HYDROLOGIC CONDITIONS

Rainfall

Annual rainfall statistics were computed from rainfall data measured at the NOAA weather station (pl. 1; fig. 1). The 37-year (1965–2001) average annual rainfall is 36.31 inches with a minimum of 15.49 inches in 1989 and a maximum of 59.46 inches in 1983. The standard deviation of annual rainfall, 10.09 inches, indicates relatively large differences in annual rainfall.

Study area rainfall (average of data from NOAA and TAES weather stations [pl. 1; fig. 1] during 2000 and 2001 was 33.27 and 28.20 inches, respectively. Monthly rainfall was below normal during 16 of the 24 months. A few periods of heavy rainfall, interspersed between relatively dry periods, accounted for much of the annual rainfall. A comparison of 1965–2001 mean monthly rainfall at the NOAA station with monthly study area rainfall is shown in figure 3.

Runoff

Long-term USGS streamflow-gaging stations have been operated on the Aransas River near Skidmore (about 17.6 miles northwest of the study area) and on the Mission River at Refugio (about 15.6 miles northeast of the study area). The Aransas River station (08189700, fig. 1) has operated since 1964 and has a drainage area of 247 square miles. The Mission River station (08189500, fig. 1) has operated since 1939 and has a drainage area of 690 square miles. The rivers are unregulated and unaffected by diversions; both watersheds are predominantly rangeland. The average annual runoff for the period of record at these two stations is 25,120 acre-feet (1.91 inches) for the Aransas River station and 87,200 acre-feet (2.37 inches) for the Mission River station. On the basis of these historical data, average annual runoff in the study area is about 2 inches, but like rainfall, can vary substantially from year to year.

Runoff in the study area during 2000–2001 corresponded to the rainfall pattern, with runoff events interspersed between long periods of no runoff. Nine runoff events (runoff at one or more of the stations) occurred. The dates of the runoff events, rainfall, runoff volume, and runoff coefficients (ratio of runoff volume, in inches, to rainfall volume, in inches) are listed in table 1. Most of the events were relatively small (in terms of runoff volume). There were three major runoff events. During Mar. 13–16, 2000, 6.11 inches of rain produced 0.59 inch of runoff¹. During Aug. 28–Sept. 1, 2001, 7.46 inches of rain produced 1.08 inches of runoff. During Nov. 16–17, 2001, 4.17 inches of rain produced 0.34 inch of runoff. These three events produced 29 percent of the total rainfall and 86 percent of the total runoff for 2000–2001.

Generally, rain events of 2 inches or less resulted in little or no runoff depending on antecedent rainfall. The event of Oct. 6, 2000, produced 4.67 inches of rain after 6 months of very dry conditions. Runoff occurred at two of the three stations; total runoff was relatively small (less than 0.01 inch).

During 2000–2001, 61.47 inches of rain on the study area produced 2.46 inches (2,800 acre-feet) of runoff. The runoff coefficient during this period was 0.038. The runoff coefficients for the nine individual events that produced runoff ranged from 0.002 to 0.145.

WATER QUALITY

Rainfall

Five rainfall samples collected at the Moody Creek station during June 2000–August 2001 represent 8.76 inches of rain. During the same period, 22.80 inches of rain fell on the study area; therefore, the samples represent about 38 percent of the study area rainfall during the 15-month period.

Nutrients

Concentrations of selected nutrients analyzed in each sample are listed in table 2; summary statistics computed for the nutrient concentrations are listed in table 3. Most of the rainfall nitrogen is in the form of dissolved ammonia and dissolved nitrate, which were detected in all of the samples. Phosphorus was detected in less than one-half of the samples and at relatively small concentrations (near laboratory minimum reporting levels).

Rainfall Deposition of Nitrogen

The deposition of rainfall constituents (in pounds per acre) can be defined as the product of the EMC and the rainfall volume. For rainfall events during which rainfall samples were collected and analyzed, thus providing EMCs, daily deposition of total nitrogen was computed as

                   D_TN = C_TN x R x Cf,             (1)

where

  D_TN  = daily deposition of total nitrogen, in pounds per acre;

   C_TN  = total nitrogen rainfall EMC, in milligrams per liter;

         R = daily rainfall, in inches; and

        Cf = conversion factor of 0.2266.

For unsampled rainfall events, for which nitrogen concentration data were not available, daily deposition of total nitrogen was estimated using regression equations that relate daily rainfall and daily nitrogen deposition. Regression equations from a previous study of rainfall deposition of nitrogen (Ockerman and Livingston, 1999) were modified using data from this study. The resulting equation is

                  D_TN = 0.038 x R^0.586,            (2)

where

  D_TN    = estimated daily deposition of total nitrogen, in pounds per acre; and

         R = daily rainfall, in inches.

A comparison of total nitrogen deposition computed from sample analysis and estimated by regression is shown in figure 4.

The daily values for rainfall deposition of total nitrogen were aggregated to produce the monthly and annual totals for 2000–2001 listed in the following table.

Monthly and annual rainfall deposition of total nitrogen, 2000–2001

[In pounds per acre]

¹ Gaging equipment had not been installed when this event occurred. Runoff was estimated at 0.59 inch by regression of rainfall-runoff coefficients of subsequent measured events at each watershed.

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