This digital dataset defines the spring 1961 hydraulic head in the lower pumped zone of California's Central
Valley. It was used to initiate the water-level altitudes for the lower zones of the transient hydrologic model of
the Central Valley flow system. The Central Valley encompasses an approximate 50,000 square-kilometer
region of California. The complex hydrologic system of the Central Valley is simulated using the USGS
numerical modeling code, MODFLOW-FMP (Schmid and others, 2006). This simulation is referred to here as
the Central Valley Hydrologic Model (CVHM) (Faunt, 2009). Utilizing MODFLOW-FMP, the CVHM simulates
groundwater and surface-water flow, irrigated agriculture, land subsidence, and other key processes in the
Central Valley on a monthly basis from 1961-2003. The starting heads are based on the water-table and
potentiometric surface developed by Williamson and others (1989). Maps of the spring 1961 water-table and
hydraulic head in the lower pumped zone are shown in Figure 31 of Williamson and others (1989). The CVHM is
the most recent regional-scale model of the Central Valley developed by the U.S. Geological Survey (USGS).
The CVHM was developed as part of the USGS Groundwater Resources Program (Faunt, 2009. See
"Foreword", Chapter A, page iii, for details).
