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What is Evapotranspiration (ET)? And what is ETo?

We'll quote the Australian Bureau of Meteorology (BoM) here, "Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation and into the atmosphere by the processes of both transpiration and evaporation." If you are interested, the BoM goes into more detail here.

The evapotranspiration will depend on the climatic conditions (temperature, solar radiation, wind and humidity), water availability and crop type, e.g. the amount of water extracted by a football oval covered in grass will differ from the amount used by a vineyard. Another factor is the crop stage; typically larger, more mature crops will extract more water than young crops.

It would be impractical to forecast and publish evapotranspiration for a wide variety of crops due to the factors mentioned previously. In most contexts evapotranspiration can be calculated by using "reference evapotranspiration" or  ETo , which refers to the amount of evapotranspiration estimated for a reference surface of green grass about 12cm (5 inches) high, and multiplying it by a "crop factor" to adjust ETo for different crop types, growth stages and seasonal variation. SWAN Systems has an extensive library of crop factors for common crops, or you can use crop factors that you have derived yourself.