Hydrologic conditions and water quality in an agricultural area in Kleberg and Nueces Counties, Texas, 1996-98

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.