of the Arroyo Colorado, Texas, 

1989–99

Water-Resources Investigations Report 02–4110

U.S. Department of the Interior

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

U.S. Geological Survey

Simulation of Flow and Water Quality 

of the Arroyo Colorado, Texas, 

1989–99

By Timothy H. Raines and Roger M. Miranda

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02–4110

In cooperation with the Texas Natural Resource Conservation Commission 

and the Nueces River Authority

Austin, Texas

2002
ii

U.S. DEPARTMENT OF THE INTERIOR

Gale A. Norton, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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

descriptive purposes only and does not imply 

endorsement by the U.S. Government.

For additional information write to

District Chief

U.S. Geological Survey

8027 Exchange Dr.

Austin, TX 78754–4733

E-mail: dc_tx@usgs.gov

Copies of this report can be purchased from

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225–0286

E-mail: infoservices@usgs.gov
CONTENTS iii

CONTENTS

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

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

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

Description of Study Area ........................................................................................................................................ 3

Description of Simulation Model ............................................................................................................................ 4

Acknowledgments .................................................................................................................................................... 5

Simulation of Flow and Water Quality of the Arroyo Colorado .......................................................................................... 5

Model Setup .............................................................................................................................................................. 5

Calibration and Testing of Flow ............................................................................................................................... 9

Error Analysis of Flow ............................................................................................................................................. 12

Calibration and Testing of Water Quality ................................................................................................................. 18

Error Analysis of Water Quality ............................................................................................................................... 26

Simulated Point- and Nonpoint-Source Loads ......................................................................................................... 26

Summary .............................................................................................................................................................................. 35

References Cited .................................................................................................................................................................. 35

FIGURES

1–6. Maps showing:

1. Arroyo Colorado Basin, Texas ................................................................................................................... 2

2. Arroyo Colorado subbasins and locations of selected streamflow-gaging and precipitation stations 

and water-quality sampling sites ................................................................................................................ 6

3. Locations of point-source discharges in the Arroyo Colorado subbasins ................................................. 7

4. Irrigation districts in and adjacent to the Arroyo Colorado Basin ............................................................. 8

5. Land use in the Arroyo Colorado Basin, 1995 .......................................................................................... 10

6. Major soil groups in the Arroyo Colorado Basin ...................................................................................... 11

7–11. Graphs showing measured and simulated daily streamflow at Arroyo Colorado at: 

 7. Weslaco, 1989–95 ...................................................................................................................................... 13

 8. Harlingen, 1989–95 ................................................................................................................................... 14

9. Mercedes, 1989–95 .................................................................................................................................... 15

10. Mercedes, 1996–99 .................................................................................................................................... 16

11. Harlingen, 1996–99 ................................................................................................................................... 17

12–25. Graphs showing measured and simulated concentrations (or appropriate measure) at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet: 

12. Suspended sediment, 1989–95 .................................................................................................................. 19

13. Water temperature, 1989–95 ...................................................................................................................... 20

14. Biochemical oxygen demand, 1989–95 ..................................................................................................... 21

15. Dissolved oxygen, 1989–95 ....................................................................................................................... 22

16. Nitrate nitrogen, 1989–95 .......................................................................................................................... 23

17. Ammonia nitrogen, 1989–95 ..................................................................................................................... 24

18. Orthophosphate, 1989–95 .......................................................................................................................... 25

 19. Suspended sediment, 1996–99 .................................................................................................................. 27

 20. Water temperature, 1996–99 ...................................................................................................................... 28

21. Biochemical oxygen demand, 1996–99 ..................................................................................................... 29

 22. Dissolved oxygen, 1996–99 ....................................................................................................................... 30

23. Nitrate nitrogen, 1996–99 .......................................................................................................................... 31

24. Ammonia nitrogen, 1996–99 ..................................................................................................................... 32

25. Orthophosphate, 1996–99 .......................................................................................................................... 33

26. Pie diagrams showing simulated total point- and nonpoint-source loads in percent by land-use type for 

selected properties and constituents, Arroyo Colorado Basin, 1989–99 .............................................................. 34
iv 

TABLES

1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN ............................... 37

2. Basin-related model parameters for the Hydrological Simulation Program—FORTRAN .................................. 41

3. Selected physical and hydrologic characteristics of the Arroyo Colorado subbbasins ........................................ 41

4. Permitted point-source daily effluent limits, Arroyo Colorado Basin .................................................................. 42

5. Irrigation schedule for row crops, citrus, and sugar cane for dry, normal, and wet years, Arroyo Colorado 

Basin, 1989–99 ..................................................................................................................................................... 42

6. Basin-related parameters, Arroyo Colorado Basin ............................................................................................... 43

7. Annual PERLND and IMPLND process-related parameters for flow, Arroyo Colorado Basin .......................... 44

8. Monthly PERLND and IMPLND process-related parameters for flow, Arroyo Colorado Basin ........................ 45

9. Selected calibration and testing results using HSPEXP, Arroyo Colorado Basin ................................................ 46

10. Annual PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin ............ 47

11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin .......... 48

12. Annual RCHRES process-related parameters for water quality, Arroyo Colorado Basin .................................... 54

13. Selected nonpoint-source event-mean concentrations for calibration and testing of water quality, Arroyo 

Colorado Basin ..................................................................................................................................................... 56
Abstract 1

Abstract

A model parameter set for use with the 

Hydrological Simulation Program—FORTRAN 

watershed model was developed to simulate flow 

and water quality for selected properties and constituents for the Arroyo Colorado from the city of 

Mission to the Laguna Madre, Texas. The model 

simulates flow, selected water-quality properties, 

and constituent concentrations. The model can be 

used to estimate a total maximum daily load for 

selected properties and constituents in the Arroyo 

Colorado. The model was calibrated and tested for 

flow with data measured during 1989–99 at three 

streamflow-gaging stations. The errors for total 

flow volume ranged from -0.1 to 29.0 percent, and 

the errors for total storm volume ranged from -15.6 

to 8.4 percent. The model was calibrated and tested 

for water quality for seven properties and constituents with 1989–99 data. The model was calibrated 

sequentially for suspended sediment, water temperature, biochemical oxygen demand, dissolved 

oxygen, nitrate nitrogen, ammonia nitrogen, and 

orthophosphate. The simulated concentrations of 

the selected properties and constituents generally 

matched the measured concentrations available for 

the calibration and testing periods. The model was 

used to simulate total point- and nonpoint-source 

loads for selected properties and constituents for 

1989–99 for urban, natural, and agricultural landuse types. About one-third to one-half of the biochemical oxygen demand and nutrient loads are 

from urban point and nonpoint sources, although 

only 13 percent of the total land use in the basin is 

urban.

INTRODUCTION

The Arroyo Colorado is located in the Lower Rio 

Grande Valley of South Texas and extends from near 

Mission, Tex., eastward to the Laguna Madre (fig. 1). 

Streamflow in the Arroyo Colorado primarily is sustained by effluent from municipal wastewater-treatment 

plants. Additional streamflow results from irrigation 

return flow, storm runoff, and other point-source 

discharges. The Arroyo Colorado is used as a floodway, 

an inland waterway, and a recreational area for swimming, boating, and fishing, and is an important nursery 

and foraging area for shrimp, crab, and several types of 

marine fish.

The Texas Natural Resource Conservation Commission (TNRCC) has classified two reaches of the 

Arroyo Colorado on the basis of the physical characteristics of the stream (fig. 1). Segment 2201, from the port 

of Harlingen to the confluence with the Laguna Madre, 

is tidally influenced and has designated uses of contact 

recreation and high aquatic life. The nontidal segment 

of the Arroyo Colorado, Segment 2202, has designated 

uses of contact recreation and intermediate aquatic life. 

The tidal segment of the Arroyo Colorado, Segment 

2201, has failed to meet the water-quality criteria 

required for its designated uses and is included on the 

State 303(d) list of impaired water bodies for dissolved 

oxygen (DO) levels below the criteria specified in the 

Texas Surface-Water-Quality Standards (Texas Natural 

Resource Conservation Commission, 1997).

A total maximum daily load (TMDL) must be 

determined for the Arroyo Colorado as required by 

the 1972 Federal Clean Water Act because Segment 

2201 is identified on the 303(d) list for failure to meet 

the DO water-quality standards. The TMDL process is 

1 U.S. Geological Survey

2 Texas Natural Resource Conservation Commission

Simulation of Flow and Water Quality of the Arroyo 

Colorado, Texas, 1989–99

By Timothy H. Raines1 and Roger M. Miranda2
2 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Figure 1. Arroyo Colorado Basin, Texas.

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator projectionZone 14

ARROYO COLORADO BASIN

Segment 2202 Segment 2201

HIDALGO

WILLACY

CAMERON

UNITED STATES

MEXICO

TEXAS

LOCATION MAP

Area enlarged

0 10 5 MILES

RIO GRANDE

Arroyo Colorado

Cayo Atascoso

North Floodway

Elsa

Donna

Alamo

Pharr

Alton

Lyford

Hidalgo

Bayview

Weslaco

Primera

Mission

Penitas

La Joya La Homa

Edcouch

Progreso

La Feria

Mercedes

San Juan

Palmview McAllen

Edinburg

Mila Doce

Harlingen

Rio Hondo

Sebastian

San Benito

Lopezville

Perezville

Santa Rosa

Brownsville

Indian Lake

Los Fresnos

Rangerville

Palm Valley

Rancho Viejo

Raymondville

Progreso Lakes

98 ̊30' 98 ̊ 97 ̊30'

26 ̊30'

26 ̊
INTRODUCTION 3

designed to specify how existing and future loadings 

need to be allocated to meet water-quality standards. 

Quantitatively, a TMDL is the sum of all existing and 

future point-source waste-load allocations, existing and 

future nonpoint-source pollutant or background loadings, and a margin of safety to account for loading 

uncertainty.

Simulation models typically are used to estimate 

TMDLs because the models are developed to represent 

the cause-and-effect relations between natural inputs to 

an aquatic ecosystem and the resulting water quality. 

Several best-management practice (BMP) alternatives 

can be evaluated objectively using simulation models to 

determine what changes will be needed to meet the 

water-quality standards.

The U.S. Geological Survey, in cooperation 

with the TNRCC and the Nueces River Authority, 

began a study in 1998 to simulate the flow and the water 

quality of selected constituents in the Arroyo Colorado. 

Working jointly with the TNRCC, specific objectives of 

the study were to (1) set up a simulation model of the 

Arroyo Colorado Basin that would allow different 

BMPs to be used by the TNRCC to estimate the TMDL 

for selected constituents to meet DO water-quality 

standards, and (2) calibrate and test a set of processrelated model parameters with available streamflow and 

water-quality data for the Arroyo Colorado.

Purpose and Scope

This report describes the setup, calibration, and 

testing of a model to simulate the flow and water quality 

of the Arroyo Colorado. The basin was subdivided into 

12 subbasins—four in Segment 2201 and eight in Segment 2202. The basin was characterized by a set of 26 

pervious land segments that were defined on the basis of 

types of land use and soil and a set of three impervious 

land segments that were defined on the basis of types of 

land use only. Eight reaches were defined for the eight 

subbasins in the non-tidal segment. No reaches were 

used for the tidal segment. Six basin-related parameters 

were defined for each stream reach. For flow, a total of 

18 process-related parameters were defined and calibrated for each land segment. For water quality, a total 

of 31 process-related parameters were defined and calibrated for each land segment, and 75 process-related 

parameters were defined and calibrated for each stream 

reach.

Eleven years (1989–99) of precipitation, evaporation, air temperature, dewpoint temperature, wind 

speed, cloud cover, solar radiation, streamflow, and 

water-quality data were used for model calibration and 

testing. Precipitation data from three stations were used; 

evaporation data from one station were used; streamflow data from three stations were used; and suspended 

sediment, biochemical oxygen demand (BOD), DO, 

nitrate nitrogen, ammonia nitrogen, and orthophosphate 

from three stations were used. Additional streamflow 

and water-quality data were collected at several locations along the Arroyo Colorado during low flow in 

June 1998 and during high flow after a storm in March 

1999.

Description of Study Area

The study area is in South Texas in the Lower Rio 

Grande Valley (fig. 1). The Arroyo Colorado flows 

about 90 miles from west to east and drains about 700 

square miles, excluding the North Floodway. The basin 

is located in parts of Hidalgo, Cameron, and Willacy 

Counties.

The study area is located in the neotropical Southern Coastal Plain physiographic region and is characterized by long, hot summers and short, mild winters. The 

mean annual temperature is 73 degrees Fahrenheit (°F) 

with mean monthly temperatures ranging from 58 °F in 

January to 84 °F in July. Mean annual evaporation in the 

study area is about 58 inches. Mean annual precipitation 

ranges from about 21 to 27 inches, generally from west 

to east, in the basin (National Oceanic and Atmospheric 

Administration, 1996). Most of the annual precipitation 

results from frontal storms and tropical storms. Widely 

scattered convective thunderstorms also produce precipitation during the summer.

The soils are clays, clay loams, and sandy loams. 

The major soil series comprise the Harlingen, Hidalgo, 

Mercedes, Raymondville, Rio Grande, and Willacy 

(U.S. Department of Agriculture, Soil Conservation 

Service, 1977, 1981–82). Most soil depths range from 

about 63 to 78 inches. The Harlingen, Mercedes, and 

Raymondville soil series consist predominantly of clay 

soils with low permeability classified as hydrologic soil 

group D. A representative soil profile consists of about 
4 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

71 to 78 inches of clay. The Hidalgo, Rio Grande, and 

Willacy soil series consist predominantly of sandy loam 

and sandy clay loam soils with moderate permeability 

classified as hydrologic soil group B. A representative 

soil profile consists of about 14 to15 inches of sandy 

loam overlying 48 to 60 inches of sandy clay loam.

The flat terrain is extensively cultivated and irrigated for agriculture. Water for irrigation is taken from 

the Rio Grande and moved through canals to the fields 

by numerous irrigation districts located in the basin. 

The irrigation districts provide water for irrigation of 

citrus, sugar cane, and several types of row crops 

including corn, grain sorghum, and cotton. Irrigation 

practices consist of flooding fields with a specified 

depth of water during periods of insufficient precipitation to produce desired crop yields.

Urbanization is extensive in the areas directly 

adjacent to the main stem of the Arroyo Colorado, particularly in the western and central parts of the basin. 

Principal urban areas include the cities of Mission, 

McAllen, Pharr, Donna, Weslaco, Mercedes, Harlingen, 

and San Benito (fig. 1). Recent (1995) 1:24,000-scale 

land-use data were obtained for this study from highresolution aerial photography (M.P. Stier, U.S. Geological Survey, written commun., 1999).

Of the 21 permitted dischargers in the Arroyo 

Colorado Basin, 16 are municipal, three are industrial, 

and two are shrimp farms. The discharge permit limits 

of the municipal plants range from 0.4 to 10 million gallons per day. The shrimp farms discharge infrequently.

Description of Simulation Model

The Hydrological Simulation Program—

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

continuous-simulation model using a conceptual framework to represent infiltration, evaporation, interception 

storage, surface runoff, interflow, and base flow on a 

pervious land segment (PERLND) and retention storage 

and surface runoff on an impervious land segment 

(IMPLND). Each user-defined land segment represents 

its own unique hydrologic response system on the basis 

of land cover, soil type, watershed slope, or other basin 

characteristic. These land segments do not need to be 

contiguous. The runoff from each land segment is 

moved through a system of channel or reservoir reaches 

(RCHRES) using storage routing. In addition to runoff, 

water-quality concentrations for several constituents 

can be simulated for each land segment and reach.

HSPF uses input from three types of data: time 

series, process-related parameters, and basin-related 

parameters. At a minimum, continuous time series of 

precipitation and potential evaporation are needed for 

model simulations. Point-precipitation data, measured 

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

segment. Potential evaporation data can be estimated 

from measured pan evaporation or from minimum and 

maximum air temperatures. Time series of measured 

streamflow are needed for model calibration and testing. Time series of air temperature, dewpoint temperature, wind speed, cloud cover, and solar radiation are 

needed for simulation of water-quality constituents. 

Measured water-quality data for selected constituents 

are needed for calibration and testing.

Eighteen process-related parameters are used for 

flow and 106 process-related parameters are used for 

water quality for suspended sediment, water temperature, BOD, DO, nitrate nitrogen, ammonia nitrogen, 

and orthophosphate (table 1, at end of report). The 

18 process-related parameters for flow represent the 

physical processes of infiltration, evapotranspiration, 

interception storage, surface runoff, interflow, and base 

flow for each land segment, and the 106 process-related 

parameters for water quality represent the physical processes of buildup and washoff of constituents, sediment 

transport, heat flux, BOD and DO kinetics, and the 

nutrient cycle. Some of the process-related parameters 

can be varied by month to represent seasonal variations. 

The HSPF users manual (Bicknell and others, 1997) 

provides a more complete description of each parameter. The process-related model parameters are adjusted 

to calibrate the model.

The six basin-related model parameters define the 

areal extent of each land segment, the reach length, and 

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

discharge as a function of depth for each reach of the 

basin (table 2, at end of report). These parameters represent the physical characteristics of each subbasin and 

associated reach and remain unchanged during model 

calibration and testing. These parameters can be 

changed to represent BMPs or other changes to the 

physical characteristics of the basin.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 5

One set of process-related model parameters was 

developed to account for all unique land segments to 

provide confidence in the model results. The model 

parameters were calibrated with 1989–95 data for two 

stations and were tested temporally with 1996–99 data 

for the two stations. The model parameters were tested 

spatially with 1989–99 data for an additional station. 

The model calibration and testing of flow was facilitated by using an expert system interface developed 

by Lumb and others (1994) that provided graphics, error 

statistics, and advice on which parameters to modify. 

The model calibration and testing of water-quality 

constituents was done using Generation and Analysis of 

Model Simulation Scenarios (GenScn) developed by 

Kittle and others (1998).

Acknowledgments

The members of the Arroyo Colorado TMDL 

Science and Technical Advisory Committee provided 

valuable comments and suggestions during the course 

of the study. The streamflow data were provided by 

the International Boundary and Water Commission 

(IBWC). Tom Jobes of Aqua Terra Consultants provided valuable support and guidance during the setup 

and calibration of the HSPF water-quality modules.

SIMULATION OF FLOW AND WATER 

QUALITY OF THE ARROYO COLORADO

Time series of precipitation, evaporation, streamflow, point-source, and irrigation data were used for 

model input and calibration. Land-use and soils data 

were used to define PERLNDs and IMPLNDs. Basinrelated and process-related parameters were estimated 

for each subbasin. The model was tested after calibration. An error analysis was done to identify sources of 

error. 

Model Setup

The Arroyo Colorado Basin was subdivided into 

12 subbasins, eight in the nontidal segment and four in 

the tidal segment, at locations of selected streamflowgaging stations, water-quality sampling sites, or point 

sources (fig. 2). Streamflow data were available for 

three streamflow-gaging stations located at Weslaco, 

Mercedes, and Harlingen (fig. 2) that are operated by 

the IBWC. Daily precipitation data were available at 

three National Oceanic and Atmospheric Administration stations in or near the basin (McAllen, Weslaco, 

and Harlingen) and were disaggregated to an hourly 

time step using hourly precipitation data available at 

Brownsville. Evaporation data were available at the 

Weslaco station. Air temperature, dewpoint temperature, cloud cover, wind speed, and solar radiation data 

were available at the Brownsville station. Water-quality 

data for selected constituents collected periodically 

were available at three sites (fig. 2). Selected characteristics for each subbasin are listed in table 3 (at end of 

report).

Time series of monthly self-reporting data were 

available for the 21 point sources (fig. 3). The permitted 

limits of each point source are listed in table 4 at end of 

report. Generally the point-source dischargers were 

operating at about 60 percent of permitted capacity during 1989–99.

Water used for irrigation in the Arroyo Colorado 

Basin is diverted from the Rio Grande by about 20 irrigation districts (fig. 4) and conveyed to land owners. 

Monthly time series of the amount of water diverted for 

each irrigation district are available from TNRCC 

water-master reports. Because the actual amount and 

time that each farmer irrigated was unknown, a set irrigation schedule was used. Irrigation time series were 

estimated on the basis of annual crop needs corresponding to a wet, normal, or dry year and monthly distribution determined from the available monthly TNRCC 

water-master data from the irrigation districts. The 

amount of water diverted by each irrigation district was 

area-weighted by the percentage of the irrigation district 

within the Arroyo Colorado Basin. That amount was 

divided by the amount of irrigated land in the subbasin 

and summed for each year to determine an annual unit 

irrigation depth in inches. Thus, each year of 1989–99 

was classified as either a wet, normal, or dry year by the 

amount of water per acre diverted each year. The irrigation schedules for each type of year and type of crop are 

listed in table 5 (at end of report). 

Some of the water diverted for irrigation is lost 

to inefficiencies in the conveyance system. Daily time 

series of monthly irrigation losses were estimated using 

values for the irrigation efficiency of each irrigation 

district provided by Guy Fipps, Texas A&M University 

(written commun., 1999) and the available monthly 

TNRCC water-master data for each irrigation district. 

The sum of the water applied using the irrigation 
6 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Figure 2. Arroyo Colorado subbasins and locations of selected streamflow-gaging and precipitation stations and water-qua

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator projectionZone 14

ARROYO COLORADO BASIN

HIDALGO

WILLACY

CAMERON

UNITED STATES

MEXICO

0 10 5 MILES

RIO GRANDE

Arroyo Colorado

Cayo Atascoso

North Floodway

Weslaco

Mercedes

McAllen

Harlingen

EXPLANATION Brownsville

Streamflow-gaging station and International Boundary and Water Commission numberPrecipitation stationWater-quality sampling site and Texas Natural Resource Conservation Commission number

Subbasin boundary

 08-4770.50 

 13081

 08-4770.50 08-4703.00 

 08-4704.00

 13079

 13074

 13081

98 ̊30' 98 ̊ 97 ̊30'

26 ̊30'

26 ̊
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 7 Base from U.S. Geological Survey digital data, 1:100,000 Universal Transverse Mercator projection Zone 14 ARROYO COLORADO BASIN

HIDALGO

WILLACY

CAMERON UNITED STATES 0 10 5 MILES MEXICO

RIO GRANDE

Arroyo Colorado

Cayo Atascoso North Floodway

98 ̊30' 98 ̊ 97 ̊30'

26 ̊30'

26 ̊

9 8

7

5 6 4 3 2

1

21 20

19

18

17

16

15

14

13

12

11

10

EXPLANATION

Point-source discharge

 1. City of Rio Hondo 2. City of Mercedes 3. City of San Benito 4. City of Mission 5. City of Harlingen no. 1 6. City of Harlingen no. 2 7. City of Donna 8. City of Pharr 9. City of McAllen 10. City of La Feria Utility Board 11. City of Hidalgo 12. City of San Juan 13. City of Alamo 14. City of Weslaco 15. Winter Garden Park Association16. Military Highway Water Supply Corp. 17. Donna Independent School District 18. Central Power and Light Co. Bates 19. Central Power and Light Co. La Palma 20. Southern Star Inc.21. Arroyo Aquaculture

Subbasin boundary

Figure 3. Locations of point-source discharges in the Arroyo Colorado subbasins.
8 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 HIDALGO WILLACY

0 10 5 MILES

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator projectionZone 14

98 ̊30' 98 ̊ 97 ̊30'

26 ̊30'

26 ̊

EXPLANATION

Irrigation district

ARROYO COLORADO BASIN

CAMERON

MEXICO

UNITED STATES

RIO GRANDE

Arroyo Colorado

Cayo Atascoso

North Floodway

Figure 4. Irrigation districts in and adjacent to the Arroyo Colorado Basin.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 9

schedules and the water lost during conveyance was 

compared to the total water diverted from the Rio 

Grande by the irrigation districts for a gross check of the 

water balance.

Eight major land-use types (low-density urban, 

high-density urban, natural vegetation, pasture and hay, 

row crops, citrus, sugar cane, and water and wetlands) 

were selected from the land-use data (fig. 5). All except 

water/wetlands were used to represent PERLNDs in the 

basin; three major land-use types (low-density urban, 

high-density urban, and water/wetlands) were used to 

represent IMPLNDs in the basin. The row crops were 

further subdivided into irrigated and nonirrigated row 

crops. The citrus and sugar cane were further subdivided into tile drained and nontile drained on the basis 

of information from Andy Garza, Texas State Soil and 

Water Conservation Board (written commun., 1999). 

The low-density urban land use was further subdivided 

into areas of land application of wastewater, colonias, 

and septic.

Soils in the basin were represented by two soil 

groups determined from county soil surveys (U.S. 

Department of Agriculture, Soil Conservation Service, 

1977, 1981–82). The basin was divided into the two 

major soil groups on the basis of permeability and 

hydrologic soil group (fig. 6). Soil group 1 consists predominantly of clay soils with low permeability classified as hydrologic soil group D. Soil group 2 consists 

predominantly of clay loam and sandy loam soils with 

moderate permeability classified as hydrologic soil 

group B. 

From the combinations of the two soil groups and 

the 14 land-use types, 26 unique PERLNDs (2 soil 

groups X 13 pervious land-use types) and three unique 

IMPLNDs (three impervious land-use types) were used. 

Because all fields are not irrigated at the same time, 

additional PERLNDs were used to account for the temporal variations in irrigation: The irrigated row crops 

were divided into 10 pieces, and the citrus and sugar 

cane (all irrigated) each were divided into three pieces. 

Therefore, an additional 34 PERLNDs (10 for row 

crops plus 12 each for citrus and sugar cane [2 soil 

groups X 2 land-use types X 3 pieces]) were used to 

account for all the irrigated agricultural land. All the 

PERLNDs associated with each major land-use type 

and soil group shared the same set of process-related 

parameters.

The basin-related parameters (table 6, at end 

of report) were estimated for each subbasin from geographic information system (GIS) coverages of the soils 

and land use, available channel cross-section data, 

and available rating tables for the streamflow-gaging 

stations. 

Calibration and Testing of Flow

The HSPF model was calibrated for flow 

using data measured at the Weslaco and Harlingen 

streamflow-gaging stations (fig. 2) during January 1, 

1989–December 31, 1995. Initial estimates for the 

18 process-related parameters were (1) based on the 

physical properties of the land use or soil in the subbasins (Chow and others, 1988); (2) assigned the default 

values listed in table 1; or (3) taken from a previous 

study (Dean and others, 1984). The 15 calibrated annual 

parameters are listed in table 7 (at end of report), and the 

three calibrated monthly parameters are listed in table 8 

(at end of report). Each parameter varied by land-use 

type or soil group corresponding to the physical process 

the parameter represents. For example, the parameters 

LZSN and INFILT varied by soil group and represented 

the different storage and infiltration capacities of the 

soil, whereas the parameters CEPSC, LZETP, and 

NSUR varied by land-use type and represented the different interception storages, evapotranspiration potentials, and surface roughness of the land-use types. The 

parameter AGWRC was assumed to be uniform for all 

land segments. The parameters INTFW, IRC, and 

NSUR were not varied monthly because iterative model 

simulations showed that the parameter values had little 

or no effect in explaining seasonal variations in runoff 

for this study area. For each land segment, the default 

values were used for the parameters AGWETP, 

BASETP, DEEPFR, INFEXP, INFILD, and KVARY. 

Values of the annual parameters AGWRC, INFILT, 

INTFW, IRC, LSUR, LZSN, NSUR, RETSC, and 

SLSUR and values of the monthly parameters CEPSC, 

LZETP, and UZSN were adjusted during the calibration 

process using the software program HSPEXP (Lumb 

and others, 1994). The starting values for the initial conditions were estimated from Dean and others (1984) and 

were revised during calibration.

The calibrated process-related parameter set 

for flow (tables 7–8, at end of report) was developed 

from 1989–95 data for the Weslaco and Harlingen 
10 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Figure 5. Land use in the Arroyo Colorado Basin, 1995 (M.P. Stier, U.S. Geological Survey, written commun., 1999).

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator projectionZone 14

HIDALGO

WILLACY

CAMERON

UNITED STATES

MEXICO

0 10 5 MILES

98 ̊30' 98 ̊ 97 ̊30'

26 ̊30'

26 ̊

Subbasin boundary

EXPLANATION

Low-density urbanHigh-density urbanNatural vegetationPasture/hayRow cropsCitrusSugar caneWater/wetlands

Land use

RIO GRANDE

Arroyo Colorado

Cayo Atascoso

North Floodway
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 11Figure 6. Major soil groups in the Arroyo Colorado Basin. Base from U.S. Geological Survey digital data, 1:100,000 Universal Transverse Mercator projection Zone 14

HIDALGO

WILLACY

CAMERON UNITED STATES MEXICO

0 10 5 MILES

98 ̊30' 98 ̊ 97 ̊30'

26 ̊30'

26 ̊

EXPLANATION

Soil group 1Soil group 2

Subbasin boundary

LA

RIO GRANDE

Arroyo Colorado

Cayo Atascoso North Floodway ARROYO COLORADO BASIN
12 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

streamflow-gaging stations (fig. 2). Data collected during 1989–95 at the Mercedes streamflow-gaging station 

were used to test the parameters spatially, and data collected during 1996–99 at the Mercedes and Harlingen 

streamflow-gaging stations were used to test the parameters temporally. The parameter set was tested spatially 

to assess the transferability of the model parameters to 

ungaged subbasins including the subbasins located in 

the tidal segment. The spatial testing was done for the 

same time period that was used for model calibration. 

The parameter set was tested temporally to assess 

the ability of the model to accurately simulate under 

climatic conditions different from those for which the 

model was calibrated.

The measured and simulated flow volumes 

and errors for calibration and testing using HSPEXP 

(Lumb and others, 1994) are listed in table 9 (at end 

of report). The errors for total flow volume for calibration at Weslaco and Harlingen were both -0.1 percent, 

and the errors for total storm volume were -8.0 percent 

at Weslaco and -5.6 percent at Harlingen. The errors 

for the total of highest 10-percent and lowest 50-percent 

flows were all less than 6 percent for the two stations. 

Only the errors for summer flow and summer storm 

volumes at Weslaco exceeded 10 percent. Time series, 

flow-duration curves, and graphs of measured and 

simulated flow are shown in figures 7 and 8 for the 

Arroyo Colorado at Weslaco and at Harlingen. There 

appears to be a bias for low flows less than 100 ft3

/s 

(cubic feet per second) and high flows more than 600 

ft3

/s at Weslaco (fig. 7); that is, there is a good match 

between measured and simulated flow between about 

100 and 600 ft3

/s (between about the 90- and 1-percent 

exceedance probabilities). There is a good match 

between the measured and simulated flows at Harlingen 

except for discharges greater than about 1,000 ft3

/s 

(about 1-percent exceedance probability), which are 

undersimulated (fig. 8).

Data collected during 1989–95 at the Arroyo 

Colorado at Mercedes were used to test the parameters 

spatially (table 9). The error for total flow volume was 

7.5 percent, and the error for total storm volume was 

-1.3 percent. The results are similar to the calibration 

results for Weslaco and Harlingen. The only errors that 

exceeded 10 percent were for summer flow and summer 

storm volumes. A time series, a flow-duration curve, 

and a graph of measured and simulated flow are shown 

in figure 9 for Arroyo Colorado at Mercedes. There is a 

good match between the measured and simulated flows 

at Mercedes except for discharges greater than about 

700 ft3/s (about 1-percent exceedance probability), 

which are undersimulated (fig. 9).

Data collected during 1996–99 at the Arroyo 

Colorado at Mercedes and at Harlingen were used to 

test the parameters temporally (table 9). The errors 

for total flow volume for the two stations were 29 and 

11.2 percent, respectively, and the errors for total storm 

volume were 8.4 and -15.6 percent, respectively. The 

errors for the total of highest 10-percent flows were 13.2 

and -12.5 percent, respectively; and the errors for summer flow and summer storm volumes were less than 20 

percent. Time series, flow-duration curves, and graphs 

of measured and simulated flow are shown in figures 10 

and 11 for the Arroyo Colorado at Mercedes and at 

Harlingen. There is a consistent bias for oversimulating 

discharges less than about 600 ft3

/s and undersimulating 

discharges greater than about 600 ft3/s. The oversimulation appears to occur more frequently for the 1998–99 

period.

Error Analysis of Flow

The types of error from the model calibration and 

testing can be classified as measurement errors or systematic errors. Measurement errors are introduced as a 

result of missing data and inaccurate rating tables of 

stage and discharge. Some daily precipitation data were 

missing and had to be estimated from other daily data. 

The spatial variability of precipitation for some storms 

might not be represented adequately with the existing 

network of rain gages. The streamflow-gaging station 

data were limited to only daily mean flows with no 

hourly data available for calibration and testing of the 

instantaneous peak flows. The main source of error in 

the model is associated with the distribution of irrigation water. The amount of water applied to the basin for 

irrigation is a substantial component of the water balance. It is impossible to accurately characterize the 

actual irrigation on every field in a time series for model 

input. The point-source data were limited to monthly 

data, which do not adequately represent the daily fluctuations of the effluent. Another substantial source of 

error in the water balance is evapotranspiration. More 

accurate estimates of actual evapotranspiration might 

reduce model error.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 13

Line of equal value

Measured

Simulated

0

500

1,000

1,500

2,000

2,500

FLOW, IN CUBIC FEET PER SECOND

1989 1990 1991 1992 1993 1994 1995

YEAR

Measured

Simulated

10

100

1,000

10,000

FLOW, IN CUBIC FEET PER SECOND

0.5 2 10 50 80 95 99.5

EXCEEDANCE PROBABILITY, IN PERCENT

5 20 30 70 90 98

10

100

1,000

10,000

SIMULATED FLOW, IN CUBIC FEET PER SECOND

10 100 1,000

MEASURED FLOW, IN CUBIC FEET PER SECOND

10,000

Figure 7. Measured and simulated daily streamflow at Arroyo Colorado at Weslaco, 1989–95.
14 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Line of equal value

Measured

Simulated

1989 1990 1991 1992 1993 1994 1995

YEAR

0

1,600

3,200

4,800

6,400

8,000

FLOW, IN CUBIC FEET PER SECOND

Measured

Simulated

0.5 2 10 50 80 95 99.5

EXCEEDANCE PROBABILITY, IN PERCENT

5 20 30 70 90 98

10

100

1,000

10,000

FLOW, IN CUBIC FEET PER SECOND

10 100 1,000

MEASURED FLOW, IN CUBIC FEET PER SECOND

10,000

10

100

1,000

10,000

SIMULATED FLOW, IN CUBIC FEET PER SECOND

Figure 8. Measured and simulated daily streamflow at Arroyo Colorado at Harlingen, 1989–95.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 15

Line of equal value

Measured

Simulated

1989 1990 1991 1992 1993 1994 1995

YEAR

0

500

1,000

1,500

2,000

2,500

FLO

W, IN CUBIC FEET PER SECOND

Measured

Simulated

0.5 2 10 50 80 95 99.5

EXCEEDANCE PROBABILITY, IN PERCENT

5 20 30 70 90 98

10

100

1,000

10,000

FLO

W, IN CUBIC FEET PER SECOND

10 100 1,000

MEASURED FLOW, IN CUBIC FEET PER SECOND

10,000

10

100

1,000

10,000

SIMULATED FLO

W, IN CUBIC FEET PER SECOND

Figure 9. Measured and simulated daily streamflow at Arroyo Colorado at Mercedes, 1989–95.
16 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Line of equal value

Measured

Simulated

0

600

1,200

1,800

2,400

3,000

FLOW, IN CUBIC FEET PER SECOND

1996 1997 1998 1999

YEAR

Measured

Simulated

10

100

1,000

10,000

FLOW, IN CUBIC FEET PER SECOND

0.5 2 10 50 80 95 99.5

EXCEEDANCE PROBABILITY, IN PERCENT

5 20 30 70 90 98

10

100

1,000

10,000

SIMULATED FLOW, IN CUBIC FEET PER SECOND

10 100 1,000

MEASURED FLOW, IN CUBIC FEET PER SECOND

10,000

Figure 10. Measured and simulated daily streamflow at Arroyo Colorado at Mercedes, 1996–99.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 17

Line of equal value

Measured

Simulated

0

800

1,600

2,400

3,200

4,000

FLOW, IN CUBIC FEET PER SECOND

1996 1997 1998 1999

YEAR

Measured

Simulated

10

100

1,000

10,000

FLOW, IN CUBIC FEET PER SECOND

0.5 2 10 50 80 95 99.5

EXCEEDANCE PROBABILITY, IN PERCENT

5 20 30 70 90 98

10

100

1,000

10,000

SIMULATED FLOW, IN CUBIC FEET PER SECOND

10 100 1,000

MEASURED FLOW, IN CUBIC FEET PER SECOND

10,000

Figure 11. Measured and simulated daily streamflow at Arroyo Colorado at Harlingen, 1996–99.
18 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Systematic errors are associated with the inability 

of the simulation model to represent the physical 

processes of runoff. These errors are contained in the 

model parameters and model equations, as the model 

parameters and equations inherently fall short of accurately and precisely accounting for all the variations in 

runoff. The PERLNDs used in this model are general 

representations of the different hydrologic response 

units of the study area. Also, the values of FTABLES are 

somewhat uncertain for the reach volume and the corresponding discharge. 

The measurement and systematic errors account 

for some of the error and bias of the simulated flow. 

From the results listed in table 9 and shown in figures 

7–11, the calibrated parameter set adequately simulates 

flow in the Arroyo Colorado. There could be a bias for 

undersimulating peak flows; but because low DO typically occurs during low flows, more emphasis was 

placed on fitting the model to low flows than to peak 

flows.

Calibration and Testing of Water Quality 

The HSPF model was calibrated for selected 

water-quality properties and constituents primarily 

using data measured at the Weslaco and Harlingen 

streamflow-gaging stations (fig. 2) during January 1, 

1989–December 31, 1995. The model was used to 

simulate suspended sediment, water temperature, 

BOD, DO, nitrate nitrogen, ammonia nitrogen, and 

orthophosphate. Initial estimates for the water-quality 

process-related parameters were (1) assigned the default 

values listed in table 1 or (2) taken from a previous 

study (Dean and others, 1984). Of the 106 processrelated parameters used for water quality, the 19 calibrated annual parameters for each PERLND and 

IMPLND are listed in table 10 (at end of report), the 

12 calibrated monthly parameters for each PERLND 

and IMPLND are listed in table 11 (at end of report), 

and the 75 calibrated annual parameters for each 

RCHRES are listed in table 12 (at end of report). Each 

parameter varied by land-use type for each PERLND or 

IMPLND or by reach for each RCHRES corresponding 

to the physical process the parameter represents. For 

example, the parameters AFFIX and COVER varied by 

land-use type and by month (COVER). The two parameters represented the different fractions for detached 

sediment storage and fractions of land surface shielded 

from erosion for each PERLND. The parameters 

TAUCD and TAUCS varied by RCHRES and streambed material and represented the different critical shear 

stress needed for deposition and for scour in each reach. 

The calibrated parameter set (tables 10–12) was 

developed primarily from 1989–95 data collected at the 

Weslaco and Harlingen stations. Data collected during 

1989–95 at the Segment 2202 outlet station were used 

to test the parameters spatially, and data collected during 1996–99 at the Weslaco, Harlingen, and Segment 

2202 outlet stations were used to test the parameters 

temporally. Some intermediate calibration and testing 

results from selected PERLNDs and IMPLNDs for suspended sediment, BOD, nitrate nitrogen, ammonia 

nitrogen, and orthophosphate were compared to available event-mean concentration data from previous 

studies near the study area or in other parts of the State 

(table 13, at end of report). These comparisons were 

useful in modifying the buildup-washoff model parameters for the selected PERLNDs or IMPLNDs. However, event-mean concentrations were not available for 

all PERLNDs and IMPLNDs; in those cases, the model 

parameters were modified on the basis of professional 

judgment. The associated model parameters for each 

RCHRES were then calibrated and tested with data 

from the Weslaco, Harlingen, and Segment 2202 outlet 

stations.

The model was calibrated and spatially tested 

sequentially for seven selected properties and constituents in the following order: suspended sediment, water 

temperature, BOD, DO, nitrate nitrogen, ammonia 

nitrogen, and orthophospate for the period 1989–95. 

Because only periodic water-quality data were available 

for the calibration, the objective of the calibration and 

testing was to match the general shape of the time series 

of measured data. Instantaneous measured time series 

and daily simulated data for the seven selected properties and constituents are shown in figs. 12–18.

The simulated suspended sediment concentrations (fig. 12) for the Weslaco and Harlingen stations 

appear to match the low concentrations (less than 400 

milligrams per liter); there are no measured concentrations to calibrate the high concentrations, however. No 

measured suspended sediment data were collected at the 

Segment 2202 outlet station. 

The simulated water temperature matches the 

annual cycle of the measured water temperature fairly 

well at the three sites, although some of the higher 
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 19

(a)

(b)

(c)

Simulated

Measured

0

800

1,600

2,400

3,200

4,000

CONCENTRATION, IN MILLIGRA

MS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

800

1,600

2,400

3,200

4,000

CONCENTRATION, IN MILLIGRA

MS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

0

800

1,600

2,400

3,200

4,000

CONCENTRATION, IN MILLIGRA

MS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 12. Measured and simulated suspended sediment concentrations, 1989–95, at Arroyo Colorado at

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet (simulated only).
20 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99

(a)

(b)

(c)

Simulated

Measured

50

60

70

80

90

100

TEMPERATURE, IN DEGREES FAHRENHEIT

40

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

50

60

70

80

90

100

TEMPERATURE, IN DEGREES FAHRENHEIT

40

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

50

60

70

80

90

100

TEMPERATURE, IN DEGREES FAHRENHEIT

40

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 13. Measured and simulated water temperature, 1989–95, at Arroyo Colorado at (a) Weslaco,

(b) Harlingen, and (c) Segment 2202 outlet.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 21

(a)

(c)

(b)

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRA

MS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRA

MS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRA

MS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 14. Measured and simulated biochemical oxygen demand concentrations, 1989–95, at Arroyo Colorado at

(a) Weslaco (simulated only), (b) Harlingen, and (c) Segment 2202 outlet (simulated only).
22 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

(a)

(b)

(c)

Simulated

Measured

0

3

6

9

12

15

CONCENTRATION, IN MILLIGRAMS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

3

6

9

12

15

CONCENTRATION, IN MILLIGRAMS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

3

6

9

12

15

CONCENTRATION, IN MILLIGRAMS PER LITER

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 15. Measured and simulated dissolved oxygen concentrations, 1989–95, at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 23

(a)

(b)

(c)

Simulated

Measured

0

2

4

6

8

12

CONCENTRATION, IN MILLIGRAMS PER LITER

10

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

2

4

6

8

12

CONCENTRATION, IN MILLIGRAMS PER LITER

10

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

2

4

6

8

12

CONCENTRATION, IN MILLIGRAMS PER LITER

10

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 16. Measured and simulated nitrate nitrogen concentrations, 1989–95, at Arroyo Colorado at (a) Weslaco, 

(b) Harlingen, and (c) Segment 2202 outlet.
24 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

(a)

(b)

(c)

Simulated

Measured

0

0.4

0.8

1.6

CONCENTRATION, IN MILLIGRAMS PER LITER 1.2

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

0.4

0.8

1.6

CONCENTRATION, IN MILLIGRAMS PER LITER

1.2

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

0.4

0.8

1.6

CONCENTRATION, IN MILLIGRAMS PER LITER

1.2

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 17. Measured and simulated ammonia nitrogen concentrations, 1989–95, at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 25

(a)

(b)

(c)

Simulated

Measured

0

0.3

0.6

1.2

CONCENTRATION, IN MILLIGRAMS PER LITER

0.9

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

0.3

0.6

1.2

CONCENTRATION, IN MILLIGRAMS PER LITER

0.9

1989 1991 1993 1995

YEAR

1990 1992 1994

Simulated

Measured

0

0.3

0.6

1.2

CONCENTRATION, IN MILLIGRAMS PER LITER

0.9

1989 1991 1993 1995

YEAR

1990 1992 1994

Figure 18. Measured and simulated orthophosphate concentrations, 1989–95, at Arroyo Colorado at (a) Weslaco, 

(b) Harlingen, and (c) Segment 2202 outlet.
26 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

temperatures are undersimulated (fig. 13). The simulated BOD matches the measured data for the Harlingen 

station (fig. 14). No BOD data were measured at the 

Weslaco or Segment 2202 outlet stations during 1989–

95. The simulated DO concentrations (fig. 15) follow 

the same cyclical pattern as the simulated water temperature and BOD data. There is some scatter in the measured DO concentrations that is higher and lower than 

the simulated DO concentrations.

The simulated concentrations of nitrate nitrogen, 

ammonia nitrogen, and orthophosphate all show a general annual cyclical pattern (figs. 16–18). The simulated 

concentrations generally match the measured concentrations. The measured and simulated concentrations 

of nitrate and orthophosphate show the same range in 

magnitude for all three stations. The measured and 

simulated concentrations of ammonia are substantially 

lower at the Harlingen station than at the other two 

stations (fig. 17).

Data collected during 1996–99 at the Arroyo 

Colorado at Weslaco, Harlingen, and Segment 2202 

outlet stations were used to test the parameters temporally. The results for the 1996–99 period for the seven 

properties and constituents (figs. 19–25) are very similar to the results for the 1989–95 period. No measured 

BOD data for the Segment 2202 outlet station and no 

measured nitrate nitrogen data for the Weslaco and Segment 2202 outlet stations were available for comparison 

to simulated data.

Error Analysis of Water Quality

Because few water-quality data were available 

at the stations along the Arroyo Colorado for model 

calibration and testing, a substantial amount of error can 

be introduced. In addition, few data were available to 

calibrate and test the nonpoint-source loads prior to 

their entry into the main stem of the Arroyo Colorado. 

A continuous time series of DO would have been useful 

for model calibration. The majority of the existing 

water-quality data were collected at low flow. More 

frequent measurements for a range of flows would 

improve model calibration.

Systematic errors are introduced with the numerous parameters used to simulate water quality. As with 

the simulation of runoff, some of the inability of the 

model to represent the physical processes is in the 

model parameters and model equations. The buildup 

and washoff parameters used in this model are general 

approximations of the physical processes for the generation of nutrient loads in the basin. 

The measurement and systematic errors account 

for some of the error and bias of the simulated water 

quality. From the results listed in table 9 and shown in 

figures 7–11, the calibrated parameter set adequately 

simulates water quality in the Arroyo Colorado for 

selected properties and constituents. 

Simulated Point- and Nonpoint-Source Loads

The model can be used to simulate loads for 

selected water-quality properties and constituents. The 

TNRCC will use the model to simulate different scenarios to determine the effects of various BMPs. To provide a baseline for the current (1998) loadings relative 

to the major land-use types in the Arroyo Colorado 

Basin, the simulated loads were aggregated for point 

and nonpoint sources for 1989–99. The loads from lowdensity urban, colonias, septic, permitted land application, and high-density urban were combined into urban 

nonpoint-source loads. The loads from pasture and hay, 

nonirrigated and irrigated row crops, all citrus, and all 

sugar cane were combined into agricultural nonpointsource loads. The relative percentage of total load for 

suspended sediment, BOD, nitrate nitrogen, ammonia 

nitrogen, and orthophosphate are shown in figure 26.

The fractions of major land-use type by area, 

excluding water and wetlands (aggregated from data in 

table 6), for the Arroyo Colorado Basin is about 13 percent urban, 19 percent natural, and 68 percent agricultural. The fraction of the total load for each of the five 

properties and constituents that is agricultural nonpointsource load ranges from 41 to 87 percent (fig. 26). The 

fraction of the total load for each of the five properties 

and constituents that is urban nonpoint-source load 

ranges from 10 to 30 percent. The fraction of the total 

load for each of the five properties and constituents that 

is urban point-source load ranges from 1 to 40 percent. 

The fraction of the total load for each of the five properties and constituents that is natural nonpoint-source 

load ranges from 1 to 6 percent. A substantial fraction, 

from about one-third to one-half, of the BOD and 

nutrient loads is from urban point and nonpoint sources 

compared to the relatively small fraction of the total 

land use (13 percent) that is urban.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 27

(a)

(b)

(c)

Simulated

Measured

0

800

1,600

2,400

3,200

4,000

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Simulated

Measured

0

800

1,600

2,400

3,200

4,000

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Simulated

Measured

0

800

1,600

2,400

3,200

4,000

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Figure 19. Measured and simulated suspended sediment concentrations, 1996–99, at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet.
28 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

(a)

(b)

(c)

Simulated

Measured

1996 1997 1998

YEAR

1999

50

60

70

80

90

100

TEMPERATURE, IN DEGREES FAHRENHEIT

40

Simulated

Measured

1996 1997 1998

YEAR

1999

50

60

70

80

90

100

TEMPERATURE, IN DEGREES FAHRENHEIT

40

Simulated

Measured

1996 1997 1998

YEAR

1999

50

60

70

80

90

100

TEMPERATURE, IN DEGREES FAHRENHEIT

40

Figure 20. Measured and simulated water temperature, 1996–99, at Arroyo Colorado at (a) Weslaco, 

(b) Harlingen, and (c) Segment 2202 outlet.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 29

(a)

(b)

(c)

Simulated

Measured

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Simulated

Measured

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

0

5

10

15

20

25

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Figure 21. Measured and simulated biochemical oxygen demand concentrations, 1996–99, at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet (simulated only).
30 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

(a)

(b)

(c)

Simulated

Measured

0

3

6

9

12

15

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Simulated

Measured

0

3

6

9

12

15

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Simulated

Measured

0

3

6

9

12

15

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Figure 22. Measured and simulated dissolved oxygen concentrations, 1996–99, at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 31

(a)

(b)

(c)

0

2

4

6

8

12

CONCENTRATION, IN MILLIGRAMS PER LITER 10

1996 1997 1998

YEAR

1999

Simulated

Measured

0

2

4

6

8

12

CONCENTRATION, IN MILLIGRAMS PER LITER

10

1996 1997 1998

YEAR

1999

0

2.4

4.8

7.2

9.6

12.0

CONCENTRATION, IN MILLIGRAMS PER LITER

1996 1997 1998

YEAR

1999

Figure 23. Measured and simulated nitrate nitrogen concentrations, 1996–99, at Arroyo Colorado at (a) Weslaco 

(simulated only), (b) Harlingen, and (c) Segment 2202 outlet (simulated only).
32 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

(a)

(b)

(c)

Simulated

Measured

0

0.4

0.8

1.6

CONCENTRATION, IN MILLIGRAMS PER LITER 1.2

1996 1997 1998

YEAR

1999

Simulated

Measured

0

0.4

0.8

1.6

CONCENTRATION, IN MILLIGRAMS PER LITER 1.2

1996 1997 1998

YEAR

1999

Simulated

Measured

0

0.4

0.8

1.6

CONCENTRATION, IN MILLIGRAMS PER LITER

1.2

1996 1997 1998

YEAR

1999

Figure 24. Measured and simulated ammonia nitrogen concentrations, 1996–99, at Arroyo Colorado at 

(a) Weslaco, (b) Harlingen, and (c) Segment 2202 outlet.
SIMULATION OF FLOW AND WATER QUALITY OF THE ARROYO COLORADO 33

(a)

(b)

(c)

Simulated

Measured

0

0.3

0.6

1.2

CONCENTRATION, IN MILLIGRAMS PER LITER

0.9

1996 1997 1998

YEAR

1999

Simulated

Measured

0

0.3

0.6

1.2

CONCENTRATION, IN MILLIGRAMS PER LITER

0.9

1996 1997 1998

YEAR

1999

Simulated

Measured

0

0.48

0.72

1.20

CONCENTRATION, IN MILLIGRAMS PER LITER 0.96

0.24

1996 1997 1998

YEAR

1999

Figure 25. Measured and simulated orthophosphate concentrations, 1996–99, at Arroyo Colorado at (a) Weslaco, 

(b) Harlingen, and (c) Segment 2202 outlet.
34 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

1 10

87

20

11

68

1

40

10

49

1

23

30

41

6

22

13

64

1

2

EXPLANATION

Urban point source

Urban nonpoint source

Agricultural nonpoint source

Natural nonpoint source

Biochemical oxygen demand Nitrate nitrogen

Suspended sediment

Ammonia nitrogen Orthophosphate

Figure 26. Simulated total point- and nonpoint-source loads in percent by land-use type for selected properties 

and constituents, Arroyo Colorado Basin, 1989–99.
SUMMARY 35

SUMMARY

The purpose of this study was to set up, calibrate, 

and test a simulation model (HSPF) using a set of process-related model parameters with available flow 

and water-quality data for the Arroyo Colorado from 

Mission to the Laguna Madre, Texas. The model simulates flow, selected water-quality properties, and constituent concentrations. The model can be used by the 

TNRCC to simulate different BMPs to estimate a 

TMDL for selected constituents to meet water-quality 

standards for DO in the Arroyo Colorado. 

Time series of precipitation, evaporation, air temperature, dewpoint temperature, wind speed, cloud 

cover, solar radiation, streamflow, and concentrations of 

selected properties and constituents for January 1, 

1989–December 31, 1999, were used in this study. 

Twenty-six pervious land segments and three impervious land segments were defined for the study on the 

basis of 14 land-use types and two soil groups. Eighteen 

process-related parameters were defined and calibrated 

for each land segment for flow, 31 process-related 

parameters were defined and calibrated for each land 

segment for water quality, and 75 process-related 

parameters were defined and calibrated for each stream 

reach for water quality.

The model was calibrated for flow with 1989–95 

data for two stations, tested spatially with 1989–95 data 

for one station, and tested temporally with 1996–99 data 

for two stations using HSPEXP. The errors for total flow 

volume ranged from -0.1 to 29.0 percent, and the errors 

for total storm volume ranged from -15.6 to 8.4 percent. 

The errors for the total of highest 10-percent flows 

ranged from -12.5 to 13.2 percent, and the errors for 

lowest 50-percent flows ranged from -5.0 to 27.6 percent. The errors were larger for the 1996–99 period.

The model was calibrated for water quality for 

seven properties and constituents with 1989–95 data for 

two stations, tested spatially with 1989–95 data for one 

station, and tested temporally with 1996–99 data for 

three stations. The model was calibrated sequentially 

for suspended sediment, water temperature, BOD, DO, 

nitrate nitrogen, ammonia nitrogen, and orthophosphate. The simulated concentrations of the selected 

properties and constituents generally matched the measured concentrations available for the calibration and 

testing periods.

The model was used to simulate total pointand nonpoint-source loads for selected water-quality 

properties and constituents for 1989–99 for urban, natural, and agricultural land-use types. About one-third to 

one-half of the BOD and nutrient loads are from urban 

point and nonpoint sources, although only 13 percent of 

the total land use in the basin is urban.

REFERENCES CITED

Ambiotech Environmental Consultants, Inc., 1998, Resaca 

nonpoint-source pilot study—Aquatic resource impairment of resacas by nonpoint-source pollutions from 

urban runoff in Brownsville, Texas area: Prepared for 

Texas Natural Resource Conservation Commission 

under Texas Clean Rivers Program Contract No. 

E2550003 (55–000000–3), 69 p.

Baldys, Stanley, III, Raines, T.H., Mansfield, B.L., and 

Sandlin, J.T., 1998, Urban stormwater quality, eventmean concentrations, and estimates of stormwater pollutant loads, Dallas-Fort Worth area, Texas, 1992–93: 

U.S. Geological Survey Water-Resources Investigations 

Report 98–4158, 51 p.

Bicknell, B.R., Imhoff, J.C., Kittle, J.L., Jr., Donigian, 

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

Simulation Program—FORTRAN, user’s manual for 

version 11: Athens, Ga., U.S. Environmental Protection 

Agency, National Exposure Research Laboratory, 

EPA/600/R–97/080, 755 p.

Chow, V.T., Maidment, D.R., and Mays, L.W., 1988, Applied 

hydrology: New York, McGraw-Hill, 572 p.

Dean, J.D., Meier, D.W., Bicknell, B.R., and Donigian, A.S., 

Jr., 1984, Simulation of DDT transport and fate in the 

Arroyo Colorado watershed, Texas: Athens, Ga., U.S. 

Environmental Protection Agency, Environmental 

Research Laboratory, Contract No. 68–03–3116, 199 p.

Flowers, Joan, Easterling, Nancy, and Hauck, Larry, 1998, 

Prediction of effects of best management practices on 

agricultural nonpoint-source pollution in the Arroyo 

Colorado watershed: Texas Institute for Applied Environmental Research, Report PR 97–06, 102 p.

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.

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.
36 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Moore, A.W., 1995, Southmost Soil and Water Conservation 

District nutrient management study—Final report: San 

Benito, Tex., U.S. Department of Agriculture, Natural 

Resources Conservation Service, 4 p.

National Oceanic and Atmospheric Administration, 1996, 

Climatological data, Texas, annual summary: Asheville, 

N.C., U.S. Department of Commerce, v. 101, no. 13, 

77 p.

Newell, C.J., Rifai, H.S., and Bedient, P.B., 1992, Characterization of nonpoint sources and loadings to Galveston 

Bay: Texas Natural Resource Conservation Commission, Galveston Bay National Estuary Program, 

Technical Report GBNEP–15, v. I, 121 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.

Texas A&M University, 1995, Development of new orchard 

soil-management practices for improving water-use 

efficiency and for reducing water-quality degradation—

Final report: Texas A&M University, Kingsville Citrus 

Center at Weslaco, Texas Water Development Board 

Grant No. 92–483–331, 80 p.

Texas Natural Resource Conservation Commission, 

1997, Texas surface-water-quality standards: Texas 

Natural Resource Conservation Commission, chap. 

307.1–307.10, 125 p.

______1998, Total loads and water quality in the Corpus 

Christi Bay System: Texas Natural Resource Conservation Commission, Corpus Christi Bay National Estuary 

Program, Technical Report CCBNEP–27, 133 p.

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

1977, Soil survey of Cameron County, Texas: Soil 

Conservation Service, 92 p.

______1981, Soil survey of Hidalgo County, Texas: Soil 

Conservation Service, 171 p.

______1982, Soil survey of Willacy County, Texas: Soil 

Conservation Service, 137 p.
Table 1 37

Simulation of Flow and Water Quality of the 

Arroyo Colorado, Texas, 1989–99

Table 1

Table 1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN—Continued

Parameter Description1 Default Minimum Maximum Units

Flow

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

AGWRC Active ground-water recession rate -- .001 1.0 /d

BASETP Available ET satisfied by base flow 0 0 1.0 --

CEPSC Interception storage capacity 0 0 1.0 in.

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

INFEXP Infiltration equation exponent 2.0 0 10.0 --

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

INFILT Index to infiltration capacity of soil -- .0001 100.0 in/hr

INTFW Interflow inflow -- 0 -- --

IRC Interflow recession rate -- 0 1.0 /d

KVARY Nonexponential ground-water recession rate 0 0 -- /in.

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

LZETP Lower-zone ET 0 0 1.0 --

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

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

RETSC Retention storage capacity for impervious areas 0 0 10 ft

SLSUR Slope of assumed overland flow plane -- .000001 10.0 ft/ft

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

Water quality

Sediment

ACCSDP Rate solids are placed on land surface 0 0 -- tons/ac-d

AFFIX Fraction detached sediment storage decreases 0 0 1.0 /d

BEDWID Width of cross section for bed-sediment deposition -- 1.0 -- ft

BEDWRN Bed depth for warning message 100 .001 -- ft

COVER Fraction of land surface shielded from erosion 0 0 1.0 --

D Effective diameter of sand, silt, or clay particles -- .001 100 in.

EXPSAND Exponent of sandload equation 0 0 -- --

JEIM Exponent of solids washoff equation -- -- -- --

JGER Exponent of matrix soil scour equation -- -- -- --

JRER Exponent of soil detachment equation -- -- -- --

JSER Exponent of detached sediment washoff equation -- -- -- --

KEIM Coefficient of solids washoff equation 0 0 -- --

KGER Coefficient of matrix soil scour equation 0 0 -- --

KRER Coefficient of soil detachment equation 0 0 -- --

KSAND Coefficient of sandload equation 0 0 -- --

Table 1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN

[ET, evapotranspiration; --, not applicable; /d, per day; in., inches; in/hr; inches per hour; /in., per inch; ft, feet; ft/ft, feet per foot; tons/ac-d, 

tons per acre per day; lb/ft2-d, pounds per square foot per day; lb/ac-d, pounds per acre per day; g/cm3, grams per cubic centimeter; lb/ft2, 

pounds per square foot; in/s, inches per second; °F, degrees Fahrenheit; DO, dissolved oxygen; BOD, biochemical oxygen demand; mg C/L, 

milligrams carbon per liter; mg/L, milligrams per liter; °C, degrees Celsius; mg/m2-hr, milligrams per square meter per hour; /hr, per hour; 

lb/ac, pounds per acre; mL/g, milliliters per gram; mg/kg, milligrams per kilogram; mg/mg, milligrams per milligram; mol/mol, moles per 

mole; μg/L, micrograms per liter; ly/min, langleys per minute; /ft, per foot; L/mg-ft, liters per milligram per foot; ft3/s, cubic feet per second; 

ft/hr, feet per hour]

Footnote at end of table.
38 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

KSER Coefficient of detached sediment washoff equation 0 0 -- --

M Erodibility coefficient of sediment 0 0 -- lb/ft2-d

NVSI Rate sediment enters detached storage from atmosphere 0 -- -- lb/ac-d

POR Bed porosity .5 .1 0.9 --

REMSDP Fraction solid storage removed daily during no runoff 0 0 1.0 /d

RHO Density of sand, silt, or clay particles 2.65 1.0 4.0 g/cm3

SMPF Supporting management factor 1.0 .001 1.0 --

TAUCD Critical bed shear stress for deposition -- -- -- lb/ft2

TAUCS Critical bed shear stress for scour -- -- -- lb/ft2

W Fall velocity of sand, silt, or clay particles -- .02 500 in/s

Temperature

ASLT Intercept of surface layer temperature equation 32 0 100 °F

BSLT Slope of surface layer temperature equation 1.0 .001 2.0 °F

CFSAEX Correction factor for solar radiation 1.0 .001 2.0 --

ELEV Mean RCHRES elevation above sea level 0 0 30,000 ft

ELDAT Difference between ELEV and air temperature gage elevation 0 -- 10,000 ft

KATRAD Longwave radiation coefficient 9.37 1.0 20 --

KCOND Conduction-convection heat transport coefficient 6.12 1.0 20 --

KEVAP Evaporation coefficient 2.24 1.0 10 --

LGTP1 Smoothing factor in lower layer temperature equation -- -- -- °F

ULTP1 Smoothing factor in upper layer temperature equation -- -- -- °F

ULTP2 Mean difference between upper layer soil and air temperature -- -- -- °F

DO and BOD

ACO2P Concentration of dissolved carbon dioxide in active ground water 0 0 1.0 mg C/L

ADOXP Concentration of DO in active ground water 0 0 20 mg/L

AWTF Intercept of surface-water regression equation 32 0 100 °F

BENOD Benthal oxygen demand at 20 °C 0 0 -- mg/m2-hr

BRBOD1 Benthal release of BOD at high oxygen concentrations 72 .0001 -- mg/m2-hr

BRBOD2 Increment to benthal release of BOD 100 .0001 -- mg/m2-hr

BWTF Slope of surface-water regression equation 1.0 .001 2.0 °F

ELEV Mean PERLND and IMPLND elevation above sea level 0 -1,000 30,000 ft

EXPOD Exponent of DO term in benthal oxygen demand equation 1.22 .1 -- --

EXPRED Exponent to death for reaeration coefficient equation 0 -- 0 --

EXPREL Exponent of DO term in benthal BOD release equation 2.82 .1 -- --

EXPREV Exponent to velocity for reaeration coefficient equation 0 0 -- --

ICO2P Concentration of dissolved carbon dioxide in interflow 0 0 1.0 mg C/L

IDOXP Concentration of DO in interflow 0 0 20.0 mg/L

KBOD20 Unit BOD decay rate at 20 °C -- 1.0-30 -- /hr

Table 1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN—Continued

Parameter Description1 Default Minimum Maximum Units

Footnote at end of table.
Table 1 39

KODSET Rate of BOD settling 0 0 -- ft/hr

REAK Empirical constant for reaeration coefficient equation -- 1.0-30 -- /hr

SUPSAT Allowable DO supersaturation as fraction of saturation 1.15 1.0 2.0 --

TCBEN Temperature correction coefficient for benthal oxygen demand 1.074 1.0 2.0 --

TCBOD Temperature correction coefficient for BOD decay 1.075 1.0 2.0 --

TCGINV Temperature correction coefficient for surface gas invasion 1.047 1.0 2.0 --

PERLND and IMPLND nutrients

ACQOP Rate of accumulation 0 0 -- lb/ac-d

AOQC Concentration of constituent in ground-water outflow 0 0 -- mg/L

IOQC Concentration of constituent in interflow outflow 0 0 -- mg/L

SQOLIM Maximum storage of constituent 1.0-6 1.0-6 -- lb/ac

WSQOP Rate of runoff that will remove 90 percent of storage 1.64 .01 -- in/hr

RCHRES nutrients

ADNHPM Partition coefficients for NH4 adsorbed to sand, silt, and clay 1.0-10 1.0-10 -- mL/g

ADPOPM Partition coefficients for PO4 adsorbed to sand, silt, and clay 1.0-10 1.0-10 -- mL/g

ANAER DO concentration below which anaerobic conditions exist .005 .0001 1.0 mg/L

BNH4 Constant bed concentrations of NH4 adsorbed to sand, silt, and clay 0 0 -- mg/kg

BPCNTC Percentage by weight of biomass that is carbon 1.98 1.0 5.0 --

BP04 Constant bed concentrations of PO4 adsorbed to sand, silt, and clay 0 0 -- mg/kg

BRPO41 Aerobic rate of benthic release of orthophosphate 0 0 -- mg/m2-hr

BRPO42 Anaerobic rate of benthic release of orthophosphate 0 0 -- mg/m2

-hr

BRTAM1 Aerobic rate of benthic release of total ammonia 0 0 -- mg/m2-hr

BRTAM2 Anaerobic rate of benthic release of total ammonia 0 0 -- mg/m2-hr

CVBO Conversion from milligrams biomass to oxygen 1.98 1.0 5.0 mg/mg

CVBPC Conversion from biomass as phosphorous to carbon 106 50 200 mol/mol

CVBPN Conversion from biomass as phosphorous to nitrogen 16 10 50 mol/mol

DENOXT DO concentration threshold for denitrification 2.00 0 -- mg/L

KNO220 Nitrification rate of nitrite at 20 °C -- .001 -- /hr

KNO320 Denitrification rate at 20 °C -- .001 -- /hr

KTAM20 Nitrification rate of ammonia at 20 °C -- .001 -- /hr

TCDEN Temperature correction coefficients for denitrification 1.07 1.0 2.0 --

TCNIT Temperature correction coefficients for nitrification 1.07 1.0 2.0 --

RCHRES phytoplankton

ALDH High algal unit death rate .01 1.0-6 -- /hr

ALDL Low algal unit death rate .001 1.0-6 -- /hr

ALR20 Algal unit respiration rate at 20 °C .004 1.0-6 -- /hr

ALNPR Fraction of nitrogen required for phytoplankton satisfied by nitrate 1.0 .01 1.0 --

CLALDH Chlorophyll A concentration above which high algal death rate occurs 50 .01 -- μg/L

Table 1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN—Continued

Parameter Description1 Default Minimum Maximum Units

Footnote at end of table.
40 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

1

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

parameter.

CMMLT Michaelis-Menton constant for light-limited growth 0.033 1.0-6 -- ly/min

CMMN Nitrate Michaelis-Menton constant for nitrogen-limited growth .045 1.0-6 -- mg/L

CMMNP Nitrate Michaelis-Menton constant for phosphorous-limited growth .0284 1.0-6 -- mg/L

CMMP Phosphate Michaelis-Menton constant for phosphorous-limited 

growth

.015 1.0-6 -- mg/L

EXTB Base extinction coefficient for light -- .001 -- /ft

LITSED Multiplication factor for sediment contribution to light extinction 0 0 -- L/mg-ft

MALGR Maximal unit algal growth rate .3 .001 -- /hr

MXSTAY Plankton concentration not subject to advection at very low flow 0 0 -- mg/L

NALDH Inorganic nitrogen concentration below which high algal death rate 

occurs

0 0 -- mg/L

NONREF Nonrefractory fraction of algae and zooplankton biomass .5 .01 1.0 --

OREF Outflow at which plankton concentration is midway between SEED 

and MXSTAY

.0001 .0001 -- ft3/s

OXALD Increment to phytoplankton unit death rate (anaerobic) .03 1.0-6 -- /hr

PALDH Inorganic phosphorous concentration below which high algal death 

rate occurs

0 0 -- mg/L

PHYSET Rate of phytoplankton settling 0 0 -- ft/hr

RATCLP Ratio of chlorophyll A of biomass to phosphorous content .6 .01 -- --

REFSET Rate of settling for dead refractory organics 0 0 -- ft/hr

SEED Minimum plankton concentration not subject to advection 0 0 -- mg/L

TALGRH Temperature above which algal growth ceases 95 50 212 °F

TALGRL Temperature below which algal growth ceases 43 32 212 °F

TALGRM Temperature below which algal growth is retarded 77 32 212 °F

Table 1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN—Continued

Parameter Description1 Default Minimum Maximum Units
Table 3 41

Table 3

Table 2

Table 2. Basin-related model parameters for the Hydrological Simulation Program—FORTRAN 

[PERLND, pervious land segment; IMPLND, impervious land segment; ac, acres; mi, miles; FTABLE, table of depth, surface area, volume, 

and discharge for each reach; ft, feet; ac-ft, acre-feet; ft3/s, cubic feet per second] 

1

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

parameter.

Table 3. Selected physical and hydrologic characteristics of the Arroyo Colorado subbasins

[mi, miles; mi2, square miles]

1

 Station operated by National Oceanic and Atmospheric Administration.

2

 Station operated by International Boundary and Water Commission.

3 Station operated by Texas Natural Resource Conservation Commission.

Parameter Description1

(unit)

AREA Drainage area of each PERLND or IMPLND (ac)

LEN Reach length (mi)

DEPTH FTABLE depth (ft)

SAREA FTABLE surface area (ac)

VOL FTABLE volume (ac-ft)

DISCH FTABLE discharge (ft3/s)

Subbasin/

reach

Location of

subbasin outlet

Reach 

length

(mi)

Drainage 

area 

(m2)

Precipitation 

station 

name1

Streamflowgaging

station no.2

Water-quality

sampling

site no.3

Segment 2202 (nontidal segment)

1 McAllen 10.0 26.9 -- -- --

2 Donna 6.6 28.1 McAllen -- --

3 Pharr 9.8 43.8 -- -- --

4 Weslaco 5.2 46.1 Weslaco 08–4770.50 13081

5 Mercedes 6.0 33.0 -- 08–4703.00 --

6 La Feria 6.5 45.1 -- -- --

7 Harlingen 11.5 62.4 Harlingen 08–4704.00 13079

8 Segment outlet -- 82.0 -- -- 13074

Segment 2201 (tidal segment)

9 Rio Hondo -- 80.2 -- -- --

10 La Leona -- 97.1 -- -- --

11 Laguna Atascosa -- 71.1 -- -- --

12 Segment outlet -- 86.7 -- -- --
42 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Table 5

Table 4

Table 4. Permitted point-source daily effluent limits, Arroyo Colorado Basin

[Mgal/d, million gallons per day; mg/L, milligrams per liter; --, no limit]

Table 5. Irrigation schedule for row crops, citrus, and sugar cane for dry, normal, and wet years, Arroyo Colorado 

Basin, 1989–99

[ac-in/ac, acre-inches per acre; in., inches; >, greater than; <, less than]

Permit no. Name of discharger Subbasin/

reach

Permitted limit

Flow

(Mgal/d)

 Suspended 

solids

(mg/L)

Biochemical

oxygen

demand

(mg/L)

Ammonia

nitrogen

(mg/L)

WQ0001254–000 Central Power and Light Co. Bates 1 2.00 48 -- --

WQ0010484–001 City of Mission 1 4.60 15 10 3.0

WQ0010633–003 City of McAllen 2 10.00 15 10 3.0

WQ0011080–001 City of Hidalgo 2 .41 90 30 --

WQ0010596–001 City of Pharr 3 5.00 15 10 3.0

WQ0011512–001 City of San Juan 3 1.15 20 20 --

WQ0013633–001 City of Alamo 3 2.00 90 30 --

WQ0013680–001 Donna Independent School District 4 .02 20 20 --

WQ0010504–001 City of Donna 4 2.30 20 20 --

WQ0010619–005 City of Weslaco 4 2.00 -- 10 3.0

WQ0013462–001 Military Highway Water Supply Corp. 5 .40 90 30 --

WQ0010347–001 City of Mercedes 6 2.30 15 10 3.0

WQ0010697–001 City of La Feria Utility Board 6 .50 90 30 --

WQ0011628–001 Winter Garden Park Association 7 .01 20 20 --

WQ0001256–000 Central Power and Light Co. La Palma 8 1.12 30 -- --

WQ0010490–002 City of Harlingen no. 1 8 3.10 20 20 --

WQ0010490–003 City of Harlingen no. 2 8 7.50 15 10 3.0

WQ0010473–002 City of San Benito 8 2.16 30 30 --

WQ0010475–002 City of Rio Hondo 9 .40 20 20 --

WQ0003457–000 Southern Star Inc. (Hung Group) 12 50.00 15 20 1.0

WQ0003596–000 Arroyo Aquaculture (formerly Taiwan Farms) 12 100.00 30 4 1.0

Year

Water diverted from RIo Grande

(ac-in/ac) No. of irrigations Depth of irrigation(in.)

Annual irrigation

(in.)

Row crops

Dry >25 3 6 18

Normal 13–25 2 6 12

Wet <13 1 6 6

Citrus

Dry >25 6 5 30

Normal 13–25 5 5 25

Wet <13 4 5 20

Sugar cane

Dry >25 8 6 48

Normal 13–25 6 6 36

Wet <13 4 6 24
Table 6 43

Table 6

Table 6. Basin-related parameters, Arroyo Colorado Basin[ac, acres; mi, miles; --, not applicable; ft, feet; ac-ft, acre-feet; ft3/s, cubic feet per second] 

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

Parameter1 Subbasin/reach

1 2 3 4 5 6 7 8 9 10 

PERLND AREA (ac)

Low-density urban–1 0 82.0 886.8 242.8 487.5 411.6 1,034.5 3,403.3 1,294.3 12Low-density urban–2 2,340.9 1,892.4 808 1,694.5 1,976.4 918.6 1,530.7 2,189.7 2,386.0 50Land application–1 0 0 0 0 0 0 29.0 0 0 Land application–2 1.4 0 0 0 0 60.0 0 59.0 151.7 Colonias–1 0 0 3.3 14.5 10.4 4.4 2.5 6.6 2.2 Colonias–2 55.2 2.4 10.7 69.8 52.7 30.3 6.4 15.5 49.6 Septic–1 0 0 0 32.5 0 58.0 72.4 0 1.1 Septic–2 0 0 0 18.8 0 10.4 2.0 0 46.0 High-density urban–1 0 99.2 63.4 8.6 23.5 31.3 121.5 654.3 675.9 High-density urban–2 278.7 603.5 69.2 139.6 253.8 146.3 160.1 237.3 282.1 2Natural vegetation–1 0 1,394.9 2,755.4 2,161.4 1,256.7 1,984.0 3,392.2 3,574.4 1,544.2 1,57Natural vegetation–2 4,443.9 2,647.9 1,637.3 1,739.3 1,138.0 2,322.0 3,342.5 2,700.3 2,476.3 4,77Pasture/hay–1 0 175.8 725.0 254.6 566.1 713.1 988.0 2,238.5 123.4 7Pasture/hay–2 602.0 202.4 255.2 361.3 390.5 680.4 733.6 2,132.6 1,183.2 200Row crops nonirrigated–1 0 0 0 0 0 0 0 0 0 5,17Row crops nonirrigated–2 0 0 0 0 0 0 0 0 3,305.1 16,54Row crops irrigated–1 0 1,146.3 13,971.7 11,473.3 5,663.2 9,467.1 9,931.2 16,364.2 10,535.9 6,14Row crops irrigated–2 7,066.4 5,212.2 3,510.8 5,744.9 3,118.8 7,168.9 10,477.0 9,601.0 19,206.0 20,21Citrus–1 0 0 6.3 58.2 94.2 0 0 0 0 Citrus–2 274.8 220.5 69.3 916.2 664.8 368.1 570.9 67.5 187.2 32Citrus tile drain–1 0 0 0 0 0 133.8 26.1 0 72.3 Citrus tile drain–2 0 0 0 0 0 64.2 1,034.1 125.7 246.6 18Sugar cane–1 0 62.7 1,155.9 2,050.5 678.6 825.9 0 0 0 Sugar cane–2 117.0 415.8 641.4 362.1 2,509.2 448.5 948.6 390.9 692.4 1,28Sugar cane tile drain–1 0 0 0 0 0 665.7 981.3 149.7 639.9 Sugar cane tile drain–2 0 0 0 0 0 0 918.9 1,119.0 539.1 71

IMPLND AREA (ac)

Low-density urban 793.4 658.9 577.9 684.8 840.7 475.8 881.2 1,871.5 1,310.3 21High-density urban 840.2 2,108.2 398.0 446.5 832.5 535.0 844.7 2,674.8 2,873.9 7Water/wetlands 396.2 1,027.7 453.4 1,039.2 572.1 1,366 1,888.4 2,871.7 1,472.4 2,18

LEN (mi) 10.0 6.6 9.8 5.2 6.0 6.5 11.5 -- -- -FTABLE

DEPTH (ft) 0–16.0 0–16.0 0–20.0 0–20.0 0–22.0 0–24.0 0–24.0 0–24.0 -- -

SAREA (ac) 0–132 0–143 0–164 0–100 0–400 0–200 0–551 0–443 -- -VOL (ac-ft) 0–1,880 0–1,910 0–2,740 0–2,770 0–5,000 0–6,080 0–8,520 0–10,400 -- -

DISCH (ft3/s) 0–4,280 0–3,580 0–7,150 0–7,590 0–24,100 0–16,900 0–20,500 0–23,500 -- -
44 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 Table 7 Table 7. Annual PERLND and IMPLND process-related parameters for flow, Arroyo Colorado Basin [Parameter definitions in table 1; units below parameter except where no units. /d, per day; in/hr, inches per hour; /in., per inch; ft, feet; in., inches; ft/f

1 Default value used.2 Initial estimates from Dean and others (1984).3 Parameter revised during calibration.

Land

segment AGWETP1 AGWRC2,3 (/d) BASETP1 DEEPFR1 INFEXP1 INFILD1 INFILT2,3 (in/hr) INTFW2,3 IRC2,3 (/d) KVARY1 (/in.) LSUR2,3 (ft) LZSN2,3 (in.) 

PERLND

Low-density urban–1 0 0.995 0 0 2.0 2.0 0.10 2.0 0.60 0 300 1.0 Low-density urban–2 0 .995 0 0 2.0 2.0 .60 2.0 .60 0 300 3.0 Land application–1 0 .995 0 0 2.0 2.0 .10 2.0 .60 0 300 1.0 Land application–2 0 .995 0 0 2.0 2.0 .60 2.0 .60 0 300 3.0 Colonias–1 0 .995 0 0 2.0 2.0 .10 2.0 .60 0 300 1.0 Colonias–2 0 .995 0 0 2.0 2.0 .60 2.0 .60 0 300 3.0 Septic–1 0 .995 0 0 2.0 2.0 .10 2.0 .60 0 300 1.0 Septic–2 0 .995 0 0 2.0 2.0 .60 2.0 .60 0 300 3.0 High-density urban–1 0 .995 0 0 2.0 2.0 .10 2.0 .60 0 300 1.0 High-density urban–2 0 .995 0 0 2.0 2.0 .60 2.0 .60 0 300 3.0 Natural vegetation–1 0 .995 0 0 2.0 2.0 .10 3.0 .60 0 500 2.0 Natural vegetation–2 0 .995 0 0 2.0 2.0 .60 3.0 .60 0 500 4.0 Pasture/hay–1 0 .995 0 0 2.0 2.0 .10 3.0 .60 0 500 2.0 Pasture/hay–2 0 .995 0 0 2.0 2.0 .60 3.0 .60 0 500 4.0 Row crops nonirrigated–1 0 .995 0 0 2.0 2.0 .10 3.0 .60 0 500 3.0 Row crops nonirrigated–2 0 .995 0 0 2.0 2.0 .60 3.0 .60 0 500 5.0 Row crops irrigated–1 0 .995 0 0 2.0 2.0 .10 3.0 .60 0 500 3.0 Row crops irrigated–2 0 .995 0 0 2.0 2.0 .60 3.0 .60 0 500 5.0 Citrus–1 0 .995 0 0 2.0 2.0 .10 3.0 .60 0 500 3.0 Citrus–2 0 .995 0 0 2.0 2.0 .60 3.0 .60 0 500 5.0 Citrus tile drain–1 0 .995 0 0 2.0 2.0 .05 9.0 .80 0 300 3.0 Citrus tile drain–2 0 .995 0 0 2.0 2.0 .30 9.0 .80 0 300 5.0 Sugar cane–1 0 .995 0 0 2.0 2.0 .10 3.0 .60 0 500 3.0 Sugar cane–2 0 .995 0 0 2.0 2.0 .60 3.0 .60 0 500 5.0 Sugar cane tile drain–1 0 .995 0 0 2.0 2.0 .05 9.0 .80 0 300 3.0 Sugar cane tile drain–2 0 .995 0 0 2.0 2.0 .30 9.0 .80 0 300 5.0 

IMPLND

Low-density urban -- -- -- -- -- -- -- -- -- -- 300 -- High-density urban -- -- -- -- -- -- -- -- -- -- 300 -- Water/wetlands -- -- -- -- -- -- -- -- -- -- 300 -- 
Table 8 45

Table 8 Table 8. Monthly PERLND and IMPLND process-related parameters for flow, Arroyo Colorado Basin

[Parameter definitions in table 1; units below parameter except where no units. in., inches]

1 Initial estimates from Dean and others (1984).

2 Parameter revised during calibration.

Land

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

CEPSC1,2

(in.)

Low-density urban 0.03 0.03 0.05 0.06 0.10 0.15 0.15 0.12 0.10 0.06 0.03 0.03

High-density urban .03 .03 .05 .06 .10 .15 .15 .12 .10 .06 .03 .03

Natural vegetation .03 .03 .05 .06 .10 .15 .15 .12 .10 .06 .03 .03

Pasture/hay .03 .03 .05 .06 .10 .15 .15 .12 .10 .06 .03 .03

Row crops nonirrigated .13 .13 .15 .16 .20 .25 .25 .25 .15 .16 .03 .03

Row crops irrigated .13 .13 .15 .16 .20 .25 .25 .25 .15 .16 .03 .03

Citrus .20 .20 .20 .25 .25 .25 .25 .25 .20 .20 .20 .20

Sugar cane .15 .15 .15 .20 .25 .25 .25 .25 .20 .15 .15 .15

LZETP1,2

Low-density urban .30 .30 .50 .60 .70 .75 .85 .85 .70 .45 .30 .30

High-density urban .30 .30 .50 .60 .70 .75 .85 .85 .70 .45 .30 .30

Natural vegetation .40 .40 .50 .60 .70 .75 .85 .85 .70 .55 .40 .40

Pasture/hay .40 .40 .50 .60 .70 .75 .85 .85 .70 .55 .40 .40

Row crops nonirrigated .40 .45 .50 .65 .70 .75 .85 .85 .75 .65 .40 .40

Row crops irrigated .40 .45 .55 .65 .70 .75 .85 .85 .75 .65 .45 .40

Citrus .40 .45 .55 .65 .75 .80 .90 .90 .75 .65 .45 .45

Sugar cane .45 .45 .55 .60 .75 .80 .90 .90 .75 .65 .45 .45

UZSN1,2

(in.)

Low-density urban .15 .15 .15 .20 .20 .20 .15 .15 .15 .15 .15 .15

High-density urban .15 .15 .15 .20 .20 .20 .15 .15 .15 .15 .15 .15

Natural vegetation .15 .20 .25 .20 .25 .20 .20 .20 .25 .25 .15 .15

Pasture/hay .15 .20 .35 .30 .25 .20 .20 .20 .25 .25 .15 .15

Row crops nonirrigated .15 .20 .35 .30 .25 .20 .20 .20 .25 .25 .15 .15

Row crops irrigated .15 .20 .35 .30 .25 .20 .20 .20 .25 .25 .15 .15

Citrus .20 .25 .35 .35 .35 .35 .35 .35 .25 .25 .25 .20

Sugar cane .20 .25 .30 .35 .35 .35 .35 .35 .25 .25 .25 .20
46 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 Table 9 Table 9. Selected calibration and testing results using HSPEXP, Arroyo Colorado Basin [Error, ([simulated–measured]/measured)*100; in., inches; --, not used] Weslaco Mercedes 

Measured (in.) Simulated (in.) Error (percent) Measured (in.) Simulated (in.) Error (percent) Measured (in.) 

Calibration 1989–95 Total flow volume 87.8 87.7 -0.1 -- -- -- 88.6 

Total of highest 10-percent flows 22.0 21.5 -2.3 -- -- -- 20.3 Total of lowest 50-percent flows 30.3 30.5 .7 -- -- -- 32.2 Summer flow volume 23.1 25.9 12.1 -- -- -- 24.7 Winter flow volume 19.0 18.8 -1.1 -- -- -- 18.4 Total storm volume 8.7 8.0 -8.0 -- -- -- 8.9 

Summer storm volume 2.4 2.8 16.7 -- -- -- 2.5 

Spatial testing 1989–95 Total flow volume -- -- -- 81.4 87.5 7.5 -- 

Total of highest 10-percent flows -- -- -- 19.2 21.0 9.4 -- Total of lowest 50-percent flows -- -- -- 29.9 30.4 1.7 -- Summer flow volume -- -- -- 22.2 26.3 18.5 -- Winter flow volume -- -- -- 17.3 18.6 7.5 -- Total storm volume -- -- -- 8.0 7.9 -1.3 -- Summer storm volume -- -- -- 2.5 2.9 16.0 -- 

Temporal testing 1996–99

Total flow volume -- -- -- 36.5 47.1 29.0 43.6 

Total of highest 10-percent flows -- -- -- 10.6 12.0 13.2 14.4 Total of lowest 50-percent flows -- -- -- 12.3 15.7 27.6 14.1 Summer flow volume -- -- -- 8.7 10.2 17.2 8.8 

Winter flow volume -- -- -- 6.9 10.1 46.4 8.3 Total storm volume -- -- -- 10.7 11.6 8.4 14.7 

Summer storm volume -- -- -- 2.3 2.5 8.7 2.1 
Table 10 47

Table 10

Table 10. Annual PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin

[Parameter definitions in table 1; units below parameter except where no units. tons/ac-d, tons per acre per day; /d, per day; --, not used or not 

applicable; lb/ac-d, pounds per acre per day; mg C/L, milligrams carbon per liter; °F, degrees Fahrenheit; ft, feet; in/hr, inches per hour] 

Land

segment

ACCSDP

(tons/ac-d)

AFFIX

(/d)

JEIM JGER JRER JSER KEIM KGER KRER KSER

PERLND

Low-density urban -- 0.001 -- 1.0 2.2 2.5 -- 0 0.40 1.6

Land application -- .001 -- 1.0 2.2 2.5 -- 0 .40 1.6

Colonias -- .001 -- 1.0 2.2 2.5 -- 0 .40 1.6

Septic -- .001 -- 1.0 2.2 2.5 -- 0 .40 1.6

High-density urban -- .001 -- 1.0 2.2 2.5 -- 0 .40 1.8

Natural vegetation -- .001 -- 1.0 2.2 2.5 -- 0 .40 1.6

Pasture/hay -- .001 -- 1.0 2.0 2.0 -- 0 .35 2.0

Row crops nonirrigated -- .005 -- 1.0 2.0 2.0 -- 0 .35 2.0

Row crops irrigated -- .005 -- 1.0 2.0 2.0 -- 0 .35 1.2

Citrus -- .005 -- 1.0 2.0 2.0 -- 0 .30 3.5

Citrus tile drain -- .005 -- 1.0 2.0 2.0 -- 0 .30 3.0

Sugar cane -- .005 -- 1.0 2.0 2.0 -- 0 .35 2.5

Sugar cane tile drain -- .005 -- 1.0 2.0 2.0 -- 0 .35 2.0

IMPLND

Low-density urban 0.005 -- 2.0 -- -- -- 0.02 -- -- --

High-density urban .005 -- 2.0 -- -- -- .02 -- -- --

Water/wetlands -- -- -- -- -- -- -- -- -- --

Land

segment

NVSI

(lb/ac-d)

REMSDP

(/d)

SMPF

ACO2P

(mg C/L)

AWTF

(°F)

BWTF

(°F)

ELEV

(ft)

ICO2P

(mg C/L)

WSQOP

(in/hr)

PERLND

Low-density urban 0 -- 1.0 -- -- -- 100 -- 0.50

Land application 0 -- 1.0 -- -- -- 100 -- .50

Colonias 0 -- 1.0 -- -- -- 100 -- .50

Septic 0 -- 1.0 -- -- -- 100 -- .50

High-density urban 0 -- 1.0 -- -- -- 100 -- .50

Natural vegetation 0 -- 1.0 -- -- -- 100 -- .50

Pasture/hay 0 -- 1.0 -- -- -- 100 -- .50

Row crops nonirrigated 0 -- 1.0 -- -- -- 100 -- .50

Row crops irrigated 0 -- 1.0 -- -- -- 100 -- .50

Citrus 0 -- 1.0 -- -- -- 100 -- .50

Citrus tile drain 0 -- 1.0 -- -- -- 100 -- .50

Sugar cane 0 -- 1.0 -- -- -- 100 -- .50

Sugar cane tile drain 0 -- 1.0 -- -- -- 100 -- .50

IMPLND

Low-density urban -- 0.01 -- -- 34 0.6 100 -- .50

High-density urban -- .01 -- -- 34 .6 100 -- .50

Water/wetlands -- -- -- -- -- -- -- -- --
48 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Table 11

.

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin—

Continued

Land segment (PERLND 

unless labeled IMPLND) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

COVER1,2

Low-density urban3 0.70 0.70 0.70 0.70 0.70 0.70 0.70 0.70 0.70 0.70 0.70 0.70

High-density urban .70 .70 .70 .70 .70 .70 .70 .70 .70 .70 .70 .70

Natural vegetation .80 .80 .80 .80 .80 .80 .80 .80 .80 .80 .80 .80

Pasture/hay .60 .60 .60 .60 .60 .60 .60 .60 .60 .60 .60 .60

Row crops nonirrigated .21 .17 .24 .36 .58 .65 .72 .72 .36 .24 .21 .21

Row crops irrigated .31 .27 .34 .46 .58 .65 .72 .72 .46 .34 .31 .31

Citrus .45 .45 .45 .45 .45 .45 .45 .45 .45 .45 .45 .45

Citrus tile drain .50 .50 .50 .50 .50 .50 .50 .50 .50 .50 .50 .50

Sugar cane .31 .27 .34 .46 .58 .65 .72 .72 .46 .34 .31 .31

Sugar cane tile drain .45 .42 .49 .61 .73 .0 .82 .82 .61 .49 .45 .45

ASLT1,2 (°F)

All pervious land uses 34 34 37.5 43 50 60 60 60 53.5 45 40 35.5

BSLT1,2 (°F)

All pervious land uses .50 .50 .50 .50 .50 .50 .50 .50 .50 .50 .50 .50

LGTP11,2 (°F)

All pervious land uses 45 45 50 55 55 60 60 60 60 55 50 45

ULTP11,2 (°F)

All pervious land uses 34.5 34.5 41 51 62.5 74 81 81 70 53.5 45 37.5

ULTP21,2 (°F)

All pervious land uses .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10

ADOXP1,2 (mg/L)

All pervious land uses 11 10 10 9 8 8 7 7 8 8 10 11

IDOXP1,2 (mg/L)

All pervious land uses 11 10 10 9 8 8 7 7 8 8 10 11

ACQOP1,2 (lb/ac-d) -BOD

Low-density urban3 .25 .25 .25 .25 .25 .25 .25 .25 .25 .25 .25 .25

High-density urban .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20

Natural vegetation .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20

Pasture/hay .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20 .20

Row crops nonirrigated .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Row crops irrigated .50 .50 .50 .50 .50 .50 .50 .50 .50 .50 .50 .50

Citrus .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Citrus tile drain .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Sugar cane .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Sugar cane tile drain .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Low-density urban IMPLND .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

High-density urban IMPLND .35 .35 .35 .35 .35 .35 .35 .35 .35 .35 .35 .35

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin

[Parameter definitions in table 1; units shown for parameter except where no units. °F, degrees Fahrenheit; mg/L, milligrams per liter; 

lb/ac-d, pounds per acre per day; BOD, biochemical oxygen demand; lb/ac, pounds per acre]

Footnotes at end of table.
Table 11 49

AOQC1,2 (mg/L) - BOD

Low-density urban3 16 16 14 10 10 10 10 10 12 14 16 16

Land application 20 20 20 20 20 20 20 20 20 20 20 20

Colonias 20 20 20 20 20 20 20 20 20 20 20 20

Septic 20 20 20 20 20 20 20 20 20 20 20 20

High-density urban 16 16 14 10 10 10 10 10 12 14 16 16

Natural vegetation 14 14 14 10 10 10 10 10 12 14 14 14

Pasture/hay 14 14 14 10 10 10 10 10 12 14 14 14

Row crops nonirrigated 6 6 6 4 4 4 4 4 6666

Row crops irrigated 6 6 6 4 4 4 4 4 6666

Citrus 6 6 6 4 4 4 4 4 6666

Citrus tile drain 6 6 6 4 4 4 4 4 6666

Sugar cane 6 6 6 4 4 4 4 4 6666

Sugar cane tile drain 6 6 6 4 4 4 4 4 6666

IOQC1,2 (mg/L) - BOD

Low-density urban3 16 16 14 10 10 10 10 10 12 14 16 16

Land application 20 20 20 20 20 20 20 20 20 20 20 20

Colonias 20 20 20 20 20 20 20 20 20 20 20 20

Septic 20 20 20 20 20 20 20 20 20 20 20 20

High-density urban 16 16 14 10 10 10 10 10 12 14 16 16

Natural vegetation 14 14 14 10 10 10 10 10 12 14 14 14

Pasture/hay 14 14 14 10 10 10 10 10 12 14 14 14

Row crops nonirrigated 6 6 6 4 4 4 4 4 6666

Row crops irrigated 6 6 6 4 4 4 4 4 6666

Citrus 6 6 6 4 4 4 4 4 6666

Citrus tile drain 6 6 6 4 4 4 4 4 6666

Sugar cane 6 6 6 4 4 4 4 4 6666

Sugar cane tile drain 6 6 6 4 4 4 4 4 6666

SQOLIM1,2 (lb/ac) - BOD

Low-density urban3 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

High-density urban 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

Natural vegetation 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

Pasture/hay 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

Row crops nonirrigated 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Row crops irrigated 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

Citrus 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Citrus tile drain 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Sugar cane 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Sugar cane tile drain 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Low-density urban IMPLND 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

High-density urban IMPLND 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin—

Continued

Land segment (PERLND 

unless labeled IMPLND) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Footnotes at end of table.
50 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

ACQOP1,2 (lb/ac-d) - NO3

Low-density urban3 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08

High-density urban .08 .08 .08 .08 .08 .08 .08 .08 .08 .08 .08 .08

Natural vegetation .08 .08 .08 .08 .08 .08 .08 .08 .08 .08 .08 .08

Pasture/hay .08 .08 .08 .08 .08 .08 .08 .08 .08 .08 .08 .08

Row crops nonirrigated .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Row crops irrigated .90 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Citrus .30 .90 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Citrus tile drain .30 .90 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Sugar cane .90 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Sugar cane tile drain .30 .30 .30 .30 .30 .30 .30 .30 .90 .30 .30 .30

Low-density urban IMPLND .016 .016 .016 .016 .016 .016 .016 .016 .016 .016 .016 .016

High-density urban IMPLND .015 .015 .015 .015 .015 .015 .015 .015 .015 .015 .015 .015

AOQC1,2 (mg/L) - NO3

Low-density urban3 .4 .4 .4 .4 .3 .3 .3 .3 .3 .3 .4 .4

Land application 10 10 10 10 10 10 10 10 10 10 10 10

Colonias 10 10 10 10 10 10 10 10 10 10 10 10

Septic 10 10 10 10 10 10 10 10 10 10 10 10

High-density urban .4 .4 .4 .4 .3 .3 .3 .3 .3 .3 .4 .4

Natural vegetation .5 .5 .5 .5 .4 .4 .4 .4 .4 .4 .5 .5

Pasture/hay .4 .4 .4 .4 .3 .3 .3 .3 .3 .3 .4 .4

Row crops nonirrigated 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5

Row crops irrigated 6.0 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5

Citrus 4.0 6.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

Citrus tile drain 4.0 6.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

Sugar cane 4.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

Sugar cane tile drain 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 4.0 2.5 2.5 2.5

IOQC1,2 (mg/L) - NO3

Low-density urban3 .4 .4 .4 .4 .3 .3 .3 .3 .3 .3 .4 .4

Land application 10 10 10 10 10 10 10 10 10 10 10 10

Colonias 10 10 10 10 10 10 10 10 10 10 10 10

Septic 10 10 10 10 10 10 10 10 10 10 10 10

High-density urban .4 .4 .4 .4 .3 .3 .3 .3 .3 .3 .4 .4

Natural vegetation .5 .5 .5 .5 .4 .4 .4 .4 .4 .4 .5 .5

Pasture/hay .4 .4 .4 .4 .3 .3 .3 .3 .3 .3 .4 .4

Row crops nonirrigated 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5

Row crops irrigated 6.0 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5

Citrus 4.0 6.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

Citrus tile drain 4.0 6.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

Sugar cane 4.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

Sugar cane tile drain 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 4.0 2.5 2.5 2.5

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin—

Continued

Land segment (PERLND 

unless labeled IMPLND) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Footnotes at end of table.
Table 11 51

SQOLIM1,2 (lb/ac) - NO3

Low-density urban3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

High-density urban .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Natural vegetation .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Pasture/hay .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Row crops nonirrigated 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Row crops irrigated 3.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Citrus 1.0 3.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Citrus tile drain 1.0 3.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Sugar cane 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 3.0 1.0 1.0 1.0

Sugar cane tile drain 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Low-density urban IMPLND .16 .16 .16 .16 .16 .16 .16 .16 .16 .16 .16 .16

High-density urban IMPLND .15 .15 .15 .15 .15 .15 .15 .15 .15 .15 .15 .15

ACQOP1,2 (lb/ac-d) - NH3

Low-density urban3 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01

High-density urban .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01

Natural vegetation .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01

Pasture/hay .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01

Row crops nonirrigated .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10

Row crops irrigated .15 .15 .15 .15 .15 .15 .15 .15 .15 .15 .15 .15

Citrus .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10

Citrus tile drain .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10

Sugar cane .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10

Sugar cane tile drain .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10 .10

Low-density urban IMPLND .006 .006 .006 .006 .006 .006 .006 .006 .006 .006 .006 .006

High-density urban IMPLND .005 .005 .005 .005 .005 .005 .005 .005 .005 .005 .005 .005

AOQC1,2 (mg/L) - NH3

Low-density urban3 .25 .25 .25 .25 .20 .10 .10 .10 .10 .20 .25 .25

Land application 5 5 5 5 5 5 5 5 5555

Colonias 5 5 5 5 5 5 5 5 5555

Septic 5 5 5 5 5 5 5 5 5555

High-density urban .25 .25 .25 .25 .20 .10 .10 .10 .10 .20 .25 .25

Natural vegetation .20 .20 .20 .20 .15 .10 .10 .10 .10 .15 .20 .20

Pasture/hay .20 .20 .20 .20 .15 .10 .10 .10 .10 .15 .20 .20

Row crops nonirrigated 1.0 1.0 1.0 .8 .7 .6 .6 .6 .6 .7 .8 .8

Row crops irrigated 1.7 1.7 1.7 1.7 1.7 1.6 1.6 1.6 1.6 1.7 1.7 1.7

Citrus 1.4 1.3 1.3 1.3 1.2 1.2 1.2 1.2 1.2 1.3 1.3 1.3

Citrus tile drain 1.4 1.3 1.3 1.3 1.2 1.2 1.2 1.2 1.2 1.3 1.3 1.3

Sugar cane 2.0 2.0 2.0 1.8 1.7 1.6 1.6 1.6 1.6 1.7 1.8 2.0

Sugar cane tile drain 2.0 2.0 2.0 1.8 1.7 1.6 1.6 1.6 1.6 1.7 1.8 2.0

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin—

Continued

Land segment (PERLND 

unless labeled IMPLND) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Footnotes at end of table.
52 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

IOQC1,2 (mg/L) - NH3

Low-density urban3 0.25 0.25 0.25 0.25 0.20 0.10 0.10 0.10 0.10 0.20 0.25 0.25

Land application 5 5 5 5 5 5 5 5 5555

Colonias 5 5 5 5 5 5 5 5 5555

Septic 5 5 5 5 5 5 5 5 5555

High-density urban .25 .25 .25 .25 .20 .10 .10 .10 .10 .20 .25 .25

Natural vegetation .20 .20 .20 .20 .15 .10 .10 .10 .10 .15 .20 .20

Pasture/hay .20 .20 .20 .20 .15 .10 .10 .10 .10 .15 .20 .20

Row crops nonirrigated 1.0 1.0 1.0 .8 .7 .6 .6 .6 .6 .7 .8 .8

Row crops irrigated 1.7 1.7 1.7 1.7 1.7 1.6 1.6 1.6 1.6 1.7 1.7 1.7

Citrus 1.4 1.3 1.3 1.3 1.2 1.2 1.2 1.2 1.2 1.3 1.3 1.3

Citrus tile drain 1.4 1.3 1.3 1.3 1.2 1.2 1.2 1.2 1.2 1.3 1.3 1.3

Sugar cane 2.0 2.0 2.0 1.8 1.7 1.6 1.6 1.6 1.6 1.7 1.8 2.0

Sugar cane tile drain 2.0 2.0 2.0 1.8 1.7 1.6 1.6 1.6 1.6 1.7 1.8 2.0

SQOLIM1,2 (lb/ac) - NH3

Low-density urban3 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05

High-density urban .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05

Natural vegetation .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05

Pasture/hay .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05

Row crops nonirrigated .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Row crops irrigated .40 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Citrus .30 .40 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Citrus tile drain .30 .40 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Sugar cane .40 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30 .30

Sugar cane tile drain .30 .30 .30 .30 .30 .30 .30 .30 .40 .30 .30 .30

Low-density urban IMPLND .06 .06 .06 .06 .06 .06 .06 .06 .06 .06 .06 .06

High-density urban IMPLND .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05

ACQOP1,2 (lb/ac-d) - PO4

Low-density urban3 .03 .03 .03 .03 .03 .03 .03 .03 .03 .03 .03 .03

High-density urban .012 .012 .012 .012 .012 .012 .012 .012 .012 .012 .012 .012

Natural vegetation .012 .012 .012 .012 .012 .012 .012 .012 .012 .012 .012 .012

Pasture/hay .012 .012 .012 .012 .012 .012 .012 .012 .012 .012 .012 .012

Row crops nonirrigated .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Row crops irrigated .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Citrus .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Citrus tile drain .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Sugar cane .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Sugar cane tile drain .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2

Low-density urban IMPLND .004 .004 .004 .004 .004 .004 .004 .004 .004 .004 .004 .004

High-density urban IMPLND .003 .003 .003 .003 .003 .003 .003 .003 .003 .003 .003 .003

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin—

Continued

Land segment (PERLND 

unless labeled IMPLND) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Footnotes at end of table.
Table 11 53

1 Initial estimates from Dean and others (1984).

2 Parameter revised during calibration.

3 Low-density urban not subdivided into land application, colonias, septic.

AOQC1,2 (mg/L) - PO4

Low-density urban3 0.10 0.10 0.17 0.20 0.40 0.45 0.45 0.45 0.40 0.30 0.17 0.10

Land application 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Colonias 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Septic 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

High-density urban .03 .03 .04 .06 .08 .10 .10 .10 .08 .06 .04 .03

Natural vegetation .03 .03 .04 .06 .08 .10 .10 .10 .08 .06 .04 .03

Pasture/hay .03 .03 .04 .06 .08 .10 .10 .10 .08 .06 .04 .03

Row crops nonirrigated .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Row crops irrigated .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Citrus .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Citrus tile drain .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Sugar cane .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Sugar cane tile drain .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

IOQC1,2 (mg/L) - PO4

Low-density urban3 .10 .10 .17 .20 .40 .45 .45 .45 .40 .30 .17 .10

Land application 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Colonias 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Septic 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

High-density urban .03 .03 .04 .06 .08 .10 .10 .10 .08 .06 .04 .03

Natural vegetation .03 .03 .04 .06 .08 .10 .10 .10 .08 .06 .04 .03

Pasture/hay .03 .03 .04 .06 .08 .10 .10 .10 .08 .06 .04 .03

Row crops nonirrigated .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Row crops irrigated .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Citrus .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Citrus tile drain .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Sugar cane .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

Sugar cane tile drain .10 .10 .20 .30 .40 .40 .40 .40 .40 .30 .20 .10

SQOLIM1,2 (lb/ac) - PO4

Low-density urban3 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05 .05

High-density urban .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02

Natural vegetation .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02

Pasture/hay .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02

Row crops nonirrigated .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Row crops irrigated .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Citrus .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Citrus tile drain .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Sugar cane .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Sugar cane tile drain .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40 .40

Low-density urban IMPLND .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04

High-density urban IMPLND .03 .03 .03 .03 .03 .03 .03 .03 .03 .03 .03 .03

Table 11. Monthly PERLND and IMPLND process-related parameters for water quality, Arroyo Colorado Basin—

Continued

Land segment (PERLND 

unless labeled IMPLND) Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
54 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 Table 12 Table 12. Annual RCHRES process-related parameters for water quality, Arroyo Colorado Basin [Parameter definitions in table 1; units below parameter except where no units. ft, feet; in., inches; lb/ft2-d, pounds per square foot per day; g/cm3, gram

lb/ft2, pounds per square foot; in/s, inches per second; mg/m2-hr, milligrams per square meter per hour; /hr, per hour; ft/hr, feet per hour; mL/g, millilit

milligrams per liter; mg/kg, milligrams per kilogram; mg/mg, milligrams per milligram; mol/mol, moles per mole; 

μg/L, micrograms per liter; ly/min, 

per foot; L/mg-ft, liters per milligram per foot; ft3/s, cubic feet per second; ft/hr, feet per hour; °F, degrees Fahrenheit] 

Land

segment

BEDWID

(ft)

BEDWRN

(ft)

D

sand

(in.)

D

silt

(in.)

D

clay

(in.)

EXPSND KSAND

M

silt

(lb/ft2-day)

M

clay

(lb/ft2-day)

POR

RH

san

(g/cm

RCHRES

1 20.0 6.0 0.01 0.0006 0.00006 2.0 1.9 0.004 0.0006 0.8 2.5 2 20.0 6.0 .01 .0006 .00006 2.0 2.8 .003 .001 .8 2.5 3 20.0 6.0 .01 .0006 .00006 2.0 4.6 .008 .001 .8 2.5 4 20.0 6.0 .01 .0006 .00006 2.0 11.5 .012 .001 .8 2.5 5 20.0 6.0 .01 .0006 .00006 2.0 13.3 .025 .0006 .8 2.5 6 20.0 6.0 .01 .0006 .00006 2.0 11.4 .020 .004 .8 2.5 7 20.0 6.0 .01 .0006 .00006 2.0 3.6 .025 .003 .8 2.5 8 20.0 6.0 .01 .0006 .00006 2.0 4.9 .033 .002 .8 2.5 

Land

segment

TAUCD

silt 

(lb/ft2)

TAUCD

clay

(lb/ft2)

TAUCS

silt

(lb/ft2)

TAUCS

clay

(lb/ft2)

W

sand

(in/s)

W

silt

(in/s)

W

clay

(in/s)

CFSAEX ELEV (ft) ELDAT (ft) KATR

RCHRES

1 0.016 0.016 0.019 0.019 0.03 0.0003 0.000006 0.85 100.0 0 6.32 .037 .037 .049 .049 .03 .0003 .000006 .85 100.0 0 6.33 .063 .063 .077 .077 .03 .0003 .000006 .85 100.0 0 6.34 .076 .075 .080 .080 .03 .0003 .000006 .85 100.0 0 6.35 .040 .038 .053 .053 .03 .0003 .000006 .85 100.0 0 6.36 .075 .079 .095 .095 .03 .0003 .000006 .85 100.0 0 6.37 .070 .077 .087 .087 .03 .0003 .000006 .85 100.0 0 6.38 .096 .096 .100 .100 .03 .0003 .000006 .85 100.0 0 6.3

Land

segment

BENOD

(mg/m2-hr)

BRBOD1 

(mg/m2-hr)

BRBOD2 

(mg/m2-hr) EXPOD EXPRED EXPREL EXPREV KBOD20 (/hr) KODSET (ft/hr) REAK (/hr) SUPSAT TC

RCHRES

1–4 60 72 100 1.22 0 2.82 0 0.03 0.004 0.20 1.15 5–7 50 72 100 1.22 0 2.82 0 .005 .004 .20 1.15 8 100 72 100 1.22 0 2.82 0 .08 .004 .10 1.15 
Table 12 55

Table 12. Annual RCHRES process-related parameters for water quality, Arroyo Colorado Basin—Continued

Land

segment

ADNHPM

sand

(mL/g)

ADNHPM

silt

(mL/g)

ADNHPM

clay

(mL/g)

ADPOPM

sand

(mL/g)

ADPOPM

silt

(mL/g)

ADPOPM

clay

(mL/g)

ANAER

(mg/L)

BNH4

sand

(mg/kg)

BNH4

silt

(mg/kg)

BNH4

clay

(mg/kg)

BPCN

RCHRES

1–4 10 100 100 100 1,000 1,000 0.2 10 50 50 49 5–7 10 100 100 100 1,000 1,000 .2 10 50 50 49 8 10 100 100 100 1,000 1,000 .2 10 50 50 49 

Land

segment

BPO4

clay

(mg/kg)

BRPO41

(mg/m2-hr)

BRPO42

(mg/m2-hr)

BRTAM1

(mg/m2-hr)

BRTAM2

(mg/m2-hr)

CVBO

(mg/mg)

CVBPC

(mol/mol)

CVBPN

(mol/mol)

DENOXT

(mg/L)

KNO220

(/hr)

KNO

(/h

RCHRES

1–4 100 1 4 10 20 1.98 106 16 4 0.25 05–7 100 1 4 10 20 1.98 106 16 4 .25 8 100 1 4 10 20 1.98 106 16 4 .25 

Land

segment TCNIT ALDH (/hr) ALDL (/hr) ALR20 (/hr) ALNPR CLALDH (μg/L) CMMLT (ly/min) CMMN (mg/L) CMMNP (mg/L) CMMP (mg/L) EXT

(/ft

RCHRES

1–4 1.07 0.01 0.001 0.005 1.0 50 0.03 0.045 0.0284 0.015 0.1 5–7 1.07 .01 .001 .005 1.0 50 .03 .045 .0284 .015 .1 8 1.07 .01 .001 .005 1.0 50 .03 .045 .0284 .015 .1 

Land

segment

MXSTAY

(mg/L)

NALDH

(mg/L) NONREF OREF (ft3/s) OXALD (/hr) PALDH (mg/L) PHYSET (ft/hr) RATCLP REFSET (ft/hr) SEED (mg/L) TALG

(°F

RCHRES

1–4 2 0 0.5 130 0.03 0 0.06 0.60 0.10 1.0 95 5–7 2 0 .5 130 .03 0 .06 .60 .10 1.0 95 8 2 0 .5 130 .03 0 .08 .60 .10 1.0 95 
56 Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

Table 13

Table 13. Selected nonpoint-source event-mean concentrations for calibration and testing of water quality, Arroyo 

Colorado Basin

[mg/L, milligrams per liter; --, no local or regional data available] 

1

 Baldys and others (1998).

2

 Newell and others (1992).

3 Texas Natural Resource Conservation Commission (1998).

4

 Ockerman and Petri (2001).

5

 Flowers and others (1998).

6 Ambiotech Environmental Consultants, Inc. (1998).

7

 Moore (1995).

8

 Texas A&M University (1995).

9

 Guy Fipps, Texas A&M University (written commun., 1999).

Land segment

Suspended

sediment

(mg/L)

Biochemical

oxygen demand

(mg/L)

Nitrate

nitrogen

(mg/L)

Ammonia

nitrogen

(mg/L)

Orthophosphate

(mg/L)

Low-density urban -- 17.3 10.58 10.18 10.21

High-density urban -- 17.0 1.52 1.16 1.08

Natural vegetation -- 26.0 3.54 3.96 3.03

Pasture/hay -- 36.0 3.40 3.30 3.03

Row crops nonirrigated 4366 54 5.24 5.04 5.32

Row crops irrigated -- 62.8 77.6 -- 7.06

Citrus -- -- 834 -- --

Citrus tile drain -- -- -- -- --

Sugar cane -- -- -- -- --

Sugar cane tile drain -- -- 93.5 -- --
Raines and Miranda— Simulation of Flow and Water Quality of the Arroyo Colorado, Texas, 1989–99 

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

–4733