CalSIMETAW was designed to estimate daily soil water balance to determine potential crop evapotranspiration (ETc) and evapotranspiration of applied water (ETaw) for132individual crops, 20 crop categories, and four land-use categories by DAU/county for use in California Water Plan. The model requires weather data, soils, crop coefficients, rooting depths, seepage, etc., that influence crop water balance. The model uses daily weather data, derived from monthly PRISM climate data and daily US National Climate Data Center climate station data to cover California with 4×4 km grid spacing. From the PRISM data, reference evapotranspiration (ETo) is estimated using the Hargreaves-Samani equation that was calibrated to estimate regional Penman-Monteith equation ETo to account for spatial climate differences. In addition to using historical data, CalSIMETAW can use near-real-time data from Spatial CIMIS, which is a model that combines weather station data and remote sensing to provide a statewide grid of ETo information.