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In cooperation with the Texas Agricultural Experiment Station,
Caesar Kleberg Wildlife Research Institute, and
Coastal Bend Bays and Estuary Program

Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98

By Darwin J. Ockerman and Brian L. Petri

U.S. Geological Survey
Water-Resources Investigations Report 01–4101


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Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Texas Water-Quality Standards

Acknowledgments

Data-Collection Methods

Rainfall

Runoff

Water Quality

Fertilizer and Pesticide Application

Hydrologic Conditions

Rainfall

Runoff

Water Quality

Rainfall

Event-Mean Nutrient Concentrations

Rainfall Deposition

Runoff

Event-Mean Concentrations

Nutrients and Major Inorganic Ions

Pesticides

Suspended Sediment

Constituent Loads and Yields

Nutrients

Pesticides

Sediment

Runoff-Weighted Average Concentrations

Nutrients

Pesticides

Summary

Selected References

Appendix I–Rainfall Sample Analyses

Appendix II–Runoff Event-Mean Concentrations

Figures

1–2. 
Maps showing:

1. Location of agricultural study area in Kleberg and Nueces Counties, Texas

2. Location of data-collection sites in agricultural study area

3. 
Diagram showing typical timing of agricultural activities in Nueces and Kleberg Counties, Texas
4. 
Hydrograph showing rainfall, discharge, and subsample-collection timing at site 4 during storm event of September 22–24, 1998
5–7. 
Graphs showing:
5. Annual rainfall at National Oceanic and Atmospheric Administration Kingsville weather station, 1969–98
6. Mean monthly rainfall at National Oceanic and Atmospheric Administration Kingsville weather station, 1969–98
7. Deviation of monthly study area rainfall from mean monthly rainfall at National Oceanic and Atmospheric Administration Kingsville weather station, 1969–98
8. 
Hydrograph showing cumulative rainfall and runoff in study area, 1996–98
9. 
Boxplots showing distribution of sulfate concentrations in runoff samples from sites 1–4 and site 5, 1996–98
10. 
Graphs showing herbicide and insecticide detection frequency in 29 runoff samples, 1996–98
11. 
Boxplots showing seasonal distribution of atrazine concentrations in runoff samples, 1996–98
12–14. 
Graphs showing:
12. 
Relation between suspended sediment concentration (C) and discharge (Q) in runoff samples collected at site 5
13. 
Comparison of rainfall deposition and runoff yield of nitrogen during April 2–13, 1997, lower study area
14. 
Relation between suspended sediment load and storm event runoff at site 5, 1996–98

Tables

1. 
Selected characteristics of data-collection sites
2. 
Applications of fertilizers and selected pesticides on 40,540-acre lower study area, 1996–98
3. 
Rainfall, runoff volumes, and runoff coefficients for runoff events, 1996–98
4. 
Summary statistics of selected nutrient concentrations for 19 rainfall samples, 1996–98
5. 
Monthly and annual rainfall deposition of nitrogen and phosphorus, 1996–98
6. 
Summary statistics of selected major ion and nutrient concentrations in runoff samples, 1996–98, and selected regulatory water-quality criteria
7. 
Pesticides applied on lower study area, 1996–98
8. 
Summary statistics of selected pesticide concentrations in runoff samples, 1996–98, and selected regulatory water-quality criteria
9. 
Median concentrations of selected pesticides in runoff samples grouped by seasonal category
10. 
Monthly and annual loads of selected nutrients in runoff, 1996–98
11. 
Annual and average annual yields of selected nutrients in runoff, 1996–98
12. 
Monthly and annual loads of selected pesticides in runoff, 1996–98
13. 
Annual and average annual yields of selected pesticides in runoff, 1996–98
14. 
Monthly and annual loads of sediment in runoff, study area, 1996–98
15. 
Annual and average annual yields of sediment in runoff, study area, 1996–98
16. 
Study area runoff-weighted concentrations of selected nutrients, 1996–98
17. 
Study area runoff-weighted concentrations of selected pesticides, 1996–98

Abstract

During 1996–98, rainfall and runoff were monitored on a 49,680-acre agricultural watershed in Kleberg and Nueces Counties in South Texas. Nineteen rainfall samples were analyzed for selected nutrients, and runoff samples from 29 storms were analyzed for major ions, nutrients, and pesticides. Loads of nutrients in rainfall and loads of nutrients and pesticides in runoff were computed. For a 40,540-acre part of the watershed (lower study area), constituent loads entering the watershed in rainfall, in runoff from the upper study area, and from agricultural chemical applications to the lower study area were compared with runoff loads exiting the lower study area.

Total rainfall for 1996–98 averaged 25.86 inches per year, which is less than the long-term annual average rainfall of 29.80 inches for the area. Rainfall and runoff during 1996–98 were typical of historical patterns, with periods of below average rainfall and runoff interspersed with extreme events. Five individual storms accounted for about 38 percent of the total rainfall and 94 percent of the total runoff.

During the 3-year study, the total nitrogen runoff yield from the lower study area was 1.3 pounds per acre per year, compared with 49 pounds per acre per year applied as fertilizer and 3.1 pounds per acre per year from rainfall. While almost all of the fertilizer and rainfall nitrogen was ammonia and nitrate, most of the nitrogen in runoff was particulate organic nitrogen, associated with crop residue. Total nitrogen exiting the lower study area in surface-water runoff was about 2.5 percent of the nitrogen inputs (fertilizer and rainfall nitrogen). Annual deposition of total nitrogen entering the lower study area in rainfall exceeded net yields of total nitrogen exiting the watershed in runoff because most of the rainfall does not contribute to runoff.

During the study, the total phosphorus runoff yield from the lower study area was 0.48 pound per acre per year compared with 4.2 pounds per acre per year applied as fertilizer and 0.03 pound per acre per year from rainfall.

Twenty-one pesticides were detected in runoff with varying degrees of frequency during the study. The herbicide atrazine was detected in all runoff samples. All of the most frequently detected pesticides (atrazine, trifluralin, simazine, pendimethalin, and diuron) exhibited higher concentrations during the pre-harvest period (March–May) than during the post-harvest period (August–October).

During 1996–98, an average of 0.37 pound per acre per year of atrazine was applied to the lower study area. During the same period, 0.0027 pound per acre per year of atrazine and its breakdown product deethylatrazine exited the lower study area in runoff (about 0.7 percent of the total atrazine applied to the cropland). During 1997, when heavy rainfall occurred during the months of April and May, the atrazine plus deethylatrazine exiting the lower study area was 1.8 percent of the applied atrazine.

The 1996–98 average sediment yield was 610 pounds per acre per year. Sediment loads from the study area are associated with large storm events. Of the 45,300 tons of sediment transported from the study area during 1996–98 about 87 percent was transported during the three largest runoff events (April 1997, October 1997, and October 1998).

Runoff-weighted average concentrations were computed for selected nutrients and pesticides. The 1996–98 runoff-weighted concentrations for total nitrogen and total phosphorus were 1.3 and 0.50 milligrams per liter, respectively. The 1996–98 runoff-weighted concentration for atrazine plus deethylatrazine was 2.7 micrograms per liter.


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