Guide for data collection to calibrate a predictive digital ground-water model of the unconfined aquifer in and near the city of Modesto, California

The city of Modesto encompasses about 12 square miles in the northeastern part of the San Joaquin Valley, Calif. The ground-water model encompasses about 542 square miles. In the Modesto area, ground water occurs in an unconfined aquifer a confined aquifer. both of which are composed of unconsolidated materials, and a consolidated-rock aquifer. Only the unconfined aquifer was modeled, using several simplifying assumptions concerning hydrologic conditions in the ground-water basin. A program is used that computes the net rate of recharge and discharge under steady-state conditions. The model was then modified until reasonable values of recharge and discharge were computed. Testing of the model indicated that simulated water levels were especially sensitive to tansmissivity, storage coefficient, irrigation return, and riverbed hydraulic conductivity; amond the parameters that affected water levels least were the vertical hydraulic conductivity and specific storage of the confining bed, the so-called E-clay. (Woodard-USGS)