Multi-decadal trends in precipitation and streamflow for selected Texas River basins

Water resource planners have long been interested in how climate change and variability impact the prediction of future water resources, relationships between precipitation and streamflow and how streamflow has changed over the past several decades. This study analyzes trends in precipitation and streamflow for the past 60 years (1950-2009) and analyzes patterns in zero and non-zero streamflow and precipitation days for three south-central Texas river basins; Nueces River basin, San Antonio River basin, Guadalupe River basin. Once trends and correlations in precipitation and streamflow were identified for each river basin (annual averages and seasonal time-scales), the correlation of precipitation and streamflow to climatic indexes such as the El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) were also identified. For the trend and correlation analysis, seasonal as well as annual average datasets sub-divided the streamflow, precipitation and climate index data. Results showed that there is an overall increasing trend in summertime precipitation only; this applied to all three river basins over the past 60 years. Annual average streamflow has shown increasing trends in the San Antonio and Guadalupe River basins. The Nueces River basin did not show increasing trends in streamflow; however this may be due to a bias resulting from the physical location of the streamflow station analyzed and the complex relationship between groundwater and surface water due to the multiple aquifers found within the Nueces River Basin. The San Antonio River basin indicated increasing streamflow trends in all season except for the fall season, which reported values slightly over the confidence level (no significant indication) which defines the statistical significance of the trend. A longer study period might indeed reveal an increasing trend in streamflow for the San Antonio River basin fall months. Precipitation and streamflow show strong correlations within each river basin while correlations to climatic indices were not as strong. Of the two indices, ENSO had the only significant correlation with respect to precipitation, particularly in the winter months for all three river basins.

Days with recorded precipitation were found to be increasing as well as the maximum recorded precipitation value for each year in all river basins. Days with zero streamflow were identified for all three river basins as well as what days were most probable to have a zero streamflow value recorded during the 60 year study period. These results were mixed for each river basin, however the streamflow stations in the San Antonio River basin were found to have continuous streamflow values for each day of the study period.