Reference Crop Evapotranspiration (ETo) Calculator

Online reference crop evapotranspiration (ET0) calculator to calculate measurement of the combined effect of temperature, humidity, windspeed and sunshine on the reference crop evapotranspiration.
If the water depth in the pan drops too much (due to lack of rain), water is added and the water depth is measured before and after the water is added. If the water level rises too much (due to rain) water is taken out of the pan and the water depths before and after are measured.

The principle of the evaporation pan is the following:

• the pan is installed in the field
• the pan is filled with a known quantity of water (the surface area of the pan is known and the water depth is measured)
• the water is allowed to evaporate during a certain period of time (usually 24 hours). For example, each morning at 7'o clock a measurement is taken. The rainfall, if any, is measured simultaneously
• after 24 hours, the remaining quantity of water (i.e. water depth) is measured
• the amount of evaporation per time unit (the difference between the two measured water depths) is calculated;
• this is the pan evaporation: E pan (in mm/24 hours)
• the E pan is multiplied by a pan coefficient, K pan, to obtain the ET_o.