Figure 1: Nueces River Basin and precipitation gauges. 



Trends in precipitation should be described within an a priori selected time period. For example, a short-term 

 trend following a drought would be increasing precipitation. The duration of such trends is of great interest 

 and the ability to forecast them is constandy sought. For purposes of the Rincon project, precipitation trends 

 over 1940-99, a 60-year period, are the main concerns of study. Additionally, Rincon area precipitation is 

 compared among the periods 1940-57, 1958-81, and 1982-99, that correspond to different construction 

 influences on Nueces River flows. 



On the National basis, the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric 

 Administration routinely develops temperature and precipitation trend estimates (available on the Internet, 

 address ftp://ftp.ncep.noaa.gov/charts.htinl/') . CPC uses 102 climate regions of near equal area for the lower 

 48 states. Each climate region is composed of one or more climate divisions. Monthly data are assembled into 

 time series for three-month periods and an annual average. Baseline values were developed from data to 1966. 

 The linear change per decade is computed from the base line value through the current year. Current trends are 

 based on 1941-1998 data for temperatures and 1931-1998 data for precipitation. The CPC trends indicate 

 National increases of 0.15 °F and 0.9 inches of precipitation per decade. For south Texas, no trend in mean 

 annual temperatures is indicated, but a 0.2 to 0.6 inch increase in precipitation per decade is revealed. 



The following covers a discussion of precipitation in the Nueces River watershed based on data from several 

 gauges located there. A summary is given of some study results on the El Nino-Southem Oscillation (ENSO) 

 and other climate variables, and their relationship to tropical cyclone occurrence in the Atlantic basin and in 

 particular, the Gulf of Mexico. Nueces River watershed precipitation anomalies stratified by ENSO type are 

 presented. Finally, daily precipitation distributions for one watershed gauge are developed and discussed in the 

 context that for different periods there can occur different precipitation distributions causing differing runoff 

 while maintaining s imilar annual precipitation. 



NUECES RIVER WATERSHED PRECIPITATION SURVEY 



The 30-year mean precipitation for gaviges shown in Figure 1 portray differences across the Rincon area. Mean 

 annual preapitation (1961-1990 record) for Corpus Christi WSO AP, CotuUa, Carrizo Springs, BeeviUe 5 NE, 

 and Sabinal are the respective 30.14, 22.95, 21.22, 31.97, 25.45 inches. These values support lesser 

 precipitation occurs in the southwestern Rincon area and higher amounts along the coastline. Figures 2 

 through 5 present annual precipitation totals for Cotulla, Corpus ChristL, Beeville 5 NE, and Sabmal through 

 1999, with a Lowess (Cleveland, 1979; 1981) smoothing line (average of weighted values in a moving window 



D-2 *♦♦ Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas 



