The California Irrigation Management Information System (CIMIS) currently manages over 145 active weather stations throughout the state. Archived data is also available for 85 additional stations that have been disconnected from the network for various reasons. CIMIS stations provide hourly records of solar radiation, precipitation, air temperature, air humidity, and wind speed. Most of the CIMIS stations produce estimates of reference evapotranspiration (ETo) for the station location and their immediate surroundings, often in agricultural areas. The Department of Water Resources operates CIMIS as a free resource to help California to manage water resources more efficiently. CIMIS weather stations collect weather data on a minute-by-minute basis. Hourly data reflects the previous hour's 60 minutes of readings. Hourly and daily values are calculated and stored in the dataloggers. A computer at the DWR headquarters in Sacramento calls every station starting at midnight Pacific Standard Time (PST) and retrieves data at predetermined time intervals. At the time of this writing, CIMIS data is retrieved from the stations every hour. When there is a communication problem between the polling server and any given station, the server skips that station and calls the next station in the list. After all other stations have reported, the polling server again polls the station with the communication problem. The interrogation continues into the next day until all of the station data have been transmitted. CIMIS data processing involves checking the accuracy of the measured weather data for quality, calculating reference evapotranspiration (ETo/ETr) and other intermediate parameters, flagging measured and calculated parameters, and storing the data in the CIMIS database. Evapotranspiration (ET) is a loss of water to the atmosphere by the combined processes of evaporation from soil and plant surfaces and transpiration from plants. Reference evapotranspiration is ET from standardized grass or alfalfa surfaces over which the weather stations are sitting. The standardization of grass or alfalfa surfaces for a weather station is required because ET varies depending on plant (type, density, height) and soil factors and it is difficult, if not impossible, to measure weather parameters under all sets of conditions. Irrigators have to use crop factors, known as crop coefficients (Kc), to convert ET from the standardized reference surfaces into an actual evapotranspiration (ETc) by a specific crop. For more information go to https://cimis.water.ca.gov/.  The associated data are considered DWR enterprise GIS data, which meet all appropriate requirements of the DWR Spatial Data Standards, specifically the DWR Spatial Data Standard version 3.3, dated April 13, 2022. DWR makes no warranties or guarantees —either expressed or implied — as to the completeness, accuracy, or correctness of the data. DWR neither accepts nor assumes liability arising from or for any incorrect, incomplete, or misleading subject data. Comments, problems, improvements, updates, or suggestions should be forwarded to GIS@water.ca.gov.