Environmental Engineering Reference
In-Depth Information
mean air temperature, mean air humidity and mean wind speed. The data are needed at
only one level above the ground surface and are recorded at most meteorological stations.
The Penman approach has been modified by Monteith to allow for the nature of
vegetation with either optimal or restricted water supply. Although likely to be more
realistic, the formula does require information about the resistance of the plants' stomata
to water vapour flow. This is not readily available, so the use of the formula has been
restricted to research applications.
MOISTURE BUDGET METHODS
An alternative method of obtaining actual evapotranspiration is the
moisture balance
equation
. At a site the moisture balance can be expressed as:
where
P
is precipitation,
I
is irrigation water added,
E
is evapotranspiration,
R
is run-off,
D
is drainage to bedrock and ∆
S
is the change in soil moisture content. As before, if we
knew all the other elements in this equation we could calculate
E
by difference. Several
of the components present little problem, for precipitation and irrigation inputs can easily
be measured, as can run-off. But drainage to bedrock (
D
) and changes in soil moisture
content (∆
S
) are rarely known. As a result, the method can be used only on a large-scale,
long-term (e.g. annual) basis where it can be assumed that drainage to bedrock is
balanced by release from spring seepage, and where changes in soil moisture content are
negligible.
It is clear from what we have said that evapotranspiration remains one of the most
difficult aspects of the hydrological system to measure. For this reason, if for no other,
our knowledge of the processes involved remains uncertain. For the same reason, it is
difficult to give precise figures for the global pattern of evapotranspiration. None the less,
it is useful to consider the role of evapotranspiration within the global system.
EVAPOTRANSPIRATION IN THE HYDROLOGICAL CYCLE
As we have noted, evapotranspiration provides the main output of moisture from the
surface hydrological system,
