Evapotranspiration is the invisible antagonist of precipitation. More than 70 % of annual precipitation return to the atmosphere by evapotranspiration or directly from the soil (evaporation) or by the intermediary of vegetation (transpiration) through a physiologically controlled water transport from root to canopy. Evapotranspiration is a cooling agent and can lower the temperature by a few degrees – the reduction of evapotranspiration in urban environments can increase temperatures by about 2-3 degrees Celsius. Evaporation is a slow but mighty process: It consumes between 0.5 (winter) to 5 Liters per square meter per day. The steady process, however, can lead to a total evapotranspiration of 450 to 550 Liters per square meter per day on land surfaces and even higher amounts on lakes or water bodies.

Canal between Lübeck and the Elbe river
Evaporation can be measured with a pan (class A pan) in which the water level is monitored. The station needs to comprise a precipitation gauge (to account for precipitation falling into the pan) and should also have temperature, humidity, wind speed and if possible radiation measurements.

Key factors for evaporation are:

  • energy – short and long wave radiation
  • humidity deficit of the air – the capacity of the atmosphere to absorb moisture
  • wind – the capacity to transport saturated air away from the site and to renew the vertical transfer process

Class A pan data can be found on: https://openmeteo.org/stations/d/1480/

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