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1
0 . 8
E
0 . 6
( c m d 1 )
0 . 4
0 . 2
0
0
2 0 0
4 0 0
6 0 0
8 0 0
D e p t h t o s i m u l a t e d w a t e r t a b l e ( c m )
Fig. 9.22 Comparison between the experimental rate of steady evaporation from a column of clay soil
and the curve calculated by means of Equation (9.103) with (8.37) in the form
k
p w ) 2 ]cmd 1 , where p w is in hPa. (After Gardner and Fireman, 1958.)
=
1100
/
[565
+
(
The first and second stages of evaporation from soil
Ever since the studies of Fisher (1923) and Sherwood and Comings (1933), among others,
it has been customary to classify bare soil evaporation under constant external conditions
in the laboratory into several stages of drying; from the hydrologic point of view, the first
two of these stages are the more important ones. In the first stage, which prevails as long as
the soil is sufficiently moist, the evaporation rate is primarily controlled by the atmospheric
conditions. It is therefore best expressed in terms of measurements in the atmosphere. For
a moist surface under natural conditions, several well-known formulations are available,
which make use of atmospheric variables and which are treated in Chapter 4. Obviously,
for constant atmospheric conditions, the rate of drying is constant. The duration of the first
stage depends on the rate of evaporation and the ability of the soil profile to supply this
rate.
As the soil near the surface dries out, the water supply to the surface eventually falls
below that required by the atmospheric conditions. In this second or falling-rate stage, the
rate of evaporation is mainly limited by the soil moisture conditions and the soil properties,
and much less by the available energy. The transition from the first stage to the second stage
may be quite abrupt at a given point on the surface; on a wider scale it is usually more
gradual, because local transitions at different locations tend to occur at different times. It
was noted by Jackson et al. (1976) that the transition from the first to the second stage can
be observed visually by changes in color and in albedo.
In the second stage of drying, water moves also through the profile by diffusion of water
vapor. And especially after the soil has become quite dry, the water transport through the
profile is sensitive to the temperature gradients in the soil as well. Nevertheless, at least
initially in the falling rate stage, it appears that the water moves primarily as a liquid.
Although the matter is more complicated (Philip, 1957c; Cary, 1967), just like for the
steady case, experimental evidence has shown that some of the more important features
of the falling-rate stage can be obtained by means of the isothermal flow description with
Richards's equation.
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