Historically, California’s Central Valley has been one of the most productive agricultural regions in the world. The Central Valley also is rapidly becoming an important area for California’s expanding urban population. During 1980–2007, the population nearly doubled in the Central Valley, increasing the competition for water. Because of the importance of ground water in the Central Valley, the U.S. Geological Survey (USGS) Ground-Water Resources Program is evaluating ground-water conditions in the valley on the basis of historical and anticipated water use. This study updates the USGS Central Valley Regional Aquifer System and Analysis (CVRASA) model that was originally? calibrated to observed conditions for the period 1961-77. The model developed for this study utilizes MODFLOW-2000, and was calibrated to observed conditions for the period 1961-2003. Key updates include characterization of the aquifer system using a detailed textural analysis of more than 8,500 drillers’ logs; use of the MODFLOW subsidence package (SUB) to simulate aquifer-system compaction; and, most importantly, use of the newly developed MODFLOW Farm Process (FMP) for simulating irrigation and other landscape processes. The FMP provides coupled simulation of the ground-water and surface-water components of the hydrologic cycle for irrigated and non-irrigated areas. A dynamic allocation of ground-water recharge and ground-water pumping is simulated on the basis of residual crop-water demand after surface-water deliveries and root uptake from shallow ground water. The FMP links with the Streamflow Routing Package SFR1) to facilitate the simulated conveyance of surface-water deliveries. Ground-water Pumpage through both single-aquifer and multi-node wells, irrigation return flow, and variable irrigation efficiencies also are simulated by the FMP. The simulated deliveries and ground-water pumpage in the updated model reflect climatic differences, differences among defined water-balance regions, and changes in the waterdelivery system, during the 1961–2003 simulation period. The model is designed to accept forecasts from Global Climate Models (GCMs) to simulate the potential effects on surface-water delivery, ground-water pumpage, and ground-water storage in response to climate change. The model provides a detailed transient analysis of changes in ground-water availability in relation to climatic variability, urbanization, and changes in irrigated agriculture.