ALTERATIONS TO RECHARGE TO THE EDWARDS AQUIFER, BARTON SPRINGS SEGMENT, CAUSED BY URBANIZATION

Barton Springs are the major discharge feature of the Barton Springs segment of the karstic Edwards aquifer. Monthly records of springflow and precipitation (at nearby Camp Mabry) reach back to 1917. These data show much variability as indicated by visual inspection of the data and autoregressive moving averages. However, some general trends are evident (i.e., months of high or low monthly precipitation are followed by periods of low or high spring flow). We smoothed the data (linearly-weighted arithmetic means over periods of 4 to 8 months) and inferred from this that there is more springflow, relative to precipitation, in years since the 1960s. To test this hypothesis we assume that, for the first 60 months of data, springflow (Q) is a function of precipitation (P) and that there are no changes in storage. Using this equation (Q=CP), we extrapolate to the present. The analysis (C is constant) shows little change until about 1960. Since 1960, the summed monthly Q continually increases over the summed monthly CP. 1960 corresponds to the time when Austin population and urban sprawl began its significant increase. Correlagrams show a 1-month lag in correlating Q with P before and a 2-month lag after 1960. This suggests another source of recharge has become significant that dampens/delays the response of discharge compared to precipitation in the time-series data. We infer that leakage from utility systems, urban irrigation, and the increase in impervious cover are the sources of the additional recharge. Impervious cover may increase both flows from the catchment area to the losing streams that are a major source of recharge and direct recharge in in urbanized areas. We suggest that urbanization has significantly increased recharge and this must be considered in any future management plans and can be extended to other karstic subsystems in areas of urbanization.