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Evapotranspiration

Evapotranspiration, known as ET, a process that transfers water from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plant tissues, is one of the components of the global water cycle. Observations and models of evapotranspiration often become inputs to hydrological models. Yet another common area for application of evapotranspiration is agroclimatology. In the catchment scale, evapotranspiration generates significant loss of water which is returned to the atmosphere and does not participate in other components of the hydrological cycle.<

Evapotranspiration is a complex process which is influenced by both meteorological factors – such as radiation, humidity, temperature and wind – as well as the parameters associated with vegetation, namely aerodynamic drag and the resistance of the pore vegetation.

The amount of water vapor passed into the atmosphere under current meteorological conditions with a limited amount of water in the soil, along with the evaporation of rainwater retained on plants as a result of interception, is called the actual evapotranspiration. It is difficult to measure its values, therefore the notions of potential and reference evapotranspiration, denoted as ETp and ET0 respectively, have been proposed. The former corresponds to a maximum amount of water that under existing meteorological conditions and optimum supply of plants and the soil in water could be brought into the atmosphere, assuming that vegetation is uniform and fits the broad class of short green crops, thus without relating the calculation to a specific crop. However, the reference evapotranspiration offers more specific assumption on vegetation by introducing a hypothetical reference crop (height of 0.12 m, fixed surface resistance of 70 sm⁻¹ and albedo of 0.23).

There are many ways of calculating and modeling of ETp and ET0. The most popular method, outlined by Monteith (1965) who extended the approach of Penman (1948), is to apply the Penman-Monteith equation, recommended by WMO (World Meteorological Organization) and the FAO (U.N Food and Agriculture Organization) as an international standard. Following the FAO 56 paper (Allen et al., 1998), the Penman-Monteith governing equation is:

$$ET_{PM} = {{\Delta}(R_n - G)+{\rho}_a c_p {(e_s-e_a)\over r_a} \over {\Delta}+{\gamma}(1+{r_s\over r_a})},$$

where:

ETPM – evapotranspiration based on the Penman-Monteith [mm day⁻¹]
Δ – slope of the saturation vapour pressure-temperature relationship [kPa °C⁻¹]
γ – psychometric constant [kPa °C⁻¹]
Rn – net radiation [MJ m⁻² day⁻¹]
G – soil heat flux density [MJ m⁻² day⁻¹]
ρₐ – mean atmospheric density [kg m⁻³]
cp – specific heat of air at constant pressure 1.013 * 10⁻³ [MJ kg⁻¹ °C⁻¹]
es – saturation vapour pressure [kPa]
ea – actual vapour pressure [kPa]
rs – canopy resistence
ra – aerodynamic resistance [sm⁻¹]

References

Allen R.G., Pereira L.S., Raes D., Smith M., 1998. Crop evapotranspiration – Guidelines for computing crop water requirements. FAO Irrigation and Drainage, paper 56, FAO – Food and Agriculture Organization of the United Nations, Rome.

Monteith J.L., 1965. Evaporation and environment. In: Fogg G.E.(ed.), Symposium of the Society for Experimental Biology, The State and Movement of Water in Living Organisms 19, Academic Press, Inc., NY., 205-234.

Penman H.L., 1948. Natural evaporation from open water, bare soil, and grass. Proceedings of the Royal Society of London A193, 120-146.

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