller Ranch subbasin. Increased 

channel losses and ground-water recharge offset the 

increased precipitation in the other five subbasins. The 

simulated volume of selected storms increased from 

2 to 87 percent; interflow ranged from no change to 

SIMULATION OF BEST-MANAGEMENT PRACTICES 21 
an increase of 78 percent; and total surface runoff 

increased 36 to 101 percent for the six subbasins. Simulated summer flow volume ranged from no change to 

an increase of 115 percent; winter flow volume ranged 

from no change to an increase of 45 percent; and summer storm volume ranged from no change to an increase 

of 189 percent. 

Simulated results for the weather-modification 

scenario were comparable to the results for the brushmanagement scenario. The total runoff increased in all 

subbasins, but only base flow increased in the Seco 

Creek at Miller Ranch subbasin. 

SUMMARY 

The purpose of the study was to assess the effects 

of two best-management practices on selected hydrologic processes in the upper Seco Creek Basin, a primarily rangeland area of about 165 mi2

 that overlies parts 

of the Trinity and Edwards aquifers in south-central 

Texas. This report describes the development of a 

parameter set for use with a model (HSPF) to simulate 

flows in six gaged subbasins in the upper Seco Creek 

Basin and presents an assessment on the basis of simulation of the changes in surface-water quantity that 

could result from brush management (removal of 

woody species locally known as cedar) and weather 

modification (rainfall enhancement). 

A model parameter set for use with HSPF 

was developed to simulate surface-water-budget 

components for six gaged subbasins. Rainfall and 

runoff data were collected during January 1, 1991– 

September 30, 1998. Data from 60 storms were used 

for the simulations. Twenty-one pervious land segments 

were defined for the study on the basis of geology and 

land cover. Sixteen annual parameters, one monthly 

parameter, and three initial-condition parameters were 

defined for each land segment. 

The model was calibrated with data from 33 

storms (in two subbasins) and tested spatially with data 

from 27 storms (in four subbasins). The final parameter 

set was assumed to represent average subbasin conditions during the simulation periods. An error analysis 

and a sensitivity analysis were done on each subbasin, 

and the results were used to develop the final parameter 

set. 

The calibrated and tested model was used to 

assess the effects of cedar removal and rainfall enhancement. Simulating the effects of cedar removal by 

decreasing ET 5 to 6 percent resulted in simulated 

increases in annual runoff of 1 to 47 percent and 

increases in surface runoff of 14 to 48 percent. Simulated increases in rainfall totals and intensities of 10 percent from weather modification yielded increases of 5 to 

6 percent in ET, increases in annual runoff of 2 to 92 

percent, and increases in surface runoff of 36 to 101 percent for the six subbasins. 

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

Simulation Program—FORTRAN, user’s manual for 

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22 Simulation of Flow and Effects of Best-Management Practices in the Upper Seco Creek Basin, South-Central Texas, 1991–98 
Brown and Raines— Simulation of Flow and Effects of Best-Management Practices in the 

Upper Seco Creek Basin, South-Central Texas, 1991–98 

District Chief U.S. Geological Survey 8027 Exchange Dr. Austin, TX 78754–4733