Daily reference evapotranspiration for California using satellite imagery and weather station measurement interpolation

Important water resources in California's agricultural and urban landscapes are at risk without more efficient management strategies. Improved monitoring can increase the efficiency of water use and mitigate these potential risks. The California Irrigation Management Information System (CIMIS) programme helps farmers, turf managers, and other resource managers develop water budgets that improve irrigation scheduling and monitor water stress. The CIMIS system is a repository of meteorological data collected at over 130 computerised weather stations. These are located at key agricultural and municipal sites throughout California and provide comprehensive, timely, weather data collected hourly and daily. In this article, the CIMIS sensor system is combined with hourly NOAA Geostationary Operational Environmental Satellite (GOES) visible satellite data to develop a methodology to extend reference evapotranspiration ($ET_0$) station estimations to spatial daily ET0 maps of California. The maps are calculated on a (2 km)2 grid, a high spatial resolution compared with the density of CIMIS stations. The hourly GOES satellite images are used to estimate cloud cover, which are used in turn to modify clear sky radiation estimates. These are combined with interpolated CIMIS weather station meteorological data to satisfy the Penman–Monteith $ET_0$ equation