The ET is calculated for a grass reference crop using a modified Penman Monteith equation, which is the standard method recommended by the UN Food and Agriculture Organization.
Calculated evapotranspiration depends on a number of factors including temperature, solar radiation, vapour pressure, and wind speed. For most of the stations only temperature data is available daily. ET can be calculated using only temperature data but on very windy days or cloudy days the ‘daily’ calculated ET value may over or under estimated the actual ET. However, it is known that over a longer time period (i.e. week) the temperature calculated ET values average out to be close to the actual ET.
The sites that use radiation information have more accurate daily readings but it is still best to use an average daily value for scheduling.
Please refer to the FAO Penman-Monteith Equation details.