Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 25

• wind and atmospheric pressure;
• nature and state of vegetation;
• topography and exposure.
2.3.1 Calculating evapotranspiration
a) Theoretical calculation
On the scale of Europe, it is considered that approximately 60% of water in
precipitation returns to the atmosphere through evapotranspiration.
Various equations exist to calculate evapotranspiration. It is important,
at this point, to differentiate between potential evapotranspiration (PET),
which is the amount of water that an open water surface could theoretically
evaporate, and actual evapotranspiration (AET), which is the amount of
water that actually evaporates, and is dependent on natural conditions
(vegetation, nature of the soil) and on the amount of available water in
the soil, called available water capacity (AWC). This amount includes
easily used available water or fi eld capacity that is relatively easy for plants to
extract from the soil. It also includes water that is more diffi cult for plants to
extract. It can range from 0 (empty shallow reserve) during the dry season,
to a maximum value that depends on the porosity and thickness of the soil
cover. It corresponds to a layer of water that can range from a few tens of
mm over compact rock, to 400 mm in deep soils.
To link these concepts to those used in agrometeorology, the empty
reserve corresponds to less than the permanent wilting point, and the full
reserve to more than the fi eld capacity (c. chap. A5.1).
If (P d +UR d-1 ) ≥ PET d , then AET d = PET d
If (P d +UR d-1 ) < PET d , then AET d < PET d
where d = day
Desert areas therefore have a very high PET but a very low AET, given
their UR equalling zero.
The reference evapotranspiration (ET 0 ) used in agronomy, is the
evapotranspiration of a reference surface, under a given climate in well-
defi ned conditions, corresponding to a homogeneous lawn in active growth
covering the entire surface, with a plentiful water supply.
PET can be calculated with the help of a number of equations
(Thornthwaite, Turc, Primault, Penman, FAO-Penman-Monteith, etc.).