Abstracts Geography

Simulating the Predevelopment Hydrologic Condition of the San Joaquin Valley, California

by Benjamin Luke Bolger

The San Joaquin Valley is part of the Great Central Valley of California, a major agricultural centre and food supplier for the United States. This area has significant water management concerns given the very high water demand for an increasing state population and for intense irrigation in a hot, temperate to semi-arid climate where the overall rate of evapotranspiration (ET) is high, and the overall rate of precipitation is low. Irrigation heavily relies upon groundwater and surface water extractions. Through the historical and current concerns of regional water resources reliability, land surface subsidence, water quality issues, and the health of ecosystems, a need for regional-scale water resource management and planning has developed. The physically-based surface-subsurface HydroGeoSphere (HGS) model is used to examine the regional-scale hydrologic budget of a large portion of the San Joaquin Valley. The objective of this investigation is to develop a steady-state groundwater-surface water model of the San Joaquin Valley representative of predevelopment hydrologic conditions. The groundwater-surface water system has undergone drastic changes since the employment of groundwater and surface water extractions for irrigation and mining, and is still responding to past and present stresses. The only certain stable initial condition must therefore be that of the natural system. The model input parameters were constrained by all relevant available hydrologic data. The model was not calibrated to subsurface hydraulic heads or river flows. However, the model does provide a fair match between simulated and actual estimated water table elevations. Historic river flow estimates were not used to calibrate the model, because data consistent with that collected by Hall (1886) and representative of the natural system were not available. For this investigation, water enters through precipitation and the inflow of major rivers only. The subsurface domain is bounded by no-flow boundaries, and groundwater is therefore only able to exit the subsurface through discharge to surface water features or through ET. Surface water is only able to exit the model through discharge via the San Joaquin River and through ET. Average river inflows circa 1878 to 1884 documented by Hall (1886) were applied where the rivers enter into the valley. The spatially variable average rate of precipitation (years 1971 to 2000) from a PRISM dataset was applied to the top of the model. The spatially variable long term average potential ET rates from the California Department of Water Resources (DWR) et al. (1999) were applied to the top of the model. Averaged overland flow parameters and vegetation factors needed to calculate actual ET were specified at the top of the model based on literature values and the 1874 spatial distribution of natural vegetation provided by California State University at Chico et al. (2003). Hydrogeological data including hydraulic conductivities, porosities, specific storage, and unsaturated zone properties…