Potential evapotranspiration or PET (size for evaporation)

In addition to the precipitation, the water availability for humans, animals and plants is also determined by the evapotranspiration. This evapotranspiration is a collective term for the water discharge through the soil and vegetation to the atmosphere. This includes all precipitation that comes directly into the atmosphere by evaporation or indirectly via ecosystems. The ‘potential evapotranspiration’ or PET is expressed in mm of precipitation per year. This is the maximum possible evapotranspiration that occurs if sufficient water is always available on the surface or in the soil. If the latter is not the case, as during dry summer periods, the actual evapotranspiration is lower than the potential. Nevertheless, this indicator describes the potential and not the actual evapotranspiration because the latter is very difficult to quantify. Reasons for this are that the evapotranspiration is highly region-dependent and function of the water availability on the surface and in the soil. A forest area or a highly urbanised area therefore have a very different evapotranspiration.