Application of MODFLOW’s Farm Process to California’s Central Valley Claudia C. Faunt 1 , Randall T. Hanson 2 , Wolfgang Schmid 3 , and Kenneth Belitz 4 1 U.S. Geological Survey, ccfaunt@usgs.gov, San Diego, CA, USA 2 U.S. Geological Survey, rthanson@usgs.gov, San Diego, CA, USA 3 University of Arizona, w_schmid@hwr.arizona.edu, Tucson, AZ, USA 4 U.S. Geological Survey, kbelitz@usgs.gov, San Diego, CA, USA ABSTRACT California’s Central Valley has been one of the most productive agricultural regions in the world for more than 50 years. The Central Valley also is rapidly becoming an important area for California’s expanding urban population. During 1980–2007 the population nearly doubled, increasing the competition for water within the Central Valley and statewide. Because of the importance of ground water in the Central Valley, the U.S. Geological Survey’s (USGS) Ground-Water Resources Program is evaluating ground-water conditions in the valley based on historical and anticipated future water use. This study is updating the USGS’s Central Valley Regional Aquifer System and Analysis (CV-RASA) model and supporting information previously developed by the USGS to quantitatively address ground-water issues in the valley. The Central Valley aquifer system was characterized using a detailed textural analysis of more than 8,500 driller’s logs. The updated model utilizes the Farm Process (FMP) for MODFLOW that provides coupled simulation of the ground-water and surface-water components of the hydrologic cycle mainly in irrigated but also in non-irrigated areas. The FMP provides a dynamic allocation of ground-water recharge and ground-water pumping based on 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. The FMP also simulates ground-water pumpage through both single-aquifer and multi-node wells, irrigation return flow, and variable irrigation efficiencies. The Subsidence (SUB) package was used to simulate land subsidence, a consequence of overdraft. The simulated deliveries and ground-water pumpage reflect climatic differences, differences among the water-balance regions, and changes in the water-delivery system during the 1961–2003 simulation period. The model is designed to be coupled with forecasts from Global Climate Models (GCMs) to simulate the potential variabilities of 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. INTRODUCTION California’s Central Valley covers about 20,000 mi 2 and is one of the most productive agricultural regions in the world (fig. 1). More than 250 different crops are grown in the valley with an estimated value of $17 billion/yr. This irrigated agriculture relies heavily on surface-water diversions and ground-water pumping. Approximately one-sixth of the Nation’s irrigated land is in the Central Valley, and about one-fifth of the Nation’s pumping is from its aquifers (Great Valley Center, 2005). The Central Valley also is rapidly becoming an important area for California’s expanding urban population. Since 1980, the population of the valley has nearly doubled from 2 to 3.8 million people (California Department of Finance, 2007), and the U.S. Census Bureau projects that the valley’s population will increase to 6 million people by 2020. This increase in population has increased the competition for water within the Central Valley and statewide. Competition for water is likely to be exacerbated by reduced deliveries of Colorado River water to southern California and reduced deliveries through the Sacramento-San Joaquin Delta owing to environmental flow requirements. As a result, a number of issues have gained prominence: conservation of agricultural land, conjunctive use, aquifer storage and recovery, hydrologic implications of land-use change, and effects of climate variability. To help address these issues, the U.S. Geological Survey (USGS) Ground-Water Resources Program initiated a study in 2005 to assess ground-water availability of the Central Valley, including: 1) quantifying the present status of ground-water resources; 2) evaluating how the availability and allocation of these resources have changed over time, and; 3) developing a tool MODFLOW and More 2008: Ground Water and Public Policy - Conference Proceedings, Poeter, Hill, & Zheng - www.mines.edu/igwmc/ 496