Time Series Analysis of Freshwater Inflows in an Urbanizing Coastal River Basin

Estuaries are the connecting link between terrestrial and marine ecosystems, and provide a critical coastal habitat that is essential ecologically and economically to the world economy. Therefore, it is important to maintain the productivity and ecological integrity of estuarine ecosystems. The productivity of these systems depends on the timing and magnitude of freshwater inflow along with the associated nutrients such as N and P, metals, and organic matter from the terrestrial environment. Freshwater inflow, nutrient, and metal delivery are influenced by the land use/land cover and water management practices in the contributing watershed, particularly in watersheds that are experiencing rapid human induced disturbances. San Antonio, TX, the 8th largest city in the US, is situated in the San Antonio River basin. Rapid urbanization has changed the land use and land cover in this river basin. Studies in the river basin suggest that change in land use has primarily been an increase in impervious surface. Increase in impervious surface can change the flow regime by altering timing, magnitude, and frequency of freshwater inflows. This study analyzed trends in the San Antonio River flow regime using 63 years of average daily discharge obtained from USGS gauging station number 8188500 situated most downstream of the river and 16 years of average daily discharge data from USGS gauging station number 08178565, situated on the San Antonio River at Loop 410 in San Antonio. This portion of the river basin is highly urbanized. Datasets for USGS station 8188500 were divided into two time series: 1940-1970 (pre-development), and 1971-2003 (post-development). The time scale was also divided seasonly. Probability of exceedence of peak freshwater inflow was investigated. Correlograms were used to detect the flow relationship between the upstream and most downstream gauging stations using 16 years of data (1987-2003). Preliminary investigation suggested a decreasing trend in total annual flow in the pre-development years. However, an increasing trend was observed in total annual flow in the post development years. Similar trends were observed in yearly average flow (derived from monthly average flow) for the two periods. Pre-development seasonal flow analysis suggested a decreasing trend during spring-summer, and fall and an increasing trend in winter flow for the period. Post-development seasonal flow analysis suggested an increasing trend during winter and fall; and decreasing trend during spring-summer. Interestingly, the spring-summer period during post development experienced the largest number of peak flows. Analysis of total monthly flow suggested an increase in the magnitude of peak flow in the post development period.