Evapotranspiration: General Concepts

Evaporation process: It is the loss of water from a wet surface through its convection into its gaseous state. Surface can be bare soil (soil evaporation), open water (including river, lakes and oceans) or intercepted water held upon plant surfaces (interception evaporation).

Transpiration process: based on the concept of a continuos pumping (by growing plants) of water from the soil which is then moved up to leaves and lost, through biological processes in vegetation, into the atmosphere (DIN 4049,1994).

Evapotranspiration: the sum of soil, interception and open water evaporation and transpiration.

Potential evapotranspiration (Maximal) (Etp) : is “the rate of evapotranspiration from an extended surface of an 8 - 15cm tall green grass cover, actually growing, completely shading the ground and not short of water” (DIN 4049, 1994). This maximal evapotranspiation is also called reference evapotranspiation.

Actual evapotranspiration (Eta): is “the rate of the evapotranspiration from a surface under field conditions and with limited water supply”.

The evapotranspiration rate depends on different parameters, which are defined considering:

  • Atmosphere
  • Soil
  • type of vegetation cover

 

Calculation of evapotranspiration

To calculate the rate of evapotranspiration the following methods are considered:

  • Aerodynamic method (Dalton-process)
  • Energy balance methods: or thermodynamic approach.
  • Combination of the above mentioned methods

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Hydrological modelling
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