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2007 Ground Water Summit |
Tuesday, May 1, 2007 : 10:40 a.m.
Estimation of Spatially Varying Recharge for a Regional Scale Ground Water Model in the Texas Gulf Coast
A regional scale groundwater model is being developed for a 9800 square mile region along the Texas Gulf Coast. Our first step in estimating recharge was to estimate the magnitude of baseflow, which is the major source of discharge in the surficial system. Multiple hydrograph separation analyses were performed on several basins in the region, where unregulated gage data were available. In addition, a low-flow study for the Colorado River (where unregulated gages are unavailable) provided an estimate of baseflow in that basin. Annual estimates of precipitation for each basin were compared to the annual baseflow estimates, and good positive correlation was observed. A simple semi-log regression yielded an equation for variation of annual baseflow with annual precipitation. The spatial variability of recharge was conceptualized to be primarily dependent on the variation in precipitation in the region and the local topography. The underlying geology was added as a secondary effect. The precipitation versus baseflow relationship was used to create a rough distribution of recharge across the region, and the topographical variation was used as a driver to increase recharge in the topographically high areas, and decrease recharge in the topographically low areas. These relationships were quantified using simple equations. The parameters for the equations were calibrated using PEST, a parameter estimation and optimization code. The calibration targets were measured heads, the estimates of baseflow, and an estimate of submarine discharge to the gulf. The resulting recharge array satisfied both our conceptualization and the constraints of the calibration.
John Waugh, San Antonio Water System John Waugh is director of SAWS, San Antonio Water System.