Geoscience Reference
In-Depth Information
conditions. The rate of evapotranspiration from this instrument is obtained by solution of
Equation (4.58). In order to produce the same rates of evapotranspiration as the surrounding
area, a lysimeter should be representative of the conditions of the natural soil profile and
of the vegetation around it. In other words, when designing and installing a lysimeter, care
must be taken to insure the same water fluxes at the soil surface and the same developments
of the plant roots in the soil profile. This means that its surface should be flush with the
surrounding ground surface, and that it should be at least as deep as the rooting depth of the
vegetation; moreover, the profiles of the soil structure, soil texture, soil water content and
soil temperature in the lysimeter must be made as similar as possible to those on the out-
side. These conditions are not always easily met (Brutsaert, 1982). Va r ious methods can be
used to determine the different terms in Equation (4.58). One of the more complete designs
(Pruitt and Angus, 1960) has a circular surface area of 6.1mdiameter and a depth of
0.91 m and it is equipped with temperature control; the integral term in(4.58) is deter-
mined by continuous monitoring of weight changes and
q
z
d
is measured as the outflow by
maintaining soil water suction control at the bottom of the container.
Other parameterizations
Water budget considerations can also be used to express soil surface evaporation as a
capillary rise phenomenon in terms of soil properties and other variables beside soil water
content. However, such derivations require some understanding of the physics of flow in
partly saturated soils and also the solution of the Richards equation for various boundary
conditions. This topic is therefore delayed until Chapter 9, after the principles of flow in
porous media have been presented in Chapter 8.
4 . 5
EVAPORATION CLIMATOLOGY
In Chapter 1 it was pointed out that on a global basis the annual evaporation is of the order
of1m,which balances the annual precipitation. Table 1.1 indicates that the evaporation
from the land surfaces of the Earth is around 0.5m,which is roughly two-thirds of the
mean annual precipitation.
Interestingly, practical experience and folk wisdom suggest values similar to those
given in Tables 1.1 and 1.2. For example, irrigation engineers, when lacking better
information, sometimes use the rule of thumb that the duty of water for a well-irrigated
crop is around 1 l s
−
1
ha
−
1
. Similarly, farmers in the northeastern United States are said
to require a weekly rainfall of about one inch, that is2.5 cm, to maintain field crops in
good condition during their active growing period. With typical irrigation efficiencies
of 25% to 40%, and a growing season of 4-5 months, both practical estimates of the
evaporative requirements in agriculture are consistent with global climatological values
of around 0.50my
−
1
.
However, because precipitation and the radiative energy supply are highly variable
over the surface of the Earth, the actual evaporation is usually quite different from these
long-term climatological mean values. Over periods shorter than a year, deviations of
evaporation from the mean can be characterized by a cyclicorperiodic behavior, namely
withadaily and with a seasonal time scale. In the extreme case of an arid, warm climate,
with a pronounced dry and wet season, the seasonal evaporation cycle issimilar to the
