U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5027

In cooperation with the San Antonio River Authority, the Evergreen Underground Water 

Conservation District, and the Goliad County Groundwater Conservation District

Simulation of Streamflow, Evapotranspiration, and 

Groundwater Recharge in the Lower San Antonio River 

Watershed, South-Central Texas, 2000–2007
Front cover: Falls on the San Antonio River near Falls City, Texas (photograph courtesy of San Antonio 

River Authority).

Back cover: Cibolo Creek near Farm Road 775, Wilson County, Texas (photograph courtesy of San Antonio 

River Authority).
Simulation of Streamflow, 

Evapotranspiration, and Groundwater 

Recharge in the Lower San Antonio 

River Watershed, South-Central Texas, 

2000–2007

By Joy S. Lizárraga and Darwin J. Ockerman

In cooperation with the San Antonio River Authority, the Evergreen Underground 

Water Conservation District, and the Goliad County Groundwater Conservation 

District

Scientific Investigations Report 2010–5027

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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

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Box 25286, Denver Federal Center

Denver, CO 80225

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1-888-ASK-USGS

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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:

Lizárraga, J.S., and Ockerman, D.J., 2010, Simulation of streamflow, evapotranspiration, and groundwater recharge in 

the lower San Antonio River watershed, south-central Texas, 2000–2007: U.S. Geological Survey Scientific Investigations Report 2010–5027, 41 p. 
iii

Contents

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

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

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

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

Previous Studies ..................................................................................................................................6

Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge ....................................7

Functional Description of Hydrological Simulation Program—FORTRAN .................................7

Model Development ............................................................................................................................8

Subwatershed and Stream Reach Delineations ...................................................................8

Classification of Hydrologic Response Units .........................................................................8

Time-Series Development .......................................................................................................13

Streamflow ........................................................................................................................13

Meteorological Data .......................................................................................................19

Model Calibration and Testing .........................................................................................................19

Streamflow .................................................................................................................................19

Evapotranspiration and Groundwater Recharge ................................................................20

Simulated Streamflow, 2000–2007 ...................................................................................................32

Simulated Evapotranspiration, 2000–2007 .....................................................................................34

Estimated Groundwater Recharge, 2000–2007 .............................................................................35

Water-Budget Summary, 2000–2007 ...............................................................................................35

Sensitivity Analysis ............................................................................................................................37

Model Limitations ...............................................................................................................................38

Summary .......................................................................................................................................................39

References ...................................................................................................................................................40

Figures

1–2. Maps showing: 

1. Location of data-collection stations that provided data for the Hydrological 

Simulation Program—FORTRAN model of the lower San Antonio River 

watershed, south-central Texas ......................................................................................2

2. Aquifer systems, surficial geology, and delineation of water-budget zones for 

the Hydrological Simulation Program—FORTRAN model of the lower San 

Antonio River watershed, south-central Texas .............................................................5

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

for hydrologic processes for (A) impervious and (B) pervious land segments .................9

4–8. Maps showing: 

4. Subwatershed and stream/reservoir reach delineation for the Hydrological 

Simulation Program—FORTRAN model of the lower San Antonio River 

watershed, south-central Texas ....................................................................................10

5. Relative soil infiltration rates in the lower San Antonio River watershed, 

south-central Texas ..........................................................................................................12
iv

6. Land cover in the lower San Antonio River watershed, south-central Texas ........14

7. Location of National Weather Service meteorological stations and 

associated Thiessen rainfall areas for the Hydrological Simulation Program—

FORTRAN model of the lower San Antonio River watershed, south-central 

Texas ...................................................................................................................................15

8. Location of wastewater treatment plant discharge sites, lower San Antonio 

River watershed, south-central Texas ..........................................................................16

9–15. Graphs showing: 

9. Measured and simulated daily mean streamflow at 08183500 San Antonio 

River near Falls City, Texas, 2000–2007 .........................................................................21

10. Measured and simulated daily mean streamflow at 08185100 Martinez Creek 

near St. Hedwig, Texas, 2006–07 ....................................................................................22

11. Measured and simulated daily mean streamflow at 08186000 Cibolo Creek 

near Falls City, Texas, 2000–2007 ....................................................................................23

12. Measured and simulated daily mean streamflow at 08186500 Ecleto Creek 

near Runge, Texas, 2003–07 ............................................................................................24

13. Measured and simulated daily mean streamflow at 08188500 San Antonio 

River at Goliad, Texas, 2000–2007 ..................................................................................25

14. Measured and simulated daily mean streamflow at 08188570 San Antonio 

River near McFaddin, Texas, 2006–07 ...........................................................................26

15. Measured weekly evapotranspiration at 290810099212100 SW Medina County 

meteorological station near D’Hanis, Texas, and Hydrological Simulation 

Program—FORTRAN simulated weekly evapotranspiration for the Carrizo Sand 

outcrop area of the lower San Antonio River watershed, south-central Texas, 

October 2006–December 2007 ........................................................................................27

16. Map showing estimated annual mean groundwater recharge in water-budget zones 

of the lower San Antonio River watershed, 2000–2007 .......................................................36

Tables

1. Description of U.S. Geological Survey and National Weather Service stations from 

which data were obtained for the Hydrological Simulation Program—FORTRAN 

model of the lower San Antonio River watershed, south-central Texas ............................3

2. Rainfall at five National Weather Service stations in the lower San Antonio River 

watershed, south-central Texas, 2000–2007 ...........................................................................6

3. Parameters for hydrologic processes in the Hydrological Simulation Program—

FORTRAN model of the lower San Antonio River watershed, south-central Texas .......11

4. Land-cover categories represented in the Hydrological Simulation Program—

FORTRAN model of the lower San Antonio River watershed, south-central Texas .......17

5. Rainfall calculated for water-budget zones of the Hydrological Simulation 

Program—FORTRAN model of the lower San Antonio River watershed, southcentral Texas, 2000–2007 ..........................................................................................................17

6. Wastewater discharges included in the Hydrological Simulation Program—

FORTRAN model of the lower San Antonio River watershed, south-central Texas .......18

7. Streamflow calibration and testing results, Hydrological Simulation Program—

FORTRAN model of the lower San Antonio River watershed, south-central Texas .......28

8. Calibrated values for selected parameters, by water-budget zone, for the 

Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas ...................................................................................33
v

9. Simulated streamflow volumes and basin yields generated from subwatersheds 

in the Hydrological Simulation Program—FORTRAN model of the lower San 

Antonio River watershed, south-central Texas, 2000–2007 ................................................34

10. Annual mean streamflow volumes and basin yields at streamflow-gaging stations 

at upstream boundary of the lower San Antonio River watershed, south-central 

Texas, 2000–2007 ........................................................................................................................34

11. Estimated evapotranspiration for pervious land in water-budget zones of the 

lower San Antonio River watershed, south-central, Texas, 2000–2007 ............................35

12. Estimated groundwater recharge for pervious land in water-budget zones of the 

lower San Antonio River watershed, south-central Texas, 2000–2007 .............................37

13. Sensitivity of the water balance in water-budget zone 3 to changes in selected 

process-related parameters of the Hydrological Simulation Program—Fortran 

(HSPF) model of the lower San Antonio River watershed, south-central Texas, 

2000–2007 ....................................................................................................................................38

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

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)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Datum

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

The U.S. Geological Survey (USGS), in cooperation with 

the San Antonio River Authority, the Evergreen Underground 

Water Conservation District, and the Goliad County Groundwater Conservation District, configured, calibrated, and tested 

a watershed model for a study area consisting of about 2,150 

square miles of the lower San Antonio River watershed in 

Bexar, Guadalupe, Wilson, Karnes, DeWitt, Goliad, Victoria, 

and Refugio Counties in south-central Texas. The model simulates streamflow, evapotranspiration (ET), and groundwater 

recharge using rainfall, potential ET, and upstream discharge 

data obtained from National Weather Service meteorological 

stations and USGS streamflow-gaging stations. Additional 

time-series inputs to the model include wastewater treatmentplant discharges, withdrawals for cropland irrigation, and 

estimated inflows from springs.

Model simulations of streamflow, ET, and groundwater 

recharge were done for 2000–2007. Because of the complexity 

of the study area, the lower San Antonio River watershed was 

divided into four subwatersheds; separate HSPF models were 

developed for each subwatershed. Simulation of the overall 

study area involved running simulations of the three upstream 

models, then running the downstream model. The surficial 

geology was simplified as nine contiguous water-budget zones 

to meet model computational limitations and also to define 

zones for which ET, recharge, and other water-budget information would be output by the model. The model was calibrated 

and tested using streamflow data from 10 streamflow-gaging 

stations; additionally, simulated ET was compared with measured ET from a meteorological station west of the study area. 

The model calibration is considered very good; streamflow 

volumes were calibrated to within 10 percent of measured 

streamflow volumes. 

During 2000–2007, the estimated annual mean rainfall 

for the water-budget zones ranged from 33.7 to 38.5 inches 

per year; the estimated annual mean rainfall for the entire 

watershed was 34.3 inches. Using the HSPF model it was estimated that for 2000–2007, less than 10 percent of the annual 

mean rainfall on the study watershed exited the watershed as 

streamflow, whereas about 82 percent, or an average of 28.2 

inches per year, exited the watershed as ET. Estimated annual 

mean groundwater recharge for the entire study area was 3.0 

inches, or about 9 percent of annual mean rainfall. Estimated 

annual mean recharge was largest in water-budget zone 3, the 

zone where the Carrizo Sand outcrops. In water-budget zone 

3, the estimated annual mean recharge was 5.1 inches or about 

15 percent of annual mean rainfall. Estimated annual mean 

recharge was smallest in water-budget zone 6, about 1.1 inches 

or about 3 percent of annual mean rainfall. The Cibolo Creek 

subwatershed and the subwatershed of the San Antonio River 

upstream from Cibolo Creek had the largest and smallest 

basin yields, about 4.8 inches and 1.2 inches, respectively. 

Estimated annual ET and annual recharge generally increased 

with increasing annual rainfall. Also, ET was larger in zones 8 

and 9, the most downstream zones in the watershed.

Model limitations include possible errors related to model 

conceptualization and parameter variability, lack of data to 

quantify certain model inputs, and measurement errors. Uncertainty regarding the degree to which available rainfall data 

represent actual rainfall is potentially the most serious source 

of measurement error.

Introduction

The San Antonio River is in south-central Texas (fig. 1), 

and is within Region L in the State’s regional water plans. 

Region L is expected to increase in population by 75 percent 

between 2010 and 2060, and water demands are expected to 

increase by 29 percent (Texas Water Development Board, 

2006). Most of this anticipated population growth is upstream 

from the lower San Antonio River watershed in the San 

Antonio, Tex., area. Most of the water supplied to the San 

Antonio area comes from outside the lower San Antonio River 

watershed, but part of the water supply for San Antonio might 

be met with exported groundwater resources from Wilson 

County, which is within the study area. The lower San Antonio 

River watershed, defined as the contributing area to the San 

Antonio River south of U.S. Geological Survey (USGS) 

streamflow-gaging station 08181800 San Antonio River 

near Elmendorf, Tex. (site 8; fig. 1, table 1) in southeastern 

Simulation of Streamflow, Evapotranspiration, and 

Groundwater Recharge in the Lower San Antonio River 

Watershed, South-Central Texas, 2000–2007

By Joy S. Lizárraga and Darwin J. Ockerman
2 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

#

#

#

#

#

#

#

#

#

#

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Cibolo Creek

San Antonio River

Cabeza Creek

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San ntonio

Cibolo Creek

Guadalupe

River

 River

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

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REFUG OI

COUNTY

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

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90 £¤

281 £¤

77 £¤

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77 £¤

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181 £¤

77 £¤

181

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Poth

Floresville

Stockdale

Nixon

Seguin

Cibolo

New

Braunfels

San

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Goliad

Kenedy

Runge

Karnes

City

Yorktown

St. Hedwig

Elmendorf

Selma

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14

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7

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Falls City

McFaddin

Sutherland

Springs

97°0'

97°30'

98°0'

29°30'

29°0'

28°30' 0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

EXPLANATION

Lower San Antonio River watershedU.S. Geological Survey streamflow-gagingstation and site number—See table 1National Weather Service meteorologicalstation and site number—See table 1

8

#

1

TEXAS

Study area

LOCATION MAP

Figure 1. Location of data-collection stations that provided data for the Hydrological Simulation Program—FORTRAN model of the lower San Antonio River watershed, southcentral Texas. 
Introduction 3

Bexar County and south of USGS streamflow-gaging station 

08185000 Cibolo Creek at Selma, Tex. (site 11) in Guadalupe 

County, also receives a large amount of the wastewater discharged from the growing San Antonio metropolitan area. 

To better understand the hydrology, including the relative 

contribution of the various water-budget components to the 

overall water budget, the USGS in cooperation with the San 

Antonio River Authority, the Evergreen Underground Water 

Conservation District, and the Goliad County Groundwater 

Conservation District, developed a watershed model for the 

lower San Antonio River watershed. As the region develops, 

the lower San Antonio River watershed model can be modified 

to simulate future scenarios of land-cover change and water 

use. The model-derived estimates of evapotranspiration (ET) 

Table 1. Description of U.S. Geological Survey and National Weather Service stations from which data were obtained for the 

Hydrological Simulation Program—FORTRAN model of the lower San Antonio River watershed, south-central Texas.

[dd, degrees; mm, minutes; ss, seconds; NWS, National Weather Service; --, not available; USGS, U.S. Geological Survey]

Site number

(fig. 1) Station number and name Latitude(dd mm ss)

Longitude

(dd mm ss) Type of data Period of record used

1 NWS 417422 Randolph Field 29°33'--" 98°16'--" Rainfall 2000 to 2007 

2 NWS 417945 San Antonio International Airport 29°32'--" 98°28'--" Rainfall, air temperature1

2000 to 2007

3 NWS 418658 Stockdale 4 N 29°17'--" 98°58'--" Rainfall 2000 to 2007 

4 NWS 413201 Floresville 29°08'--" 98°10'--" Rainfall, air temperature12000 to 2007

5 NWS 417836 Runge 28°50'--" 97°43'--" Rainfall 2000 to 2006

6 NWS 414696 Karnes City 2 N 29°54'--" 97°53'--" Rainfall 2000 to 2007 

7 NWS 413618 Goliad 28°40'--" 97°23'--" Rainfall, air temperature1

2000 to 2007

8 USGS station 08181800 San Antonio River near 

Elmendorf, Tex.

29°13'19" 98°21'20" Streamflow 2000 to 2007 

9 USGS station 08183200 San Antonio River near 

Floresville, Tex.

29°06'36" 98°10'28" Streamflow 2005 to 2007 

10 USGS station 08183500 San Antonio River near 

Falls City, Tex.

29°57'05" 98°03'50" Streamflow 2000 to 2007 

11 USGS station 08185000 Cibolo Creek at Selma, 

Tex.

29°35'38" 98°18'39" Streamflow 2000 to 2007 

12 USGS station 08185065 Cibolo Creek near St. 

Hedwig, Tex.

29°30'05.2" 98°11'10.5" Streamflow 2005 to 2007 

13 USGS station 08185100 Martinez Creek near 

St. Hedwig, Tex.

29°26'38" 98°10'08" Streamflow 2005 to 2007 

14 USGS station 08185500 Cibolo Creek at 

Sutherland Springs, Tex. 

29°16'47" 98°03'12" Streamflow 2005 to 2007 

15 USGS station 08186000 Cibolo Creek near 

Falls City, Tex. 

29°00'50" 97°55'48" Streamflow 2000 to 2007 

16 USGS station 08186500 Ecleto Creek near 

Runge, Tex.

28°55'12" 97°46'19" Streamflow 2002 to 2007 

17 USGS station 08188500 San Antonio River at 

Goliad, Tex.

28°38'58" 97°23'04" Streamflow 2000 to 2007 

18 USGS station 08188570 San Antonio River near 

McFaddin, Tex.

28°31'52.5" 97°02'33.7" Streamflow 2005 to 2007 

19 

(not on fig. 1)

USGS 290810099212100 SW Medina County 

meteorological station near D’Hanis, Tex.

29°08'10.3" 99°21'20.5" Evapotranspiration2 2006 to 2007 

1

 Air temperature data were used to derive estimates of potential evaporation using the Hamon method in Basins 4.0 (Paul Hummel, Aqua Terra Consultants, 

written commun., 2008). 

2

 Evapotranspiration measured by eddy covariance method (Bidlake, 2002). 
4 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

and groundwater recharge could be used as inputs to regional 

groundwater flow models of the Gulf Coast aquifer system, 

Carrizo-Wilcox aquifer, Queen City aquifer, or Sparta aquifer 

(Texas Water Development Board, 2009). Additionally, the 

modular nature of the model will accommodate the simulation 

of water-quality constituents not reported here.

Purpose and Scope

The purpose of this report is to describe the simulation 

of streamflow, ET, and groundwater recharge in the lower San 

Antonio River watershed using a watershed model. The model 

was developed using input data collected during 2000–2007 

to simulate streamflow, ET, and groundwater recharge first for 

four subwatersheds and nine contiguous water-budget zones 

and then for the overall area of the lower San Antonio River 

watershed. The functionality of the model and the input data 

are described, followed by the configuration, calibration, and 

testing of the model. The hydrologic and meteorological conditions in the four subwatersheds and nine contiguous waterbudget zones of the lower San Antonio River watershed and 

the iterative process of developing the model are summarized. 

Annual mean inflows and outflows of the major water-budget 

components for the entire study area are presented, and finally, 

limitations of model-simulated estimates of streamflow, ET, 

and groundwater recharge are described. 

Description of the Study Area

The San Antonio River extends about 240 miles from 

northwest of San Antonio, Tex., to the confluence of the San 

Antonio and Guadalupe Rivers. The drainage area of the lower 

San Antonio River is about 2,150 square miles (fig. 1); it is 

characterized by gently sloping topography and land cover 

consisting mostly of brush and grassland (Multi-Resolution 

Land Characteristics Consortium, 2008). The lower San Antonio River watershed encompasses parts of Bexar, Guadalupe, 

Wilson, Karnes, DeWitt, Goliad, Victoria, and Refugio Counties in south-central Texas.

The northern tip of the lower San Antonio River watershed overlies Cretaceous rocks of the Edwards-Trinity aquifer 

system. The remainder of the watershed overlies the Texas 

Coastal Uplands and Coastal Lowlands aquifer systems 

(Ryder, 1996) (fig. 2). The Texas Coastal Uplands aquifer 

system (Sparta, Queen City, and Carrizo-Wilcox aquifers) is 

composed of formations of Paleocene and Oligocene age, and 

the Texas Coastal Lowlands aquifer system (Chicot, Evangeline, and Jasper aquifers) is composed of younger formations 

from Oligocene through Holocene age. The Coastal Lowlands 

aquifer system comprises the same aquifers as the Gulf Coast 

aquifer system (Kasmarek and Robinson, 2004) and is equivalent to the Gulf Coast aquifer as defined by Ashworth and 

Hopkins (1995). In the Coastal Uplands aquifer system, the 

sediments, in order of dominance, consist mostly of sand, silt, 

and clay. The sediment deposits are distributed as relatively 

uniform sequences of predominantly fine- or coarse-grained 

material. In the Coastal Lowlands aquifer system, the aquifers 

dip and thicken toward the Gulf, and sediments exist in complex, overlapping mixtures of sand, silt, and clay as a result of 

numerous oscillations of ancient shorelines.

Rainfall amounts for the study area were derived from 

measured rainfall at five National Weather Service (NWS) 

meteorological stations (sites 1, 3, 4, 6, 7; fig. 1, table 1). 

Rainfall varied from year to year and throughout the lower 

San Antonio River watershed, which is typical of south-central 

Texas. During 2000–2007, annual mean rainfall measured at 

the five NWS stations in the lower San Antonio River watershed varied from 33.5 to 40.2 inches per year (table 2), similar 

to the long-term average rainfall of 31 to 39 inches per year 

for this area of Texas (Narasimhan and others, 2005). During the study, the smallest annual rainfall (18.4 inches) was 

recorded in 2005 at NWS 413201 Floresville, Tex. (site 4) 

and the largest (51.8 inches) was recorded in 2007 at NWS 

413618 Goliad, Tex. (site 7). Years with above-average rainfall 

included 2002, 2004, and 2007.

During 2000–2007, annual mean streamflow volume 

measured at 08181800 San Antonio River near Elmendorf 

(site 8; fig. 1, table 1) ranged from 0.160 million acre-feet in 

2006 to 1.41 million acre-feet in 2002. The average of daily 

streamflow during 2000–2007 was 886 cubic feet per second. According to Ockerman and McNamara (2003, p. 28), 

streamflow at site 8 averaged 0.40 million acre-feet annually 

during 1997–2001. During this period, streamflow at site 8 

consisted primarily of base flow and runoff from the upstream 

drainage area (about 75 percent), treated wastewater discharge 

(about 20 percent), and Edwards aquifer springflow (about 5 

percent). During 2000–2007, the average of daily streamflow 

increased slightly at the downstream USGS streamflow-gaging 

station 08183500 San Antonio River near Falls City, Tex. (site 

10; fig. 1, table 1). Site 10 is upstream from the confluence 

of the San Antonio River and Cibolo and Ecleto Creeks. The 

average of daily streamflow during 2000–2007 at site 10 was 

891 cubic feet per second. The streamflow data used for this 

report are available from the USGS National Water Information System (NWISWeb) (U.S. Geological Survey, 2009).

During 2000–2007, annual mean streamflow volume 

at USGS streamflow-gaging station 08185000 Cibolo Creek 

at Selma (site 11; fig. 1, table 1) ranged from no flow in 

2003 and 2006 to 151,050 acre–feet in 2002. Flow at site 11 

consists mostly of stormwater runoff from the Cibolo Creek 

contributing area upstream from Selma. The average of daily 

streamflow at site 11 during 2000–2007 was 59.4 cubic feet 

per second. About 10 miles downstream from site 11, Cibolo 

Creek starts to receive inflows of treated wastewater. These 

inflows are from one wastewater treatment plant on the main 

stem of Cibolo Creek and from three wastewater treatment 

plants on Martinez Creek, a tributary of Cibolo Creek. Downstream from the wastewater inflow and crossing the Carrizo 

Sand outcrop (fig. 2), Cibolo Creek gains flow from multiple 

springs, including those known collectively as Sutherland 

Springs. USGS streamflow-gaging station 08186000 Cibolo 
Introduction 5

TEXAS COASTAL LOWLANDS

AQUIFER SYSTEM

TEXAS COASTAL UPLANDS

AQUIFER SYSTEM

EDWARDS-TRINITY

AQUIFER SYSTEM

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San Antonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

Zone 4

Zone 3

Zone 8

Zone 9

Zone 1

Zone 2

Zone 5

Zone 6

Zone 7

Mg

Ql

Mo

Oc

Em

Ecm

Ey

Ecd

Eqc

Es

Ec

Ew

Qle

Er

Emi

Ewi

Kknm

Qt

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Qbc

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Kau

Qt

Qt

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97°0'

97°30'

98°0'

29°30'

29°0'

28°30'

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

Surficial geology modified from Proctor and others (1974), Brown and others (1976, 1983), and Aronow and others (1987)Aquifer systems modified from Ryder (1996)

Whitsett FormationManning FormationWellborn FormationCaddell FormationYegua FormationCook Mountain FormationSparta SandWeches FormationQueen City SandRecklaw FormationCarrizo SandWilcox GroupMidway GroupNavarro GroupPecan Gap ChalkAustin ChalkWater bodyHydrological Simulation Program—FORTRAN waterbudget zone boundary (zones 1–9)

Lower San Antonio Riverwatershed boundaryAquifer system boundary

Surface geologyAlluviumFluviatile terrace depositsDeweyville FormationBeaumont Formation (sand)Beaumont Formation (clay)Lissie FormationLeona FormationWillis FormationGoliad FormationFleming FormationOakville SandstoneCatahoula Formation

SYSTEM

QUATERNARY TERTIARY

Qal

Qt

Qd

Qbs

Qbc

Ql

Qle

PoW

Mg

Mf

Mo

EXPLANATION

TERTIARY UPPER 

CRETACEOUS

Ecd

Em

Ewb

Eca

Ey

Ecm

Es

Ew

Eqc

Er

Ec

Ewi

Emi

Kknm

Kpg

Kau

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Figure 2. Aquifer systems, surficial geology, and delineation of water-budget zones for the Hydrological Simulation Program—FORTRAN model of the lower San Antonio River watershed, south-central Texas. 
6 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Creek near Falls City, Tex. (site 15; fig. 1, table 1) records 

streamflow upstream from the confluence of Cibolo Creek and 

the San Antonio River. During 2000–2007, the annual mean 

volume of streamflow at site 15 ranged from 25,416 acre-feet 

in 2006 to 398,262 acre-feet in 2002, whereas the average of 

daily streamflow was 250 cubic feet per second.

Streamflow data are recorded near the Ecleto Creek 

outlet to the lower San Antonio River at USGS streamflowgaging station 08186500 Ecleto Creek near Runge, Tex. (site 

16; fig. 1, table 1). Ecleto Creek originates in the outcrop of 

the Carrizo Sand and joins the San Antonio River over the 

Texas Coastal Lowlands aquifer system, downstream from 

the confluence of the San Antonio River and Cibolo Creek. 

Although the streamflow record at site 16 began in March 

1962, the station was not in service during October 1989–

September 2002. For the period when the station was in 

operation relevant to this report (October 2002–December 

2007), the average of daily streamflow was 52.3 cubic feet per 

second. 

Downstream from the confluence of Cabeza Creek and 

the lower San Antonio River, streamflow in the lower San 

Antonio River is recorded at USGS streamflow-gaging station 

08188500 San Antonio River at Goliad, Tex. (site 17; fig. 1, 

table 1). During 2000–2007, annual mean streamflow volume 

at site 17 ranged from 226,068 acre-feet in 2006 to 2.06 million acre-feet in 2002, whereas the average of daily streamflow 

was 1,370 cubic feet per second. 

Previous Studies

The lower San Antonio River and its tributaries 

include gaining and losing reaches. North of the study area, 

over the Edwards aquifer recharge area, streams in the San 

Antonio River watershed typically lose water to the groundwater system. Rainfall on the Edwards aquifer outcrop and 

instream channel losses contribute to recharge of the Edwards 

aquifer (Ockerman, 2002). Conversely, the Carrizo Sand 

outcrop is an area where Anders (1957) reported the San 

Antonio River and Cibolo Creek gained “large” amounts of 

groundwater. Water was considered to be discharging from 

the groundwater system into the river system in many places. 

However, large losses from this discharged groundwater also 

were suspected to be occurring because of ET in the riparian 

zones. 

Evaporation and transpiration are major components of 

the water budget in Texas. Evaporation occurs directly from 

free-water surfaces, such as lakes, streams, and temporary 

rainfall accumulations (for example, puddles in depressions 

or droplets on leaves); transpiration occurs as plant roots 

extract water from the soil and release water vapor into the 

atmosphere through plant-leaf stomata. ET is a combination 

of evaporation and transpiration. ET rates can vary depending 

on meteorological conditions, the type of land cover (paved, 

wetland, wooded, agricultural, and others), the time of day, the 

time of year, and soil moisture.

Strategic water management requires quantification of 

ET for reliable hydrologic analyses and calibration of hydrologic models (Sumner and Tihansky, 2007). In spite of the 

relative importance of ET within the hydrologic cycle—after 

rainfall, it is the largest component of the water budget—reliable data for actual ET have historically been scarce in Texas 

(Scanlon and others, 2005). Information on ET in Texas is 

generally limited to measured pan evaporation and computed 

estimates of potential evapotranspiration (PEVT) derived from 

meteorological data. The meteorological data used to compute PEVT are obtained from NWS meteorological stations 

throughout Texas and from other ET networks, such as the 

Texas High Plains Evapotranspiration Network (Texas A&M 

University, 2005). Pan evaporation and PEVT are measures 

of the ability of the atmosphere to remove water from the 

surface assuming an unlimited water supply (Pidwirny, 2006). 

These estimates are generally used as input to hydrological 

models, and then the models use other spatial and temporal 

factors such as rainfall and land-cover information to simulate 

actual ET. Actual ET data have rarely been available for model 

calibration. Recently, advanced micrometeorological stations 

have been instrumented to measure actual ET data at some 

locations in Texas, including several on the Edwards Plateau 

since the early 2000s. In 2006, the USGS installed a station 

(290810099212100 SW Medina County meteorological station near D’Hanis, Tex.) where data are collected to measure 

actual ET continuously using the eddy covariance method 

(Bidlake, 2002). This station (not shown in fig. 1) is in Medina 

County about 70 miles west of the study area on the Carrizo 

Table 2. Rainfall at five National Weather Service stations in 

the lower San Antonio River watershed, south-central Texas, 

2000–2007.

Year

or period

National Weather Service station (fig. 1)

Randolph 

Field

(site 1)

Stockdale 

4 N

(site 3)

Floresville

(site 4)

Karnes 

City 2 N

(site 6)

Goliad

(site 7)

2000 33.9 34.4 31.6 35.6 37.1

2001 36.7 36.7 31.3 35.9 45.9

2002 40.0 40.1 41.1 39.0 42.4

2003 25.7 27.0 29.6 26.8 34.5

2004 1

48.1 146.0 41.3 44.4 47.9

2005 20.2 19.9 18.4 21.5 28.9

2006 24.1 1

27.0 26.0 121.7 32.7

2007 1

48.0 146.4 48.9 144.9 51.8

2000–2007

annual 

mean

34.6 34.7 33.5 33.7 40.2

1

 Rainfall records during these years at these stations were supplemented 

with rainfall data from nearby National Weather Service stations—San 

Antonio International Airport and Runge (sites 2 and 5, respectively; fig. 1, 

table 1). 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 7

Sand, one of the formations that outcrop in the lower San 

Antonio River watershed. 

The University of Texas Bureau of Economic Geology has compiled published information on groundwater 

recharge rates to the major aquifers of Texas for the Texas 

Water Development Board (Scanlon and Dutton, 2003). These 

compiled estimates of recharge rates range from 0.1 to 5.8 

inches per year in the Carrizo-Wilcox aquifer. Higher recharge 

rates occur in the sandy parts of the aquifer (such as the Carrizo Sand). Reported recharge rates for the Gulf Coast aquifer 

system (0.0004 to 2 inches per year) are generally lower than 

those for the Carrizo-Wilcox aquifer. 

Simulation of Streamflow, 

Evapotranspiration, and Groundwater 

Recharge

To simulate streamflow, ET, and recharge in the lower 

San Antonio River watershed, a continuous simulation watershed model was needed that would take into account all of 

the water-budget components and processes. The Hydrological Simulation Program—FORTRAN (HSPF), version 12 

(Bicknell and others, 2001), was selected for modeling the 

study watershed because it is one of the most comprehensive 

watershed models available and can simulate a wide variety 

of stream and watershed conditions with reasonable accuracy 

(Donigian and others, 1995). HSPF has been used successfully in south-central Texas to represent complex hydrologic 

systems, simulate streamflow, and estimate groundwater 

recharge to the Trinity and Edwards aquifers (Ockerman, 

2002, 2007; Ockerman and McNamara, 2003).

Functional Description of Hydrological 

Simulation Program—FORTRAN

The HSPF model software is distributed as part of the 

BASINS (Better Assessment Science Integrating Point and 

Nonpoint Sources) system. BASINS 4.0 was developed by 

the U.S. Environmental Protection Agency (2007) to support 

watershed management. BASINS serves as an umbrella-like 

package, interfacing with pertinent geodatabases, ancillary 

datasets, and software programs to facilitate user interaction with the model and to help the user better understand 

the hydrological characteristics of a watershed. Time-series 

data and model output-generated time-series data are stored 

in a Watershed Data Management (WDM) file. The WDM 

database is a binary file accessed by GenScn (GENeration and 

analysis of model simulation SCeNarios) (Kittle and others, 

1998) or by WDMUtil (Hummel and others, 2001). These 

programs are provided in BASINS and are used to manage, 

display, transform, plot, and analyze time-series data stored 

in the WDM file. Time-series data are organized in the WDM 

database by dataset number (DSN). Each DSN has attribute 

information that describes the data type, time step, location, 

and other important characteristics of the data. The HSPF 

model is the primary surface-water modeling component of 

BASINS. HSPF also exists as a stand-alone program as do 

the other tools and programs included in BASINS, such as 

WDMUtil and GenScN. The HSPF users’ manual provides 

model documentation, underlying model theory, and model 

parameterization guidance (Bicknell and others, 2001). HSPF 

is an integrated basin-scale model that combines watershed 

processes with in-stream fate and transport in one-dimensional 

characterizations of stream channels. 

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 has an associated drainage area that was partitioned 

into HRUs. HRUs are areas with similar land cover, surficial 

geology, and other factors deemed important to produce a 

similar hydrologic response to rainfall and PEVT. 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. Water is moved through this network of 

HRUs and RCHRESs for each time step specified in the model 

while conserving water mass—that is, inflow equals outflow 

plus or minus any change in storage. The water budget for 

the overall model (as well as the smaller HRUs and RCHRES 

drainage areas) can be stated as

P + Qin = ET + Qout + ∆S, (1)

where 

P is precipitation (rainfall [might also include 

irrigation or other special applications]); 

Qin and Qout are water flow into and out of the model, 

respectively; 

ET is evapotranspiration; and 

∆S is change in water storage.

Individual components can be broken down into subcomponents (for example, water flow into an HRU [Qin] is the 

sum of surface flow and interflow). A simplified water-budget 

equation for the overall model to incorporate some of these 

subcomponents, and assuming that the change in storage over 

time is minimal, results in

P + Qin

sw + Qingw = ETsw + ETuz + ETgw + Qoutsw + R, (2)

where

P is precipitation (rainfall);

Qin

sw is surface-water flow from upstream and other 

surface-water discharges (such as wastewater 

treatment plants);
8 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Qin

gw is groundwater discharge to streams (such as 

springflow);

ETsw is ET from the surface water;

ETuz is ET from the unsaturated zone (upper zone; fig. 3);

ETgw is ET from the active groundwater (lower zone, fig. 3);

Qout

sw is surface-water flow out of the model as runoff and 

withdrawals; and 

R is groundwater recharge (recharge is defined as 

including any infiltrating water that reaches the 

inactive groundwater, bottom of fig. 3). 

While maintaining the overall water balance, the model 

continuously simulates the interaction among subcomponents 

of the water-budget equation and variations of these subcomponents over time. The conceptualization of the complex 

hydrologic processes is depicted in figure 3. The hydrologic 

processes are described by empirical equations in the model 

code. Model parameters used in the empirical equations 

(table 3) are estimated and then adjusted during the calibration of the model. Typical values and ranges of model parameters from Donigian and others (1984), as well as watershed 

characteristics, were used to develop initial values for model 

parameters. 

HSPF has some limited functionality for characterizing 

groundwater and surface-water interactions. HSPF simulates 

groundwater inflow—base flow and interflow—to a RCHRES 

that originates from infiltration of rainfall. HSPF also accounts 

for groundwater leaving the system as recharge. ET simulations also are included for water stored in upper and lower 

storage zones and active groundwater. However, groundwater entering the system from springs or regional groundwater inflow can be input to HSPF only as an external time 

series.

Model output can include time series of any of the simulated subcomponents at any designated outlet or HRU. HSPF 

is calibrated by adjusting the process-related model parameters 

for each HRU or RCHRES until there is acceptable correlation 

between measured data and model output (simulated data). 

Generally, regardless of the relative magnitude of streamflow 

compared to that of precipitation and ET, streamflow is used 

for calibration because measured streamflow data are most 

readily available. However, errors in the estimation of any of 

the individual components of the water budget will affect the 

estimation of other components. 

Model Development

The HSPF model of the lower San Antonio River 

watershed was developed by (1) defining subwatersheds, 

RCHRESs, and water-budget zones for the study area; 

(2) classifying HRUs on the basis of surficial geology, land 

cover, and location of rainfall gages; (3) developing the 

input time series of meteorological and streamflow data; and 

(4) determining initial (uncalibrated) values of associated 

model parameters. Initial estimates of parameters were determined or estimated from default values, previous studies, and 

available data. 

Subwatershed and Stream Reach Delineations

Because the study area is large, the lower San Antonio 

River watershed was divided into four subwatersheds: (1) San 

Antonio River upstream from Cibolo Creek; (2) Cibolo Creek; 

(3) Ecleto Creek; and (4) San Antonio River downstream from 

Cibolo Creek (fig. 4). The subwatershed of the San Antonio 

River upstream from Cibolo Creek includes the drainage area 

extending from 08181800 San Antonio River near Elmendorf 

(site 8) to the confluence of the San Antonio River and Cibolo 

Creek. The Cibolo Creek subwatershed includes the drainage 

area extending from 08185000 Cibolo Creek at Selma (site 

11) to the confluence of Cibolo Creek and the San Antonio 

River. The Ecleto Creek subwatershed includes the drainage 

area extending from the headwaters of Ecleto Creek to the 

confluence of Ecleto Creek and the San Antonio River. The 

subwatershed of the San Antonio River downstream from 

Cibolo Creek includes the drainage area extending immediately downstream from the confluence of the San Antonio 

River and Cibolo Creek to the confluence of the San Antonio 

and Guadalupe Rivers. 

Separate HSPF models were developed for each subwatershed. The most downstream subwatershed model, San 

Antonio River downstream from Cibolo Creek, receives 

the simulated streamflow from the outlets of the three other 

subwatershed models. A simulation of the overall study area 

involves running simulations of the three upstream models, 

then running the downstream model. Each subwatershed 

model area was further subdivided into stream reaches 

(RCHRESs) with associated drainage areas. Considerations 

in developing the subwatershed and stream-reach delineations 

included (1) defining reaches with streamflows such that travel 

times through RCHRESs approximate the simulation time 

step; and (2) locating outlets of RCHRESs at strategic points, 

such as streamflow-gaging stations, tributary confluences, 

and geologic outcrop boundaries (Donigian and others, 

1984).

USGS 7.5-minute digital elevation models (U.S. Geological Survey, 2001) were used to delineate the RCHRESs and to 

calculate watershed topography (slope). Channel characteristics for each RCHRES (surface area, volume, and discharge as 

a function of depth) were entered into HSPF FTABLES (tables 

of stream-channel parameters). For gaged stream reaches, 

FTABLES parameters were based on discharge measurements 

made at USGS streamflow-gaging stations. FTABLE parameters for ungaged reaches were estimated. A 1-hour time step 

was used to accurately simulate storm events. Subwatershed 

and stream-reach delineation is shown in figure 4. 

Classification of Hydrologic Response Units

HRUs for the watershed were defined on the basis of 

surficial geology, land-cover classification, and raingage 

locations. Spatial information was compiled and analyzed 

using the geographical information system software ArcGIS 

(ESRI, 2009) to determine the acreage of each HRU within 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 9

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*

INFILT*

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

(B) Pervious land segments

(A) Impervious land segments

Model inputStorageOutflowDecision pointEvapotranspirationDesignates controllingHSPF parameter—Refer to table 3

EXPLANATION

*

ET

Figure 3. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for hydrologic processes for (A) impervious and (B) pervious land segments (modified from Wicklein and Schiffer, 2002, fig. 3). 
10 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

Manahuilla

Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San Antonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

44

80

82

86 84 88

89

92

66

68

76

20

64

54

64 52

82

78

56 62

80

76

74

58 60

68

40

42 70 75

46

48 50

72

20

16

10 14

12

24

18 30

22 28

26

36 34

2 7

32

5

66

48

98

94

92

86

84

90

82 76 74

80

78

60

62 64 68

66 72 70

56

58 54

46

50

34 42

40

44

38

36

12

10

6

14

22

21

24

32 28

30

25

16

20

17

2 4

88

96

90

8

23

52

18 19

26 5 6

52 49

46

63

25

26

34

36

33

28

27

30 32 44

31

9

10

11

12 16 13

14

19 18

67

8

2 21 22

24 40

38

42

45 47 54

56

58

57

60

61

62

65

51

50 48

71

72

73

74

70

77 78

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

EXPLANATION

Subwatershed of lower San Antonio River watershedSan Antonio River upstream from Cibolo CreekCibolo CreekEcleto CreekSan Antonio River downstream from Cibolo CreekLower San Antonio River watershed boundaryLower San Antonio River stream/reservoirreach (RCHRES) and RCHRES drainage area,identified by number

12

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

97°0'

97°30'

98°0'

29°30'

29°0'

28°30'

Figure 4. Subwatershed and stream/reservoir reach delineation for the Hydrological Simulation Program—FORTRAN model of the lower San Antonio River watershed, southcentral Texas. 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 11

a RCHRES drainage area. The surficial geology of the study 

area was simplified as nine contiguous water-budget zones 

(fig. 2). This simplification was necessary to meet model computational limitations and also to define zones for which ET, 

recharge, and other water-budget information would be output 

by the model. 

In addition to surficial geology, county soil data from 

the Natural Resources Conservation Service (2008) were 

compiled for the study area (fig. 5). As expected, the attributes 

of the soils correlated fairly closely with the surficial geology. 

Attributes associated with the soils in these geodatabases aided 

in the selection of initial estimates for HSPF parameters, such 

as the HSPF soil-infiltration rate (INFILT). As an example, the 

largest value for INFILT in the model was assigned to waterbudget zone 3, characterizing the soil-infiltration rate of the 

Carrizo Sand.

Table 3. Parameters for hydrologic processes in the Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas.

[PERLND, pervious land; IMPLND, impervious land] 

Parameter Description1 Units Land segment

AGWS Initial active groundwater storage inches PERLND

AGWETP Fraction of available potential evapotranspiration demand that can be met from 

stored groundwater

none PERLND

AGWRC Groundwater recession parameter; an index of rate at which groundwater drains 

from land

1/day PERLND

BASETP Fraction of available potential evapotranspiration demand that can be met from 

groundwater outflow; simulates evapotranspiration from riparian vegetation

none PERLND

CEPSC Interception storage capacity inches PERLND

DEEPFR Fraction of groundwater that does not discharge to surface within boundaries of 

modeled area

none PERLND

INFEXP Infiltration equation exponent; controls rate of infiltration decrease as a function 

of increasing soil moisture

none PERLND

INFILD Ratio of maximum and mean infiltration capacities none PERLND

INFILT Index to infiltration capacity of soil; also affects percolation to groundwater 

zone

inches/hour PERLND

INTFW Interflow index; controls amount of infiltrated water that flows as shallow 

subsurface runoff

none PERLND

IRC Interflow recession coefficient; index for rate of shallow subsurface runoff 1/day PERLND

KVARY Groundwater outflow modifier; index of how much effect recent recharge has on 

groundwater outflow

1/inch PERLND

LSUR Length of assumed overland flow plane feet PERLND or IMPLND

LZETP Lower-zone evapotranspiration; index value (ranging from 0 to 0.99) representing the density of deep-rooted vegetationnone PERLND

LZS Initial lower-zone storage inches PERLND

LZSN Lower-zone nominal storage; index to soil moisture holding capacity of unsaturated zone

inches PERLND

NSUR Manning’s n for assumed overland flow plane none PERLND or IMPLND

RETSC Impervious retention storage capacity inches IMPLND

SLSUR Slope of assumed overland flow plane feet PERLND or IMPLND

UZS Initial upper-zone storage inches PERLND

UZSN Upper-zone nominal storage; index to amount of surface storage in 

depressions and the upper few inches of soil

inches PERLND

1

 The user’s manual for Hydrological Simulation Program—FORTRAN (Bicknell and others, 2001) provides a detailed description of each parameter.
12 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Figure 5. Relative soil infiltration rates in the lower San Antonio River watershed, south-central Texas. 

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San Antonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

Cibolo Creek

San Antonio River

Cabeza Creek

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Zone 4

Zone 3

Zone 8

Zone 9

Zone 1

Zone 2

Zone 5

Zone 6

Zone 7

EXPLANATION

Relative soil infiltration rateRapid to very rapidModerate to moderately rapidModerate to moderately slowSlow to very slowWater bodyHydrological Simulation Program—FORTRAN 

water-budget zone boundary (zones 1–9)Lower San Antonio River watershed boundary

97°0'

97°30'

98°0'

29°30'

29°0'

28°30' 0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

Soil data modified from Natural ResourcesConservation Service (2008)
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 13

Land-cover data were also compiled to define HRUs. 

Land-cover data for 2001 were available from the “National 

Land Cover Database 2001” (Multi-Resolution Land Characteristics Consortium, 2008). The lower San Antonio River 

watershed contains 15 of the 29 possible land-cover classes in 

the database. The 15 land-cover classes were consolidated into 

eight land-cover categories to simplify the model configuration (table 4). For example, barren land (class 31) was grouped 

with developed open space (class 21) to create one land-cover 

category called open space. Figure 6 shows seven of the eight 

categories; small amounts of open space (class 21) and barren 

land (class 31) were grouped with proximate developed and 

grass land cover. In the consolidated open space category, 15 

percent of the acreage was considered impervious. The developed land-cover category includes low, medium, and high 

intensity development (classes 22, 23, and 24). The study area 

acreage was predominantly low intensity development (class 

22), listed as 20 to 49 percent impervious in the database. In 

the consolidated developed land-cover category, 25 percent 

of the acreage was considered impervious. The acreage from 

the model land-cover categories “open space” and “developed” (table 4) was assigned as PERLND and IMPLND areas, 

respectively. Area categorized as open water is not part of an 

HRU. Instead this area is considered to be modeled as part of 

the stream reach and might vary slightly during model simulation on the basis of streamflow and channel dimensions.

Finally, the definition of each HRU depends on its location relative to available rainfall data. Data from the seven 

NWS meteorological stations (sites 1–7; fig. 1, table 1) were 

consolidated into five hourly rainfall time series and then were 

assigned areal significance (fig. 7) using the Thiessen method 

(Linsley and others, 1982). Of the seven NWS meteorological 

stations, sites 2 and 5 were only used to fill in missing record 

at nearby sites, thus the consolidation of data from seven stations into five hourly rainfall time series. The Thiessen rainfall 

areas were used to determine HRU acreages, as well as to 

determine where to apply the rainfall time-series data in the 

HSPF model. Rainfall was aggregated by water-budget zone 

by overlaying the Thiessen area rainfall amounts on the waterbudget zone areas (table 5). During 2000–2007, annual mean 

rainfall estimates for the water-budget zones ranged from 33.7 

to 38.5 inches per year; for the entire watershed the estimated 

annual mean rainfall was 34.3 inches (table 5).

Using ArcGIS, a spatial data intersection of waterbudget zones, land-cover data, Thiessen rainfall areas, and 

RCHRES drainage areas was done to determine PERLND 

and IMPLND acreages for the HSPF models. The ArcGIS 

intersection resulted in a set of 77 unique PERLNDs classified using rainfall from Thiessen rainfall areas, nine waterbudget zones, and seven land-cover categories. Ten unique 

IMPLNDs were classified using rainfall from Thiessen rainfall 

areas and two impervious land-cover categories (impervious 

acreages of open space and developed land-cover categories; 

table 4). Water-budget zones are not relevant for IMPLNDs. 

For example, RCHRES 2 in the subwatershed of the San Antonio River upstream from Cibolo Creek has a drainage area of 

about 8,920 acres and contains 16 unique HRUs. One HRU in 

the drainage area of RCHRES 2 represents pervious cropland 

in water-budget zone 2 and rainfall amounts measured at NWS 

413201 Floresville (site 4; fig. 1, table 2). This same HRU also 

is present in RCHRES 7.

Time-Series Development

Streamflow from the USGS streamflow-gaging stations 

at the study area boundary, meteorological data, wastewater 

discharges, springflows, and surface-water withdrawals for 

irrigation are input to the lower San Antonio River model as 

time-series data. Streamflow data from USGS streamflowgaging stations in the study area were used for calibration 

and testing. The data for each time series were compiled from 

national databases and local agencies.

Streamflow

The streamflow data used for this report are available 

from the USGS NWISWeb (U.S. Geological Survey, 2009). 

Wastewater discharges (fig. 8, table 6) have an appreciable 

effect on streamflow in the lower San Antonio River. A 

previous study reported that wastewater discharge accounted 

for about 20 percent of streamflow at the San Antonio River 

near Elmendorf during 1997–2001 (Ockerman and McNamara, 2003). Wastewater discharge entering the San Antonio River upstream from the study area is accounted for by 

08181800 San Antonio River near Elmendorf (site 8). The 

total streamflow at this station is input to the model as a 

boundary condition. 

Downstream from 08185000 Cibolo Creek at Selma 

(site 11), discharges from three wastewater treatment plants 

on Martinez Creek and one on the main stem of Cibolo Creek 

change Cibolo Creek from an ephemeral to perennial stream. 

The wastewater discharges during 2000–2007 from these 

four plants, OJ Riedel–Martinez II (table 6), were available 

from plant operators as average monthly discharges (Daniel Flores, San Antonio River Authority, written commun., 

2009; David Humphrey and Robert Dabney, Cibolo Creek 

Municipal Authority, written commun., 2009). Locations 

and discharge amounts for eight additional smaller wastewater treatment plants (each serving populations of less than 

10,000), La Vernia–Goliad (table 6), were obtained from the 

U.S. Environmental Protection Agency (2004). 

Inflow to Cibolo Creek from springs upstream of the 

USGS streamflow-gaging station 08185500 Cibolo Creek at 

Sutherland Springs, Tex. (fig. 1) was simulated in the model 

as a time series input to stream reaches (RCHRES) 34, 42, 48, 

and 52 of the Cibolo Creek model (fig. 4). The time-series representing springflow was developed from data collected during 

gain/loss streamflow measurements in 2006–07. Streamflow 

gains, attributed to springs (Brune, 1975), were observed 

from the differences between measurements made at the 

USGS station 08185085 Cibolo Creek at Farm Road 2538 

near St. Hedwig, Tex. (not a continuous gaging station, but a 
14 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San Antonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

Zone 4

Zone 3

Zone 8

Zone 9

Zone 2

Zone 1

Zone 5

Zone 6

Zone 7

97°0'

97°30'

98°0'

29°30'

29°0'

28°30'

EXPLANATION

Land cover (table 4)CroplandDevelopedForestGrass/pasture/hayRiparian/wetlandShrub/scrubWaterHydrological Simulation Program—FORTRAN

water-budget zone boundary (zones 1–9)Lower San Antonio River watershed boundary

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

Land-cover data modified from Multi-ResolutionLand Characteristics Consortium (2001)

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

Figure 6. Land cover in the lower San Antonio River watershed, south-central Texas. 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 15 Figure 7. Location of National Weather Service meteorological stations and associated Thiessen rainfall areas for the Hydrological Simulation Program—FORTRAN model of the lower San Antonio River watershed, south-central Texas. Base modified from U.S. Geological Survey digital data 1:24,000 quadrangles Universal Transverse Mercator projection, North American Datum of 1983, Zone 14 Zone 4

Zone 3

Zone 8

Zone 9

Zone 1

Zone 2

Zone 5

Zone 6

Zone 7

Randolph Field

Floresville

Stockdale 4 N

Karnes City 2 N

Goliad

7

6

1

3

4

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San Antonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

97°0'

97°30'

98°0'

29°30'

29°0'

28°30'

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

EXPLANATION

Subwatershed of the lower San Antonio River watershedSan Antonio River upstream from Cibolo CreekCibolo CreekEcleto CreekSan Antonio River downstream from Cibolo CreekHydrological Simulation Program—FORTRAN

water-budget zone boundary (zones 1–9)Thiessen rainfall area boundaryLower San Antonio River watershed boundaryLower San Antonio River stream/reservoirreach (RCHRES) drainage area boundaryNational Weather Service meteorological stationand site number—See table 1

3
16 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Figure 8. Location of wastewater treatment plant discharge sites, lower San Antonio River watershed, south-central Texas. 

EXPLANATION

Subwatershed of lower San Antonio River watershedSan Antonio River upstream from Cibolo CreekCibolo CreekEcleto CreekSan Antonio River downstream from Cibolo CreekLower San Antonio River watershed boundaryLower San Antonio River stream/reservoir reach (RCHRES)

drainage area boundaryWastewater treatment plant discharge site and name (table 6)

") Runge

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San ntonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

87 £¤

183 £¤

281

£¤

87 £¤

90 £¤

281 £¤

77 £¤

90 £¤

59 £¤

77 £¤

281

£¤

181 £¤

77 £¤

181

£¤

37

§ ̈¦

35

§ ̈¦

35

§ ̈¦

10

¦

35

¦

410

35

¦

37

¦

10

§ ̈¦

410

¦

")

")

")

")

")

")

")

")

")

")

")

")

")

")

")

Goliad

OJ Riedel

Upper Martinez

Martinez II

Kenedy

Stockdale

Milam St

La Vernia

Salatrillo

Runge

Leon Creek

Medio Creek

Dos Rios

Karnes Main St

Floresville

97°0'

97°30'

98°0'

29°30'

29°0'

28°30'
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 17

Table 4. Land-cover categories represented in the Hydrological Simulation Program—FORTRAN model of the lower San Antonio River 

watershed, south-central Texas.

[HSPF, Hydrological Simulation Program—FORTRAN; --, not applicable]

HSPF model land-cover category 

(fig. 6)

Corresponding classification from National 

Land Cover Database 20011

Percent impervious 

land cover

Water 11 --

Open space (not on fig. 6) 21, 31 15

Developed 22, 23, 24 25

Forest 41, 42, 43 0

Shrub/scrub 52 0

Grass/pasture/hay 71, 81 0

Cropland 82 0

Riparian/wetland 90, 91 0

1

 Multi-Resolution Land Characteristics Consortium (2008).

Table 5. Rainfall calculated for water-budget zones of the Hydrological Simulation Program—FORTRAN model of the lower San 

Antonio River watershed, south-central Texas, 2000–2007.

Waterbudget 

zone

(fig. 2)

Area 

(acres)

Rainfall

(inches)

2000 2001 2002 2003 2004 2005 2006 2007 Annual 

mean 

1 108,716 33.9 36.7 40.0 25.7 48.1 20.2 24.1 48.0 34.6

2 133,447 33.9 36.2 40.2 26.4 47.0 20.0 25.1 47.7 34.6

3 97,342 32.9 33.9 40.6 28.2 43.6 19.1 26.3 47.7 34.0

4 161,944 33.0 33.9 40.7 28.4 43.6 19.1 26.5 47.8 34.1

5 158,316 33.2 34.1 40.5 28.2 43.6 19.3 26.1 47.5 34.0

6 92,595 34.9 35.3 39.3 27.1 44.1 21.0 22.6 45.5 33.7

7 236,723 35.5 35.9 39.0 26.8 44.4 21.5 21.7 44.9 33.7

8 276,464 35.5 35.9 39.0 26.8 44.4 21.5 21.7 44.9 33.7

9 82,207 36.6 43.3 41.4 32.5 46.9 27.0 30.0 49.9 38.5

Area weighted1 34.5 35.8 39.8 27.5 44.8 20.8 24.2 46.6 34.3

1

 For entire lower San Antonio River watershed (normalized to amount of pervious land in water-budget zones 1–9).
18 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

miscellaneous measurement site) and streamflow-gaging 

station 08185500 Cibolo Creek at Sutherland Springs. During 

four sets of synoptic measurements made during base-flow 

conditions (April 2006, August 2006, February 2007, and 

October 2007), streamflow gains between stations 08185085 

and 08185500 ranged between 4 and 29 cubic feet per second 

(U.S. Geological Survey, 2009). To develop a continuous daily 

time series of springflow for input to the model, the observed 

streamflow gains were related to daily water level, measured 

on the same day as the streamflow measurements, at the J–17 

regional index well (Edwards Aquifer Authority, 2009). This 

well (state well number AY–68–37–203) is located approximately 27 miles northwest of station 08185500, on the Fort 

Sam Houston military base in San Antonio (not shown in 

fig. 1). A linear regression relation of measured streamflow 

gains and J–17 daily water levels was developed and used to 

estimate daily springflow input to Cibolo Creek for 2000–

2007. The resulting regression equation was:

Qspringflow = 0.482 x L – 305.2, (3)

where 

Qspringflow is estimated mean daily spring discharge to Cibolo 

Creek, in cubic feet per second, and 

L is mean daily water level of the J–17 well, in feet.

The coefficient of determination of the regression equation was 0.868. The springflow estimated by the regression 

equation was divided among the four model RCHRESs in 

proportion to the stream length of each stream reach.

Surface-water withdrawals from the San Antonio River 

and Cibolo Creek for irrigation were determined from data 

provided by the Texas Commission on Environmental 

Quality (TCEQ) (Ceasar Alvarado, Texas Commission on 

Environmental Quality, written commun., 2009). These data 

were provided in the form of monthly volumes, for TCEQ 

defined stream reach segments, for 1997–2005. For input to 

Table 6. Wastewater discharges included in the Hydrological Simulation Program—FORTRAN model of the lower San Antonio River 

watershed, south-central Texas.

[HSPF, Hydrological Simulation Program—FORTRAN; RCHRES, stream reach or reservoir]

Wastewater

treatment plant 

(fig. 8)

Receiving stream Receiving 

HSPF RCHRES

Average discharge1

(million gallons per day) 

Medio Creek Medio Creek2 Upstream from study area 6

Leon Creek Leon Creek2 Upstream from study area 35

Dos Rios Medina River2 Upstream from study area 54

Salado Creek3 Salado Creek2 Upstream from study area 31

OJ Riedel Cibolo Creek 6 4.5

Salatrillo Cibolo Creek 17 3.8

Upper Martinez Cibolo Creek 20 1.7

Martinez II Cibolo Creek 20 1.7

La Vernia Cibolo Creek 34 .04

Stockdale Cibolo Creek 68 .10

Floresville San Antonio upstream from Cibolo Creek 26 .60

Milam St. San Antonio upstream from Cibolo Creek 76 .20

Karnes Main St. San Antonio downstream from Cibolo Creek 18 .05

Kenedy San Antonio downstream from Cibolo Creek 16 .80

Runge San Antonio downstream from Cibolo Creek 26 .06

Goliad San Antonio downstream from Cibolo Creek 78 .20

1

 Reported discharges for Medio Creek, Leon Creek, Dos Rios, and Salado Creek facilities from Pablo Martinez (San Antonio Water System, written 

commun., 2007); reported discharges from other facilities from U.S. Environmental Protection Agency (2004).

2

 Discharge is upstream from U.S. Geological Survey station 08181800 San Antonio River near Elmendorf, Texas, and included in measured streamflow at the 

station.

3

 Salado Creek facility (not on fig. 8) decommissioned in 2007; influent previously treated by Salado Creek facility now treated by Dos Rios facility.
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 19

the HSPF model the data were disaggregated to average hourly 

values, based on the monthly totals. Monthly average withdrawals for each stream reach during 1997–2005 were used to 

estimate monthly values for the period when withdrawal data 

were not available (2006–07). Also, TCEQ defined stream 

reaches do not correspond directly with the HSPF RCHRESs; 

each TCEQ reach typically includes several HSPF RCHRESs. 

Total withdrawals from each TCEQ stream reach were 

allocated to the appropriate HSPF RCHRES in proportion to 

the amount of cropland included in the area draining to each 

RCHRES. Cropland acreage (fig. 6) was assumed constant for 

the simulation period, 2000–2007.

Meteorological Data

BASINS 4.0 was used to download and pre-process 

rainfall and air temperature data from the NWS sites (table 1). 

These data were used to create the hourly time series of rainfall and PEVT for the HSPF model—downloading and formatting regional meteorological data from national datasets. The 

algorithms in this version of BASINS (U.S. Environmental 

Protection Agency, 2007) download and process national datasets through 2006. To extend the record through 2007 for this 

model, available rainfall and air temperature data for the same 

or nearby NWS sites were downloaded from the National 

Climatic Data Center (2009). These data were reviewed, 

processed, and appended to the input WDM files using the 

same processing steps available in BASINS 4.0. HSPF uses 

BASINS-computed PEVT estimates with other model input 

(rainfall, storage, lower-zone parameters) to simulate actual 

ET. Three of the NWS meteorological stations (sites 2, 4, 

and 7; table 1) had air temperature data that BASINS used 

to compute PEVT estimates through 2006. Air temperature 

data at these sites were used to extend the PEVT time series 

through 2007 for the model. The Hamon method (Bidlake, 

2002), a subroutine available in the WDMUtil program of 

BASINS 4.0, was used on the 2007 maximum and minimum 

daily air temperature data to estimate computed PEVT for 

2007. 

Model Calibration and Testing

Model calibration is an inherently iterative process of 

parameter evaluation and adjustment. Initial estimates of 

model parameters (such as INFILT and LZSN in HSPF) are 

adjusted until the simulated streamflow and ET data compare favorably to measured data, and predefined calibration 

criteria are satisfied. Various acceptance criteria are used. 

Comparisons of simulated data with measured data are facilitated through the use of descriptive statistics such as means, 

medians, and variances, and by the use of graphs. Jain and 

Sudheer (2008, p. 981) note, “Many times, the parameters 

of the hydrologic models are not measurable in the field or 

there [might] be a dearth of field measurements. In such 

cases, initial parameter values are assigned [on the basis of] 

relevant measurable catchment properties—[for example] soil 

properties, vegetation characteristics or by experience.” Model 

testing involves using the calibrated model to simulate data 

for another time period. These simulated data are compared 

with additional measured data that were not used in the initial 

calibration. 

Model parameters were adjusted to meet acceptance 

criteria for streamflow at various USGS streamflow-gaging 

stations in the watershed. Effort was also made to minimize 

the difference between simulated ET in the water-budget 

zones representing the Carrizo Sand and the measured ET at 

USGS 290810099212100 SW Medina County meteorological 

station. Model parameters were adjusted while maintaining 

recharge rates within the range of literature values reported 

by Scanlon and Dutton (2003). In addition, initial estimates 

of irrigation withdrawals were adjusted by as much as ±100 

percent. 

Streamflow

A primary goal of hydrologic model calibration is to 

adjust model-simulated streamflow to match streamflow 

measured at a nearby streamflow-gaging station. The lower 

San Antonio River model was calibrated in accordance with 

guidelines by Donigian and others (1984) and Lumb and 

others (1994). These guidelines involved comparing measured 

and simulated streamflow data and minimizing the difference 

between the total volumes of streamflow, largest 10 percent 

of streamflows, and smallest 50 percent of base flows. In 

addition, model-fit statistics generated by the software program GenScn (U.S. Environmental Protection Agency, 2007) 

were used to examine the quality of the model fit on an annual, 

monthly, daily, and hourly basis for the (1) coefficient of determination (R-squared) of the linear regression between measured and simulated streamflow; (2) Nash-Sutcliff coefficient 

of model-fit efficiency (NSE), which measures the amount 

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). The R-squared and NSE are similar; each provides 

a measure of the variability in a dataset accounted for by 

the statistical model. The NSE, however, provides a generally preferable evaluation of the fit quality because the NSE 

measures the magnitude of the differences between measured 

and simulated values, whereas the R-squared measures the 

difference between mean values (Zarriello and Ries, 2000). 

The MAE and RMSE statistics express the difference between 

measured and simulated streamflow in original units (cubic 

feet per second). Because a large NSE can be achieved with a 

less-than-adequate model, it is important to also evaluate the 

model performance using other methods (Jain and Sudheer, 

2008), such as scatter plots. 

Eleven USGS streamflow-gaging stations are in the lower 

San Antonio River watershed. Two stations—08181800 San 

Antonio River near Elmendorf and 08185000 Cibolo Creek at 

Selma (sites 8 and 11, respectively; fig. 1, table 1)—provided 

streamflow data that were used as boundary condition data to 
20 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

represent streamflow entering the study area. Data from eight 

of the nine remaining stations were used for calibration or testing, or both. Three of the eight stations—08183500 San Antonio River near Falls City, 08186000 Cibolo Creek near Falls 

City, and 08188500 San Antonio River at Goliad (sites 10, 15, 

and 17, respectively; fig. 1, table 1)—had streamflow records 

for the entire 2000–2007 study period; data for 2004–07 were 

used for model calibration and the remaining data, 2000–2003, 

for testing. 

Five of the eight stations had data for only part of the 

study period; data from these stations were used for either 

calibration or testing. Data from three stations—08185100 

Martinez Creek near St. Hedwig, Tex., 08186500 Ecleto 

Creek near Runge, Tex., and 08188570 San Antonio River 

near McFaddin, Tex. (sites 13, 16, and 18, respectively; fig. 1, 

table 1)—were used for calibration. Data from two stations—08183200 San Antonio River near Floresville, Tex., 

and 08185500 Cibolo Creek at Sutherland Springs, Tex. (sites 

9 and 14, respectively; fig. 1, table 1)—were not used for calibration but were used for additional testing of model streamflow simulation. 

The streamflow calibration process was accomplished 

beginning with the most upstream subwatersheds, using available streamflow-gaging data to adjust model process parameters. For example, data from 08183500 San Antonio River 

near Falls City were used to calibrate model streamflow for 

the drainage area upstream from the station (through RCHRES 

54) in the subwatershed of the San Antonio River upstream 

from Cibolo Creek. Similarly, data from 08185100 Martinez 

Creek near St. Hedwig, were used to calibrate the drainage 

area associated with RCHRESs 17–23 in the Cibolo Creek 

subwatershed. Using data from the next downstream stations, 

further calibration was performed by adjusting process-related 

parameters for the intervening area downstream from the 

previously calibrated drainage area. For model RCHRES 

outlets representing locations of streamflow-gaging stations, 

measured streamflow data rather than simulated streamflow 

data were routed to the next downstream RCHRES. In this 

way, simulation errors (differences between measured and 

simulated streamflows) were not propagated downstream. 

Measured and simulated streamflows and model-fit statistics 

for all stations used in the calibration and testing process are 

listed in table 7. 

Simulated flows 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 exceedanceprobability curves indicate adequate calibration over the 

range of flow conditions. Daily time series, exceedanceprobability curves, and scatter plots of measured daily 

and simulated daily streamflow are shown graphically for 

calibration stations 08183500 San Antonio River near Falls 

City (fig. 9), 08185100 Martinez Creek near St. Hedwig 

(fig. 10), 08186000 Cibolo Creek near Falls City (fig. 11), 

08186500 Ecleto Creek near Runge (fig. 12), 08188500 San 

Antonio River at Goliad (fig. 13), and 08188570 San Antonio 

River near McFaddin (fig. 14). Simulated streamflow agreed 

reasonably well with measured streamflow for the range of 

streamflow observed during the study.

Donigian and others (1984) provide general 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 

the error is within 10 to 15 percent, and fair when the error 

is within 15 to 25 percent. According to these guidelines, 

calibration results for annual streamflow volumes at all of 

the calibration stations are considered very good or good. 

The R-squared and NSE values are considered acceptable for 

annual, monthly, and daily statistics (table 7). The NSE for 

daily streamflows ranged from 0.57 to 0.93 for the calibration 

periods at all stations. Generally, R-squared and NSE values 

were lower for hourly streamflow values, especially for Cibolo 

Creek near Falls City. The NSE for hourly simulations ranged 

from 0.29 to 0.90 for the calibration periods at all stations. 

Simulated streamflow volumes, streamflow extremes, and 

the model-fit statistics were considered good at 08183200 San 

Antonio River near Floresville. The simulated streamflows 

exceeded measured streamflows at 08185500 Cibolo Creek at 

Sutherland Springs, and the model-fit statistics were considered poor, most likely because of less-than-optimal modeling 

of groundwater and wastewater contributions to flow. 

Evapotranspiration and Groundwater Recharge

Besides accurate simulation of streamflow, another 

goal of watershed model calibration is to accurately simulate 

the overall water budget in the watershed, including ET and 

groundwater recharge. The nearest measured ET data for comparison with HSPF-simulated ET values are collected at USGS 

290810099212100 SW Medina County meteorological station 

near D’Hanis, about 70 miles west of the lower San Antonio 

River study area. This station was installed in September 2006 

on shrub/scrub land on the Carrizo Sand outcrop (Richard 

Slattery, U.S. Geological Survey, written commun., 2008). ET 

data are computed by the eddy covariance method, a statistical 

method that measures and calculates vertical turbulent fluxes 

within atmospheric boundary layers on the basis of micrometeorological data, including wind and scalar atmospheric data 

series, and yields values of fluxes for these properties that are 

then used to estimate ET (Bidlake, 2002). 

HSPF-simulated ET from the Carrizo Sand outcrop in 

the lower San Antonio River watershed was compared with 

ET measured at the SW Medina County meteorological station during October 2006–December 2007 (fig. 15). Because 

local conditions such as rainfall, cloud cover, and humidity are 

highly variable, direct comparison of measured and HSPFsimulated ET on a short time scale is of limited use. Overall, 

however, HSPF-simulated ET compared fairly well with 

measured ET during the comparison period. Total HSPFsimulated ET from the pervious area of water-budget zone 3 

for October 2006–November 2007 was 38.4 inches, and total 

measured ET at the SW Medina County meteorological 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 21

Figure 9. Measured and simulated daily mean streamflow at 08183500 San Antonio River near Falls City, Texas, 2000–2007. 

10

100

1,000

10,000

10

100

10,000

100,000

SIMULATED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

100,000

1,000

10

100

10,000

100,000

1,000

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

2001 2002 2004 2006

YEAR

2000 2003 2005 2007

10 100 1,000 100,000

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

MEASURED

SIMULATED

MEASURED

SIMULATED

Line of equal value
22 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Figure 10. Measured and simulated daily mean streamflow at 08185100 Martinez Creek near St. Hedwig, Texas, 2006–07. 

10

100

1,000

1

10

1,000

10,000

SIMULATED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

100

1

10

1,000

10,000

100

1

1 10

1 10 100 10,000

2006

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

EXCEEDANCE PROBABILITY, IN PERCENT

YEAR

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

1,000

2007

MEASURED

SIMULATED

MEASURED

SIMULATED

Line of equal value
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 23

Figure 11. Measured and simulated daily mean streamflow at 08186000 Cibolo Creek near Falls City, Texas, 2000–2007. 

10

1

100

1,000

10,000

10

1

100

10,000

100,000

SIMULATED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

100,000

1,000

10

1

100

10,000

100,000

1,000

1 10

1 10 100 1,000 100,000

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

EXCEEDANCE PROBABILITY, IN PERCENT

YEAR

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

2000 2001 2002 2003 2004 2005 2006 2007

Line of equal value

MEASURED

SIMULATED

MEASURED

SIMULATED
24 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Figure 12. Measured and simulated daily mean streamflow at 08186500 Ecleto Creek near Runge, Texas, 2003–07. 

10

100

1,000

1

10

1,000

10,000

SIMULATED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

100

1

10

1,000

10,000

100

1

1 10 100 10,000

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

1,000

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

EXCEEDANCE PROBABILITY, IN PERCENT

2004

YEAR

2003 2005 2006 2007

Line of equal value

MEASURED

SIMULATED

MEASURED

SIMULATED
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 25

Figure 13. Measured and simulated daily mean streamflow at 08188500 San Antonio River at Goliad, Texas, 2000–2007. 

10

100

1,000

10,000

10

100

10,000

100,000

SIMULATED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

100,000

1,000

10

100

10,000

100,000

1,000

10 100 1,000 100,000

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

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

2001 2002 2004 2006

YEAR

2000 2003 2005 2007

MEASURED

SIMULATED

MEASURED

SIMULATED

Line of equal value
26 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Figure 14. Measured and simulated daily mean streamflow at 08188570 San Antonio River near McFaddin, Texas, 2006–07. 

10

100

1,000

10,000

10

100

10,000

100,000

SIMULATED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

100,000

1,000

10

100

10,000

100,000

1,000

2006

YEAR

2007

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

10 100 1,000 100,000

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

10,000

MEASURED

SIMULATED

MEASURED

SIMULATED

Line of equal value
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 27

station for the same time period was 37.4 inches. As a result of 

streamflow and ET calibration and testing, a final set of HSPF 

hydrologic parameters was obtained for the model; values 

for selected parameters are listed by water-budget zone in 

table 8.

Direct measurements of groundwater recharge in the San 

Antonio River watershed were not available for comparison 

with model simulations of recharge. Therefore, model simulations, or estimates, of groundwater recharge depended on 

accurate model representations of the remaining water-budget 

Figure 15. Measured weekly evapotranspiration at 290810099212100 SW Medina County meteorological station near D’Hanis, Texas, 

and Hydrological Simulation Program—FORTRAN simulated weekly evapotranspiration for the Carrizo Sand outcrop area of the lower 

San Antonio River watershed, south-central Texas, October 2006–December 2007. 

0

0.4

1.2

1.6

WEEKLY EVAPOTRANSPIRATION, IN INCHES

0.8

0

0.4

0.8

1.2

1.6

SIMULATED WEEKLY EVAPOTRANSPIRATION, IN INCHES

YEAR

2006 2007

0.4 0.8 1.2 1.6 0

MEASURED WEEKLY EVAPOTRANSPIRATION, IN INCHES

MEASURED

SIMULATED

Line of equal value
28 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Table 7. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas.

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

08183500 San Antonio River near Falls City, Texas

Calibration period 2004–07

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 2.558 2.718 6.2 10

Mean flow rate (ft3

/s) 883 938 6.2 10

Total of highest 10 percent of daily flows (million acre-ft) 1.135 1.236 8.9 10

Total of lowest 50 percent of daily flows (acre-ft) 383,000 399,000 4.2 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R-squared) .99 .98 .93 .92

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .98 .98 .93 .92

Mean absolute error (ft3

/s) 46 83 128 137

Root mean square error (ft3

/s) 78 153 426 475

Testing period 2000–2003

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 2.608 2.691 3.2 10

Mean flow rate (ft3

/s) 900 929 3.2 10

Total of highest 10 percent of daily flows (million acre-ft) 1.404 1.476 5.1 10

Total of lowest 50 percent of daily flows (acre-ft) 379,000 377,000 -.5 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R-squared) 1.00 1.00 .97 .97

Nash-Sutcliff coefficient of model-fit efficiency (NSE) 1.00 1.00 .97 .97

Mean absolute error (ft3

/s) 35 70 132 138

Root mean square error (ft3

/s) 39 116 488 536

Simulation period 2000–2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 5.167 5.363 3.8 10

Mean flow rate (ft3

/s) 891 925 3.8 10

Total of highest 10 percent of daily flows (million acre-ft) 2.557 2.723 6.5 10

Total of lowest 50 percent of daily flows (acre-ft) 758,000 769,000 1.5 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 8 96 2,922 70,128

Coefficient of determination (R-squared) .99 .99 .96 .96

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .99 .99 .96 .96

Mean absolute error (ft3

/s) 40 76 130 138

Root mean square error (ft3

/s) 61 136 458 506

1

 Error = [(simulated-measured)/measured] x 100. 

2

 Default error criteria from HSPEXP (Lumb and others, 1994). 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 29

Table 7. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas—Continued.

08186000 Cibolo Creek near Falls City, Texas

Calibration period 2004–07

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (acre-ft) 722,000 738,000 2.2 10

Mean flow rate (ft3

/s) 249 254 2.2 10

Total of highest 10 percent of daily flows (acre-ft) 526,000 492,000 -6.5 10

Total of lowest 50 percent of daily flows (acre-ft) 51,800 49,100 -5.2 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R-squared) 1.00 .99 .70 .50

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .99 .98 .58 .29

Mean absolute error (ft3

/s) 11 39 117 143

Root mean square error (ft3

/s) 13 63 584 826

Testing period 2000–2003

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (acre-ft) 728,000 778,000 6.8 10

Mean flow rate (ft3

/s) 251 268 6.8 10

Total of highest 10 percent of daily flows (acre-ft) 534,000 522,000 -2.2 10

Total of lowest 50 percent of daily flows (acre-ft) 53,400 49,300 -7.7 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,063

Coefficient of determination (R-squared) .91 .91 .68 .53

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .87 .91 .64 .47

Mean absolute error (ft3

/s) 54 94 156 175

Root mean square error (ft3

/s) 58 161 653 866

Simulation period 2000–2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 1.450 1.515 4.5 10

Mean flow rate (ft3

/s) 250 261 4.5 10

Total of highest 10 percent of daily flows (million acre-ft) 1.062 1.014 -4.5 10

Total of lowest 50 percent of daily flows (acre-ft) 105,000 98,400 -6.3 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 8 96 2,922 70,127

Coefficient of determination (R-squared) .95 .94 .69 .51

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .94 .94 .62 .39

Mean absolute error (ft3

/s) 33 67 137 159

Root mean square error (ft3

/s) 42 122 619 846

1

 Error = [(simulated-measured)/measured] x 100. 

2

 Default error criteria from HSPEXP (Lumb and others, 1994). 
30 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Table 7. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas—Continued.

08188500 San Antonio River at Goliad, Texas

Calibration period 2004–07

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 3.842 4.130 7.5 10

Mean flow rate (ft3

/s) 1,330 1,430 7.5 10

Total of highest 10 percent of daily flows (million acre-ft) 1.852 2.038 10.0 10

Total of lowest 50 percent of daily flows (acre-ft) 542,000 527,000 -2.8 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R-squared) 1.00 .99 .83 .81

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .97 .98 .82 .80

Mean absolute error (ft3

/s) 108 165 304 317

Root mean square error (ft3

/s) 146 258 1,117 1,192

Testing period 2000–2003

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 4.106 4.135 0.7 10

Mean flow rate (ft3

/s) 1,420 1,430 .7 10

Total of highest 10 percent of daily flows (million acre-ft) 2.315 2.350 1.5 10

Total of lowest 50 percent of daily flows (acre-ft) 563,000 511,000 -9.2 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 4 48 1,461 35,064

Coefficient of determination (R-squared) 1.00 .99 .86 .85

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .99 .99 .86 .85

Mean absolute error (ft3

/s) 60 140 323 336

Root mean square error (ft3

/s) 74 254 1,501 1,560

Simulation period 2000–2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 7.934 8.281 4.4 10

Mean flow rate (ft3

/s) 1,370 1,430 4.4 10

Total of highest 10 percent of daily flows (million acre-ft) 4.177 4.405 5.5 10

Total of lowest 50 percent of daily flows (million acre-ft) 1.105 1.074 -2.8 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 8 96 2,922 70,128

Coefficient of determination (R-squared) .99 .99 .85 .84

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .98 .99 .85 .84

Mean absolute error (ft3

/s) 81 153 313 327

Root mean square error (ft3

/s) 116 256 1,320 1,388

1

 Error = [(simulated-measured)/measured] x 100. 

2

 Default error criteria from HSPEXP (Lumb and others, 1994). 
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 31

Table 7. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas—Continued.

08183200 San Antonio River near Floresville, Texas

Testing period 01/05/2006–12/31/2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 1.307 1.303 -0.3 10

Mean flow rate (ft3

/s) 908 905 -.3 10

Total of highest 10 percent of daily flows (acre-ft) 720,000 724,000 .6 10

Total of lowest 50 percent of daily flows (acre-ft) 138,000 130,000 -5.8 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 1 23 726 17,424

Coefficient of determination (R-squared) -- 1.00 .98 .98

Nash-Sutcliff coefficient of model-fit efficiency (NSE) -- 1.00 .98 .97

Mean absolute error (ft3

/s) 16 35 87 102

Root mean square error (ft3

/s) 16 51 270 338

08185100 Martinez Creek near St. Hedwig, Texas

Calibration period 11/17/2005–12/31/2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (acre-ft) 67,200 71,800 6.9 10

Mean flow rate (ft3

/s) 43.7 46.7 6.9 10

Total of highest 10 percent of daily flows (acre-ft) 46,800 49,100 4.9 10

Total of lowest 50 percent of daily flows (acre-ft) 5,810 5,750 -1.0 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 2 25 775 18,600

Coefficient of determination (R-squared) 1.00 .96 .72 .50

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .99 .96 .67 .37

Mean absolute error (ft3

/s) 3.3 8.6 22 26

Root mean square error (ft3

/s) 3.4 14 71 112

08185500 Cibolo Creek at Sutherland Springs, Texas

Testing period 12/21/2005–12/31/2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (acre-ft) 256,000 355,000 39 10

Mean flow rate (ft3

/s) 174 241 39 10

Total of highest 10 percent of daily flows (acre-ft) 189,000 252,000 33 10

Total of lowest 50 percent of daily flows (acre-ft) 18,700 21,400 14 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 2 24 741 17,784

Coefficient of determination (R-squared) 1.00 .95 .42 .29

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .82 .91 .32 .18

Mean absolute error (ft3

/s) 67 72 132 146

Root mean square error (ft3

/s) 85 125 635 827

1

 Error = [(simulated-measured)/measured] x 100. 

2

 Default error criteria from HSPEXP (Lumb and others, 1994). 
32 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

components not associated with recharge (primarily streamflow and ET).

Simulated Streamflow, 2000–2007 

Boundary inflows to the study area were obtained from 

08181800 San Antonio River near Elmendorf and 08185000 

Cibolo Creek at Selma. These flows and other streamflow 

additions and withdrawals, including measured or reported 

volumes of streamflow, springflow, wastewater discharge, and 

irrigation withdrawals, are routed downstream concurrently 

with the meteorological data. The lower San Antonio River 

watershed HSPF model can then be used to simulate streamflow at the outlet of any RCHRES (fig. 4) for calibration or 

testing, or for comparison purposes. Using model input and 

output, streamflow amounts and sources can be compared for 

each subwatershed. Annual mean streamflow volumes and 

basin yields generated in each subwatershed were compiled 

(table 9). The annual mean streamflow volumes represent the 

streamflow generated in each subwatershed from all inputs 

to that subwatershed but do not include upstream inflows. 

The simulated streamflow volumes from each subwatershed 

include runoff from pervious and impervious areas and all 

streamflow additions and withdrawals that can be quantified. 

Runoff from precipitation is the largest source of streamflow generated in each subwatershed in the study area. Other 

contributions to streamflow in each subwatershed listed in 

table 9 are relatively small compared with the total simulated streamflow volumes. During 2000–2007, annual mean 

streamflow volumes from the four subwatersheds totaled 

0.381 million acre-feet. Annual mean basin yields for each 

of the subwatersheds and for the upstream watersheds were 

Table 7. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model of the lower San Antonio 

River watershed, south-central Texas—Continued.

08186500 Ecleto Creek near Runge, Texas

Calibration period 10/02/2002–12/31/2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 0.199 0.208 4.6 10

Mean flow rate (ft3

/s) 52.3 54.7 4.6 10

Total of highest 10 percent of daily flows (acre-ft) 178,000 173,000 -2.8 10

Total of lowest 50 percent of daily flows (acre-ft) 728 716 -1.6 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 5 50 1,918 35,032

Coefficient of determination (R-squared) .89 .84 .65 .52

Nash-Sutcliff coefficient of model-fit efficiency (NSE) .87 .84 .57 .43

Mean absolute error (ft3

/s) 8.7 22 37 43

Root mean square error (ft3

/s) 13 43 161 208

08188570 San Antonio River near McFaddin, Texas

Testing period 11/24/2005–12/31/2007

Comparison of streamflow 

volumes and peaks

Measured 

streamflow

Simulated 

streamflow

Error1

(percent)

Criteria2

(percent)

Total flow volume (million acre-ft) 1.971 2.032 3.1 10

Mean flow rate (ft3

/s) 1,290 1,330 3.1 10

Total of highest 10 percent of daily flows (million acre-ft) 1.083 1.116 3.0 10

Total of lowest 50 percent of daily flows (acre-ft) 201,000 221,000 10.0 10

Model-fit statistics Annual Monthly Daily Hourly

Number of years, months, days, or hours 2 25 768 18,432

Coefficient of determination (R-squared) 1.00 .97 .79 .77

Nash-Sutcliff coefficient of model-fit efficiency (NSE) 1.00 .97 .78 .76

Mean absolute error (ft3

/s) 48 180 373 388

Root mean square error (ft3

/s) 50 311 1,180 1,240

1

 Error = [(simulated-measured)/measured] x 100. 

2

 Default error criteria from HSPEXP (Lumb and others, 1994). 
Table 8. Calibrated values for selected parameters, by water-budget zone, for the Hydrological Simulation Program—FORTRAN model 

of the lower San Antonio River watershed, south-central Texas.

Parameter1 Units

Calibrated values by water-budget zone

(fig. 2)

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 Zone 9

AGWETP none 0 0.01–0.05 0.01–0.05 0.01–0.05 0.01–0.05 0.01–0.05 0.01–0.05 0.01 0.05

AGWRC 1/day .92 .92–.94 .85–.94 .92–.97 .95–.97 .95–.97 .95–.98 .98 .98

BASETP none .01 .01 .01–.20 .01–.20 .01–.15 .01–.10 .01–.15 .01 .10

CEPSC inches .1–.3 .1–.3 .1–.3 .1–.3 .1–.3 .1–.3 .1–.3 .1–.3 .1–.3

DEEPFR none .35 .40–.60 .70–.85 .60–.85 .55–.60 .10–.25 .20–.45 .32 .20

INFEXP none 2 2 2 2 2 2 2 2 2

INFILD none 2 2 2 2 2 2 2 2 2

INFILT inches/hour .50–.55 .40–.60 .45–.90 .35–.65 .15–.55 .10–.50 .25–.55 .60–.64 .37–.40

INTFW none 8.5 1.0–8.5 1.0–1.50 1.0–2.50 1.0–2.50 1.0–2.50 1.0–2.50 1.5 1.5

IRC 1/day .5 .5 .1–.5 .1–.5 .1–.5 .1–.5 .1–.5 .5 .5

KVARY 1/inch 4.0 2.0–4.0 2.0–4.0 2.0–4.0 2.0–4.0 2.0–4.0 2.0–4.0 2.0 2.0

LSUR feet 250–300 250–300 250–300 250–300 250–300 250–300 250–300 300 300

LZETP none .2–.8 .2–.8 .2–.8 .2–.8 .2–.8 .2–.8 .2–.8 .4–.8 .4–.8

LZSN inches 8.5 8.0–8.5 8.0–9.5 8.0–8.5 8.0–8.5 8.0–8.5 8.0–8.5 8.0–8.5 8.0–8.5

NSUR none .15–.20 .15–.31 .15–31 .15–.31 .15–.31 .15–.31 .15–.31 .20–.31 .20–.31

RETSC inches .1 .1 .05–.1 .05–.1 .05–.1 .05–.1 .05–.1 .1 .1

SLSUR feet .03 .03 .03 .03 .03 .03 .03 .03 .03

UZSN inches .54–.56 .54–.76 .54–.76 .54–.76 .54–.76 .54–.76 .54–.86 .84–.86 .84–.86

1

 See table 3 for description of parameters.

Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 33

computed by dividing the generated streamflow volume at the 

outlets by the corresponding subwatershed area. Of the four 

subwatersheds in the lower San Antonio River watershed, the 

Cibolo Creek subwatershed had the largest annual mean basin 

yield, about 4.8 inches per year. Springflow and wastewater 

discharges were larger in this subwatershed compared with the 

other subwatersheds. The smallest annual mean basin yield 

was from the subwatershed of the San Antonio River upstream 

from Cibolo Creek, about 1.2 inches per year. This lower 

yield might be caused partly by the way the model represents 

retention and subsequent evaporation of overland flow in 

Calaveras Lake from RCHRES 5 (fig. 4). Possible reasons 

for the differences in yields have not been fully studied. The 

measured (gaged) annual mean volume of streamflow entering 

the lower San Antonio River watershed during 2000–2007 was 

0.685 million acre-feet (table 10). The overall annual mean 

basin yield from the upstream contributing area to the model 

was 6.4 inches (table 10), or 3.1 inches higher than the overall 

annual mean basin yield of 3.3 inches (table 9) from the lower 

San Antonio River watershed. 

The simulated (modeled) annual mean volume of streamflow exiting the lower San Antonio River watershed during 

2000–2007 was 1.07 million acre-feet. This volume includes 

the annual mean inflow from the streamflow-gaging stations 

at the study area boundary (0.685 million acre-feet [table 10]) 

and the annual mean streamflow generated from the study area 

(0.381 million acre-feet [table 9]). Compared with the annual 

mean volume of streamflow entering the lower San Antonio 

River watershed from upstream (0.685 million acre-feet), 

the annual mean volume of streamflow exiting the lower San 

Antonio River watershed (1.07 million acre-feet) represents 

an increase of about 56 percent. Annual mean streamflow 
34 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

originating in the lower San Antonio River watershed during 

2000–2007 (estimated as annual mean simulated streamflow at the outlet minus annual mean streamflow at the inlet 

boundaries, 08181800 San Antonio River near Elmendorf 

and 08185000 Cibolo Creek at Selma) is less than 10 percent of the annual mean rainfall supplied. About 90 percent 

of the rainfall on the watershed is either evapotranspired or 

recharged.

Simulated Evapotranspiration, 2000–2007 

Parameters in the HSPF model representing final model 

runs were synthesized for each of the subwatersheds to 

produce estimated annual ET from the pervious land in each 

water-budget zone (table 11). Evaporation also occurred 

directly from water surfaces in IMPLNDs and RCHRESs, but 

the amount was small compared with ET from pervious land. 

About 2 percent of the study area consisted of impervious 

land; the evaporation from surficial waters in IMPLNDs and 

RCHRESs was about 1 percent of the total ET. 

For the entire study area, annual mean ET from 

PERLNDs was 28.2 inches per year (table 11). The smallest 

annual mean ET during 2000–2007 was 20.1 inches during 

2006, following a relatively dry year. The largest annual mean 

ET during 2000–2007 was 34.8 inches during 2007. Simulated 

ET was largest in water-budget zones 8 and 9, with annual 

mean amounts of 30.6 and 32.8 inches per year, respectively. 

These water-budget zones included a larger percentage of 

riparian/wetland land cover than the other zones in the study 

area. Land cover was used to adjust several HSPF parameters 

in the model. The larger percentage of riparian/wetland land 

cover in water-budget zones 8 and 9 might be partly responsible for the larger amounts of ET in these zones compared 

with ET in other zones. Also, rainfall was larger in zones 8 

and 9 than in the other zones, so more water was available to 

be evapotranspired. 

The annual mean rainfall for the study area during 

2000–2007 was 34.3 inches (table 5). An annual mean ET of 

28.2 inches indicates that, on average, about 82 percent of the 

rainfall supplied to the study area was evapotranspired. If the 

annual rainfall is above average, a smaller percentage of the 

annual rainfall usually evapotranspires. For example, 2004 and 

2007, with rainfall of 44.8 and 46.6 inches, respectively, were 

relatively wet years. In 2004 and 2007, an estimated 74 and 75 

percent, respectively, of rainfall evapotranspired on the basis 

of model simulations. The largest percentage of rainfall evapotranspired in years with lower-than-average rainfall and follow 

years with greater-than-average rainfall. For 2005, a very dry 

Table 9. Simulated streamflow volumes and basin yields generated from subwatersheds in the Hydrological Simulation Program—

FORTRAN model of the lower San Antonio River watershed, south-central Texas, 2000–2007.

[--, no flow]

Subwatershed

Drainage area 

(square 

miles)

Annual mean 

wastewater 

discharge 

(thousand 

acre-feet)1

Annual mean 

springflow 

(thousand 

acre-feet)2

Annual mean 

irrigation 

withdrawals 

(thousand 

acre-feet)3

Annual 

mean 

streamflow 

(million 

acre-feet)

Annual 

mean 

basin yield 

(inches)4

San Antonio River upstream from Cibolo Creek 554 0.86 0 3.8 0.035 1.2

Cibolo Creek 579 9.7 12.6 -- .149 4.8

Ecleto Creek 266 -- -- -- .042 3.0

San Antonio River downstream from Cibolo Creek 749 1.2 -- 1.6 .155 3.9

Total, all subwatersheds 11.8 12.6 5.4 .381 3.3

1

 Wastewater discharge input derived from reported wastewater discharges (table 6).

2

 Springflow input derived from correlation developed between streamflow gains and nearby groundwater levels.

3

 Calculated from reported surface-water withdrawals to irrigated cropland (Ceasar Alvarado, Texas Commission on Environmental Quality, written commun., 

2009).

4

 Calculated by dividing annual mean streamflow volume by drainage area.

Table 10. Annual mean streamflow volumes and basin yields at 

streamflow-gaging stations at upstream boundary of the lower 

San Antonio River watershed, south-central Texas, 2000–2007.

U.S. Geological Survey 

streamflow-gaging station 

(fig. 1)

Drainage 

area 

(square 

miles)

Annual 

mean 

streamflow 

(million 

acre-feet)

Annual 

mean 

basin 

yield 

(inches)1

08181800 San Antonio 

River near Elmendorf, Tex. 

(site 8)

1,750 0.642 6.9

08185000 Cibolo Creek near 

Selma, Tex. (site 11)

274 .043 2.9

Total at upstream boundary 2,020 .685 6.4

1

 Calculated by dividing annual mean streamflow volume by drainage area.
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 35

year (rainfall of 20.8 inches), simulated ET was 121 percent of 

the annual rainfall. Simulated ET can exceed rainfall in a given 

year when extra water that has been stored in the unsaturated 

zones during the preceding year is available to satisfy potential 

ET demand.

Estimated Groundwater Recharge, 2000–2007 

Similar to the procedure for generating ET estimates 

by water-budget zones, the output from the final model runs 

were synthesized to produce estimates of annual groundwater 

recharge into the pervious land of each water-budget zone. 

These subwatershed estimates from each model were areaweighted by the amount of pervious area of each subwatershed 

to produce estimated annual recharge rates by water-budget 

zone (fig. 16, table 12).

The largest groundwater recharge estimates were in 

water-budget zones 3 and 4, with annual estimates of 5.1 and 

4.8 inches, respectively. Water-budget zone 3 overlies the 

Carrizo Sand; zone 4 overlies the Recklaw Formation and 

Queen City Sand. The annual mean rainfall on zone 3 during 

2000–2007 was 34.0 inches (table 5). On average, an estimated 15 percent of rainfall went to groundwater recharge in 

zone 3. The smallest groundwater recharge estimates were in 

water-budget zones 6 and 9, with annual mean estimates of 

about 1.1 and 1.5 inches, respectively (fig. 16, table 12). The 

annual mean groundwater recharge estimate in zone 6 represents about 3.3 percent of the annual mean rainfall of 33.7 

inches in this zone during 2000–2007 (table 5). These recharge 

estimates are generally within the ranges reported by Scanlon 

and Dutton (2003). 

Estimated annual groundwater recharge in the lower 

San Antonio watershed during 2000–2007 varied from year 

to year but generally increased as precipitation increased. 

The smallest annual estimate of groundwater recharge was 

0.7 inch in 2006 (table 12); 2006 rainfall was about 24.2 

inches (table 5). The largest annual estimate of groundwater 

recharge was 6.1 inches in 2007 (table 12); 2007 rainfall was 

about 46.6 inches (table 5). During 2000–2007, groundwater 

recharge estimates as a percentage of rainfall varied from 

about 3 percent (2006) to 13 percent (2007). The annual 

mean estimated recharge rate for all pervious land in the study 

area was 3.0 inches per year (table 12). This recharge rate 

represents about 9 percent of annual mean rainfall for 2000–

2007.

Water-Budget Summary, 2000–2007

The inflows to the lower San Antonio River watershed 

are represented by the terms on the left-hand side of equation 

2. The modeled inflows include rainfall (P, precipitation), 

streamflow entering the study area from upstream as measured at 08181800 San Antonio River near Elmendorf (site 8) 

and 08185000 Cibolo Creek at Selma (site 11), estimated 

discharge of treated wastewater, and estimated springflow. 

Using the Thiessen diagrams, the average area-weighted 

rainfall in the lower San Antonio River watershed was computed as 34.3 inches per year (3.92 million acre-feet per 

year) during the study period. The measured streamflow 

volume entering the study area from upstream averaged 

0.685 million acre-feet per year. The combined volume of 

wastewater and springflow entering the study area averaged 

Table 11. Estimated evapotranspiration for pervious land in water-budget zones of the lower San Antonio River watershed, southcentral, Texas, 2000–2007.

Waterbudget 

zone

(fig. 2)

Pervious 

land 

(acres)

Estimated evapotranspiration 

(inches)

2000 2001 2002 2003 2004 2005 2006 2007 Annual 

mean

1 108,716 22.5 24.4 30.1 26.7 30.5 23.6 21.2 32.3 26.4

2 133,447 23.0 25.7 30.4 27.8 31.7 23.9 20.5 33.3 27.0

3 97,342 23.8 26.4 29.7 28.9 32.2 23.0 19.3 34.1 27.2

4 161,944 23.9 26.7 29.6 29.0 32.4 22.9 19.4 34.3 27.3

5 158,316 24.2 27.1 29.8 29.1 32.9 23.0 19.2 34.5 27.5

6 92,595 24.1 27.2 27.3 28.5 32.4 23.4 16.4 34.0 26.7

7 236,723 24.7 29.0 28.1 29.4 33.4 24.7 16.0 35.1 27.6

8 276,464 28.1 29.5 30.5 33.2 35.2 28.7 23.6 36.3 30.6

9 82,207 30.5 30.4 31.9 35.9 37.7 31.2 26.7 38.3 32.8

Area weighted1 25.1 27.7 29.7 30.0 33.3 25.1 20.1 34.8 28.2

1 For entire lower San Antonio River watershed (normalized to amount of pervious land in water-budget zones 1–9).
36 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Figure 16. Estimated annual mean groundwater recharge in water-budget zones of the lower San Antonio River watershed, 2000–2007. 

Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

0 10 15 20 MILES 5

0 10 15 20 KILOMETERS 5

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORIA COUNTY

DE WITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOL AI

D

COUNTY

GOL AI

D

COUNTY

REFUG OI

COUNTY

Manahuilla Creek

Ecleto Creek

San Antonio River

San Antonio River

Martinez Creek

Calaveras Lake

Guadalupe River

San Antonio

Cibolo Creek

Guadalupe

River

 River

Cibolo Creek

San Antonio River

Cabeza Creek

97°0'

97°30'

98°0'

29°30'

29°0'

28°30'

Hydrological Simulation Program—FORTRAN 

model water-budget zone boundary (zones 1–9) andcorresponding annual mean recharge, in inchesLower San Antonio River watershed boundaryBoundary of subwatershed of lower San Antonio River watershed

Groundwater recharge, in inches

0 6.0

EXPLANATION

Zone 4

4.8 inches

Zone 3

5.1 inches

Zone 8

2.5 inches

Zone 9

1.5 inches

Zone 2

3.5 inches

Zone 1

2.5 inches

Zone 5

3.6 inches

Zone 6

1.1 inches

Zone 7

2.3 inches
Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge 37

24,400 acre-feet per year. Total inflow volumes from measured or estimated inputs were 4.63 million acre-feet per 

year.

The simulated outflows from the lower San Antonio 

River watershed are represented by the terms on the right side 

of equation 2 and can be summarized as total ET, streamflow 

exiting the study area, and groundwater recharge. Total ET 

averaged 3.20 million acre-feet per year and includes ET from 

the surface, ET from the unsaturated zone, and ET derived 

from groundwater discharging to streams. The average volume 

of simulated streamflow outflows from the study area was 1.07 

million acre-feet per year, which included simulated surfacewater diversions for irrigation. Simulated surface-water withdrawals for irrigation averaged about 5,470 acre-feet per year. 

Simulated groundwater recharge averaged 3.0 inches per year 

across the watershed, which is equivalent to about 340,000 

acre-feet per year. Total outflows equaled 4.61 million acrefeet per year, obtained by adding the simulated total volumes 

of ET, streamflow exiting the study area, and groundwater 

recharge. 

For the overall water budget, the largest inflow to the 

study area is rainfall; the largest outflow is ET. Wastewater 

discharges, springflow, and irrigation withdrawals in the study 

area make up only a small percentage of the overall water 

budget in the study area. Despite the relatively small contribution of wastewater discharges, springflow, and irrigation to 

the overall water budget, taking their contributions to local 

streamflow into account proved necessary to achieve acceptable model calibration results. 

Sensitivity Analysis

A sensitivity analysis of selected HSPF model parameters was performed to determine the effects of systematic 

changes to the values of the parameters on simulated recharge, 

ET, and surface runoff from the PERLND areas in waterbudget zone 3, the Carrizo Sand. Zone 3 crosses three subwatersheds of the model—San Antonio River upstream from 

Cibolo, Cibolo Creek, and Ecleto Creek (fig. 4) . Each parameter was changed by a hydrologically reasonable amount while 

keeping the other parameters unchanged, and the simulations 

were run for each subwatershed. The results were areally 

weighted by the total PERLND area in each subwatershed. 

The resulting areally weighted changes in recharge, ET, and 

surface runoff exiting the PERLND area of the zone are listed 

in table 13. 

The parameters to which simulated water balance components of zone 3 were most sensitive for the given changes 

were lower-zone ET (LZETP) and the fraction of ground 

water that does not discharge to the surface within the boundaries of the modeled area (DEEPFR). Increasing the LZETP 

values by between 12.5 and 50 percent resulted in a 12-percent 

decrease in recharge, a 15-percent decrease in surface runoff 

from PERLNDs, and a 3.3-percent increase in ET. Reducing the DEEPFR values by between 23.5 and 28.5 percent 

resulted in a 25-percent decrease in recharge and an 85-

percent increase in runoff. Reducing the initial amount of 

water in the RCHRESs does not change the water balance 

components of zone 3. 

Table 12. Estimated groundwater recharge for pervious land in water-budget zones of the lower San Antonio River watershed, southcentral Texas, 2000–2007.

Waterbudget 

zone

(fig. 2)

Pervious 

land 

(acres)

Estimated recharge 

(inches)

2000 2001 2002 2003 2004 2005 2006 2007 Annual mean

1 108,716 1.4 3.5 3.3 1.3 4.0 1.1 0.4 5.4 2.5

2 133,447 1.8 4.6 4.6 1.8 5.3 1.7 .6 7.2 3.5

3 97,342 2.4 5.4 7.1 3.4 7.3 2.9 1.3 11.3 5.1

4 161,944 2.4 5.0 6.7 3.2 6.7 2.7 1.3 10.3 4.8

5 158,316 1.9 3.8 4.9 2.4 5.1 2.0 .9 7.4 3.6

6 92,595 .8 1.2 1.4 0.7 1.8 .6 .2 2.1 1.1

7 236,723 1.6 2.9 2.8 1.3 3.9 1.2 .3 4.3 2.3

8 276,464 1.2 3.3 3.2 1.5 3.5 1.6 .8 4.8 2.5

9 82,207 .7 2.0 1.8 .9 2.1 1.1 .6 2.9 1.5

Area weighted1 1.6 3.6 4.0 1.8 4.4 1.7 .7 6.1 3.0

1

 For entire lower San Antonio River watershed (normalized to amount of pervious land in water-budget zones 1–9).
38 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

Model Limitations

Model limitations include possible errors related to model 

conceptualization and parameter variability, lack of data to 

quantify certain model inputs, and measurement errors. HSPF 

is a complex watershed model that can handle multiple hydrological scenarios; however, the model that was developed 

still represents a simplified understanding of the hydrological 

processes of the lower San Antonio River watershed. Natural 

hydrological processes are infinitely more complex than the 

simulations possible using empirical equations embedded in 

modeling software such as HSPF. The modelers’ conceptualization of the watershed—FTABLES, stream dimensions, and 

so forth, and the variation in model parameters among water 

budget zones—based on decisions as to which watershed 

factors drive the hydrologic responses of the watershed might 

not be accurate or might be oversimplified. HSPF distributes 

inflows and outflows to maintain a balanced water budget as 

calibration parameters are changed. The accuracy of the modeled distribution of water within the watershed depends on the 

adequacy of the measured data used to calibrate the model. 

ET is by far the most dominant part of the water budget yet 

few ET data are available for most places, including the study 

area. The lack of measured ET in the study area for the different surficial geologic units, land covers, vegetative types, and 

seasons is particularly limiting, because it is not always clear 

how model parameters for ET should be varied. The lack of 

adequate ET data could cause systematic errors in representing 

the hydrological processes of the watershed (Raines, 1996).

Groundwater and surface-water interactions are modeled 

in a relatively simple way by HSPF. Over the past decades, 

this has led to the coupling of HSPF with groundwater models 

to better represent the complexity of groundwater and surfacewater interactions. Within the limited functionality of HSPF 

for determining recharge, the authors have defined waterbudget zones that they believe vary in soil infiltration and 

other modeled parameters available in HSPF. This variability 

has been introduced and is maintained by the overall calibration. However, across individual water-budget zones and on 

smaller spatial scales, measured streamflow or ET data are not 

sufficient to further verify the resulting gradients in ET and 

recharge rates. 

Measurement errors are introduced as a result of inaccurate or missing data. Because large, isolated storms are common in south-central Texas, rainfall can vary greatly over a 

short distance. The degree to which available rainfall data represent the actual rainfall is potentially the most serious source 

of measurement error for the study. Rainfall input to the study 

area, derived from measured rainfall at five NWS meteorological stations, is represented by five areas of assumed homogeneous rainfall; each meteorological station represents, on 

average, an area of about 430 square miles. Also, four of the 

five NWS meteorological stations record daily rainfall data. 

Because of the highly localized nature of rainfall in southcentral Texas, the disaggregation of daily rainfall data to 

hourly data does not always accurately represent rainfall duration or intensity. 

The emphasis of the watershed-model calibration was 

accurate simulation of streamflow. Streamflow accounts for 

a relatively small percentage of the water budget in the study 

area and, in the main stem of the lower San Antonio River, is 

largely determined from upstream flows. Although an accurate 

simulation of the hydrograph relates to the accurate simulation of all the components of the water cycle, the accuracy of 

groundwater recharge estimation depends on accurate simulation of other water-budget components as well, especially ET. 

Few or no measured data were available to calibrate or test 

ET and groundwater recharge. Much of the surficial geology 

in zone 3 consists primarily of the Carrizo Sand. Where the 

surficial geology also consists primarily of the Carrizo Sand 

(outside the study area), measured ET data were available to 

compare with simulated ET data from water-budget zone 3; 

differences between the measured and simulated ET data 

were small and the simulated ET data appear reasonable. 

Table 13. Sensitivity of the water balance in water-budget zone 3 to changes in selected process-related parameters of the 

Hydrological Simulation Program—Fortran (HSPF) model of the lower San Antonio River watershed, south-central Texas, 2000–2007.

Parameter1 Initial values Adjusted values Change in recharge 

(percent)

Change in 

evaptranspiration 

(percent)

Change in runoff from 

pervious area 

(percent)

LZSN 8.0–9.5 Increase to 12.0 -10 1.1 -15

UZSN .54–.66 Increase to 1.5 -2 .7 -15

LZETP .2–.8 Increase by 0.1 (0.3–0.9) -12 3.3 -15

INFILT .45–.90 Decrease by 20 percent -2 .4 0

DEEPFR .70–.85 Decrease by 0.2 -25 .4 85

AGWRC .85–.94 Increase by 0.05 0 .4 -15

CEPSC .1–.3 Increase by 0.05 (0.15–0.35) -3 .7 -8

1

 See table 3 for description of parameters.
Summary 39

Simulations for other types of surficial geology and land cover 

are even less certain because of the lack of measured ET data 

for comparison purposes. Additional ET datasets (if available) 

could be used to improve the calibration. To further understand the groundwater components of the model, linkages of 

this watershed model with groundwater models for the region 

would be useful. 

Summary

The U.S. Geological Survey (USGS), in cooperation 

with the San Antonio River Authority, the Evergreen Underground Water Conservation District, and the Goliad County 

Groundwater Conservation District, configured, calibrated, 

and tested a Hydrological Simulation Program—FORTRAN 

(HSPF) watershed model for the approximately 2,150-squaremile lower San Antonio River watershed in Bexar, Guadalupe, Wilson, Karnes, DeWitt, Goliad, Victoria, and Refugio 

Counties in south-central Texas. Because of the complexity 

of the study area, the lower San Antonio River watershed was 

divided into four subwatershed models; separate HSPF models 

were developed for each subwatershed. The most downstream 

subwatershed model, San Antonio River downstream from 

Cibolo Creek, receives the simulated streamflow from the 

outlets of the other three subwatershed models. Simulation 

of the overall study area involved running simulations of the 

three upstream models, then running the downstream model. 

The surficial geology was simplified as nine contiguous waterbudget zones to meet model computational limitations and 

also to define zones for which ET, recharge, and other waterbudget information would be output by the model. The model 

was used to simulate streamflow, evapotranspiration (ET), and 

groundwater recharge in the lower San Antonio River watershed in south-central Texas during 2000–2007 to gain a better 

understanding of the water budget. HSPF was used to simulate 

streamflow, ET, and groundwater recharge in each water-budget zone and for the watershed as a whole. 

Rainfall data used as input for the model were obtained 

from seven National Weather Service (NWS) meteorological 

stations in or near the study area. Air temperature data from 

three of the NWS stations were used to estimate potential ET 

in the model. Other time-series datasets for the model were 

developed for wastewater discharges, surface-water withdrawals for irrigation, and springflow at Sutherland Springs. 

The model was calibrated and tested using streamflow 

data obtained from 10 of the 11 USGS streamflow-gaging 

stations in the study area. Using various graphical and statistical methods, the calibration was characterized as very good; 

streamflow volumes were calibrated to within 10 percent of 

the measured streamflow volumes. Additionally, for calibration, ET simulations were compared with ET measured continuously at a USGS meteorological station in Medina County, 

about 70 miles west of the study area. The total HSPFsimulated ET from the pervious area of water-budget zone 3 

for October 2006–November 2007 was 38.4 inches, and total 

measured ET at the Medina County station for this same time 

period was 37.4 inches.

Streamflow volumes and basin yields for the four subwatersheds in the study area were compiled. The measured 

annual mean volume of streamflow entering the study area 

from upstream during 2000–2007 was 0.685 million acre-feet. 

The simulated annual mean volume of streamflow exiting at 

the downstream outlet of the study area during 2000–2007 was 

1.07 million acre-feet, an increase of 56 percent between the 

upstream contributing area and the downstream outlet of the 

study area. Of the four subwatersheds in the lower San Antonio River watershed, the Cibolo Creek subwatershed had the 

largest annual mean basin yield, about 4.8 inches per year. The 

annual mean basin yield of 6.4 inches from the San Antonio 

River drainage area upstream from the study area is 3.1 inches 

higher than the annual mean basin yield of 3.3 inches from the 

lower San Antonio River watershed. 

During 2000–2007, annual mean rainfall estimates for 

the nine water-budget zones ranged from 33.7 to 38.5 inches 

per year; for the entire watershed the estimated annual mean 

rainfall was 34.3 inches. Most of the rainfall does not become 

streamflow but is either lost to the atmosphere as ET or stored 

as recharge. Using the HSPF model, it was estimated on the 

basis of simulation results that, for 2000–2007, less than 10 

percent of the annual mean rainfall on the study watershed 

exited the watershed as streamflow. Using the HSPF model, it 

was also estimated that about 82 percent, or an average of 28.2 

inches per year, exited the watershed as ET, primarily from 

pervious land. The Cibolo Creek subwatershed and the subwatershed of the San Antonio River upstream from Cibolo Creek 

had the largest and smallest basin yields, about 4.8 inches 

and 1.2 inches, respectively. Estimated annual ET and annual 

recharge generally increased with increasing annual rainfall. 

Also, ET was larger in zones 8 and 9, the most downstream 

zones in the watershed. These zones included larger percentages of riparian/wetland land cover, which exhibit larger ET 

rates than other land covers simulated in the model. Zones 8 

and 9 also had more rainfall than the other zones, thus more 

water to satisfy potential ET demand.

The HSPF model also was used to estimate groundwater 

recharge for nine selected water-budget zones. The largest estimated annual mean groundwater recharge, about 5.1 inches, 

was in water-budget zone 3, the zone where the Carrizo Sand 

outcrops. On average, an estimated 15 percent of annual mean 

rainfall in water-budget zone 3 was converted to recharge. The 

smallest estimated annual mean recharge, about 1.1 inches 

(about 3 percent of annual mean rainfall), was in water-budget 

zone 6. For the entire watershed study area, annual mean 

recharge was about 3.0 inches or about 9 percent of annual 

mean rainfall.

Model limitations include possible errors related to model 

conceptualization and parameter variability, lack of data to 

quantify certain model inputs, and measurement errors. The 

conceptualization of the watershed and the variation in model 

parameters among water-budget zones, as well as the decisions 

as to which watershed factors drive the hydrologic responses 
40 Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed

of the watershed, might not be accurate or might be oversimplified. The lack of measured ET data for different surficial 

geologic units, land covers, vegetative types, and seasons is 

limiting because it is not always clear how model parameters 

for ET should be varied. Rainfall can vary greatly over a short 

distance; uncertainty regarding the degree to which available 

rainfall data represent actual rainfall is potentially the most 

serious source of measurement error.

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Lizárraga and Ockerman—Simulation of Streamflow, Evapotranspiration, and Groundwater Recharge in the Lower San Antonio River Watershed—SIR 2010–5027

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