The dataset contains daily grass-reference evapotranspiration (ETo) maps stored as ASCII files. ETo at a 2 km spatial resolution are calculated statewide using the American Society of Civil Engineers version of the Penman-Monteith equation (ASCE-PM). Required input parameters for the ASCE-PM ETo equation are solar radiation, air temperature, relative humidity, and wind speed at two meters height. These parameters are estimated for each 2 km pixel using various methods. Daily solar radiation is generated from the visible band of the National Oceanic and Atmospheric Administration's (NOAA) Geostationary Operational Environmental Satellite (GOES) using the Heliosat-II model. This model is designed to convert images acquired by the Meteosat satellite into maps of global (direct plus diffused) irradiation received at ground level.
The ETo maps are presented as compressed ASCII files (ETo.asc.gz). The ETo maps are contained in folders defined by Year, Month, and Day. ETo data is expressed in millimeters.The ASCIII files can be converted to conventional raster formats (TIF, img) using commercial and open source software. The spatial reference of the maps are defined as EPSG Projection 3310 - NAD83 / California Albers (http://spatialreference.org/ref/epsg/3310/). .