This digital dataset contains monthly reference evapotranspiration (ETo) data 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 USGS numerical modeling code MODFLOW-FMP (Schmid and others, 2006b). 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 that 50 percent of the cells are active. The CVHM 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). The main climatic contributors to the CVHM are ETo and precipitation. Data from Parameter-Elevation Regressions on Independent Slopes Model (PRISM) (Climate Source, 2006) provide the temperature and precipitation on a monthly basis for a 2-kilometer grid. Mapping land-use type and distribution is integral to calculating the crop irrigation demand and, ultimately, the water use in the Central Valley. For a given land use, the demand can be calculated from two variables: crop coefficient (Kc) and reference evapotranspiration (ETo). For the CVHM, ETo is calculated from the daily minimum and maximum air temperatures derived from PRISM data and the extraterrestrial solar radiation (Faunt, 2009, p. 151). 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 (see "Foreword", Chapter A, page iii, for details).