A measurement of the amount of water vapor returned to the air in a given area through both evaporation and the amount of water vapor returned through transpiration (exhaling of moisture through plant stomata). The Vantage Pro2 GroWeather uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate evapotranspiration or ET, which is calculated once an hour on the hour. A GroWeather or optional solar radiation sensor is required.