Hydrologic Trends and Correlations in South Texas River Basins: 1950–2009

The identification of seasonal and annual trends in precipitation and streamflow volumes at the regional scale contributes to the understanding of global climate change and variation and is essential to the development of hydrologic models, hydrologic forecasting, and water resources planning and management for basins. This study examines such trends over the past 60 years (1950–2009) for the Nueces River basin, the San Antonio River basin, and the Guadalupe River basin, all in south Texas. The correlation of precipitation and streamflow with the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) climatic indices is also evaluated, with a view toward incorporating these two relatively predictable cyclical patterns into modeling and forecasting of precipitation and streamflow. Trends and correlations were measured using the Kendall and Mann-Kendall test statistics, while the more commonly used Pearson $r$ coefficient was shown not to be consistently appropriate for the data analyses. The influence of PDO on annual precipitation is substantially less in these basins than it was found to be in a basin approximately 1,000 km northwest of them. The majority of trends and correlations were found to be statistically significant at the level of 0.05, and all such relationships were positive (or increasing). The analyses suggest that forecasting and management practices require a deterministic hydrologic model capable of accommodating natural features, management practices, and meteorological input for estimating evapotranspiration potential. Calibration and uncertainty analysis must be of a sophistication to accommodate nonnormal residuals, so that confidence intervals may be reliably used in forecasting and in management decisions.