Simulating the pre-development hydrologic conditions in the San Joaquin Valley, California

The San Joaquin Valley (SJV) in central California has significant water management concerns given the high water demand for an increasing state population and for intense irrigation. The groundwater-surface water system in the area 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. In this study, we develop a pre-development hydrologic model of the SJV to serve as an appropriate initial state to analyze the influence of historic anthropogenic stresses. Specifically, the physically-based surface–subsurface numerical HydroGeoSphere model is used to examine the regional-scale hydrologic budget of the SJV at pre-development conditions, constrained by available historical data. As a result, complex hydrologic processes, including groundwater-surface water interaction along the major rivers and within wetland areas formed by flooded surface water, as well as evapotranspiration (ET) and impacted root zone processes were identified in the area. The presence and path of the major rivers in the domain are well defined in the model output. The general location and formation of the major wetlands simulated by the model, and the hydrologic processes that occurred within them have a fair agreement with historical records. There is also a fair match between simulated and estimated water table elevations. ET is a significant sink of both surface water and groundwater (44.8% of the water balance input). Successful simulation of the complex hydrologic processes and features, and the water balance of the natural system underscores the importance and necessity of using an integrated model, especially when available data is limited for input and calibration. This pre-development hydrologic condition should serve as a reasonable initial state for future transient runs that bring the model up to current hydrologic conditions capable of estimating present and future water budgets.