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ASSESSMENT OF WATER AVAILABILITY IN REGIONAL AQUIFER SYSTEMS USING MODFLOW’s FARM PROCESS

Congress: 2008
Author(s): Randall T Hanson, Claudia C Faunt, Steven P Phillips, Diane L Rewis
Randall T Hanson & Claudia C Faunt: US Geological Survey, San Diego, California, USA Steven P Phillips & Diane L Rewis: US Geological Survey, Sacramento, California, USA
AbstractThe new Farm Process (FMP) allows simulation of land-use processes affecting the movement and use of surface and ground water with the U.S. Geological Survey’s hydrologic model, MODFLOW (MF2K). The MF2K-FMP is used to assess water availability for developed and undeveloped land use. Because FMP is fully coupled with simulations of ground-water and surface-water flow, it can be used as a tool to investigate linkages between management decisions and the availability of water. The FMP has been applied to regions in the State of California at scales ranging from the entire 50,000- square-kilometers (km2) Central Valley to the 400-km2 Pajaro Valley, a coastal watershed of Monterey Bay. A medium scale application is the 2,700-km2 Modesto Region near Sacramento located within the Central Valley. The FMP integrates simulation of water supply and demand, which facilitates the simulation of water movement in complex and heavily developed water-resource systems. The FMP is especially useful in regions where historical ground-water pumpage is unknown, or where estimates of future pumpage on the basis of anticipated natural or anthropogenic demands are needed. The Central Valley and Pajaro Valley are two very productive agricultural regions in California. Expansion of irrigated agriculture and growth of urban population has increased the competition for surface and ground water. The MF2K-FMP was used to simulate the conjunctive use of surface and ground water in these two valleys. The FMP was used to estimate historical pumpage and delivery of surface water (1960s to present) for 21 water-balance subregions in the Central Valley, 63 water budget areas in the Modesto Region, and 12 subregions in Pajaro Valley. The Central Valley Model indicates that surface-water delivery provides most of the agricultural demand early in the growing season, augmented with ground-water pumpage later in the season. The relative proportions of surface water and ground water used for irrigation in the Central Valley also vary from year to year in response to climate and import availability. In contrast, the ground water used for irrigation in the Pajaro Valley is significantly augmented by precipitation early in the growing season. These models are being used to evaluate the effects of development on water levels, ground-water storage, and ground-water discharge to streams and other surface-water bodies. The models also are being used to assess the adequacy of monitoring networks.
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