Summary

Description

Abstract: This digital dataset contains the virtual wells used for pumpage for the Central Valley Hydrologic Model (CVHM). 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 U.S. Geological Survey (USGS) numerical modeling code MODFLOW-FMP (Schmid and others, 2006). This simulation is referred to here as the 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 total active modeled area is 20,334 square-miles on a finite difference grid comprising 441 rows and 98 columns. Slightly less than 50 percent of the cells are active. The CVHM model grid has a uniform horizontal discretization of 1x1 square mile and is oriented parallel to the valley axis, 34 degrees west of north (Faunt, 2009). Groundwater pumpage is a major part of the groundwater budget of the Central Valley, and is grouped into two categories for this study: agricultural and urban (which includes municipal and industrial sources). Discharge from agricultural wells rarely is metered in the Central Valley (Diamond and Williamson, 1983), and therefore must be estimated by indirect means. If consumptive use can be quantified, groundwater pumpage may be estimated by taking into account surface-water supply, irrigation efficiency, and effective precipitation. Irrigation efficiency, as used in this report, is the percentage of water delivered to the Water Balance Subregion (WBS) that is available for consumptive use. The newly developed MODFLOW-FMP uses this method (Schmid and others, 2006). Wells were simulated as a combination of farm wells (Schmid and others, 2006) and multi-node wells (Halford and Hanson, 2002) (Faunt, 2009; fig. C3). Farm wells are simulated in a manner similar to the WEL package (Harbaugh and others, 2000) and the pumpage is distributed among each of the farm wells (Schmid and others, 2006). Agricultural pumpage is estimated through the FMP. A single well that represents the composite of all wells within a model cell is referred to here as a virtual well. For the FMP, in each WBS, a virtual well was placed in each model cell where an irrigated crop was the predominant land use for a given time frame. Because the extent of irrigated agriculture changes through time, wells were added and deleted accordingly in the model during the simulation period. In general, wells were added through time because the extent of irrigated agriculture generally increases through time. In some areas, however, agricultural wells were replaced by urban wells in the model as the land use changed from agricultural to urban. The CVHM is the most recent regional-scale model of the Central Valley developed by the USGS.The CVHM was developed as part of the USGS Groundwater Resources Program (see "Foreword", Chapter A, page iii, for details).