ETo is simply a reference number which represents an estimate of evapotranspiration (ET) from an extended surface of 3 to 6 inch (8 - 15 cm) tall green grass cover of uniform height, actively growing, completely shading the ground and not short on water. All of the CIMIS weather stations throughout the state are situated within in a small grass field which is optimally irrigated. Thus, instruments attached to weather station device measure weather parameters that would directly affect ETo estimates such as solar radiation, air temperature, humidity, wind and rain. This data is incorporated within the weather station's database and calculates a reference evapotranspiration (ETo) number every hour (modified Penman).