USGS streamflow-gaging and
water-quality sampling stations were installed at the outlets of each of the
study area watersheds (pl. 1; fig.
1) in May 2000 and operated through December
2001. The Texas Agricultural Experiment Station (TAES) in Corpus Christi has
operated a weather station near watershed 1 since November 15, 2000. The
wildlife refuge staff monitors a National Oceanic and Atmospheric Administration
(NOAA) weather station (also near watershed 1) from which rainfall data have
been collected since 1965.
Water-surface elevation
(stage) was continuously recorded during runoff events at all three
streamflow-gaging stations using a gas-bubbler and pressure transducer system (Rantz
and others, 1982). At Moody Creek (station 08189710), tidal backwater conditions
result in a variable relation between stage and discharge. An acoustic doppler
velocimeter also was used to measure stream velocity. Correlations between
stage, velocity, and measured discharge were used to develop the discharge
rating and to compute continuous discharge at this station (Patino and Ockerman,
1997). Relations between stage and runoff (discharge) were developed at
watersheds 1 and 2 (stations 08189711 and 08189714, respectively) by making
independent discharge measurements (Buchanan and Somers, 1969; Kennedy, 1984).
Water-quality samples were
collected from two sources. Rainfall samples were collected primarily to
determine rainfall nitrogen delivered to the study area. Runoff samples were
collected to characterize runoff quality and to estimate constituent loads and
yields transported to receiving waters.
Rainfall samples were
collected at the Moody Creek station by an automatic rainfall collector. The
collector is equipped with polyethylene buckets that are covered when rainfall
is not occurring to prevent contamination and evaporation of the sample. A
moisture sensor activates a mechanism to uncover the collection bucket when
rainfall begins and to cover the sample when rainfall ends. About 0.2 inch of
rain was required to provide sufficient sample volume for analysis. Rainfall
samples were collected as single event-composite samples during rainfall events
and therefore represent rainfall event-mean concentrations (EMCs). The samples
were retrieved as soon as possible after a rainfall event, chilled, and shipped
overnight to the USGS National Water Quality Laboratory (NWQL) in Denver, Colo.,
for analysis.
Rainfall samples were analyzed
for the following forms of nitrogen: ammonia, dissolved organic, total organic,
and nitrite plus nitrate. Total nitrogen concentrations were computed for each
sample as the sum of ammonia, total organic, nitrite, and nitrate nitrogen. The
samples also were analyzed for dissolved phosphorus, total phosphorus, and
dissolved orthophosphate phosphorus. Field measurements of pH, conductivity, and
alkalinity also were made for selected samples.
Automatic water samplers
collected runoff samples during storm events. When streamflow-monitoring
equipment detected runoff, automatic samplers were activated to collect discrete
aliquots (subsamples). Aliquots were collected at a pre-programmed rate
depending on the station. Aliquots were collected at 30-minute intervals at
watersheds 1 and 2. Aliquots were collected hourly at the Moody Creek watershed,
where runoff duration was longer. At the end of the runoff event, the aliquots
from each station were combined into a single discharge-weighted composite
sample (one sample from each station). The volume of each aliquot added to the
composite sample was proportional to the stream discharge at the time of the
aliquot collection. Thus, the analysis of the composite samples yielded EMCs
that represent the discharge-weighted average concentrations during the runoff
event. Figure 2 shows a rainfall-discharge
hydrograph of a runoff event at watershed 2 on Aug. 30, 2001, and shows the
timing of subsample collection.
After runoff samples were
collected, they were chilled and bottled, necessary preservatives were added,
and the samples were shipped overnight to the NWQL. Samples were analyzed for
nutrients, major inorganic ions, trace elements, and dissolved pesticides.
In addition to the subsamples,
discrete grab samples were collected. These samples were analyzed for bacteria
at the USGS office in San Antonio, Tex.