During 2000–2001, rainfall and runoff were monitored in one mixed
agricultural watershed and two rangeland watersheds in San Patricio County,
located in the Coastal Bend area of South Texas. During this period, five
rainfall samples were collected and analyzed for selected nutrients. Ten runoff
samples from nine runoff events were collected at the three watershed monitoring
stations. Runoff samples were analyzed for selected nutrients, major ions, trace
elements, pesticides, and bacteria.
Study area rainfall during 2000 and 2001 was 33.27 and 28.20 inches,
respectively, less than the long-term average annual of 36.31 inches. Total
runoff from the study area watersheds during 2000–2001 was 2.46 inches; the
regional average is about 2 inches per year. Rainfall and runoff during the
study period was typical of historical patterns, with periods of below average
rainfall interspersed with extreme events. Three individual storm events
accounted for about 29 percent of the total rainfall and 86 percent of the total
runoff during 2000–2001.
Runoff concentrations of nutrients, major ions, and trace elements
generally were larger in the mixed agricultural watershed than runoff
concentrations in the rangeland watersheds. Pesticides were detected in two of
eight runoff samples. Three pesticides (atrazine, deethylatrazine, and
trifluralin) were detected in very small concentrations; only deethylatrazine
was detected in a concentration greater than the laboratory minimum reporting
level.
Bacteria in agricultural and rangeland runoff is a potential water-quality
concern as all fecal coliform and E. coli densities in the runoff samples
exceeded Texas Surface Water Quality Standards for receiving waters. However,
runoff and relatively large bacteria densities represent very brief and
infrequent conditions, and the effect on downstream water is not known.
Rainfall deposition is a major source of nitrogen delivered to the study
area. Rainfall nitrogen (mostly ammonia and nitrate) exceeded the runoff yield.
The average annual rainfall deposition of total nitrogen on the study area
watersheds was 1.3 pounds per acre. In contrast, an average annual yield of 0.57
and 0.21 pound per acre of total nitrogen in runoff exited the mixed
agricultural watershed and the rangeland watersheds, respectively.
The Coastal Bend bays and
estuaries system of Texas is one of 28 estuaries in the United States that have
been designated as “Estuaries of National Significance” (Texas Natural
Resource Conservation Commission, 1996). The Coastal Bend bays and estuaries are
affected by nonpoint-source runoff from agricultural land uses within the
12-county Coastal Bend area of South Texas (fig.
1). Agricultural land uses are predominant on about 88 percent of the
Coastal Bend area and range from cattle grazing to row crop farming. About
two-thirds of this agricultural land use is rangeland (Quenzer and others,
1998), the largest single land use category in the Coastal Bend area.
Because information on the
characteristics of runoff from rangeland in the Coastal Bend area is meager, the
U.S. Geological Survey (USGS) studied the hydrology and water quality of three
watersheds consisting of 14,264 acres of mixed agricultural land and coastal
plains rangeland in San Patricio County during January 2000–December 2001. The
study was done in cooperation with the U.S. Department of Agriculture, Natural
Resources Conservation Service; San Patricio Soil and Water Conservation
District; and The Welder Wildlife Foundation.
This report presents the
results of a study to describe hydrologic conditions, to characterize the
quality of rainfall, and to characterize the quantity and quality of stormwater
runoff in one mixed agricultural watershed and two rangeland watersheds in San
Patricio County in the Coastal Bend area of South Texas. Rainfall samples were
collected in the mixed agricultural watershed at a streamflow-gaging and
water-quality station equipped with an automatic rainfall collector. Rainfall
samples were analyzed for selected nutrients. Runoff samples were collected in
each of the three watersheds at a streamflow-gaging and water-quality station
equipped with automatic water samplers. Runoff samples were analyzed for
selected nutrients, major inorganic ions, trace elements, and pesticides. Loads
and yields of selected constituents entering the receiving bays and estuaries
from these watersheds were estimated for 2000–2001.
The
study area watersheds are delineated on the aerial photograph on
plate 1. The Moody Creek mixed agricultural
watershed is the largest (13,818 acres). It is mostly rangeland, but also includes
about 2,500 acres of cropland in the upper part of the watershed as well as
U.S. highway right-of-way. Soils in the Moody Creek watershed are largely a
mixture of sandy loams and clay loams (Soil Conservation Service, 1979). Watersheds
1 and 2 are entirely rangeland and located within the Welder Wildlife Refuge.
Watershed 1 is about 97 acres of loamy sands. Watershed 2 is about 349 acres
of clay soils. The topography of the area is relatively flat with altitudes
ranging from about 25 feet above mean sea level near the outlets of the watersheds
to about 75 feet above mean sea level in the upper Moody Creek watershed. Vegetation
on the rangelands is a mixture of grass and varying densities of brush and woody
vegetation (Drawe, 1997). The study area rangelands are used for grazing domestic
livestock and to provide wildlife habitat.
Creeks in the study area are
ephemeral, producing runoff only after heavy rains. The streamflow-gaging
stations in watersheds 1 and 2 are located at the edge of fields (grassed
ditches) that typically are dry. The lower reach of Moody Creek is inundated by
backwater from the tidal reach of the Aransas River. All study area watersheds
drain to the tidal segment of the Aransas River and then to Copano Bay (fig.
1).
The climate of the area is
classified as subtropical (short, mild winters and long, hot and humid summers).
Prevailing winds are from the southeast throughout the year (Baird and others,
1996).
The
Texas Natural Resource Conservation Commission (TNRCC) has designated water-quality
standards and appropriate uses (such as aquatic life, contact or non-contact
recreation, or drinking water) for specific stream, estuary, and bay segments
(Texas Natural Resource Conservation Commission, 2002). To support the designated
use of the water-body segments, standards for common water-quality indicators
such as dissolved oxygen, temperature, pH, dissolved minerals, and bacteria
have been established for some stream and bay segments. The TNRCC has not developed
segment-specific standards for any of the creeks monitored during this study.
However, some segment-specific standards have been established for the tidal
segment of the Aransas Rivewhich receive rr and Copano Bay, unoff from the study
area. The tidal segment of the Aransas River is designated for contact recreation
and high aquatic life. Similarly, the Copano Bay segment is designated for contact
recreation, high aquatic life, and oyster waters (State of Texas, 2000).
Special thanks are extended to Dr. D. Lynn Drawe, Director of The Welder Wildlife Foundation, for valuable project oversight and technical assistance and to Leroy Wolff, U.S. Department of Agriculture, Natural Resources Conservation Service, Sinton, Tex., for indispensable administrative and technical support.