Hydroclimatic trend detection in a rapidly urbanizing semi-arid and coastal river basin

SummaryDetection of past trends, changes, and variability in the time series of hydroclimatic variables is important for understanding the potential impact of future change. The objective of this study was to analyze trends in several hydroclimatic variables in the rapidly urbanizing semi-arid San Antonio River Basin, particularly trends in freshwater inflows to the Guadalupe Estuary. Seasonal time series for 27 hydroclimatic variables from nine USGS stream gauging stations and five NCDC weather stations were analyzed for trend using the Mann–Kendall non-parametric test after removing serial and cross-correlations. The significance of detected trends was analyzed using a permutation method. Variability in total annual and seasonal precipitation was characterized and the effect of precipitation on regional streamflow trends during coinciding periods was analyzed. Results of the analysis revealed a definitive line just below the City of San Antonio above which the influence of the increase in impermeable surfaces can be clearly seen. Above this line nearly all significant trends were negative (decreasing). The percent contribution of baseflow to total streamflow in the upper watershed decreased for almost every season for average, high, and low precipitation events. Below this line all significant trends were positive (increasing). A greater number of significant trends was seen in all hydroclimatic variables during all seasons at stations in the lower watershed and the percent contribution of baseflow to streamflow is increasing across most seasons for average, high, and low precipitation. Some significant positive trends were found in precipitation, but only in the winter season and with no significant spatial pattern. Streamflow has been increasing at the outlet of the watershed, however, freshwater inflow contributions from this watershed are only being met during years that have higher than average precipitation. This has very real implications for management in this watershed.