U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5194

In cooperation with the U.S. Army Corps of Engineers, Fort Worth District; City of Corpus Christi; 

Guadalupe-Blanco River Authority; San Antonio River Authority; and San Antonio Water System

Simulation of Streamflow and Suspended-Sediment 

Concentrations and Loads in the Lower Nueces River 

Watershed, Downstream from Lake Corpus Christi to the 

Nueces Estuary, South Texas, 1958–2008
Cover: Aerial photograph of the Nueces River and Nueces Estuary downstream from Interstate Highway 

37, near Corpus Christi, Texas. 
Simulation of Streamflow and SuspendedSediment Concentrations and Loads in 

the Lower Nueces River Watershed, 

Downstream from Lake Corpus Christi 

to the Nueces Estuary, South Texas, 

1958–2008 

By Darwin J. Ockerman and Franklin T. Heitmuller

In cooperation with the U.S. Army Corps of Engineers, Fort Worth District; 

City of Corpus Christi; Guadalupe-Blanco River Authority; San Antonio River 

Authority; and San Antonio Water System

Scientific Investigations Report 2010–5194

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/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ockerman, D.J., and Heitmuller, F.T., 2010, Simulation of streamflow and suspended-sediment concentrations and 

loads in the lower Nueces River watershed, downstream from Lake Corpus Christi to the Nueces Estuary, South Texas, 

1958–2008: U.S. Geological Survey Scientific Investigations Report 2010–5194, 50 p.
iii

Contents

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

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

Purpose and Scope .............................................................................................................................2

Description of the Lower Nueces River Watershed ......................................................................2

Suspended-Sediment Concentrations and Loads of the Lower Nueces River ..................................5

Review of Historical Suspended-Sediment Data and Studies in the Lower Nueces River ....5

Recent (2006–07) Suspended-Sediment Sampling ........................................................................8

Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower 

Nueces River ....................................................................................................................................9

Functional Description of Hydrological Simulation Program—FORTRAN .................................9

Input Data for the Lower Nueces River Watershed Model ........................................................12

Model Development ..........................................................................................................................18

Model Calibration ...............................................................................................................................18

Hydrology ...................................................................................................................................19

Suspended Sediment ...............................................................................................................22

Estimation of Suspended-Sediment Loads from Lake Corpus Christi .....................23

Sediment Yields for Selected Land-Use Types and Land Covers ............................24

Model Calibration ............................................................................................................26

Parameter Calibration Values .................................................................................................27

Sensitivity Analysis ............................................................................................................................27

Model Limitations ...............................................................................................................................31

Estimated Suspended-Sediment Loads to the Nueces Estuary, 1958–2008 ............................33

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

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

Appendix 1. Hydrological Simulation Program—FORTRAN (HSPF) Parameters Used to 

Simulate Hydrologic and Sediment Processes in the Lower Nueces River 

Watershed, South Texas ...................................................................................................45

Figures

1–2. Maps showing: 

1. Location of Nueces River Basin, including lower Nueces River study area, 

South Texas ..........................................................................................................................3

2. Location of lower Nueces River watershed, South Texas ...........................................4

3. Graph showing annual suspended-sediment loads for the Nueces River near 

Mathis, Texas, 1942–57 and 1962–82 ........................................................................................6

4. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for hydrologic 

processes on (A) impervious and (B) pervious land segments .........................................10

5. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for sediment 

processes on (A) impervious land segments, (B) pervious land segments, and 

(C) stream reaches ....................................................................................................................11

6–10. Maps showing: 

6. Land-cover categories in the lower Nueces River watershed, South Texas .........13
iv

7. Surficial geology of the lower Nueces River watershed, South Texas ...................14

8. Relative soil infiltration rates in the lower Nueces River watershed, South 

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

9. Subwatershed delineation for the lower Nueces River watershed model, 

South Texas ........................................................................................................................16

10. Locations of U.S. Geological Survey streamflow-gaging stations and National 

Weather Service rainfall stations (with associated Thiessen rainfall areas) 

providing data for the lower Nueces River watershed model, South Texas ..........17

11–16. Graphs showing: 

11. Measured and simulated daily mean streamflow at U.S. Geological Survey 

streamflow-gaging station 08211200 Nueces River at Bluntzer, Texas, 

2005–06 ...............................................................................................................................20

12. Measured and simulated 7-day mean streamflow at U.S. Geological Survey 

streamflow-gaging station 08211500 Nueces River at Calallen, Texas, 

2001–08 ...............................................................................................................................21

13. Relation between streamflow and suspended-sediment load and plot of 

regression error residuals based on 22 streamflow-load data pairs from 

08211000 Nueces River near Mathis, Texas, 1964–71 and 2006–07 ..........................25

14. Relation between streamflow and suspended-sediment load and plot of 

regression error residuals based on 10 streamflow-load data pairs from 

08211500 Nueces River at Calallen, Texas, 2006–07 ...................................................28

15. Estimated (by regression) and simulated monthly suspended-sediment loads 

for 08211500 Nueces River at Calallen, Texas, 2001–08 .............................................29

16. Estimated annual streamflow and suspended-sediment loads to the Nueces 

Estuary, South Texas, 1958–2008 ....................................................................................34

Tables

1. Annual suspended-sediment loads for the Nueces River near Mathis, Texas, 

1942–58 and 1961–82 ...................................................................................................................7

2. Streamflow and suspended-sediment loads for selected flow releases from 

Wesley E. Seale Dam on the Nueces River near Mathis, Texas, October 1964–

September 1971 ............................................................................................................................8

3. Data-collection sites providing data for the lower Nueces River watershed 

model, South Texas ....................................................................................................................18

4. Streamflow calibration and testing results for the lower Nueces River watershed 

model, South Texas ....................................................................................................................22

5. Measured and simulated streamflows, measured suspended-sediment particle 

size, and measured and simulated suspended-sediment concentrations and loads 

for selected samples collected at 08211000 Nueces River near Mathis, Texas, 

2006–07 ........................................................................................................................................26

6. Comparison of literature estimates and simulation results for sediment yields 

from selected land covers and land uses in the lower Nueces River watershed 

and South Texas area ................................................................................................................26

7. Measured and simulated streamflows, measured suspended-sediment particle 

size, and measured and simulated suspended-sediment concentrations and loads 

for selected samples collected at 08211200 Nueces River at Bluntzer, Texas, 

2006–07 ........................................................................................................................................30
v

8. Suspended-sediment calibration results for the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, South Texas, 

2001–08 ........................................................................................................................................30

9. Measured and simulated streamflows, measured suspended-sediment particle 

size, and measured and simulated suspended-sediment concentrations and loads 

for selected samples collected at 08211500 Nueces River at Calallen, Texas, 

2006–07 ........................................................................................................................................31

10. Summary of calibrated values for selected hydrologic parameters for the 

Hydrological Simulation Program—FORTRAN model of the lower Nueces River 

watershed, South Texas ...........................................................................................................32

11. Summary of calibrated values for selected sediment-related parameters for the 

Hydrological Simulation Program—FORTRAN model of the lower Nueces River 

watershed, South Texas ...........................................................................................................33

12. Sensitivity of simulated streamflow volumes and suspended-sediment loads to 

changes in selected process-related parameters for the lower Nueces River 

watershed model, South Texas ...............................................................................................34

13. Estimated annual streamflows and suspended-sediment loads to the Nueces 

Estuary, South Texas, 1958–2008 .............................................................................................35

14. Estimated annual suspended-sediment loads at selected stations, lower Nueces 

River watershed, South Texas, 1958–2008 .............................................................................37

15. Estimated annual streamflows and suspended-sediment loads, by sediment 

source, simulated by the Hydrological Simulation Program—FORTRAN model of 

the lower Nueces River watershed, South Texas, 1958–2008 ............................................38
vi

Conversion Factors, Datum, and Water-Quality Abbreviation

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

cubic foot per second (ft3

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

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

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

Pressure

pound per square foot (lb/ft2

) 0.04788 kilopascal (kPa) 

Hydraulic gradient

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

Yield

pound per acre (lb/acre) 1.121 kilogram per hectare kg/ha)

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

°C=(°F-32)/1.8

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

Volume

milliliter (mL) 0.03382 ounce, fluid (fl. oz)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

Datum

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

Water-Quality Abbreviation

mg/L, milligrams per liter
Abstract

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

with the U.S. Army Corps of Engineers–Fort Worth District, 

City of Corpus Christi, Guadalupe-Blanco River Authority, 

San Antonio River Authority, and San Antonio Water System, 

developed, calibrated, and tested a Hydrological Simulation 

Program—FORTRAN (HSPF) watershed model to simulate 

streamflow and suspended-sediment concentrations and loads 

during 1958–2008 in the lower Nueces River watershed, 

downstream from Lake Corpus Christi to the Nueces Estuary 

in South Texas. Data available to simulate suspended-sediment 

concentrations and loads consisted of historical sediment data 

collected during 1942–82 in the study area and suspendedsediment concentration data collected periodically by the 

USGS during 2006–07 at three USGS streamflow-gaging 

stations, Nueces River near Mathis, Nueces River at Bluntzer, 

and Nueces River at Calallen. The Nueces River near Mathis 

station is downstream from Wesley E. Seale Dam, completed 

in 1958 to impound Lake Corpus Christi. Suspended-sediment 

data collected before and after completion of Wesley E. 

Seale Dam provide insights to the effects of the dam and reservoir on suspended-sediment loads transported by the lower 

Nueces River from downstream of the dam to the Nueces 

Estuary. Annual suspended-sediment loads at a site near the 

Nueces River at Mathis station were considerably lower, for 

a given annual mean discharge, after the dam was completed 

than before the dam was completed.

Most of the suspended sediment transported by the 

Nueces River downstream from Wesley E. Seale Dam 

occurred during high-flow releases from the dam or during 

floods. During October 1964–September 1971, about 532,000 

tons of suspended sediment were transported by the Nueces 

River near Mathis. Of this amount, about 473,000 tons, or 

about 89 percent, were transported by large runoff events 

(mean streamflow exceeding 1,000 cubic feet per second).

To develop the watershed model to simulate suspendedsediment concentrations and loads in the lower Nueces River 

watershed during 1958–2008, streamflow simulations were 

calibrated and tested with available data for 2001–08 from 

the Nueces River at Bluntzer and Nueces River at Calallen 

stations. Streamflow data from the Nueces River near Mathis 

station were used as input to the model at the upstream 

boundary of the model. Simulated streamflow volumes for the 

Bluntzer and Calallen stations showed good agreement (within 

6 percent) with measured streamflow volumes.

The HSPF model was calibrated to simulate suspended 

sediment using suspended-sediment data collected at the 

Mathis, Bluntzer, and Calallen stations during 2006–07. The 

calibrated watershed model was used to estimate streamflow 

and suspended-sediment loads for 1958–2008, including loads 

transported to the Nueces Estuary. During 1958–2008, on 

average, an estimated 307 tons per day of suspended sediment were delivered to the lower Nueces River; an estimated 

297 tons per day were delivered to the estuary. The annual 

suspended-sediment load was highly variable, depending on 

the occurrence of storm events and high streamflows. During 

1958–2008, the annual total sediment loads to the estuary varied from an estimated 3.8 to 2,490 tons per day. On average, 

117 tons per day, or about 38 percent of the estimated annual 

suspended-sediment contribution, originated from cropland 

in the study watershed. Releases from Lake Corpus Christi 

delivered an estimated 98 tons per day of suspended sediment 

or about 32 percent of the 307 tons per day estimated to have 

been delivered to the lower Nueces River. Erosion of streamchannel bed and banks accounted for 55 tons per day or about 

18 percent of the estimated total suspended-sediment load. 

All other land categories, except cropland, accounted for an 

estimated 37 tons per day, or about 12 percent of the total. An 

estimated 9.6 tons per day of suspended sediment or about 3 

percent of the suspended-sediment load delivered to the lower 

Nueces River were removed by water withdrawals before 

reaching the Nueces Estuary.

Introduction

The Nueces River extends approximately 315 miles from 

its headwaters in the southern Edwards Plateau in South Texas 

to Nueces Bay near Corpus Christi, Tex., and has a drainage 

Simulation of Streamflow and Suspended-Sediment 

Concentrations and Loads in the Lower Nueces River 

Watershed, Downstream from Lake Corpus Christi to the 

Nueces Estuary, South Texas, 1958–2008

By Darwin J. Ockerman and Franklin T. Heitmuller
2 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

area of approximately 16,700 square miles (fig. 1). The river 

exits the Edwards Plateau near Uvalde, Tex., and enters the 

South Texas Plains, also referred to as the South Texas Brush 

Country (Texas Parks and Wildlife Department, 2007), where 

the majority of its length and drainage area are located. The 

Frio River, a major tributary, joins the Nueces River near 

Three Rivers, Tex. Major impoundments in the Nueces River 

watershed in the South Texas Plains include Upper Nueces 

Reservoir, formed in 1948 by Upper Nueces Dam; Choke 

Canyon Reservoir, formed in 1982 by Choke Canyon Dam 

(on the Frio River); and Lake Corpus Christi, impounded by 

Mathis Dam in 1935 (surface area 5,493 acres, storage volume 

43,800 acre-feet) (Texas Water Development Board, 2002) 

and impounded since 1958 by Wesley E. Seale Dam (surface 

area 19,251 acres, storage volume 257,260 acre-feet) (City 

of Corpus Christi, 2010). Downstream from Wesley E. Seale 

Dam (fig. 2), the Nueces River flows about 50 miles through 

an alluvial valley to the Nueces Bay. 

The Nueces Estuary (fig. 2) consists of two areas of 

nearly equal size, Nueces Bay and the Nueces River delta. 

Nueces Bay is a shallow, 27-square-mile secondary bay of 

Corpus Christi Bay. Nueces Bay has a mean depth of about 2.5 

feet and a volume of about 40,000 acre-feet. Bottom sediments 

in Nueces Bay are deposited mostly by the Nueces River 

(Yeager and others, 2006). The Nueces River delta in southern 

San Patricio County is a 28-square-mile area of vegetated salt 

and brackish marshes, land subject to inundation from river or 

tidal flooding, and open water formed where the Nueces River 

flows into Nueces Bay (fig. 2). Currently (2010), the Nueces 

River channel is located along the southern margins of the 

delta. The distributary network includes a man-made overflow 

channel in the northwestern part of the delta that connects the 

Nueces River to Rincon Bayou (U.S. Bureau of Reclamation, 

2000; Ockerman, 2001).

In November 2005, during a resource agency meeting in San Antonio, Tex., the U.S. Army Corps of Engineers 

(USACE) highlighted 12 ecological problems in the Nueces 

River watershed. One of these was a “loss of sediment 

loading and nutrient loads to estuaries” (Marcia Hackett, 

U.S. Army Corps of Engineers–Fort Worth District, written 

commun., 2005). A number of the other ecological problems were directly related, including a reduction of overbank 

flows downstream from reservoirs and decrease in freshwater 

inflows to the Nueces Estuary. The reduction in sediment 

loads to the Nueces Estuary is the result of sedimentation in 

large impoundments, notably Lake Corpus Christi (Leibbrand, 

1987). Downstream from the reservoirs, ecological problems caused by sedimentation impoundment are expected 

to include river channel incision (Williams and Wolman, 

1984; Salant and others, 2006), channel bed armoring 

(Williams and Wolman, 1984; Vericat and others, 2006), 

and deltaic and shoreline erosion (Jaffe and others, 1998; Fan 

and others, 2006; Yang and others, 2006). Reductions in the 

extent of marshland and vegetated areas in the Nueces River 

delta occurred following the initial impoundment of Lake 

Corpus Christi (Morton and Paine 1984; White and Calnan, 

1991). The decreased sediment loads of the Nueces River, 

combined with relative sea level rise and subsidence are 

responsible for deltaic erosion and the conversion of wetland 

habitat to open water and shallow flats (Day and others, 1995; 

White and others, 2002; Yeager and others, 2006).

The USACE–Fort Worth District began a study in 2002 

to identify opportunities for flood-damage reduction, ecosystem restoration, and implementation of multipurpose projects 

in the Nueces River Basin (U.S. Army Corps of Engineers, 

2009). The purpose of the USACE study was to participate 

with other (Federal and non-Federal) sponsor agencies to 

identify and conduct detailed studies of water-resource 

problems in the Nueces River Basin (fig. 1), including 

documenting existing hydrologic, engineering, and environmental conditions of the study area. One of the specific 

feasibility investigations outlined in the USACE study 

was related to defining the existing conditions and opportunities for ecosystem restoration in the Nueces Estuary, 

namely an investigation of the current conditions of suspended-sediment concentrations and loads delivered by 

the Nueces River to the estuary. As part of this feasibility 

study, the U.S. Geological Survey (USGS), in cooperation with the USACE–Fort Worth District, City of Corpus 

Christi, Guadalupe-Blanco River Authority, San Antonio 

River Authority, and San Antonio Water System, developed, 

calibrated, and tested a watershed model of the lower Nueces 

River watershed to simulate existing hydrologic conditions 

and suspended-sediment concentrations and loads to the 

Nueces Estuary. 

Purpose and Scope

The purpose of this report is to estimate suspendedsediment concentrations and loads in the lower Nueces 

River watershed downstream from Lake Corpus Christi to 

the Nueces Estuary during 1958–2008. To accomplish this, 

(1) previous suspended-sediment data and studies for the 

study area were reviewed; (2) historic estimates of suspendedsediment loads were compiled; and (3) a watershed model to 

simulate streamflow and suspended-sediment concentrations 

and loads in the lower Nueces River watershed was developed 

and calibrated. Using the watershed model, estimates of suspended-sediment loads to the Nueces Estuary for 1958–2008 

were prepared. Limitations of model-simulated estimates of 

sediment loads are described.

Description of the Lower Nueces River 

Watershed

The lower Nueces River study area comprises about 216 

square miles of the nontidal part of the Nueces River watershed, from the outlet of Wesley E. Seale Dam near Mathis 

(fig. 2) to the tidal reach of the river that flows into the Nueces 

Estuary. The study area encompasses parts of Bee, Jim Wells, 

Live Oak, Nueces, and San Patricio Counties in South Texas. 
Introduction 3Figure 1. Location of Nueces River Basin, including lower Nueces River study area, South Texas. 

Corpus 

 Christi 

 Bay

Nueces

Bay

Nueces Estuary

TEXAS

Study area

LOCATION MAP

Base modified from U.S. GeologicalSurvey digital data 1:24,000 quadrangles Universal Transverse Mercator Projection, zone 14, North American Datum of 1983

EXPLANATION

Nueces River Basin watershedLower Nueces River watershed 

 (study area) Boundary between Edwards Plateau and South Texas Plains

Hondo

Creek

Frio River

Frio River

Nueces River

Nueces River

San Miguel Creek

Sabinal River

Atascosa River

Gulf of Mexico

San Antonio

Corpus Christi

Laredo

Uvalde

ThreeRivers

97°0'

98°0'

99°0'

100°0'

30°0'

29°0'

28°0'

Lake Corpus Christi

Choke Canyon Reservoir

UpperNueces Reservoir

0 60 120 KILOMETERS

0 60 120 MILES

WEBB

COUNTY

DUVAL

COUNTY

FRIO

COUNTY

BEE

COUNTY

EDWARDS

COUNTY

KERR

COUNTY

UVALDE

COUNTY

BEXAR

COUNTY

DIMMIT

COUNTY

KINNEY

COUNTY

ZAVALA

COUNTY

MEDINA

COUNTY

LA SALLE

COUNTY

NUECES

COUNTY

REAL

COUNTY

KLEBERG

COUNTY

MAVERICK

COUNTY

LIVE OAK

COUNTY

ATASCOSA

COUNTY

GOLIAD

COUNTY

DE WITT

COUNTY

GILLESPIE

COUNTY

MCMULLEN

COUNTY

WILSON

COUNTY

VICTORIA

COUNTY

KARNES

COUNTY

REFUGIO

COUNTY

BANDERA

COUNTY

JIM WELLS

COUNTY

COMAL

COUNTY

KENDALL

COUNTY

ARANSAS

COUNTY

GUADALUPE

COUNTY

SAN PATRICIO

COUNTY

United States

Mexico

Edwards Plateau

South Texas Plains
4 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Figure 2. Location of lower Nueces River watershed, South Texas. 

08211000

08211200

08211500

!(

35 UV

22 UV

361

UV

188

UV

44 UV 44 UV

361

UV

234

UV

624

UV

286

UV

358

UV

181

£¤

77 £¤

281

£¤

181

£¤

77 £¤

37

§ ̈¦

37

§ ̈¦

BEE COUNTY

REFUGIO COUNTY

LIVE OAK

COUNTY

JIM WELLS 

COUNTY

NUECES COUNTY

SAN PATRICIO COUNTY

KLEBERG COUNTY

Corpus Christi Bay

Nueces Bay

Lake Corpus Christi

Nueces River

97°10'

97°40' 97°50'

28°10'

28°

27°50'

27°40'

98° 97°30' 97°20'

08211000

Sinton

Mathis

Robstown

Corpus Christi

Odem

Taft

Bluntzer

Calallen

EXPLANATION

0 4 8 2 MILES0 4 8 2 KILOMETERS

Lower NuecesRiver watershed(study area)MarshLand subject to inundationStreamflow and suspendedsediment stationand number

Nueces Estuary

Base modified from U.S. Geological Survey digital data 1:24,000 quadrangles Universal Transverse Mercator Projection, zone 14, North American Datum of 1983 

RinconBayou

Wesley E. Seale Dam

6

6

RinconBayouoverflowchannel

#

#

#

#

Nueces River delta
Suspended-Sediment Concentrations and Loads of the Lower Nueces River 5

The largest town in the study area is Mathis, Tex., which had a 

population of about 5,000 in 2000 (Handbook of Texas Online, 

2009).

The study area has a subtropical, subhumid climate 

characterized by hot summers and mild, dry winters (Larkin 

and Bomar, 1983). The average monthly low temperatures 

range from 46.2 degrees Fahrenheit (°F) in January to 74.5 °F 

in August; average daily high temperatures range from 66.0 

°F in January to 93.4 °F in August (National Climatic Data 

Center, 2009). The average annual rainfall (1971–2000) in the 

study area is about 32.6 inches (32.2 inches at Mathis National 

Weather Service [NWS] station COOP ID 415661 and 32.9 

inches at Robstown NWS station COOP ID 417677). Daily 

rainfall greater than 0.01 inch occurs, on average, 72 days 

per year (National Climatic Data Center, 2009); daily rainfall 

equal to or greater than 1.0 inch occurs, on average, every 39 

days (Asquith and Roussel, 2003). Although most rainfall 

occurs in spring, early summer, and fall, amounts greater than 

about 1.0 inch can occur anytime during the year (Larkin and 

Bomar, 1983). 

Most land use is rangeland and cropland. Elevation in the 

lower Nueces River watershed ranges from about 3 to 230 feet 

above sea level (U.S. Geological Survey, 2001). Land slopes 

are generally low, mostly less than 5 percent. Overall, the 

stream-channel slope of the lower Nueces River is about 1 foot 

per mile over the 38 miles from the outlet of Wesley E. Seale 

Dam to the river crossing at Interstate Highway 37, north of 

Corpus Christi (fig. 2).

Suspended-Sediment Concentrations 

and Loads of the Lower Nueces River

Characterizing suspended-sediment concentrations and 

loads in the lower Nueces River included the following major 

steps: (1) A review of the available suspended-sediment concentration and load data and studies, including a compilation 

of historical estimates of suspended-sediment loads; (2) development and calibration of a watershed model to simulate 

suspended-sediment concentrations and loads in the lower 

Nueces River watershed; and (3) estimation of suspendedsediment loads to the Nueces Estuary for 1958–2008. 

Review of Historical Suspended-Sediment Data 

and Studies in the Lower Nueces River

Various studies have investigated the problems associated 

with the reduction in sediment loads to the Nueces Estuary 

and the associated loss of wetland habitats in the Nueces River 

delta. Morton and Paine (1984) used aerial photographs to 

show that the propagation of deltaic wetlands in the Nueces 

River delta ceased sometime between 1930 and 1959. These 

findings were substantiated by White and Calnan (1990), who 

also showed an additional decrease in net vegetated areas 

by 1979. As further support to the importance of sediment 

transport in maintaining the Nueces River delta, Leibbrand 

(1987) showed that almost all sediment entering Lake Corpus 

Christi between 1972 and 1985 was trapped. Recent studies showed that loss of wetlands in the Nueces Estuary not 

only was the result of upstream impoundments but also was 

influenced by gradual subsidence of the delta (White and others, 2002). As a consequence of these two factors, White and 

others (2002) also predicted that wetland areas would continue 

to decrease in the Nueces River delta. Finally, Yeager and 

others (2006) showed that sediment in Nueces Bay is supplied 

mostly by the river rather than by marine sources (transport 

from the Gulf of Mexico by wind or tidal currents), supporting claims that a reduction of suspended-sediment inflow from 

the Nueces River will contribute to loss of Nueces Estuary 

wetland habitat.

In 1924, the Texas Board of Water Engineers (whose 

name and functions were subsequently changed several times 

by the Texas Legislature and whose functions unrelated to 

water rights were transferred to the Texas Water Development 

Board [TWDB] in 1965) initiated a program to evaluate 

the economic life of reservoirs, which included monitoring 

suspended-sediment loads in selected Texas rivers (Stout 

and others, 1961; Mirabal, 1974). During the program, the 

USGS (in 1942) established one site in the lower Nueces 

River study area (fig. 2), streamflow-gaging station 08210000 

Nueces River near Mathis, Tex. (hereinafter, Mathis gage). 

Suspended-sediment concentration samples were collected 

on a daily basis at this site by TWDB and its predecessor 

agencies; monthly and annual suspended-sediment concentration data and sediment loads (using streamflow data from 

the Mathis gage) were computed until 1982 and published by 

various authors, including Stout and others (1961), Adey and 

Cook (1964), Cook (1967, 1970), Mirabal (1974), Dougherty 

(1979), and Quincy (1988). 

Records for June 1958–June 1961 were invalid (Adey and 

Cook, 1964) and were not used for interpretation in this report. 

Suspended-sediment samples were collected during 1942–82 

using an 8-ounce (236.3-milliliter) narrow-neck bottle which 

was held in a 10-pound (4.54-kilogram) torpedo-shaped sampler frame positioned approximately 1 foot (0.3 meter) below 

the water surface. Samples were collected either daily or 

throughout the day if the stage changed considerably. Samples 

were collected at one-sixth, one-half, and five-sixths of the 

water-surface width using a sampling device called the “Texas 

sampler” (Stout and others, 1961; Welborn, 1967). To account 

for increasing suspended-sediment concentrations with depth, 

the measured percentage of suspended sediment by weight 

was multiplied by a correction factor of 1.102 to obtain the 

mean percentage of suspended sediment in the vertical profile 

(Farris, 1933). Suspended-sediment loads were computed 

with the assumptions that 1 acre-foot of streamflow weighed 

1,361.25 tons and 1 acre-foot of sediment weighed 1,524.60 

tons (Stout and others, 1961). Since 1982, most sediment data 

have been collected as part of relatively short-term studies for 

selected river reaches (for example, Phillips and others, 2005; 

Yeager and others, 2005). 
6 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

To verify the accuracy of the suspended-sediment sampling method described in the previous paragraph, referred to 

hereinafter as the “Texas sampler method,” Welborn (1967) 

compared the results of samples collected from various Texas 

streams by the Texas sampler method with results of concurrent samples collected by a depth-integrated method (Edwards 

and Glysson, 1999). Correlations between the two methods for 

sand-bed rivers in East Texas were poor but were very good 

(differing by 15 percent or less) for mixed-bed and gravel-bed 

rivers. The lower Nueces River is a mixed sand- and gravelbed channel; therefore, suspended-sediment data collected 

using the Texas sampler method were deemed reasonably 

accurate for computation of historical loads.

Suspended-sediment data collected before and after 

completion of Wesley E. Seale Dam provide insights to the 

effects of the dam and reservoir on suspended-sediment loads 

transported by the lower Nueces River from downstream of 

the dam to the Nueces Bay. Suspended-sediment loads (fig. 3; 

table 1) measured by TWDB and its predecessor agencies 

at the Mathis gage decreased appreciably in the years after 

completion of Wesley E. Seale Dam in 1958. During 9 of the 

16 years before completion of the dam, annual suspendedsediment loads exceeded 100,000 tons. The maximum annual 

suspended-sediment load for the period of record was 561,739 

tons in 1957, an amount that likely was larger compared with 

loads for all previous years because of the erosion of sediment 

during construction of Wesley E. Seale Dam immediately 

upstream from the suspended-sediment sampling site. In the 

21 years following completion of the dam, for which reliable 

suspended-sediment data were available (1962–82), more than 

100,000 tons of suspended sediment were transported in only 

2 years—1967 and 1971. These 2 years were characterized by 

relatively high annual mean streamflows at the Mathis gage—

2,167 and 2,140 cubic feet per second in 1967 and 1971, 

respectively. During 1962–82, annual suspended-sediment 

loads were considerably lower, for a given annual mean discharge, than in the years before the dam was completed (fig. 

3; table 1). The relation between annual suspended-sediment 

loads and annual mean streamflow before and after completion 

of Wesley E. Seale Dam is shown on figure 3. As an indication 

of the sediment-retention capacity of the dam, for comparable 

annual mean discharges, annual suspended-sediment loads 

Figure 3. Annual suspended-sediment loads for the Nueces River near Mathis, Texas, 1942–57 and 1962–82. 

0 500 1,000 1,500 2,000 2,500 3,000

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

0

100,000

200,000

300,000

400,000

500,000

ANNUAL SUSPENDED-SEDIMENT LOAD, IN TONS

600,000

1942–57

1962–82
Suspended-Sediment Concentrations and Loads of the Lower Nueces River 7

Table 1. Annual suspended-sediment loads for the Nueces River near Mathis, Texas, 1942–58 and 1961–82 (from Stout and others, 

1961; Adey and Cook, 1964; Cook, 1967, 1970; Mirabal, 1974).

Hydrologic year1,2 Number of days measured 

during hydrologic year

Annual suspendedsediment load 

(tons)

Annual mean suspendedsediment load 

(tons per day)

Annual mean streamflow

(cubic feet per second)

1942 241 546,504 2,268 2,517

1943 365 44,790 123 345

1944 366 323,550 884 1,020

1945 365 125,070 343 378

1946 365 350,430 960 1,294

1947 365 244,730 670 1,273

1948 366 15,170 41 148

1949 365 212,770 583 1,226

1950 365 29,160 80 340

1951 365 106,740 292 583

1952 366 25,670 70 244

1953 365 159,200 436 741

1954 365 67,020 184 465

1955 365 7,269 20 135

1956 366 12,165 33 184

1957 365 561,739 1,539 1,962

1958 243 395,791 1,629 2,179

1961 92 2,088 23 373

1962 365 2,845 8 111

1963 365 2,769 8 110

1964 366 3,445 9 104

1965 365 33,642 92 787

1966 365 23,400 64 452

1967 365 177,600 48 2,167

1968 366 73,640 201 1,232

1969 365 4,810 13 136

1970 365 70,290 193 718

1971 365 149,100 408 2,140

1972 366 67,530 185 1,780

1973 365 14,030 38 714

1974 365 31,370 86 1,244

1975 365 11,450 31 552

1976 366 12,460 34 550

1977 365 34,830 95 1,455

1978 365 4,280 12 307

1979 365 8,210 22 499

1980 366 26,560 73 771

1981 365 35,190 96 1,268

1982 365 9,790 27 479

1

 A hydrologic year begins October 1 of previous calendar year and ends September 30 of reported year.

2

 Data for June 1958–June 1961 are invalid (Adey and Cook, 1964) and are not included.
8 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

after completion of the dam have been consistently lower compared with annual suspended-sediment loads before completion of the dam (fig. 3).

Most of the suspended sediment transported by the 

Nueces River downstream from Wesley E. Seale Dam 

occurred during high-flow releases from the dam or during 

floods. During October 1964–September 1971, about 532,000 

tons of suspended sediment were transported by the Nueces 

River near Mathis (table 1). Of this amount, about 473,000 

tons (table 2), or about 89 percent, were transported by large 

runoff events (mean streamflow exceeding 1,000 cubic feet 

per second). Furthermore, the suspended-sediment transport 

rate increases with higher magnitude flows (fig. 3). The largest 

flow event listed in table 2 (September 21–October 11, 1967) 

transported a mean of 8,872 tons of suspended sediment per 

day.

Recent (2006–07) Suspended-Sediment 

Sampling

In addition to historical suspended-sediment data collected daily by TWDB and its predecessor agencies at the 

Mathis gage during 1942–82, the USGS periodically collected 

suspended-sediment samples during 2006–07 at the Mathis 

gage, station 08211200 Nueces River at Bluntzer, Tex. (hereinafter, Bluntzer gage), and station 08211500 Nueces River at 

Calallen, Tex. (hereinafter, Calallen gage) (fig. 2). Whereas 

the historical suspended-sediment data-collection program 

typically involved daily sample collection to compute longterm sediment loads, 2006–07 suspended-sediment samples 

represent data only for selected streamflow and sediment-load 

conditions. Eight suspended-sediment samples were collected 

at the Mathis gage, 11 at the Bluntzer gage, and 10 at the 

Calallen gage.

Using standard USGS protocols and quality-control (QC) 

procedures (U.S. Geological Survey, 2006), an isokinetic 

sampler was used to collect suspended-sediment samples in 

2006 and 2007. Ockerman and Fernandez (2010, p. 7) noted 

other investigators’ findings that “an isokinetic sampler collects a water-sediment sample from the stream at a rate such 

that the velocity of the intake nozzle is equal to the incident 

stream velocity at the nozzle entrance. The water-sediment 

sample collected is thus representative of the suspendedsediment load throughout the channel cross section and is 

appropriate for use in estimating sediment load carried by the 

stream (Davis, 2005).” 

Samples were collected at five or more equally spaced 

intervals across the stream channel and were depth-integrated 

by lowering and raising the sampler through the water at a 

constant rate. The samples from each equal-width segment 

were then combined into a single composite sample for 

analysis. A composite water-sediment sample is horizontally 

and vertically averaged throughout the stream cross section 

and is assumed to represent the average streamflow-weighted 

suspended-sediment concentration (Edwards and Glysson, 

1999; U.S. Geological Survey, 2006). 

Suspended-sediment samples were collected by wading 

when streamflow conditions permitted. During higher flow 

conditions, when wading was not possible, samples were collected from a bridge or by boat. Suspended-sediment samples 

collected by the USGS were analyzed by the USGS sediment 

laboratory in Iowa City, Iowa. Samples were analyzed for 

suspended-sediment concentration and sand-separation analysis. Sand-separation analysis gives the percentage of sediment, 

by weight, that is finer and coarser than 0.0625 millimeter. 

Particle sizes smaller than 0.0625 millimeter are defined as silt 

and clay; particle sizes 0.0625 millimeter or larger are defined 

as sand (Guy, 1969). 

Each suspended-sediment sample collected by the USGS 

was associated with a streamflow value. Streamflow data were 

usually obtained from the stage-discharge rating curve at the 

streamflow-gaging station where suspended-sediment samples 

were collected (Rantz and others, 1982). For some high-flow 

conditions, streamflow measurements were made when the 

suspended-sediment samples were collected. The availability 

of streamflow data and suspended-sediment concentration data 

allowed computation of suspended-sediment discharge, or 

load, according to equation 1:

Ls = Q x Cs x ks, (1)

Table 2. Streamflow and suspended-sediment loads for selected 

flow releases from Wesley E. Seale Dam on the Nueces River near 

Mathis, Texas, October 1964–September 1971 (from Cook, 1970; 

Mirabal, 1974).

Event

Mean 

streamflow 

(cubic 

feet per 

second)

Suspendedsediment 

load 

(tons)

Mean 

suspendedsediment 

load 

(tons per 

day)

October 5–16, 1964 7,425 7,506 682

February 24–March 11, 1965 2,727 3,939 246

May 20–June 11, 1965 4,465 15,541 676

May 2–June 10, 1966 2,980 18,653 466

September 21–October 11, 1967 35,129 186,302 8,872

October 16–30, 1967 1,545 4,318 288

January 21–February 11, 1968 5,492 14,802 673

May 8–June 5, 1968 4,970 15,806 545

July 12–16, 1968 2,544 4,909 982

October 19–November 19, 1969 1,968 5,438 170

May 25–June 14, 1970 5,543 50,447 2,402

July 8–18, 1971 9,894 17,480 1,589

August 8–September 6, 1971 11,006 74,537 2,485

September 10–30, 1971 14,432 53,578 2,551
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 9

where

Ls is the instantaneous suspended-sediment load, in tons per 

day;

Q is the streamflow, in cubic feet per second;

Cs is the suspended-sediment concentration, in milligrams 

per liter; and

ks is a conversion factor of 0.0027, which results in a 

sediment load in tons per day, given streamflow 

in cubic feet per second and suspended-sediment 

concentration in milligrams per liter (Porterfield, 

1972, p. 46–47).

Simulation of Streamflow and 

Suspended-Sediment Concentrations 

and Loads of the Lower Nueces River

Although historical suspended-sediment loads during 

1942–82 are available for the Nueces River near Mathis 

(at the Mathis gage), suspended-sediment concentrations 

and loads downstream from the Mathis gage have not been 

documented, and suspended-sediment concentrations and 

loads at the Mathis gage have possibly changed since they 

were last documented. To better understand suspendedsediment conditions in the lower Nueces River and to estimate the amount of suspended sediment transported to the 

Nueces Estuary, a watershed model was developed to simulate 

streamflow and suspended-sediment loads in the lower Nueces 

River.

Streamflow and suspended-sediment concentrations 

and loads were simulated with the Hydrological Simulation 

Program—FORTRAN (HSPF) (Bicknell and others, 2001). 

HSPF was selected for the study watershed because it is 

one of the more comprehensive watershed models available, 

can simulate a variety of stream and watershed conditions 

with reasonable accuracy, and enables flexibility in adjusting the model to simulate alternative conditions or scenarios 

(Donigian and others, 1995). To simulate the hydrologic and 

sediment processes that occur in a watershed, different data 

sources are used as input to the HSPF model including rainfall 

data, potential evapotranspiration, and other meteorological 

parameters; land cover and land use; and soil characteristics. 

The outputs of an HSPF model are simulated time series of 

suspended concentrations, sediment loads, or both, as well as 

streamflow; the time series are for a user-specified interval, or 

time step. A 1-hour time step was used for this study. HSPF 

also can simulate other water-quality constituents, including 

nutrients, metals, and organic compounds. Simulations for this 

study were limited to streamflow and suspended sediment.

Continuous (hourly) models enable simulation of 

important watershed processes for a full range of streamflows. 

Ockerman and Roussel (2009, p. 4) noted other investigators’ 

findings that “the relative importance of various processes 

and factors influencing water quality can vary considerably 

with the magnitude of streamflow. For example, processes that 

appreciably affect water-quality conditions during low flows 

might have relatively minor effects on water-quality conditions 

during high flows. For assessment of peak-streamflow characteristics, continuous simulation models can provide a more 

realistic evaluation of antecedent soil-moisture conditions than 

is generally possible with event-based models (Martin and others, 2001, p. 66).”

Functional Description of Hydrological 

Simulation Program—FORTRAN1

HSPF, a continuous, semi-lumped parameter model 

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

by tracking rainfall and water-quality constituents through 

the conceptual pathways of the hydrologic cycle. 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). HRUs are areas in a 

subwatershed that have similar hydrologic and water-quality 

characteristics that are determined on the basis of land use, 

surficial geology, soil characteristics, and other factors that 

are deemed to produce similar hydrologic responses to rainfall 

and potential evapotranspiration. 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. 

Each RCHRES is associated with a particular subwatershed 

and receives the runoff, sediment, and chemical loads from the 

PERLNDs and IMPLNDs in the subwatershed. The hydraulics of a RCHRES are simulated by a storage routing method 

(Donigian and others, 1995). 

HSPF is composed of a series of computational routines 

that separately simulate processes of the hydrologic cycle. 

Specifically, HSPF simulates the hydrologic cycle as an interconnected series of storage (and processing) segments with 

water fluxes (volume per unit area per unit time) and constituent fluxes (mass [weight] per unit area per unit time) moving 

between the various storages. Figure 4 shows a flowchart of 

HSPF hydrologic processes for IMPLNDs and PERLNDs. 

Figure 5 shows a flowchart of HSPF sediment processes for 

IMPLNDs, PERLNDs, and RCHRESs. The movement of 

water and suspended sediment from IMPLNDs and PERLNDs 

and between storage zones is controlled by various processrelated parameters. Although some parameters are directly 

measurable, most are estimated during model calibration 

(Martin and others, 2001).

The HSPF model of the lower Nueces River watershed 

was developed by (1) compiling and processing required input 

data, (2) configuring the model to represent the watershed, 

and (3) calibrating the model to improve simulation accuracy. 

1

 This section modified from Ockerman and Roussel (2009, p. 4–7).
10 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed Figure 4. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for hydrologic processes on (A) impervious and (B) pervious land segments (modified from Wicklein 

and Schiffer, 2002, fig. 3). 

Interception

storage

To

stream

Directinfiltration

Rainfall

Potential ET

Rainfall

Potential ET

Surface retention

storage

Surface detention

storage

Actual

evaporation

(A) IMPERVIOUS LAND SEGMENTS

(B) PERVIOUS LAND SEGMENTS

Actual

ET

ETETETETET

Overland

flow

To

stream

RETSC

Potentialrunoff

*

CEPSC*

LZETP*

LZSN*

Lowerzone

storage

INFILT*

INFILD*

INFEXP*

INTFW*

AGWETP*

BASETP*

DEEPFR*

UZSN*

LSUR*

NSUR*

SLSUR*

LSUR*

NSUR*

SLSUR*

IRC*

MODEL INPUTSTORAGEOUTFLOWDECISION POINTDESIGNATESCONTROLLING HSPFPARAMETER. REFERTO APPENDIX 1. 

*

EXPLANATION

ET EVAPOTRANSPIRATION

Delayedinfiltration

Upper-zone

storage

Inactive groundwater 

Active groundwater

storage

Surface

detention

storage

Interflow

storage

Overland

flow

Shallow

subsurface

flow

Groundwater

flow

KVARY

AGWRC*

*
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 11 Figure 5. Hydrological Simulation Program—FORTRAN (HSPF) flowchart for sediment processes on (A) impervious land segments, (B) pervious land segments, and (C) stream reaches (modified from Bicknell and others, 2001). 

Accumulation

Solids storage Lateral input of solids

(A) IMPERVIOUS LAND SEGMENTS

(B) PERVIOUS LAND SEGMENTS

To

stream

ACCSDP*

MODEL INPUTSTORAGEOUTFLOWDESIGNATESCONTROLLING HSPFPARAMETER. REFERTO APPENDIX 1. 

*

EXPLANATION

Washoff of solids

KEIM*, JEIM*

of solids

Washoff of solids

REMSDP*

Net vertical

Detached sediment Lateral input 

of sediment

To

stream

NVSI*

Total removal  of sediment 

KSER*, JSER*

sediment input

 storage

Soil matrix

Detachment of soil by rainfall

KRER*, JRER*

Washoff of detached 

sediment by water 

Sediment attachment

AFFIX*

Scour of matrix 

soil by water 

KGER*, JGER*

Suspended Sediment inflow Sediment outflow from RCHRES sediment

Bed

Deposition

TAUCD*, TAUCS* Scour

to RCHRES

storage

sand 

clay 

silt storage

clay and silt

sand

sediment

KSAND*, EXPSND* (C) STREAM REACHES

RCHRES STREAM/RESERVOIR 

REACH
12 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

The definitions of the HSPF model process parameters used in 

the lower Nueces River watershed model are listed in appendix 1. A complete description of the computational processes 

and required input model parameters is provided in the HSPF 

users’ manual (Bicknell and others, 2001). 

Input Data for the Lower Nueces River 

Watershed Model

Input data for the lower Nueces River watershed model 

included spatial data (land cover, geology, soils, topography, 

and drainage characteristics such as subwatershed boundaries and stream-reach length and cross-section data) and 

times-series data, including meteorological data (rainfall and 

potential evapotranspiration), streamflow data, and suspendedsediment concentrations. Spatial data were used to create the 

model HRUs and RCHRESs. Of all the input data, meteorological data had the most pronounced effect on the results 

of the time-series simulations. Streamflow and suspendedsediment data were used to calibrate the model.

Land-cover data for the study area were obtained from 

the U.S. Environmental Protection Agency’s Multi-Resolution 

Land Characteristics Consortium (Homer and others, 2004). In 

the lower Nueces River watershed, there were 15 land covers 

and land uses as classified by the 2001 National Land Cover 

Database (NLCD) (Multi-Resolution Land Characteristics 

Consortium, 2008). To simplify the model configuration, the 

15 classes were reclassified into 7 land-cover categories shown 

on figure 6. Areas classified as open water were modeled as 

part of a RCHRES. Developed land use was principally classified as low-intensity development, of which about 15 percent 

was simulated as impervious area. Barren land was grouped 

with developed open space and the resulting acreage also was 

considered to be 15-percent impervious. 

Surficial geology data (fig. 7) were obtained from areal 

geologic maps published in the “Geologic Atlas of Texas” 

(GAT) by the University of Texas Bureau of Economic 

Geology for the following areas within the lower Nueces River 

study area: Beeville-Bay City sheet (Aronow and others, 1975) 

and Corpus Christi sheet (Aronow and Barnes, 1975). The 

most upstream parts of the study area include the Pleistoceneage Lissie and Deweyville Formations. Most of the rest of the 

study area is covered by the Beaumont Formation (Pleistocene 

age) or alluvium (recent age). In the Lissie and Deweyville 

Formations, the sediments, in order of dominance, consist 

mostly of sand, silt, and clay. In the Beaumont Formation and 

alluvium, the sediments, in order of dominance, consist mostly 

of clay, silt, and sand (Minzenmayer, 1979). The data obtained 

from the Beeville-Bay City sheet (northern part of the study 

area) provide a more detailed breakdown of the areas of the 

Beaumont Formation that are dominated by clay or sand, compared with the data available from the Corpus Christi sheet 

(southern part of the study area). 

Soils data for the study area were compiled from the U.S. 

Department of Agriculture, Natural Resources Conservation 

Service (2009) “Soil Survey Geographic (SSURGO) 

Database.” These data were used to provide initial estimates 

for selected HSPF process-related parameter values, primarily 

soil infiltration rate (INFILT). Figure 8 shows the relative soil 

infiltration rates for the study area. 

Topography (slope) data for the study area were 

obtained from USGS 7.5-minute digital elevation models 

(U.S. Geological Survey, 2001). The digital elevation models 

also were used to delineate subwatersheds as part of the 

HSPF model development. The study area was subdivided 

into 64 subwatersheds as shown on figure 9. The average 

size of each subwatershed is 2,000 acres. Also shown on 

figure 9 is the stream network used in the model configuration. The stream segment (RCHRES) that is associated with 

each subwatershed is identified with the same identification 

number as the subwatershed identification number shown in 

figure 9. Spatial data for streams (location and reach length) 

were obtained from the “National Hydrography Dataset” (U.S. 

Geological Survey, 2009). Stream-channel cross-section data 

were obtained from streamflow discharge measurements made 

at USGS streamflow-gaging stations in the study area (U.S. 

Geological Survey, 2010).

Streamflow data from the Mathis gage were used as a 

boundary condition input to the model. Location information, type of data collected, and period of record for sites that 

provided streamflow data for the lower Nueces River watershed are listed in table 3. Locations of the streamflow-gaging 

stations are shown on figure 10.

Water is withdrawn from the Nueces River for municipal and industrial uses in RCHRES 82 and 84 (fig. 9). These 

withdrawals averaged 64,500 acre-feet per year or the flow 

volume equivalent to a continuous flow of about 89 cubic feet 

per second during 2000–2008 (Nueces River Authority, 2009); 

withdrawal data are available as monthly totals. Within HSPF, 

the monthly total withdrawals were disaggregated to hourly 

values and input to the model.

Rainfall and air temperature data were obtained from 

NWS stations Mathis, Mathis 4 SSW, Robstown, Robstown 

Airport (rainfall only), Corpus Christi International Airport, 

and Corpus Christi Maus Field (sites 1–6, respectively, 

fig. 10; table 3). The NWS has operated rainfall stations in 

Mathis and Robstown since 1957 and 1947, respectively. 

The Mathis 4 SSW station replaced the Mathis station in 

1977. The Robstown station has been in operation since 1947 

and the Robstown Airport station since 2003. The Mathis 

and Mathis 4 SSW stations (sites 1 and 2, respectively, fig. 

10; table 3) and the Robstown and Robstown Airport stations 

(sites 3 and 4, respectively, fig. 10; table 3) were the primary 

sources of rainfall data. For modeling purposes, the data 

from the two Mathis stations were combined and the location 

of the rainfall data time series was considered to be the same 

as the newer Mathis 4 SSW station. Similarly, data from the 

two Robstown stations were combined into a single time 

series that was considered to be located at the newer Robstown 

Airport station. These time series and associated locations 

were then used for the entire simulation period. The spatial 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 13Figure 6. Land-cover categories in the lower Nueces River watershed, South Texas. Nueces Bay Lake Corpus Christi

Nue s ce ve i R rNue s ce ve i R r 97°40'

97°50'

28°10'

28°5'

28°0'

27°55'

0 4 8 2 MILES0 4 8 2 KILOMETERS 6

6

EXPLANATION

Land-cover categoryOpen space, barren land DevelopedForestShrub/scrubGrassland/pastureCultivated cropWetland

Land-cover data modified from Multi-ResolutionLand Characteristics Consortium, 2008

BEE COUNTY

LIVE OAK COUNTY

JIM WELLS COUNTY

NUECES COUNTY

SAN PATRICIO COUNTY

Base modified from U.S. Geological Survey digital data1:24,000 quadrangles, Universal Transverse MercatorProjection, zone 14, North American Datum of 1983 

Mathis

Robstown

Corpus Christi

Odem

Lower Nueces River watershed boundary
14 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Figure 7. Surficial geology of the lower Nueces River watershed, South Texas. 

Nueces Bay

Lake Corpus Christi

 

ueces N

River 

N

97°40'

97°50'

28°10'

28°5'

28°0'

27°55'

0 4 8 2 MILES

0 4 8 2 KILOMETERS

Lower Nueces Riverwatershed boundary

BEE COUNTY

LIVE OAK COUNTY

JIM WELLS COUNTY

NUECES COUNTY

SAN PATRICIO COUNTY

6

6

Mathis

Robstown

Corpus Christi

Odem

SYSTEM Surficial geology

QUATERNARY

EXPLANATION

AlluviumDeweyville FormationBeaumont FormationBeaumont Formation (sand)Beaumont Formation (clay)Lissie FormationGoliad Formation

Base modified from U.S. Geological Survey digital data 1:24,000 quadranglesUniversal Transverse Mercator Projection, zone 14, North American Datum of 1983

Qal

Qd

Qb

Qbs

Qbc

Ql

Mg

Mg

Ql

Qbc

Qbs

Qb

Qd

Qb

Qd

Qal

Ql

Ql

Surficial geology from Aronow and Barnes, 1975; Aronow and others, 1975

Mg
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 15Figure 8. Relative soil infiltration rates in the lower Nueces River watershed, South Texas. 97°40'

97°50'

28°10'

28°5'

28°0'

27°55'

0 4 8 2 MILES0 4 8 2 KILOMETERS

6

6

EXPLANATION

Relative soil infiltration ratesModerately rapidModerateModerately slowSlowVery slowLower Nueces River 

watershed boundary

BEE COUNTY

LIVE OAK COUNTY

JIM WELLS COUNTY

NUECES COUNTY

SAN PATRICIO COUNTY

Base modified from U.S. Geological Survey digital data 1:24,000 quadrangles Universal Transverse Mercator Projection, zone 14, North American Datum of 1983

Mathis

Robstown

Corpus Christi

Odem

Nueces Bay

Lake Corpus Christi

eu N sec 

v Ri r e

eu N sec 

v Ri r e

Soil data modified from U.S. Departmentof Agriculture, Natural Resources Conservation Service, 2009
16 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Figure 9. Subwatershed delineation for the lower Nueces River watershed model, South Texas. 

97°40'

97°50'

28°10'

28°5'

28°0'

27°55'

EXPLANATION

Lower NuecesRiver watershedStream/reservoir reach

(RCHRES) and RCHRES subwatershed identifiedby number

BEE COUNTY

REFUGIO 

COUNTY

JIM WELLS COUNTY

NUECES COUNTY

SAN PATRICIO COUNTY

Mathis

Corpus Christi

Odem

9

12

14 16

18

6

8 2 5

3

20

24

22 32 36

30 38 34

90

57

42 40 39

92

93

48

50

63

54

73

65

95

74 94

53

64

58

59

61

67

66

70

71

72

76

80

78

97

26

44

46

62

56

60

10

55

91

82

4

68

84

96

98

52

22

LIVE OAK COUNTY

Base modified from U.S. Geological Survey digital data 1:24,000 quadrangles Universal Transverse Mercator Projection, zone 14, North American Datum of 1983 

0 4 8 2 MILES

0 4 8 2 KILOMETERS 6

6

Nueces Bay

Lake Corpus Christi

eu N s ec 

 River

eu N s ec 

 River
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 17 Figure 10. Locations of U.S. Geological Survey streamflow-gaging stations and National Weather Service rainfall stations (with associated Thiessen rainfall areas) providing data for the lower Nueces River watershed model, South Texas. 

9

12

14 16

18

6

8 2 5

3

20

24

22 32 36

30 38 34

90

57

42 40 39

92

93

48

50

63

54

73

65

95

74 94

53

64

58

59

61

67

66

70

71

72

76

80

78

97

26

44

46

62

56

60

10

55

91

82

4

68

84

96

98

52

BEE COUNTY

LIVE OAK COUNTY

JIM WELLS COUNTY

NUECES COUNTY

SAN PATRICIO COUNTY

Nueces Bay

Lake Corpus Christi

ceu N se

er vi R

Mathis

Alice

Robstown

Corpus Christi

Odem

7

3

2

#

#

#

4

8

1

10

5 6

9

#11

0 4 8 2 KILOMETERS0 4 8 2 MILES Base modified from U.S. Geological Survey digital data 1:24,000 quadrangles, 

Universal Transverse Mercator Projection, zone 14, North American Datum of 1983

22

#

4

9

Lower NuecesRiver watershedStream/reservoir reach 

(RCHRES) and RCHRESsubwatershed identifiedby numberTheissen boundaryNational Weather Servicerainfall stationU.S. Geological Survey streamflow-gaging station

EXPLANATION

97°30' 97°40' 97°50' 98°0'

28°10'28°5'28°0'27°55'27°50'27°45'

Data-collection site and number—See table 3
18 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

application of rainfall time-series data to the model is based 

on a Theissen polygon distribution (Linsley and others, 

1982). Two rainfall areas were defined by the Theissen analysis as shown on figure 10. Rainfall to the upstream area of 

the watershed was simulated using the Mathis station data, 

and rainfall to the downstream area of the watershed was 

simulated using the Robstown station data. Rainfall data 

from the Corpus Christi International Airport, Corpus Christi 

Maus Field, and Alice stations (sites 5, 6, and 7, respectively, 

fig. 10; table 3) were used to fill periods of missing data 

when data from the Mathis or Robstown stations were not 

available. 

HSPF uses time series of potential evapotranspiration 

(PEVT) data to set the upper limit of actual evapotranspiration (ET) that can be simulated for any of the HRUs. PEVT is 

the observed ET if there is an unlimited supply of water 

to satisfy the potential ET rate. PEVT was computed from 

maximum and minimum daily air temperature (from NWS 

station data) using the Hamon method (Bidlake, 2002). 

Similar to the sources of rainfall data, the primary sources 

of air temperature data were the Mathis and Robstown NWS 

stations. PEVT time-series data were applied to delineated 

subwatersheds according to the Theissen boundary shown on 

figure 10.

Model Development 

To develop the model, the stream network of the lower 

Nueces River watershed was segmented into subwatersheds 

(fig. 9), generally on the basis of (1) similar streamflow 

travel times that approximated the model time step (1 hour); 

(2) homogeneous channel properties such as slope and conveyance; and (3) outlets of subwatersheds at points of interest 

such as streamflow-gaging stations, major tributary confluences, and points of water withdrawals.

In each subwatershed, unique pervious and impervious 

HRUs were defined according to three factors: (1) land cover 

and land use; (2) surficial geology and soil characteristics; and 

(3) the location of the nearest rainfall station (to spatially distribute meteorological input data [rainfall and PEVT]). Spatial 

information describing these three factors were compiled and 

analyzed using the Geographic Information System software 

ArcGIS (ESRI, 2009) to compute the acreage of each HRU 

within a given subwatershed. 

Model Calibration

A primary goal of model calibration is to adjust the 

process-related parameter values to minimize the differences 

Table 3. Data-collection sites providing data for the lower Nueces River watershed model, South Texas.

[ddmmss, degrees minutes seconds; NWS, National Weather Service; ID, identifier; --, not available; max, maximum; min, minimum; temp, temperature; USGS, 

U.S. Geological Survey]

Site 

number

(fig. 10)

Station number and name Latitude 

(ddmmss)

Longitude 

(ddmmss)

Type of data 

(period of record available)

1 NWS station Coop ID 415661, Mathis, Jim Wells 

County, Tex.

28°06'--" 97°49'--" Daily rainfall; daily max and min air temp 

(1957–77)

2 NWS station Coop ID 415661, Mathis 4 SSW, Jim 

Wells County, Tex.

28°02'14" 97°52'21" Daily rainfall; daily max and min air temp 

(1977–2008)

3 NWS station Coop ID 417677, Robstown, Nueces 

County, Tex.

27°47'22" 97°39'43" Daily rainfall; daily max and min air temp 

(1947–2008)

4 NWS station WBAN ID 12984, Robstown Airport, 

Nueces County, Tex.

27°46'43" 97°41'26" Hourly rainfall (2003–08) 

5 NWS station Coop ID 412015, Corpus Christi International Airport, Nueces County, Tex.27°46'27" 97°30'44" Daily rainfall; daily max and min air temp 

(1960–2008)

6 NWS station Coop ID 412015, Corpus Christi Maus 

Field, Nueces County, Tex.

27°46'--" 97°27'--" Daily rainfall; daily max and min air temp 

(1934–60)

7 NWS station Coop ID 410144, Alice, Jim Wells 

County, Tex.

27°43'42" 98°04'04" Daily max and min air temp (1942–2008)

8 USGS streamflow-gaging station 08211000 Nueces 

River near Mathis, Tex.

28°02'17" 97°51'36" Streamflow (1952–2008); suspended sediment 

(1942–82, 2006–08)

9 USGS streamflow-gaging station 08211200 Nueces 

River at Bluntzer, Tex. (partial-record station)

27°56'15" 97°46'32" Streamflow (2005–08); suspended sediment 

(2006–08)

10 USGS streamflow-gaging station 08211500 Nueces 

River at Calallen, Tex.

27°52'58" 97°37'30" Streamflow (2001–08); suspended sediment 

(2006–08)

11 USGS streamflow-gaging station 08211502 Nueces 

River near Odem, Tex. (partial-record station)

27°53'42" 97°37'43" Streamflow (flood discharge measurements, 

2001–08)
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 19

between measured and simulated flows at a streamflow-gaging 

station. The model was calibrated in accordance with guidelines by Donigian and others (1984) and Lumb and others 

(1994). The calibration of the model proceeded in two steps. 

First, parameters related to hydrologic processes were calibrated. Second, parameters related to suspended-sediment 

processes were calibrated. Calibration of hydrologic processes 

included adjusting appropriate model parameters to minimize 

differences between measured and simulated streamflow at 

streamflow-gaging stations during 2001–08 over a wide range 

of hydrologic conditions. Model parameters that control landsurface erosion and washoff processes and instream sediment 

transport processes were adjusted to minimize differences 

between measured and simulated suspended-sediment concentrations and loads.

Hydrology

To evaluate the goodness-of-fit between measured and 

simulated streamflows, criteria such as error in total streamflow volume for the calibration period and low-flow and highflow distribution were used. Simulation errors were evaluated 

by comparing total streamflow volume, 50-percent lowest 

daily flows, and 10-percent highest daily flows. Donigian 

and others (1984) presented general guidelines for characterizing the goodness-of-fit of 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 10–15 percent, and fair when the error is 15–25 

percent. 

Additionally, model-fit statistics generated by the software program GenScn (GENeration and analysis of model 

simulation SCeNarios for watersheds) (Kittle and others, 

1998) 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 [R2

]) of the linear regression between measured and simulated streamflow (Ott and 

Longnecker, 2001); (2) Nash-Sutcliffe coefficient of modelfit efficiency (NSE) (Nash and Sutcliffe, 1970); (3) mean 

absolute error (MAE) (StatSoft, Inc., 2010); and (4) root mean 

square error (RMSE) (StatSoft, Inc., 2010). The R2

 and NSE 

are similar because each provides a measure of the variation in 

the measured values that is explained by the simulated values. 

The NSE, however, provides a generally preferable evaluation of the fit quality compared with the R2

 because the NSE 

measures the magnitude of the differences between measured 

and simulated values, whereas the R2 measures the difference 

between mean values (Zarriello and Ries, 2000, p. 44). MAE 

and RMSE express the difference between measured and 

simulated streamflow in original units (cubic feet per second) 

(StatSoft, Inc., 2010).

At selected calibration sites, depending on the availability of streamflow data, the calibration process included 

a separate, post-calibration test of the model fit. For sites 

where the testing process was performed, some of the 

observed streamflow-gaging data were withheld from the 

calibration. After calibration, the withheld data were then 

used for testing the model fit between measured and simulated 

streamflow. 

Most of the streamflow in the study area originated as 

releases from Lake Corpus Christi (fig. 1). The Mathis gage 

is located immediately downstream from the lake and monitors stream discharge from the lake. Daily discharges for this 

station were available beginning in 1939 and were input to the 

model at the inlet of RCHRES 4 (fig. 9) as a boundary condition. Because actual gaged streamflow data were used as input 

to the model at this site, no model calibration was necessary at 

this station.

The Bluntzer gage (outlet of RCHRES 60, fig. 9) is 

operated as a partial-record station. Streamflows of more than 

2,750 cubic feet per second are not measured because streamflow-gaging conditions are not favorable at higher flows. 

Because streamflow data from the Bluntzer gage were not suitable for calibration of the entire range of streamflow conditions, calibration of hydrologic parameters upstream from the 

Bluntzer gage was not based on data from this gage. During 

2005–06, streamflow at the Bluntzer gage did not exceed 

2,750 cubic feet per second and the 2005–06 data were used 

as a test of the calibrated model. Streamflow testing results for 

the Bluntzer gage during 2005–06 (fig. 11; table 4) indicate 

very good agreement between measured and simulated streamflow volumes; the error in total simulated streamflow volume 

compared with measured streamflow volume during 2005–06 

at this gage was less than 2 percent. 

The Calallen gage was operated as a partial-record station 

during 1989–2000; daily streamflows were not reported when 

instantaneous streamflow exceeded 2,750 cubic feet per second because of difficulty measuring higher streamflows at this 

gage. A complete record of streamflow for the Calallen gage 

is available for 2001–08. In June 2000, the Calallen gage was 

moved about 0.4 mile downstream to its present location and 

converted to a continuous streamflow-gaging station capable 

of measuring the full range of streamflow. Streamflow data 

measured during 2005–08 at the Calallen gage were used to 

calibrate the model and streamflow data from 2001–04 were 

used to test the calibrated model. Streamflow calibration and 

testing results for the Calallen gage (fig. 12; table 4) indicate very good agreement between measured and simulated 

streamflow volumes. The error in total simulated streamflow 

volume at the Calallen gage compared with measured streamflow volume during the calibration period (2005–08) is less 

than 3 percent; for the testing period (2001–04), the error in 

total simulated streamflow volume compared with measured 

streamflow volume is less than 6 percent. 

Using evaluation criteria by Donigian and others (1984), 

calibration and testing results for streamflow volumes at 

both the Bluntzer (testing only) and Calallen gages (table 4) 

were considered very good. The R2 and NSE values were 

considered acceptable for annual, monthly, daily, and hourly 

statistics. The NSE for hourly streamflows ranged from .90 to 

.96 for the calibration and testing periods at the Bluntzer and 

Calallen gages. The minimum NSE values for daily, monthly, 
20 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Figure 11. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08211200 Nueces 

River at Bluntzer, Texas, 2005–06. 

1 10

10 100

10

100

1,000

10,000

2005

10

1,000

10,000

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

EXCEEDANCE PROBABILITY, IN PERCENT

YEAR

MEASURED DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

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

Measured

Simulated

Measured

Simulated

1,000

10,000

100

10

1,000

10,000

100

2006

Line of equal value
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 21

Figure 12. Measured and simulated 7-day mean streamflow at U.S. Geological Survey streamflow-gaging station 08211500 Nueces 

River at Calallen, Texas, 2001–08.

1 10

1 10 100 1,000

10

1

100

1,000

10,000

100,000

10

1

100

10,000

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 7-DAY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

SIMULATED 7-DAY MEAN STREAMFLOW, 

IN CUBIC FEET PER SECOND

7-DAY MEAN STREAMFLOW, 

IN CUBIC FEET PER SECOND

7-DAY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

Measured

Simulated

Measured

Simulated

Line of equal value

10,000

100,000

1,000

10

1

100

10,000

100,000

1,000

2001 2002 2003 2004 2005 2006 2007 2008
22 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

and annual simulations for both stations were .91, .97, and .98, 

respectively (table 4).

Suspended Sediment

Suspended-sediment concentrations and loads were 

simulated using the appropriate HSPF modules: SEDMNT 

for PERLND simulation, SOLIDS for IMPLND simulation, 

and SEDTRN for RCHRES simulation. For each PERLND, 

the processes of detachment of sediment from the soil matrix 

and washoff of this sediment were simulated on the basis of 

rainfall intensity, surface runoff, and model parameters that 

control the accumulation, detachment, and transport of soils. 

For each IMPLND, the processes of accumulation and washoff 

of sediment were simulated in the SOLIDS module. In each 

RCHRES, the sediment-transport processes in the stream 

channel were simulated by the SEDTRN module. Transport 

processes in RCHRES included deposition and scour, which 

are functions of sediment size, settling velocity, density, erodibility, bed depth, and critical shear stress. RCHRES sediment 

transport is computed separately for each sand, silt, and clay 

fraction of sediment size. 

Table 4. Streamflow calibration and testing results for the lower Nueces River watershed model, South Texas.

[acre-ft, acre-feet; ft3s, cubic feet per second]

08211200 Nueces River at Bluntzer, Texas

Testing period 2005–06

Streamflow volumes and peaks Measured Simulated Error

(percent)1

Criteria

(percent)2

Total flow volume (acre-ft) 353,000 357,000 1.1 10

Mean flow rate (ft3

/s) 244 247 1.1 10

Highest 10-percent daily flows (acre-ft) 151,000 166,000 9.9 10

Lowest 50-percent daily flows (acre-ft) 72,800 71,600 -1.6 10

Model-fit statistics 2005–06 Annual Monthly Daily Hourly

Number of years, months, days, or hours 2 24 730 17,520

Coefficient of determination (R2

) 1.00 .98 .92 .91

Nash-Sutcliffe coefficient of model-fit efficiency (NSE) 1.00 .97 .91 .90

Mean absolute error (ft3

/s) 4.5 30.7 38.4 38.4

Root mean square error (ft3

/s) 5.3 57.0 117 124

08211500 Nueces River at Calallen, Texas

Calibration period 2005–08

Streamflow volumes and peaks Measured Simulated Error

(percent)1

Criteria

(percent)2

Total flow volume (acre-ft) 1,400,000 1,440,000 2.9 10

Mean flow rate (ft3

/s) 483 497 2.9 10

Highest 10-percent daily flows (acre-ft) 1,220,000 1,215,000 -.4 10

Lowest 50-percent daily flows (acre-ft) 25,400 25,100 -1.2 10

Model-fit statistics 2005–08 Annual Monthly Daily Hourly

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

Coefficient of determination (R2

) 1.00 1.00 .95 .95

Nash-Sutcliffe coefficient of model-fit efficiency (NSE) 1.00 .99 .93 .92

Mean absolute error (ft3

/s) 37 66 127 130

Root mean square error (ft3

/s) 39 152 510 523
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 23

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

SEDTRN modules (appendix 1) were based on published 

guidelines (Donigian and Love, 2003; U.S. Environmental 

Protection Agency, 2006). Calibration of sediment-related 

parameters involved the following steps: (1) Estimation of 

suspended-sediment loads carried in flows released from 

Lake Corpus Christi; (2) identification of reasonable sediment yields from the various land types in the watershed; 

(3) estimation of the soil-erosion and sediment-washoff 

parameters used to generate sediment washoff from PERLNDs 

and IMPLNDs, respectively; and (4) calibration of sedimenttransport (RCHRES) parameters by comparison of measured 

and simulated suspended-sediment concentrations and loads at 

the streamflow-gaging stations. 

Estimation of Suspended-Sediment Loads from Lake 

Corpus Christi

Estimation of suspended-sediment loads in releases 

from Lake Corpus Christi were based on 14 reported 

suspended-sediment loads during 1964–71 (table 2) and 

Table 4. Streamflow calibration and testing results for the lower Nueces River watershed model, South Texas—Continued.

08211500 Nueces River at Calallen, Texas

Testing period 2001–04

Streamflow volumes and peaks Measured Simulated Error

(percent)1

Criteria

(percent)2

Total flow volume (acre-ft) 4,225,000 4,007,000 -5.2 10

Mean flow rate (ft3

/s) 1,455 1,380 -5.2 10

Highest 10-percent daily flows (acre-ft) 3,167,000 3,066,000 -3.2 10

Lowest 50-percent daily flows (acre-ft) 21,500 23,500 9.3 10

Model-fit statistics 2001–04 Annual Monthly Daily Hourly

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

Coefficient of determination (R2

) 1.00 1.00 .96 .96

Nash-Sutcliffe coefficient of model-fit efficiency (NSE) .98 .99 .96 .96

Mean absolute error (ft3

/s) 83 130 307 314

Root mean square error (ft3

/s) 133 265 838 862

08211500 Nueces River at Calallen, Texas

Simulation period 2001–08

Streamflow volumes and peaks Measured Simulated Error

(percent)1

Criteria

(percent)2

Total flow volume (acre-ft) 5,613,000 5,422,000 -3.4 10

Mean flow rate (ft3

/s) 970 937 -3.4 10

Highest 10-percent daily flows (acre-ft) 4,720,000 4,507,000 -4.5 10

Lowest 50-percent daily flows (acre-ft) 46,000 46,200 0.4 10

Model-fit statistics 2001–08 Annual Monthly Daily Hourly

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

Coefficient of determination (R2

) 1.00 1.00 .96 .96

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

Mean absolute error (ft3

/s) 60 98 217 222

Root mean square error (ft3

/s) 98 216 694 713

1

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

2

 Default error criteria from HSPEXP (Lumb and others, 1994).
24 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

8 suspended-sediment samples collected by the USGS during 

2006–07 (table 5). All suspended-sediment data (historical and 

recent) were collected at the Mathis gage downstream from 

Lake Corpus Christi. These data represented streamflow ranging from 72 to 35,129 cubic feet per second and suspendedsediment loading rates ranging from 4.3 to 8,872 tons per day. 

Data collected during 1964–71 represented mean streamflow 

and suspended-sediment loads during relatively large releases 

that lasted days or weeks. Data collected during 2006–07 

represented instantaneous conditions (a typical suspendedsediment sample was collected during a period of about 

1 hour). A least-squares regression equation (fig. 13) was 

developed relating daily mean discharge at the Mathis gage 

and daily sediment load. Discharge and sediment load were 

log-transformed before performing the regression to improve 

the quality of the regression fit, then retransformed to original 

units (Helsel and Hirsch, 2002). The resulting equation was

L = 0.026 x Q1.195, (2)

where

L = suspended-sediment load, in tons per day; and

Q = daily mean discharge, in cubic feet per second.

The R2 for the regression equation was .94; the standard 

error of the residuals (RSE), a measure of the dispersion of 

the data around the regression line (Helsel and Hirsch, 2002, 

p. 244) was 121 pounds per day. The residual plot on figure 13 

shows the regression residuals (as a percentage of the measured suspended-sediment load) plotted as a function of the 

measured suspended-sediment load. The distribution of the 

error residuals was relatively uniform, indicating a reasonable 

regression model. 

A comparison of model-simulated input of suspendedsediment loads with suspended-sediment loads computed 

from eight samples collected at the Mathis gage during 2006–

07 is listed in table 5. These results are not considered a calibration but rather a test of the ability of the model (equation 2) 

to reasonably simulate instantaneous suspended-sediment 

concentrations and loads entering the study area through 

releases from Lake Corpus Christi. In general, measured 

and simulated suspended-sediment loads compare favorably. 

Regression-equation derived sediment loads were within 

30 percent of measured loads for all samples except that on 

November 20, 2006, for which the simulated load was 116 

percent of the measured load (table 5). The November 20, 

2006, sample was collected during a period of low flow and 

represented a relatively small sediment load, compared with 

the relatively high flows that were mostly sampled at this site. 

Therefore, the loads computed by the regression equation 

were considered reasonable estimates of the total suspendedsediment loads that were discharged from Lake Corpus 

Christi.

The time series of sediment loads from Lake Corpus 

Christi were apportioned between clay and silt particle sizes 

and input to the model in RCHRES 4 (as was done with flows 

from Lake Corpus Christi). An apportionment of 40 percent 

silt and 60 percent clay was done on the basis of literature 

synthesis work by White and Calnan (1990). 

Sediment Yields for Selected Land-Use Types and Land 

Covers

Several studies of sediment yields (pounds per acre or 

tons per acre) from specific land-use types and land covers 

have been made for the South Texas area (Baird and others, 

1996; Ockerman and Petri, 2001; Ockerman, 2002; Ockerman 

and Fernandez, 2010). These studies provided data for sediment yields for cropland, rangeland (pasture/grassland), and 

developed land. Data and analyses from these studies were 

compared with simulated sediment yields from similar land 

type PERLNDs. Sediment-related parameters for PERLNDs 

were adjusted so that simulated sediment yields compared 

reasonably with results from field data. 

Ockerman and Petri (2001, p. 19), in a study of five cropland watersheds in Nueces and Kleberg Counties, reported an 

average sediment yield of 610 pounds per acre during 1996–

98. Ockerman and Fernandez (2010, p. 20), in a study of two 

primarily cropland watersheds in the Oso Creek watershed in 

Nueces County, reported average sediment runoff yields of 

139 and 522 pounds per acre during 2005–08. In a study of 

sediment runoff from two rangeland study sites in San Patricio 

County, Ockerman (2002) reported an average sediment yield 

of 28 pounds per year during 2000–2001. 

Estimates of suspended-solids concentrations in runoff 

from developed land uses and land covers were compiled 

for the South Texas area in a study by Baird and others (1996). 

Median event-mean concentrations for residential, commercial, industrial, and transportation land uses were compiled 

from a literature review. These event-mean concentrations 

were used to develop estimates of sediment loads in the Oso 

Creek watershed for 1989–93. The average sediment yield 

for developed land (including residential, commercial, industrial, and transportation land uses) was 52 pounds per acre 

per year (Baird and others, 1996, p. 199–218). A summary 

of sediment yields for selected land-use/land-cover categories 

is listed in table 6. The yields from the literature review 

were used as calibration target values for model simulations. 

For comparison, table 6 also lists simulated sediment yields 

for selected land uses and land covers for 1989–2008 in the 

lower Nueces River watershed. HSPF parameter values related 

to sediment production (sediment detachment and washoff process) were iteratively adjusted until simulated yields 

approximated the target yields determined from the literature 

review.

Annual sediment yields simulated for cropland varied 

from 200 to 530 pounds per acre per year depending on crop, 

soil type, and rainfall. Annual sediment yields simulated for 

rangeland were 39 to 55 pounds per acre per year; for developed land were 38 to 75 pounds per acre per year; and for 

open/undeveloped land were 120 to 340 pounds per acre per 

year. In the part of the study area where sediment yields were 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 25

10 100 1,000 10,000 100,000

STREAMFLOW, CUBIC FEET PER SECOND

1

10

100

1,000

10,000

SUSPENDED-SEDIMENT LOAD, IN TONS PER DAY

Load computed from measured streamflow (Q)

and sample suspended-sediment concentration 

Load (L) estimated by least-squares regression 

-100

0

(ESTIMATED DAILY LOAD MINUS MEASURED DAILY LOAD

MEASURED DAILY SUSPENDED-SEDIMENT LOAD, TONS PER DAY

 DIVIDED BY MEASURED DAILY LOAD)

ERROR RESIDUALS OF SUSPENDED-SEDIMENT LOAD, IN PERCENT 

 100

 200

 250

 -200

 -250

1 10 100 1,000 10,000

L = 0.026 x Q 1.195

Figure 13. Relation between streamflow and suspended-sediment load and plot of regression error residuals based on 22 streamflowload data pairs from 08211000 Nueces River near Mathis, Texas, 1964–71 and 2006–07. 
26 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

simulated using NWS rainfall data from Robstown, sediment 

yields were larger for all land types compared with sediment 

yields for the part of the study area where sediment yields 

were simulated using NWS rainfall data from Mathis. The 

main reason for differences in sediment yields were differences in the amount of rainfall measured during 2000–2008 at 

the NWS stations for the two parts of the study area; average 

annual rainfall amounts recorded by the NWS during 2000–

2008 were 34.1 inches at Robstown and 26.8 inches at Mathis. 

Model Calibration

For parameters related to sediment erosion from various 

land types, model calibration was based on available studies 

and data collected during 1989–2008. Therefore, calibration of 

PERLND parameters was based on simulations from the same 

period, 1989–2008. Model calibration of RCHRES suspendedsediment processes was based primarily on data collected at 

the Bluntzer and Calallen gages during 2006–07. These data 

were used to develop estimates of suspended-sediment loads 

for 2001–08, which were then compared with model simulation results.

Downstream from Lake Corpus Christi, at the Bluntzer 

and Calallen gages, suspended-sediment concentrations and 

loads in streams were calibrated to available suspended-sediment data collected by the USGS during 2006–07. RCHRES 

parameters related to sediment transport were adjusted (calibrated) to minimize the differences between measured and 

Table 5. Measured and simulated streamflows, measured suspended-sediment particle size, and measured and simulated suspendedsediment concentrations and loads for selected samples collected at 08211000 Nueces River near Mathis, Texas, 2006–07. 

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

Sample date

Measured 

streamflow

(ft3

/s)

Simulated 

streamflow

(ft3/s)

Measured 

sediment 

particle 

diameter less 

than 0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Simulated 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment 

load 

(tons per day)

Simulated 

suspendedsediment 

load 

(tons per day)

Error in 

suspendedsediment 

load

(percent)2

May 17, 2006 139 137 98 24 28 9.0 10 11

July 20, 2006 110 117 88 26 31 7.7 10 30

Sept. 5, 2006 138 136 99 24 28 8.9 10 12

Nov. 20, 2006 72 71 94 22 48 4.3 9.3 116

May 26, 2007 461 511 99 16 12 20 17 -15

May 29, 2007 1,190 1,270 98 12 11 39 35 -10

July 1, 2007 4,830 5,120 98 16 18 208 235 13

July 11, 2007 17,800 17,600 86 44 45 2,110 2,100 -.5

1

 Percent by weight of sediment sample that passes through 0.0625-mm sieve.

2

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

Table 6. Comparison of literature estimates and simulation results for sediment yields from selected land covers and land uses in the 

lower Nueces River watershed and South Texas area.

[--, not applicable]

Land-cover/land-use type Sediment yield estimate (pounds per acre per year) 

from literature and reference

Average 1989–2008 

simulated sediment yield 

(pounds per acre per year)

Cropland (cultivated crop) 610 (Ockerman and Petri, 2001); 139 and 522 (Ockerman and 

Fernandez, 2010)

200–530

Rangeland (pasture, shrub, grassland) 28 (Ockerman, 2002) 39–55

Developed (commercial, transportation, 

residential, industrial)

52 (Baird and others, 1996) 38–75

Open/undeveloped -- 120–340
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 27

simulated sediment concentrations and computed (from measured concentrations and measured streamflows) and simulated loads. Instream transport and concentrations of sand are 

simulated by power functions that relate sand concentration 

to average RCHRES stream velocity. The power functions are 

calibrated by user-selected coefficients and exponents for each 

RCHRES. Proceeding in downstream order, the calibration 

process to determine suspended-sediment concentrations and 

loads was repeated for each RCHRES in the intervening areas 

between sediment-sampling stations. Suspended-sediment 

calibration results at the Bluntzer gage are listed in table 7. 

Table 7 also lists measured and simulated suspended-sediment 

concentrations and measured (computed) and simulated loads 

from 11 samples collected during 2006–07. Errors in simulated suspended-sediment loads at the Bluntzer gage (table 7) 

were generally larger compared with those at the Mathis gage 

(table 5), mostly because of differences in the amount of error 

associated with the simulation of streamflow. Overall, the 

amount of error associated with the simulation of streamflow 

at the Bluntzer gage was larger than the amount of error associated with the simulation of streamflow at the Mathis gage.

Although simulation errors for instantaneous or daily 

suspended-sediment loads were at times relatively large (139 

percent for the January 25, 2007, sample; table 7), simulation 

errors for longer periods (for example, monthly or yearly) 

were deemed acceptable. The sum of simulated sediment loads 

for all 11 samples (6,420 tons per day) was within 1 percent of 

the sum of the measured (computed) sediment loads for all 11 

samples (6,360 tons per day). 

Suspended-sediment data were available for 10 suspended-sediment samples collected at the Calallen gage 

during 2006–07. Using these data, suspended-sediment loads 

were computed from daily mean streamflows and measured 

suspended-sediment concentrations. Similar to the approach 

used to estimate sediment loads from Lake Corpus Christi 

(fig. 13), the computed loads at Calallen were used to develop 

a regression equation to relate daily suspended-sediment loads 

to daily mean streamflow (fig. 14). The resulting equation 

was

L = 0.119 x Q1.061, (3)

where

L = suspended-sediment load, in tons per day; and

Q = daily mean discharge, in cubic feet per second.

R2 for the regression equation was .87; RSE was 427 

pounds per day. The residual plot on figure 14 shows regression residuals (as a percentage of the measured suspended-sediment load) plotted as a function of the measured suspendedsediment load. The distribution of the error residuals was 

relatively uniform, indicating a reasonable regression model.

The regression equation was then used to estimate suspended-sediment loads during 2001–08. The estimated daily 

suspended-sediment load computed using measured streamflow and sediment data was used as a calibration target to 

compare with simulated suspended-sediment loads at Calallen. 

Model sediment-transport parameter values for RCHRESs 

downstream from the Calallen gage were adjusted on the basis 

of this comparison. 

Calibration results for the Calallen gage (table 8) include 

a comparison of the average monthly streamflow volumes 

and suspended-sediment loads simulated using the HSPF 

model with the average monthly streamflow volumes measured at the gage and average monthly suspended-sediment 

loads estimated using equation 3. Table 8 also lists the annual, 

monthly, and daily model-fit statistics for 2001–08. A graphical comparison (fig. 15) of estimated and simulated monthly 

suspended-sediment loads shows reasonable agreement over 

the range of suspended-sediment conditions during 2001–08. 

Suspended-sediment calibration results for individual samples 

collected at the Calallen gage are listed in table 9. Table 9 

compares simulated suspended-sediment concentrations and 

loads with estimated suspended-sediment concentrations and 

loads computed from measured streamflow and suspendedsediment concentrations obtained from 10 samples collected 

during 2006–07.

During 2001–08, average monthly suspended-sediment 

loads at the Calallen gage were undersimulated by 4.2 percent 

(table 8), compared with the loads estimated by regression 

(equation 3). Model-fit statistics indicated the model simulates 

daily, monthly, and yearly sediment loads reasonably well. The 

NSE was .80 for daily simulations, increasing to .95 and .97, 

respectively, for simulation of monthly and yearly suspendedsediment loads (table 8). 

Donigian and others (1984) present general guidelines 

for characterizing HSPF sediment calibrations, similar to the 

guidelines for streamflow calibration. For annual and monthly 

sediment loads, model calibration is considered very good 

when the error is less than 15 percent, good when the error is 

15–25 percent, and fair when the error is 25–35 percent. By 

these guidelines, calibration results for suspended-sediment 

loads at the Calallen gage (table 8) are considered very good. 

The R2

 and NSE values were considered acceptable for annual, 

monthly, and daily statistics. 

Parameter Calibration Values 

Calibration and testing of the HSPF model resulted 

in a final set of model parameter values for simulation of 

streamflow and suspended-sediment loads for the study area. 

Calibrated values (or ranges of values) for selected parameters 

related to hydrology simulation are listed in table 10, and 

calibrated values (or ranges of values) for selected sedimentrelated parameters used in the HSPF model are listed in 

table 11.

Sensitivity Analysis 

Calibrated values of selected HSPF process-related 

parameters were further evaluated by doing a sensitivity 

analysis to determine the effects that changes in the selected 
28 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

10 100 1,000 10,000 100,000

STREAMFLOW, CUBIC FEET PER SECOND

1

10

100

1,000

10,000

SUSPENDED-SEDIMENT LOAD, IN TONS PER DAY

Load computed from measured streamflow (Q)

and sample suspended-sediment concentration 

Load (L) estimated by least-squares regression 

10 100 1,000

-250

250

0

-100

 100

 200

 -200

(ESTIMATED DAILY LOAD MINUS MEASURED DAILY LOAD

 DIVIDED BY MEASURED DAILY LOAD)

ERROR RESIDUALS OF SUSPENDED-SEDIMENT LOAD, IN PERCENT 

MEASURED DAILY SUSPENDED-SEDIMENT LOAD, TONS PER DAY

1 10,000

L = 0.119 x Q 1.061

Figure 14. Relation between streamflow and suspended-sediment load and plot of regression error residuals based on 10 streamflowload data pairs from 08211500 Nueces River at Calallen, Texas, 2006–07. 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 29

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

10

100

1,000

1

10,000

100,000

1,000,000

10

100

1,000

1

10,000

100,000

1,000,000

2001 2002 2003 2004 2005 2006 2007 2008

ESTIMATED MONTHLY SUSPENDED-SEDIMENT LOAD, IN TONS

YEAR

SIMULATED MONTHLY SUSPENDED-SEDIMENT LOAD, IN TONS MONTHLY SUSPENDED-SEDIMENT LOAD, IN TONS

Estimated

Simulated

Figure 15. Estimated (by regression) and simulated monthly suspended-sediment loads for 08211500 Nueces River at Calallen, Texas, 

2001–08. 
30 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Table 7. Measured and simulated streamflows, measured suspended-sediment particle size, and measured and simulated suspendedsediment concentrations and loads for selected samples collected at 08211200 Nueces River at Bluntzer, Texas, 2006–07. 

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

Sample date

Measured 

streamflow

(ft3

/s)

Simulated 

streamflow

(ft3/s)

Measured 

sediment 

particle diameter less than 

0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Simulated 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment load 

(tons per day)

Simulated 

suspendedsediment load 

(tons per day)

Error in 

suspendedsediment load

(percent)2

May 17, 2006 121 136 97 48 22 16 8.3 -48

July 20, 2006 116 118 88 51 25 16 7.9 -51

Sept. 5, 2006 136 129 97 52 23 19 8.1 -57

Sep. 19, 2006 1,050 1,370 97 1,070 468 3,030 1,730 -43

Nov. 20, 2006 82 71 89 25 35 5.5 6.8 24

Jan. 4, 2007 153 344 81 293 344 121 238 97

Jan. 25, 2007 243 970 99 760 455 498 1,190 139

May 26, 2007 471 405 100 94 38 119 41 -66

May 29, 2007 1,330 1,170 97 54 44 194 140 -28

July 1, 2007 5,980 6,290 92 45 33 726 553 -24

July 11, 2007 15,000 14,400 79 40 64 1,620 2,500 54

1

 Percent by weight of sediment sample that passes through 0.0625-mm sieve.

2

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

Table 8. Suspended-sediment calibration results for the Hydrological Simulation Program—FORTRAN model of the lower Nueces 

River watershed, South Texas, 2001–08.

08211500 Nueces River at Calallen, Texas

Average monthly streamflow volumes and suspended-sediment loads Measured/

estimated1 Simulated Error(percent)2

Flow volume (acre-feet) 58,500 56,400 -3.6

Suspended-sediment load (tons) 5,910 5,660 -4.2

Model-fit statistics for suspended-sediment loads 2001–08 Annual Monthly Daily

Coefficient of determination (R2

) 0.98 0.96 0.82

Nash-Sutcliffe coefficient of model-fit efficiency (NSE) .97 .95 .80

Mean absolute error (tons) 12,200 1,640 84.4

Mean absolute error (percent) 17.1 27.8 43.4

Root mean square error (tons) 15,500 4,270 444

1

 Streamflow volumes at 08211500 are measured values; suspended-sediment loads are estimated by regression of measured streamflow and suspended-sediment loads calculated from streamflow and measured suspended-sediment concentrations for 10 sampling events during 2006–07.

2

 Error = [(simulated-measured/measured)] x100.
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 31

parameters would have on simulated streamflow and suspended-sediment loads. Each sensitivity simulation was made 

by adjusting a single parameter of the model by relatively 

large amounts (increased by 40 percent) while keeping other 

model parameters unchanged. The sensitivity analysis model 

runs were performed for 2001–08. The resulting changes in 

streamflow and suspended-sediment loads were evaluated at 

the inflow to RCHRES 97, which is considered the inflow to 

the Nueces Estuary. The changes in streamflow and suspended-sediment loads resulting from adjustments of selected 

parameters are listed in table 12.

Simulated streamflow was relatively insensitive to adjustments of any of the selected parameters. One reason is that 

most of the streamflow, which is simulated in the model as 

an input boundary condition, originates as inflow from Lake 

Corpus Christi releases. In the sensitivity analysis, the parameters that had the largest effects on simulated suspendedsediment loads were the index to infiltration capacity of soil 

(INFILT), lower-zone nominal storage (LZSN), and coefficient 

of detached-sediment washoff equation (KSER). Overall, 

changes in suspended-sediment loads when these parameters 

were adjusted were comparatively small compared with the 

changes applied to the parameter values. The INFILT parameter had the largest effect on simulated suspended-sediment 

loads; a 40-percent increase in the INFILT parameter resulted 

in a 7.1-percent decrease in the simulated suspended-sediment 

load. For the other parameters examined for sensitivity, 

increases in parameter values of 40 percent resulted in changes 

to the simulated suspended-sediment load ranging from -3.2 to 

4.9 percent (table 12).

Model Limitations

Errors in model calibration can be classified as systematic 

or measurement errors (Raines, 1996). Systematic errors are 

those that arise because of the model’s failure to adequately 

represent the hydrologic and water-quality processes of the 

study watershed. As a result, there are limits to how well 

model parameters and equations can replicate the complex 

physical properties of streamflow and water-quality processes. 

Measurement errors are those that are introduced as a 

result of inaccurate or missing data. The degree to which 

available rainfall data represent actual rainfall is potentially 

the most serious source of measurement error associated with 

this model; limitations in the amount of available rainfall data 

are often a serious source of measurement error for a watershed modeling study (Ockerman and Roussel, 2009). In the 

lower Nueces River watershed model, rainfall was applied 

to the study area using data from two locations—Mathis and 

Robstown NWS stations. Long-term rainfall totals from these 

two stations are likely representative of long-term study-area 

conditions. However, the lack of additional rainfall gages 

severely limits spatial resolution of rainfall input to the 

218-square-mile study area, especially for relatively large 

storm events. Typically, measurements of extreme values of 

rainfall at a single location, when used to simulate average 

Table 9. Measured and simulated streamflows, measured suspended-sediment particle size, and measured and simulated suspendedsediment concentrations and loads for selected samples collected at 08211500 Nueces River at Calallen, Texas, 2006–07. 

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

Sample date

Measured 

streamflow

(ft3

/s)

Simulated 

streamflow

(ft3/s)

Measured sediment particle 

diameter less 

than 0.0625 mm 

(percent)1

Measured 

suspendedsediment 

concentration 

(mg/L)

Simulated 

suspendedsediment 

concentration 

(mg/L)

Measured 

suspendedsediment load 

(tons per day)

Simulated 

suspendedsediment load 

(tons per day)

Error in 

suspendedsediment load

(percent)2

May 16, 2006 15 22 82 58 27 2.3 1.4 -39

Sept. 19, 2006 1,150 1,340 100 429 316 1,330 992 -25

Jan. 5, 2007 66 299 98 19 129 3.4 104 2,960

Jan. 25, 2007 163 455 89 32 98 14 161 1,050

Mar. 15, 2007 221 170 32 134 74 80 40 - 50

July 2, 2007 5,100 7,360 94 231 44 3,180 871 - 73

July 4, 2007 6,530 7,070 93 50 43 882 814 - 7.8

July 5, 2007 8,960 10,200 84 64 61 1,550 1,670 7.7

July 11, 2007 19,800 17,400 49 45 80 2,410 3,750 56

Aug. 16, 2007 4,050 4,610 94 22 24 241 296 23

1

 Percent by weight of sediment sample that passes through 0.0625-mm sieve.

2

 Error = [(simulated-measured)/measured] x 100.
32 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

rainfall for larger areas (such as the lower Nueces River watershed) tend to overestimate the average rainfall of the watershed (Asquith, 1999). Also, the overestimation increases as the 

size of the watershed increases; the fewer rain gages available 

for a study area, the greater the probability that rainfall in the 

study area is overestimated during large storms. 

Another limitation of the rainfall data used for the model 

was that all data from the Mathis NWS station and much of 

the data from the Robstown NWS station were only available 

as daily totals. These daily totals were disaggregated to hourly 

values based on theoretical temporal distributions. As a result, 

some loss of accuracy in the timing and intensity of rainfall 

might have been incorporated into the model results. 

Overall, streamflow was simulated with reasonable 

accuracy at the streamflow-gaging stations in the study area. 

Most of the streamflow originating in the study area was from 

Lake Corpus Christi releases and was measured by the Mathis 

gage. The Bluntzer gage was a partial-record station and 

did not record streamflows greater than 2,750 cubic feet per 

second. Similar to streamflow at the Mathis gage, streamflow 

at the Bluntzer gage was largely controlled by releases from 

Lake Corpus Christi. The accuracy of simulated streamflow at 

the Bluntzer gage might have been diminished during periods 

when additional runoff downstream from Lake Corpus Christi 

was occurring. Streamflow simulation results at the Calallen 

gage were generally very good. Low flows at the Calallen 

gage (streamflows of less than about 10 cubic feet per second) 

exhibited greater error compared with higher flows (fig. 12, 

middle graph). The errors in low flows at the Calallen gage 

were possibly a result of withdrawals upstream from the gage, 

which were only available as monthly totals. Because only 

monthly totals were available to represent the withdrawals 

upstream, hourly and daily streamflows at the Calallen gage 

during low-flow periods were subject to greater uncertainty 

compared with monthly or annual flows.

Suspended-sediment simulations also were affected 

by uncertainties in parameters associated with rainfall and 

streamflow. For example, overestimation of rainfall parameters (in particular, rainfall intensity) would increase soil 

particle detachment and washoff of sediment from PERLNDs, 

resulting in possible oversimulation of runoff and suspendedsediment concentrations and loads.

How well the model represents basin-wide suspendedsediment yields for various land types is somewhat uncertain. 

Calibration of basin-wide suspended-sediment yields depends 

on reasonable estimates of sediment yields from various 

Table 10. Summary of calibrated values for selected hydrologic parameters for the Hydrological Simulation Program—FORTRAN 

model of the lower Nueces River watershed, South Texas.

[PERLND, pervious land surface; --, none; IMPLND, impervious land surface]

Parameter Land surface Description Value Unit

AGWETP PERLND Fraction of available evapotranspiration from active groundwater 0 --

AGWRC PERLND Base groundwater recession rate .90–.92 1/day

BASETP PERLND Fraction of available evapotranspiration from baseflow .01 --

CEPSC PERLND Interception storage capacity .10–.30 inch

DEEPFR PERLND Fraction of groundwater inflow to deep recharge .20 --

INFEXP PERLND Infiltration equation exponent 2.0 --

INFILD PERLND Ratio of maximum to mean infiltration rate 2.0 --

INFILT PERLND Index to infiltration capacity of soil .20–.30 inch/hour

INTFW PERLND Index to interflow 2.0 --

IRC PERLND Interflow recession coefficient .50 1/day

KVARY PERLND Groundwater outflow modifier 2.0 1/inch

LSUR PERLND or IMPLND Average length of assumed overland-flow plane 250 foot

LZETP PERLND Lower-zone evapotranspiration .2–.80 --

LZSN PERLND Lower-zone nominal storage 3.2–3.4 inch

NSUR PERLND or IMPLND Manning’s n for assumed overland-flow plane .2 --

RETSC IMPLND Retention storage capacity of impervious areas .10 inch

SLSUR PERLND or IMPLND Average slope of assumed overland-flow plane .01–.04 --

UZSN PERLND Upper-zone nominal storage .25–.30 inch
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 33

land types. Several studies of suspended-sediment yields for 

specific land types provided guidance for establishing target 

yields for calibration. The target yields were reasonably simulated by the model—albeit the sediment-yield studies used to 

establish the targets were limited in scope compared with the 

size of the study watershed and variety of land uses and land 

covers in the lower Nueces River watershed. 

Overall, suspended-sediment calibration results were 

very good for monthly and annual suspended-sediment loads. 

Relatively large errors were associated with simulated daily 

suspended-sediment loads (table 8) and with suspendedsediment load estimated for individual samples (table 9).

Estimated Suspended-Sediment Loads to the 

Nueces Estuary, 1958–2008

The calibrated model of the lower Nueces River watershed was used to estimate suspended-sediment loads to the 

Nueces Estuary for 1958–2008, as well as sediment loading 

rates (mass per unit area) and sediment loads simulated for the 

different land types and within different geographic areas. The 

model configuration and parameter values that were used for 

the 2001–08 calibration period were retained for the 1958–

2008 simulation period. The rainfall and potential evapotranspiration data used for modeling 1958–2008 were obtained 

from the same NWS stations used for calibration.

The location chosen for modeling delivery of the 

suspended-sediment load to the Nueces Estuary was USGS 

streamflow-gaging station 08211502 Nueces River near Odem, 

Tex. (hereinafter the Odem gage) (site 11, fig. 10; table 3). The 

Odem gage is located in the tidal reach of the Nueces River at 

Interstate Highway 37, about 1.1 miles downstream from the 

Calallen gage and 0.2 mile downstream from the confluence 

of Hondo Creek and the Nueces River (near the confluence of 

RCHRESs 95 and 96, or the inflow to RCHRES 97; fig. 9). 

Estimated annual suspended-sediment loads, in tons per 

day, transported during 1958–2008 to the Nueces Estuary are 

shown on figure 16 and listed in table 13. The annual mean 

Table 11. Summary of calibrated values for selected sediment-related parameters for the Hydrological Simulation Program—

FORTRAN model of the lower Nueces River watershed, South Texas.

[PERLND, pervious land surface; IMPLND, impervious land surface; RCHRES, stream/reservoir reach]

Parameter Model unit Parameter description Value Unit

KRER PERLND Coefficient of soil-detachment equation 0.07–.45 complex

JRER PERLND Exponent of soil-detachment equation 2.0 complex

KSER PERLND Coefficient of detached-sediment washoff equation .05–.65 complex

JSER PERLND Exponent of detached-sediment washoff equation 2.0 complex

AFFX PERLND Fraction by which detached sediment decreases daily through soil 

compaction .05 1/day

COVER PERLND Fraction of land surface shielded from rainfall erosion .10–.85 none

NVSI PERLND Rate at which sediment enters detached-sediment storage from 

atmosphere .3 pound/acre-day

KEIM IMPLND Coefficient of solids washoff equation .40 complex

JEIM IMPLND Exponent of solids washoff equation 2.0 complex

ACCSDM IMPLND Solids accumulation rate .015 ton/acre-day

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

M (silt) RCHRES Erodibility coefficient of sediment .07–.12 pound/square foot-day

M (clay) RCHRES Erodibility coefficient of sediment .09–.14 pound/square foot-day

W (silt and 

clay) RCHRES Settling velocity of sediment particle in still water .00001–.0005 inch/second

TAUCD 

(silt) RCHRES Critical bed shear stress for sediment deposition .03–.08 pound/square foot

TAUCS 

(silt) RCHRES Critical bed shear stress for sediment scour .10–.22 pound/square foot

TAUCD 

(clay) RCHRES Critical bed shear stress for sediment deposition .015–.04 pound/square foot

TAUCS 

(clay) RCHRES Critical bed shear stress for sediment scour .09–.14 pound/square foot
34 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Table 12. Sensitivity of simulated streamflow volumes and suspended-sediment loads to changes in selected process-related 

parameters for the lower Nueces River watershed model, South Texas.

[DEEPFR, fraction of groundwater inflow to deep recharge; INFILT, index to infiltration capacity of soil; LZSN, lower-zone nominal storage; KRER, coefficient 

of soil-detachment equation; KSER, coefficient of detached-sediment washoff equation; M (silt), erodibility coefficient of silt sediment; M (clay), erodibility 

coefficient of clay sediment]

Parameter Initial value Adjusted value

Change in parameter 

value

(percent)1

Change in streamflow 

volume

(percent)1

Change in suspendedsediment load

(percent)1

DEEPFR 0.20 0.28 40 -0.6 -0.5

INFILT .20–.30 .28–.42 40 -.2 -7.1

LZSN 3.20–3.40 4.48–4.76 40 -1.0 -3.2

KRER .07–.45 .10–.63 40 0 0

KSER .05–.65 .07–.91 40 0 4.9

M (silt) .07–.12 .10–.17 40 0 2.1

M (clay) .09–.14 .13–.20 40 0 1.1

1

 Simulation period for sensitivity analyses, 2001–08. 

1960 1970 1980 1990 2000 2008

0

1,000

2,000

3,000

4,000

ANNUAL MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

0

 500

1,000

1,500

2,000

2,500

3,000

ANNUAL MEAN SUSPENDED-SEDIMENT LOAD, IN TONS PER DAY

Suspended-sediment load

Streamflow

Figure 16. Estimated annual streamflow and suspended-sediment loads to the Nueces Estuary, South Texas, 1958–2008. 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 35

Table 13. Estimated annual streamflows and suspended-sediment loads to the Nueces Estuary, South Texas, 1958–2008.

Year Annual mean streamflow 

(cubic feet per second)

Annual suspended-sediment load 

(tons)

Annual mean suspended-sediment load 

(tons per day)

1958 2,030 387,160 1,060

1959 591 70,100 192

1960 816 334,000 914

1961 431 108,000 296

1962 38 1,390 3.8

1963 58 2,820 7.7

1964 421 50,000 137

1965 529 48,400 133

1966 598 141,000 386

1967 2,740 909,500 2,490

1968 1,040 101,000 277

1969 486 327,000 895

1970 544 60,400 165

1971 3,820 452,910 1,240

1972 452 33,400 91

1973 1,690 292,000 799

1974 572 42,100 115

1975 507 39,000 107

1976 1,500 108,000 296

1977 716 40,000 110

1978 324 24,500 67

1979 586 48,600 133

1980 862 189,000 517

1981 1,620 158,000 433

1982 236 12,500 34

1983 123 11,100 30

1984 144 15,800 43

1985 822 304,000 832

1986 144 4,880 13

1987 1,140 138,000 378

1988 117 61,300 168

1989 91 2,220 6.1

1990 453 22,700 62

1991 276 16,300 45

1992 1,430 177,000 485

1993 290 22,300 61

1994 259 11,600 32

1995 190 11,100 30

1996 111 9,720 27

1997 491 105,000 287

1998 510 67,400 185

1999 252 14,600 40

2000 156 19,500 53

2001 371 29,000 79

2002 3,170 272,000 745

2003 692 35,700 98

2004 1,290 68,200 187

2005 292 31,700 87

2006 79 3,700 10

2007 1,550 101,000 277

2008 64 2,320 6.4

1958–2008 

average annual 739 108,600 297
36 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

suspended-sediment load was highly variable, ranging from 

an estimated 3.8 tons per day in 1962 to 2,490 tons per day 

in 1967. The daily mean suspended-sediment load during 

1958–2008 was an estimated 297 tons per day. The median 

suspended-sediment load was an estimated 133 tons per day.

Estimated annual mean sediment loads at the Nueces 

Estuary (Odem gage) were compared with those estimated at 

the Mathis gage (outflow of Lake Corpus Christi) and at the 

Bluntzer and Calallen gages (table 14). These comparisons 

give an indication of sources of sediment delivered to the 

Nueces River and Nueces Estuary. 

Estimated annual suspended-sediment loads in the lower 

Nueces River simulated by the model for each major source 

of sediment in the study area are listed by category in table 

15. On average, an estimated 307 tons per day of suspended 

sediment were delivered to the lower Nueces River; an estimated 297 tons per day were delivered to the Nueces Estuary. 

Releases from Lake Corpus Christi delivered an estimated 98 

tons per day of suspended sediment or 32 percent of the 307 

tons per day estimated to have been delivered to the lower 

Nueces River. This indicated that, on average, about 209 tons 

per day of sediment, or 68 percent of the estimated 307 tons 

per day total were generated from erosion of land surfaces and 

stream-channel bed and banks. The largest source of sediment 

originating from within the study area was generated from 

cropland, about 117 tons per day, or about 38 percent of the 

total estimated for all sources each year, on average. Erosion 

of stream-channel bed and banks accounted for, on average, 55 

tons per day, or 18 percent of the estimated total suspendedsediment load. All other land categories, except cropland, 

accounted for an estimated 37 tons per day, or 12 percent of 

the total. An estimated 9.6 tons per day of suspended sediment or 3 percent of the suspended-sediment load delivered to 

the lower Nueces River were removed by water withdrawals 

before reaching the Nueces Estuary. 

Model results indicate most of the sediment load in the 

Nueces River consists of silt and clay, defined as particle sizes 

less than 0.0625 millimeter. At the Bluntzer gage, simulated 

silt and clay loads composed about 99 percent of the total 

suspended-sediment load. At the Calallen gage, simulated 

silt and clay loads composed about 98 percent of the total 

suspended-sediment load. 

Annual streamflows and suspended-sediment loads to 

the Nueces River and Nueces Estuary varied depending 

on rainfall and streamflow conditions. Annual suspendedsediment loads delivered to the estuary ranged from 3.8 tons 

per day in 1962 to 2,490 tons per day in 1967. Large rainfall 

and runoff events contributed most of the streamflow and 

suspended-sediment loads to the Nueces River. Whether the 

runoff occurred upstream or downstream from Lake Corpus 

Christi affected the amount of annual suspended-sediment 

loads that were contributed by the each of the main sources 

shown in table 15. During some years with relatively low 

annual average streamflow (1984, 1988, 1989, and 1995) net 

stream-channel erosion was negative, indicating net annual 

sediment deposition to the stream channel. Simulation results 

indicate that the largest sediment loads transported to the 

Nueces Estuary as a result of stream-channel bed and bank 

erosion occurred during years with relatively large annual 

mean streamflows (for example, 1958, 1967, and 1971). 

During low-flow years, relatively large percentages of the total 

suspended sediment transported to the Nueces River were 

removed by water withdrawals. For example, 2008 was a lowflow year; the annual mean streamflow of the Nueces River 

to the Nueces Estuary was 64 cubic feet per second in 2008 

compared with annual median streamflow of 507 cubic feet 

per second measured during 1958–2008, and about 50 percent 

of the total suspended-sediment load was removed in 2008 by 

water withdrawals. 
Simulation of Streamflow and Suspended-Sediment Concentrations and Loads of the Lower Nueces River 37

Table 14. Estimated annual suspended-sediment loads at selected stations, lower Nueces River watershed, South Texas, 1958–2008.

Year

Suspended-sediment load 

(tons per day)

08211000 Nueces River 

near Mathis, Texas

08211200 Nueces River 

at Bluntzer, Texas

08211500 Nueces River 

at Calallen, Texas

08211502 Nueces River 

near Odem, Texas

1958 304 906 1,070 1,060

1959 70 180 200 192

1960 70 630 789 914

1961 45 236 277 296

1962 5.9 8.3 8.1 3.8

1963 6.3 13 13 7.7

1964 46 133 143 137

1965 64 126 141 133

1966 50 315 359 386

1967 616 1,650 2,170 2,490

1968 134 224 279 277

1969 33 602 756 895

1970 61 131 171 165

1971 646 805 1,190 1,240

1972 45 85 102 91

1973 203 545 726 799

1974 61 108 118 115

1975 59 100 120 107

1976 164 261 301 296

1977 93 102 121 110

1978 30 62 68 67

1979 57 106 133 133

1980 111 350 460 517

1981 202 353 416 433

1982 28 28 43 34

1983 9.0 30 31 30

1984 7.6 28 41 43

1985 72 567 709 832

1986 13 18 18 13

1987 142 290 350 378

1988 10 129 145 168

1989 10 8.8 10 6.1

1990 50 63 67 62

1991 18 33 47 45

1992 156 372 444 485

1993 23 58 62 61

1994 19 34 35 32

1995 15 26 33 30

1996 10 27 28 27

1997 33 214 256 287

1998 43 134 166 185

1999 24 38 44 40

2000 10 26 52 53

2001 39 65 80 79

2002 578 569 736 745

2003 90 90 115 98

2004 154 171 192 187

2005 26 74 82 87

2006 6.7 11 13 10

2007 221 221 282 277

2008 5.9 6.8 11.1 6.4

1958–2008 average 

average 98 223 279 297
38 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Table 15. Estimated annual streamflows and suspended-sediment loads, by sediment source, simulated by the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, South Texas, 1958–2008.

Year

Annual mean 

streamflow, 

Nueces River 

to Nueces 

Estuary 

(cubic feet 

per second)1

Suspended-sediment load 

(tons per day)

Inflow associated with 

Lake Corpus 

Christi 

releases

Erosion and 

washoff from 

cropland

Erosion and 

washoff 

from other 

land-cover 

categories

Erosion 

from streamchannel bed 

and banks2

Total 

transported 

to lower 

Nueces 

River3

Removed 

by water 

withdrawals

Total 

transported 

to Nueces 

Estuary4

1958 2,110 304 26 18 729 1,080 16 1,060

1959 617 70 31 17 87 205 12 193

1960 742 70 677 114 74 935 19 916

1961 508 45 140 44 79 308 12 296

1962 38 5.9 0 4.7 5.1 16 12 3.7

1963 58 6.3 .1 7.1 .9 14 6.7 7.7

1964 421 46 15 9.0 77 147 10 137

1965 524 64 6.2 5.0 67 142 10 132

1966 601 50 171 57 122 400 14 386

1967 2,730 616 1,240 416 232 2,500 11 2,490

1968 1,040 135 39 15 99 288 10 278

1969 472 33 668 201 7.3 909 14 895

1970 559 61 39 16 61 177 11 166

1971 3,810 647 240 74 296 1,260 16 1,240

1972 462 45 6.4 6.6 43 101 9.3 92

1973 1,690 203 384 132 95 814 14 800

1974 575 61 22 7.5 31 122 6.8 115

1975 509 59 37 11 12.9 120 13 107

1976 1,480 164 38 13 92 307 11 296

1977 737 93 4.1 3.2 18 118 8.8 109

1978 321 30 18 10 16 74 7.1 67

1979 589 58 37 15 33 143 9.3 133

1980 863 112 304 88 28 532 14 518

1981 1,610 202 124 44 73 443 11 432

1982 238 28 4.3 4.4 4.7 41 7.7 34

1983 115 9.0 19 5.2 4.5 38 7.0 31

1984 139 7.6 32 11 -1.3 49 5.5 44

1985 835 72 567 189 24 852 19 833

1986 131 13 .1 2.5 2.7 18 5.1 13

1987 1,150 142 147 52 47 388 11 377

1988 118 10 155 19 -6 178 10 168

1989 91 10 .2 2.9 -3.4 10 4.0 6.0

1990 454 50 12 4.7 2.3 69 6.6 62

1991 256 18 12 7.9 12.8 51 6.0 45

1992 1,450 156 215 75 50 496 11 485

1993 291 23 24 11 11.4 69 9.0 60
Summary 39

Summary

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

with the U.S. Army Corps of Engineers–Fort Worth District, 

City of Corpus Christi, Guadalupe-Blanco River Authority, 

San Antonio River Authority, and San Antonio Water System, 

developed, calibrated, and tested a Hydrological Simulation 

Program—FORTRAN (HSPF) watershed model to simulate 

streamflow and suspended-sediment concentrations and loads 

during 1958–2008 in the lower Nueces River watershed, 

downstream from Lake Corpus Christi to the Nueces Estuary 

in South Texas. The loss of sediment loading is an important ecological problem in the Nueces River watershed in 

South Texas. The reduction in sediment loads to the Nueces 

Estuary is the result of sedimentation in large impoundments, 

notably Lake Corpus Christi, a reservoir whose storage 

volume was greatly enlarged in 1958 compared to its original 

(1935) impoundment capacity; construction of the 1958 dam 

expanded the existing storage volume of Lake Corpus Christi 

from 43,800 to 257,260 acre-feet. 

Data available to simulate suspended-sediment concentrations and loads consisted of historical sediment data collected 

during 1942–82 by the Texas Water Development Board 

(TDWB) and its predecessor agencies at sites in the study 

area, and suspended-sediment concentration data collected 

periodically by the USGS during 2006–07 at three sites—

USGS streamflow-gaging station 08211000 Nueces River near 

Mathis, Tex. (Mathis gage), station 08211500 Nueces River at 

Bluntzer, Tex. (Bluntzer gage), and station 08211500 Nueces 

River at Calallen, Tex. (Calallen gage). 

Table 15. Estimated annual streamflows and suspended-sediment loads, by sediment source, simulated by the Hydrological Simulation 

Program—FORTRAN model of the lower Nueces River watershed, South Texas, 1958–2008—Continued.

Year

Annual mean 

streamflow, 

Nueces River 

to Nueces 

Estuary 

(cubic feet 

per second)1

Suspended-sediment load 

(tons per day)

Inflow associated with 

Lake Corpus 

Christi 

releases

Erosion and 

washoff from 

cropland

Erosion and 

washoff 

from other 

land-cover 

categories

Erosion 

from streamchannel bed 

and banks2

Total 

transported 

to lower 

Nueces 

River3

Removed 

by water 

withdrawals

Total 

transported 

to Nueces 

Estuary4

1994 254 19 6.4 4.4 7.6 37 5.2 32

1995 193 15 17 7.1 -2.1 37 6.3 31

1996 113 10 14 6.5 1.2 32 5.4 27

1997 491 33 181 49 34 297 10 287

1998 507 43 108 31 9.5 192 6.8 185

1999 255 24 8.4 5.5 7.8 46 5.8 40

2000 154 10 29 10 9.6 59 6.0 53

2001 373 39 20 8.4 19 86 6.6 79

2002 3,160 578 63 16 96 753 8.2 745

2003 703 90 .7 3.9 17.0 112 13 98

2004 1,280 154 14 8.5 20 197 10 187

2005 303 26 44 17 10.0 97 10 87

2006 73 6.7 1.6 3.9 2.2 14 4.4 10

2007 1,550 221 14 7.4 41 283 6.8 276

2008 64 5.9 .7 3.6 2.3 13 6.1 6.4

1958–2008 

average 

annual 741 98 117 37 55 307 9.6 297

1

 Simulated streamflow at Nueces River at Interstate Highway 37, near Corpus Christi, Texas.

2

 Negative number indicates net suspended-sediment deposition to stream reach.

3

 Sum of suspended-sediment loads for inflow associated with Lake Corpus Christi releases, erosion and washoff from cropland, erosion and washoff from 

other land-cover categories, and erosion from stream-channel bed and banks. In some cases rounding might result in discrepancies from the computed sum of 

these categories.

4

 Total suspended-sediment loads transported to lower Nueces River minus loads removed by water withdrawals. In some cases rounding might result in 

discrepancies from the computed difference. 
40 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

During 1942–82, TWDB and its predecessor agencies 

monitored suspended-sediment loads of the Nueces River at a 

site on the Nueces River at the Mathis gage. The Mathis gage 

is downstream from Wesley E. Seale Dam, completed in 1958 

to impound Lake Corpus Christi. Suspended-sediment data 

collected before and after completion of Wesley E. Seale Dam 

provide insights to the effects of the dam and reservoir on suspended-sediment loads transported by the lower Nueces River 

from downstream of the dam to the Nueces Bay. Suspendedsediment loads measured at the Mathis gage decreased after 

completion of the dam in 1958. Annual suspended-sediment 

loads exceeded 100,000 tons during 9 of the 16 years before 

completion of the dam but during only 2 years after completion of the dam. In the 21 years following completion of 

the dam, for which reliable suspended-sediment data were 

available (1962–82), annual suspended-sediment loads were 

considerably lower, for a given annual mean discharge, than 

before the dam was completed.

Most of the suspended sediment transported by the 

Nueces River downstream from Wesley E. Seale Dam 

occurred during high-flow releases from the dam or during 

floods. During October 1964–September 1971, about 532,000 

tons of suspended sediment were transported by the Nueces 

River near Mathis. Of this amount, about 473,000 tons, or 

about 89 percent, were transported by large runoff events 

(mean streamflow exceeding 1,000 cubic feet per second). 

Furthermore, the suspended-sediment transport rate increases 

with higher magnitude flows.

To develop the watershed model to simulate suspendedsediment concentrations and loads in the lower Nueces River 

watershed, streamflow simulations were calibrated and tested 

with available data for 2001–08. Streamflow data measured 

during 2005–08 at the Calallen gage were used to calibrate 

the model and streamflow data for 2001–04 were used to test 

the calibrated model. Streamflow data for 2005–06 from the 

Bluntzer gage were used to test streamflow simulations at 

that site. Streamflow data from the Mathis gage were used 

as input to the model at the upstream boundary of the model. 

Simulated streamflow volumes for the Bluntzer and Calallen 

gages showed good agreement (within 6 percent) with 

measured streamflow volumes. Annual, monthly, and daily 

coefficients of determination of the linear regression between 

measured and simulated streamflow and Nash-Sutcliffe coefficients of model-fit efficiency are considered acceptable for 

both gages.

The HSPF model was calibrated to simulate suspended 

sediment using suspended-sediment data collected at the 

Mathis, Bluntzer, and Calallen gages during 2006–07. Soil 

erosion and sediment washoff from various land types simulated by the model were calibrated by comparing simulated 

suspended-sediment loads from the various land types to 

estimates of suspended-sediment runoff yields determined 

from other studies. Parameters related to sediment transport 

were calibrated primarily by comparing measured suspendedsediment concentrations and loads at the three gages with 

simulated suspended-sediment concentrations and loads. 

The calibrated watershed model was used to estimate 

streamflow and suspended-sediment loads for 1958–2008, 

including suspended-sediment loads transported to the Nueces 

Estuary. During 1958–2008, on average, an estimated 307 tons 

per day of suspended sediment were delivered to the lower 

Nueces River; an estimated 297 tons per day were delivered to 

the estuary. The annual suspended-sediment load was highly 

variable, depending on the occurrence of storm events and 

high streamflows. During 1958–2008, the annual total sediment loads to the estuary varied from an estimated 3.8 to 2,490 

tons per day. On average, 117 tons per day, or about 38 percent 

of the estimated annual suspended-sediment contribution, 

originated from cropland in the study watershed. Releases 

from Lake Corpus Christi delivered an estimated 98 tons per 

day of suspended sediment or about 32 percent of the 307 tons 

per day estimated to have been delivered to the lower Nueces 

River. Erosion of stream-channel bed and banks accounted 

for 55 tons per day or about 18 percent of the estimated total 

suspended-sediment load. All other land categories, except 

cropland, accounted for an estimated 37 tons per day, or about 

12 percent of the total. An estimated 9.6 tons per day of suspended sediment or about 3 percent of the suspended-sediment 

load delivered to the lower Nueces River were removed by 

water withdrawals before reaching the Nueces Estuary. 

Annual streamflows and suspended-sediment loads to 

the Nueces River and Nueces Estuary varied depending on 

rainfall and streamflow conditions. Large rainfall and runoff 

events contributed most of the streamflow and suspendedsediment loads to the Nueces River. Whether the runoff 

occurred upstream or downstream from Lake Corpus Christi 

affected the amount of annual suspended-sediment loads that 

were contributed by inflows or erosion processes. Simulation 

results indicate that the largest sediment loads transported 

to the Nueces Estuary as a result of stream-channel bed and 

bank erosion occurred during high-flow years and that during 

low-flow years, large percentages of total suspended sediment transported to the Nueces River were removed by water 

withdrawals.

References

Adey, E.A., and Cook, H.M., 1964, Suspended-sediment load 

of Texas streams—Compilation report, October 1959–

September 1961: Texas Water Commission Bulletin 6410, 

50 p.

Aronow, S., and Barnes, V.E, 1975, Geologic atlas of Texas—

Corpus Christi sheet, Alva Christine Ellisor memorial 

edition: Austin, University of Texas, Bureau of Economic 

Geology, GA0010, 1 sheet.

Aronow, S., Brown, T.E., Brewton, J.L., Eargle, D.H., and 

Barnes, V.E., 1975, Geologic atlas of Texas—BeevilleBay City sheet, Alexander Deussen memorial edition: 

Austin, University of Texas, Bureau of Economic Geology, 

GA0005, 5 p., 1 sheet.
References 41

Asquith, W.H., 1999, Areal-reduction factors for the precipitation of the 1-day design storm in Texas: U.S. Geological 

Survey Water-Resources Investigations Report 99–4267, 

81 p.

Asquith, W.H., and Roussel, M.C., 2003, Atlas of interoccurrence intervals of selected thresholds of daily precipitation in Texas: U.S. Geological Survey Water-Resources 

Investigations Report 03–4281, 204 p.

Baird, F.C., Jennings, Marshall, Dybala, T.J., and Ockerman, 

D.J., 1996, Characterization of nonpoint sources and loadings to the Corpus Christi Bay National Estuary Program 

study area: Corpus Christi Bay National Estuary Program, 

CCBNEP–05, 225 p.

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

Jr., and Johanson, R.C., 2001, Hydrological Simulation 

Program—FORTRAN, users manual for version 12: 

Research Triangle Park, N.C., U.S. Environmental 

Protection Agency, National Exposure Research Laboratory, 

Office of Research and Development, 843 p.

Bidlake, 2002, Evapotranspiration from selected fallowed 

fields on the Tule Lake National Wildlife Refuge, California, during May to October 2000: U.S. Geological Survey 

Water-Resources Investigations Report 02–4055, 59 p. 

City of Corpus Christi, 2010, City of Corpus Christi official web site—Lake Corpus Christi and Choke Canyon 

Reservoir: accessed May 29, 2010, at http://www.cctexas.

com/?fuseaction=main.view&page=1020.

Cook, H.M., 1967, Suspended-sediment load of Texas 

streams—Compilation report, October 1961–September 

1963: Texas Water Development Board Report 45, 62 p.

Cook, H.M., 1970, Suspended-sediment load of Texas 

streams—Compilation report, October 1963–September 

1965: Texas Water Development Board Report 106, 63 p.

Davis, B.E., 2005, A guide to the proper selection and use of 

federally approved sediment and water-quality samplers: 

U.S. Geological Survey Open-File Report 2005–1087, 26 p. 

Day, J.W., Jr., Pont, D., Hensel, P.F., and Ibanez, C., 1995, 

Impacts of sea-level rise on deltas in the Gulf of Mexico and 

the Mediterranean—The importance of pulsing events to 

sustainability: Estuaries, v. 18, p. 636–647.

Donigian, A.S., Jr., Bicknell, B.R., and Imhoff, J.C., 1995, 

Hydrological Simulation Program—FORTRAN (HSPF), in

Singh, V.P., ed., Computer models of watershed hydrology: 

Highlands Ranch, Colo., Water Resources Publications, 

p. 395–442.

Donigian, A.S., Jr., Imhoff, J.C., Bicknell, B.R., and Kittle, 

J.L., Jr., 1984, Application guide for Hydrological 

Simulation Program—FORTRAN (HSPF): Athens, Ga., 

U.S. Environmental Protection Agency, Environmental 

Research Laboratory, EPA–600/3–84–065, 177 p.

Donigian, A.S., Jr., and Love, J.T., 2003, Sediment calibration procedures and guidelines for watershed modeling, 

in TMDL 2003 Specialty Conference, Chicago, Illinois, 

November 16–18, 2003, Proceedings: Water Environment 

Federation, accessed January 22, 2010, at http://www.hspf.

com/pdf/TMDL2003PaperDonigian.pdf.

Dougherty, J.P., 1979, Suspended-sediment load of Texas 

streams—Compilation report, October 1971–September 

1975: Texas Water Development Board Report 233, 83 p.

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. C2, 89 p. 

ESRI, 2009, ESRI: accessed January 12, 2008, at http://www.

esri.com/.

Fan, H., Huang, H.J., and Zeng, T., 2006, Impacts of anthropogenic activity on the recent evolution of the Huanghe 

(Yellow) River delta: Journal of Coastal Research, v. 22, 

p. 919–929.

Farris, O.A., 1933, Silt load of Texas streams: U.S. Department of Agriculture Technical Bulletin 382, 71 p.

Guy, H.P., 1969, Laboratory theory and methods for sediment 

analysis: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. C1, 58 p.

Handbook of Texas Online, 2009, Mathis, Texas—Facts and 

information: accessed November 27, 2009, at http://www.

tshaonline.org/handbook/online/articles/MM/hfm3.html. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, version 

1.1, accessed online June 14, 2010, at http://pubs.usgs.gov/

twri/twri4a3/.

Homer, C., Huang, C., Yang, L., Wylie, B., and Coan, M., 

2004, Development of a 2001 national land-cover database 

for the United States: Photogrammetric Engineering and 

Remote Sensing, v. 70, no. 7, p. 829–840.

Jaffe, B.E., Smith, R.E., and Torresan, L.Z., 1998, 

Sedimentation and bathymetric change in San Pablo Bay, 

1856–1983: U.S. Geological Survey Open-File Report 

98–759, 11 maps.

Kittle, J.L., Jr., Lumb, A.M., Hummel, P.R., Duda, P.B., and 

Gray, M.H., 1998, A tool for the generation and analysis of 

model simulation scenarios for watersheds (GenScn): U.S. 

Geological Survey Water-Resources Investigations Report 

98–4134, 152 p.

Larken, T.J., and Bomar, G.W., 1983, Climatic atlas of Texas: 

Texas Department of Water Resources, Limited Printing 

Report LP–192, 151 p. 
42 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Leibbrand, N.F., 1987, Estimated sediment deposition in Lake 

Corpus Christi, Texas, 1972–85: U.S. Geological Survey 

Open-File Report 87–239, 26 p.

Linsley, R.K., Kohler, M.A., and Paulhus, J.L., 1982, 

Hydrology for engineers (3d ed.): New York, McGraw-Hill, 

512 p.

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

Users manual for an expert system (HSPEXP) for calibration of the Hydrological Simulation Program—FORTRAN: 

U.S. Geological Survey Water-Resources Investigations 

Report 94–4168, 102 p.

Martin, G.R., Zarriello, P.J., and Shipp, A.A., 2001, 

Hydrologic and water-quality characterization and modeling 

of the Chenowith Run Basin, Jefferson County, Kentucky: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4239, 197 p.

Minzenmayer, F.E., 1979, Soil survey of Jim Wells County, 

Texas: U.S. Department of Agriculture, Natural Resources 

Conservation Service (formerly Soil Conservation Service), 

111 p, 50 map sheets.

Mirabal, J., 1974, Suspended-sediment load of Texas 

streams—Compilation report, October 1965–September 

1971: Texas Water Development Board Report 184, 121 p.

Morton, R.A., and Paine, J.G., 1984, Historical shoreline changes in Corpus Christi, Oso, and Nueces Bays, 

Texas Gulf Coast: Austin, University of Texas, Bureau of 

Economic Geology Circular 84–6, 66 p.

Multi-Resolution Land Characteristics Consortium, 2008, 

National land cover database 2001: accessed June 2, 2009, 

at http://www.mrlc.gov/nlcd.php.

Nash, J.E., and Sutcliffe, J.V., 1970, River flow forecasting 

through conceptual models, part 1—A discussion of principles: Journal of Hydrology, v. 10, no. 3, p. 282–290.

National Climatic Data Center, 2009, Comparative climatic 

data for the United States through 2008: Asheville, N.C., 

National Oceanic and Atmospheric Administration, 

National Environmental Satellite, Data and Information 

Service, accessed December 13, 2009, at http://www1.ncdc.

noaa.gov/pub/data/ccd-data/CCD-2008.pdf.

Nueces River Authority, 2009, Water usage summary: 

accessed June 27, 2009, at http://www.nueces-ra.org/CP/

CITY/pipeline.php.

Ockerman, D.J., 2001, Water budget for the Nueces Estuary, 

Texas, May–October 1998: U.S. Geological Survey Fact 

Sheet 081–01, 6 p.

Ockerman, D.J., 2002, Hydrologic conditions and quality of 

rainfall and storm runoff in agricultural and rangeland areas 

in San Patricio County, Texas, 2000–2001: U.S. Geological 

Survey Open-File Report 02–291, 20 p., 1 pl.

Ockerman, D.J., and Fernandez, C.J., 2010, Hydrologic 

conditions and water quality of rainfall and storm runoff for 

two agricultural areas of the Oso Creek watershed, Nueces 

County, Texas, 2005–08: U.S. Geological Survey Scientific 

Investigations Report 2010–5136, 63 p. 

Ockerman, D.J., and Petri, B.L., 2001, Hydrologic conditions 

and water quality in an agricultural area in Kleberg and 

Nueces Counties, Texas, 1996–98: U.S. Geological Survey 

Water-Resources Investigations Report 01–4101, 36 p., 

2 app.

Ockerman, D.J., and Roussel, M.C., 2009, Simulation of 

streamflow and water quality in the Leon Creek watershed, 

Bexar County, Texas, 1997–2004: U.S. Geological Survey 

Scientific Investigations Report 2009–5191, 50 p. 

Ott, R.L., and Longnecker, M., 2001, An introduction to statistical methods and data analysis (5th ed.): Pacific Grove, 

Calif., Duxbury, 1,152 p.

Phillips, J.D., Slattery, M.C., and Musselman, Z.A., 2005, 

Channel adjustments of the lower Trinity River, Texas, 

downstream from Livingston Dam: Earth Surface Processes 

and Landforms, v. 30, p. 1,419–1,439.

Porterfield, George, 1972, Computation of fluvial-sediment 

discharge: U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. C3, 66 p.

Quincy, R.M., 1988, Suspended sediment load of Texas 

streams—Compilation report, October 1975–September 

1982: Texas Water Development Board Report 306, 153 p.

Raines, T.H., 1996, Simulation of storm peaks and storm volumes for selected subbasins in the West Fork Trinity River 

Basin, Texas, water years 1993–94: U.S. Geological Survey 

Water-Resources Investigations Report 96–4110, 41 p.

Rantz, S.E., and others, 1982, Measurement and computation 

of streamflow—Volume 1. Measurement of stage and discharge: U.S. Geological Survey Water-Supply Paper 2175, 

284 p. 

Salant, N.L., Renshaw, C.E., and Magilligan, F.J., 2006, Shortand long-term changes to bed mobility and bed composition 

under altered sediment regimes: Geomorphology, v. 76, 

p. 43–53.

Singh, V.P., 1995, Watershed modeling, in Singh, V.P., ed., 

Computer models of watershed hydrology: Highlands 

Ranch, Colo., Water Resources Publications, p. 1–22.

StatSoft, Inc., 2010, Electronic statistics textbook: accessed 

June 5, 2010, at http://statsoft.com/textbook/.

Stout, I.M., Bentz, L.C., and Ingram, H.W., 1961, Silt load 

of Texas streams—A compilation report, June 1889–

September 1959: Texas Board of Water Engineers Bulletin 

6108, 236 p.
References 43

Texas Parks and Wildlife Department, 2007, Plant guidance 

by ecoregions—Ecoregion 6–South Texas Brush Country: 

accessed June 6, 2010, at http://www.tpwd.state.tx.us/

huntwild/wild/wildscapes/guidance/plants/ecoregions/

ecoregion_6.phtml.

Texas Water Development Board, 2002, Volumetric survey of Lake Corpus Christi Reservoir: accessed May 

29, 2010, at http://www.twdb.state.tx.us/hydro_survey/

CorpusChrisrti2002/CorpusChristi_rpt.pdf.

U.S. Army Corps of Engineers, 2009, Nueces River Basin 

Reconnaissance Study home page: accessed on May 23, 

2009 at http://www.swf.usace.army.mil/pubdata/notices/

nueces_river_study.asp.

U.S. Bureau of Reclamation, 2000, Rincon Bayou 

Demonstration Project, concluding report—Volume II, 

Findings: Austin, Tex., Oklahoma-Texas Area Office, 299 p.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2009, Soil Survey Geographic 

(SSURGO) database for Texas: accessed July 23, 2009, at 

http://soildatamart.nrcs.usda.gov/Survey.aspx?State=TX.

U.S. Environmental Protection Agency, 2006, Sediment 

parameter and calibration guidance for HSPF: U.S. 

Environmental Protection Agency, EPA BASINS Technical 

Note 8, accessed October 15, 2009, at http://www.epa.gov/

waterscience/basins/docs/tecnote8.pdf.

U.S. Geological Survey, 2001, Rocky Mountain Mapping 

Center—Elevation program: accessed October 24, 2007, at 

http://rockyweb.cr.usgs.gov/elevation/index.html.

U.S. Geological Survey, 2006, National field manual for 

the collection of water-quality data—Collection of water 

samples (ver. 2.0): U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 9, chap. A4, accessed 

January 15, 2010, at http://pubs.water.usgs.gov/twri9A4/.

U.S. Geological Survey, 2009, National Hydrography Dataset 

data availability: accessed July 3, 2009, at http://nhd.usgs.

gov/data.html.

U.S. Geological Survey, 2010, National Water Information 

System (NWISWeb) data available on the World Wide Web: 

accessed September 24, 2010, at http://waterdata.usgs.gov/

tx/nwis/nwis. 

Vericat, Damia, Batalla, R.J., and Garcia, Celso, 2006, 

Breakup and reestablishment of the armour layer in a 

large gravel-bed river below dams—The lower Ebro: 

Geomorphology, v. 76, p. 122–136.

Welborn, C.T., 1967, Comparative results of sediment sampling with the Texas sampler and the depth-integrating 

samplers and specific weight of fluvial sediment deposits in 

Texas: Texas Water Development Board Report 36, 109 p.

White, W.A., and Calnan, T.C., 1990, Sedimentation in 

fluvial-deltaic wetlands and estuarine areas, Texas Gulf 

Coast—Literature synthesis: The University of Texas at 

Austin, Bureau of Economic Geology, report to Texas Parks 

and Wildlife Department, 261 p.

White, W.A., and Calnan, T.C., 1991, Submergence of vegetated wetlands in fluvial-deltaic areas, Texas Gulf Coast, 

in Coastal depositional systems in the Gulf of Mexico—

Quaternary framework and environmental issues: Houston, 

Society of Economic Paleontologists and Mineralogists, 

Gulf Coast Section, 12th Annual Research Conference 

Program with Extended Abstracts, p. 278–279.

White, W.A., Morton, R.A., and Holmes, C.W., 2002, A 

comparison of factors controlling sedimentation rates and 

wetland loss in fluvial-deltaic systems, Texas Gulf Coast: 

Geomorphology, v. 44, p. 47–66. 

Wicklein, S.M., and Schiffer, D.M., 2002, Simulation of runoff and water quality for 1990 and 2008 land-use conditions 

in the Reedy Creek watershed, east-central Florida: U.S. 

Geological Survey Water-Resources Investigations Report 

02–4018, 221 p.

Williams, G.P., and Wolman, M.G., 1984, Downstream 

effects of dams on alluvial rivers: U.S. Geological Survey 

Professional Paper 1286, 83 p.

Yang, Z., Wang, H., Saito, Y., Milliman, J.D., Xu, K., Qiao, 

S., and Shi, G., 2006, Dam impacts on the Changjiang 

(Yangtze) River sediment discharge to the sea—The past 

55 years and after the Three Gorges Dam: Water Resources 

Research, v. 42, no. W04407, 10 p., accessed June 14, 2010, 

at http://www.agu.org/journals/wr/wr0604/2005WR003970/.

Yeager, K.M., Santschi, P.H., Phillips, J.D., and Herbert, B.E., 

2005, Suspended sediment sources and tributary effects in 

the lower reaches of a coastal plain stream as indicated by 

radionuclides, Loco Bayou, Texas: Environmental Geology, 

v. 47, p. 382–395.

Yeager, K.M., Santschi, P.H., Schindler, K.J., Andres, M.J., 

and Weaver, E.A., 2006, The relative importance of terrestrial versus marine sediment sources to the Nueces-Corpus 

Christi Estuary, Texas—An isotopic approach: Estuaries and 

Coasts, v. 29, p. 443–454.

Zarriello, P.J., and Ries, K.G., III, 2000, A precipitation-runoff 

model for analysis of the effects of water withdrawals on 

streamflow, Ipswich River Basin, Massachusetts: U.S. 

Geological Survey Water-Resources Investigations Report 

02–4018, 99 p.
Blank Page
Appendix 1—Hydrological Simulation Program—FORTRAN 

(HSPF) Parameters Used to Simulate Hydrologic and Sediment 

Processes in the Lower Nueces River Watershed, South Texas

Appendix 1
Blank Page
Appendix 1 47

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas.

[--, none; HSPF, Hydrological Simulation Program—FORTRAN]

Pervious Land (PERLND)

Secondary 

module Parameter Unit Description

Water balance

Interception storage

PWATER CEPSC inch Interception storage capacity 

CEPS inch Initial interception storage

Surface and subsurface storages

UZSN inch Upper-zone nominal storage; an index to amount of depression and surface-layer storage of a 

pervious area

LZSN inch Lower-zone nominal storage; an index to soil-moisture-holding capacity

SURS inch Initial surface storage

IFWS inch Initial interflow storage

UZS inch Initial upper-zone storage

LZS inch Initial lower-zone storage

AGWS inch Initial active-groundwater storage

Evapotranspiration

LZETP -- Lower-zone evapotranspiration; an index to density of deep-rooted vegetation on a pervious area

AGWETP -- Fraction of available potential evapotranspiration demand from active groundwater

BASETP -- Fraction of available potential evapotranspiration demand from baseflow (groundwater outflow)

Recession rates

KVARY 1/inch Groundwater outflow modifier; an index of how much effect recent recharge has on groundwater 

outflow

AGWRC 1/day Basic groundwater recession rate if KVARY is zero and there is no inflow to groundwater 

IRC 1/day Interflow recession coefficient

GWVS inch Index to groundwater slope

Infiltration

INFILT inch/hour Index to infiltration capacity of soil

INFILD -- Ratio of maximum to mean infiltration rate of a pervious area

INFEXP -- Infiltration equation exponent

INTFW -- Index to amount of water that infiltrates and flows as interflow (shallow subsurface runoff)

DEEPFR -- Fraction of groundwater inflow to deep recharge
48 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas—Continued.

Pervious Land (PERLND)—Continued

Secondary 

module Parameter Unit Description

Water balance—Continued

Overland flow

LSUR foot Average length of overland-flow plane

SLSUR -- Average slope of overland-flow plane

NSUR -- Average roughness coefficient of overland-flow plane

Soil erosion

SEDMNT SMPF -- Management factor to account for use of erosion-control factors

KRER complex Coefficient of soil-detachment equation

JRER complex Exponent of soil-detachment equation

AFFIX 1/day Fraction by which detached sediment decreases daily through soil compaction

COVER -- Fraction of land surface shielded by vegetation or mulch from erosion by direct rainfall impact

NVSI pound/acre/

day

Rate at which sediment enters detached-sediment storage from atmosphere

KSER complex Coefficient of detached-sediment washoff equation

JSER complex Exponent of detached-sediment washoff equation

KGER complex Coefficient of matrix soil scour equation

JGER complex Exponent of matrix soil scour equation 
Appendix 1 49

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas—Continued.

Impervious Land (IMPLND)

Secondary 

module Parameter Unit Description

Water balance

IWATER LSUR foot Average length of overland-flow plane

SLSUR -- Average slope of overland-flow plane

NSUR -- Average roughness coefficient of overland-flow plane

RETSC inch Retention storage capacity of impervious areas

RETS inch Initial retention storage

SURS inch Initial overland-flow storage

Sediment washoff

SOLIDS KEIM complex Coefficient of solids washoff equation

JEIM complex Exponent of solids washoff equation

REMSDP 1/day Fraction of solids removed on each day without runoff

ACCSDP ton/acredaySolids accumulation rate

SLDS ton/acre Initial storage of solids

Stream Reaches (RCHRES)

Secondary 

module Parameter Unit Description

Water balance

HYDR FTABNO -- Number of F-table that contains RCHRES geometric and hydraulic properties

LEN mile Length of stream reach

DELTH foot Drop in water elevation within stream reach

STCOR foot Correction in reach depth to calculate stage

KS -- Weighting factor for flow routing

DB50 millimeter Median diameter of bed sediment

ADCALC CRRAT -- Ratio of maximum velocity to mean velocity in reach cross section under typical flow conditions
50 Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed

Appendix 1. Hydrological Simulation Program—FORTRAN parameters used to simulate the hydrologic and sediment processes in the 

lower Nueces River watershed, South Texas—Continued.

Stream Reaches (RCHRES)—Continued

Secondary 

module Parameter Unit Description

Sediment transport

SEDTRN BEDWID foot Width of streambed

BEDWRN foot Depth of streambed

POR -- Porosity of streambed

D inch Effective diameter of sediment particle

W inch/second Settling velocity of sediment particle in still water

RHO gram/cubic centimeterDensity of sediment particle

KSAND complex Coefficient of HSPF sand-load equation

EXPSND complex Exponent of HSPF sand-load equation

TAUCD pound/square foot Critical bed shear stress for sediment deposition

TAUCS pound/square foot Critical bed shear stress for sediment scour

M pound/square 

foot-day

Erodibility coefficient of sediment

BEDDEP foot Initial thickness of bed material

SSAND milligram/liter Initial concentration of sand in suspension

SSILT milligram/liter Initial concentration of silt in suspension

SCLAY milligram/liter Initial concentration of clay in suspension

FRACSAND -- Initial fraction by weight of sand in bed material

FRACSILT -- Initial fraction by weight of silt in bed material

FRACCLAY -- Initial fraction by weight of clay in bed material

Publishing support provided by

Lafayette Publishing Service Center

Information regarding water resources in Texas is available at 

http://tx.usgs.gov/
Ockerman and Heitmuller—Simulation of Streamflow and Suspended-Sediment Concentrations and Loads in the Lower Nueces River Watershed—SIR 2010–5194

Printed on recycled paper

I SBN 978- 1- 4113- 3001- 6

9 7 8 1 4 1 1 3 3 0 0 1 6