Development of a Numerical Groundwater-Flow Model to Evaluate the Effects of the San Joaquin River Restoration Program on Stream-Aquifer Interaction

The primary goal of the San Joaquin River Restoration program (SJRRP) is to reestablish the river’s natural ecology to a level that restores river flows and maintains fish populations. One likely consequence of the SJRRP is the exacerbation of existing irrigation drainage problems along the river. The U.S. Geological Survey (USGS) has developed a numerical flow model (SJRRPM) of a 5-mile buffer along the San Joaquin River from Friant Dam to the Merced River. The purpose of the SJRRPM is to provide a quantitative tool to better understand the potential effects of restoration flows on the interaction between the surface water and the groundwater systems.

The SJRRPM was constructed by using the USGS groundwater flow model MODFLOW and was built upon the framework of the USGS Central Valley Hydrologic Model (CVHM). The Farm Process (FMP2) was used to simulate the supply and demand component of irrigated agriculture. The Streamflow-Routing Package (SFR2) was used to simulate the streams and bypasses and the stream-aquifer interaction. The 1,300-square-mile model area was subdivided into 0.25 square-mile cells. Simulation of the sediment texture of the aquifer system was refined from the CVHM to better represent the natural heterogeneity of aquifer-system materials. In addition, the stream properties were updated to better simulate stream-aquifer interactions, and water budget subregions were refined to better simulate agricultural water supply and demand. Boundary conditions were provided by the CVHM.

The SJRRPM was calibrated for 1961 to 2003 by using groundwater-level observations from 133 wells and streamflow observations from 19 gages. Model calibration was accomplished in a semi-automated manner by using PEST. The SJRRPM simulated groundwater-level altitudes and trends (including seasonal fluctuations) and surface-water flow magnitudes and trends reasonably match measured data. The calibrated model will be used to evaluate the effects on the groundwater system of alternative SJRRP water-management scenarios.