Evapotranspiration (ET) is regarded as water loss from the Earth's surface from evaporation and plant transpiration. Evapotranspiration is typically expressed in mm/day or inches/day. Although all weather station parameters are required for accurate evapotranspiration monitoring, air temperature and humidity, solar radiation and windspeed and direction are required in the Penman Monteith method of computing the reference evapotranspiration (Eto). The Penman Monteith method of ETo computation is based on two equations: (i) an energy equation (solar radiation) and an aerodynamic equation (using wind and air humidity). Soil moisture probes and other sensors (like dendrometers) are also being used today to fine tune ETo. ETo stations are often used for predicting irrigation scheduling and disease monitoring in agriculture. A typical ETo station is therefore composed of the following components:
