Environmental Engineering Reference
In-Depth Information
ACTUAL EVAPOTRANSPIRATION
Actual evapotranspiration equals PE only if there is a constant and adequate supply of
water to meet the atmospheric demand. Such a situation exists over moist, vegetated
surfaces and it is also approximated over water surfaces such as the open sea or large
lakes, but most land surfaces experience significant periods when water supply is limited.
As a result, actual evapotranspiration falls below PE. We can get some idea of the
importance of surface conditions by considering evapotranspiration in a variety of
situations. Let us start by examining evapotranspiration from an open water surface.
EVAPORATION FROM WATER SURFACES
Because there is an unlimited supply of water to maintain evaporation, and because there
is no vegetation to complicate the process, the surface of oceans or large lakes provides
the simplest situation in which to study evapotranspiration. Under these conditions,
transpiration does not occur and water loss is entirely by evaporation. The main factors
determining water loss are therefore the atmospheric conditions, and there is generally a
close relationship between actual evaporation and PE.
Nevertheless, the relationship is not perfect, and the main reason is that the water is
able to absorb a large amount of energy which is not used in evaporation. This energy is
expended in heating the water, and much of it is recirculated through the water body.
Figure 5.16
Mean evaporation in millimetres per day from
the tropical Atlantic and eastern Pacific Oceans.
Source: After Hastenrath and Lamb (1978). 1.0
