According to the common equation of the water balance P = R + E for a longer period of time, we deduct the modified form E = P - R - (S - U) for shorter time frames. A substantial part of the precipitation evaporates sooner or later as water vapour.
Evaporation can be categorised as follows:
- Evaporation = soil evaporation or evaporation from free surfaces of water.
A special form of the evaporation is the interception, where a part of the precipitation is caught or absorbed by plants: the precipitation does not even reach the ground and evaporates directly from the plants.
- Evapotranspiration (ET) = sum of evaporation and transpiration of vegetation
Evaporation only depends on physical factors, in particular on solar power, air temperature, humidity and area and underground conditions.
Evapotranspiration is an auxiliary variable that comprehends the water output of the earth’s surface and of the plants into the atmosphere. The type of vegetation plays an essential role. We further differentiate between the potential (maximum amount) and the real (actual) evaporation. We assume that the air can fully be saturated with water in case of potential evaporation (there is enough water to evaporate). The real (actual) evapotranspiration is never larger than the potential evapotranspiration. Above water surfaces, there is only potential evapotranspiration.
The potential evapotranspiration can be measured and calculated with the assistance of other parameters.The most commonly used measurement procedures are Class-A-Pan, the Czeratzki-disk and the Piche evaporometer.
If there are no measurement data available, it is possible to calculate or estimate the potential evapotranspiration with existing meteorological parameters and different calculation procedures.