Geoscience Reference
In-Depth Information
Fig. 9.23 Cumulative
evaporation from
a bare soil surface
as a function of
the square root of
time obtained in
the laboratory
with a 1.0 m long
column of clay
soil. (After
Gardner, 1959.)
1 5
1 0
5
0
0
2
4
6
8
1 0
1 2
t
1 / 2
( d
1 / 2
)
Good agreement was obtained by Gardner (1959) between (9.108) and the evaporation
rate from an initially uniformly moist column of clay soil, 100 cm in length, subjected
in the laboratory to a large potential evaporation rate of about 4 cm d
−
1
. These results are
shown in Figure 9.23. Evidently, the column was long enough to be effectively semi-infinite
for about 100 d. Similar results were presented by Gardner and Hillel (1962); in addition
they observed that soon after the end of the first stage of drying, the rate of evaporation is
independent of the initial drying rate, and that it depends primarily on the water content
of the soil. These successful experimental tests of the desorption approach were obtained
under constant atmospheric conditions in the laboratory, and should lend support to the
underlying assumptions. Nevertheless, it can be expected that the neglect of gravity will
somehow result in an overestimate of the evaporation rate, whereas the neglect of the vapor
transfer must result in an underestimate of the evaporation. While these effects may be
compensating each other to some extent, this will require more study.
9.6.3
Applications in the field
Under field conditions soil evaporation is more complicated than described in the previous
section. Clearly, with a diurnal cycle of radiation, the surface boundary condition on
θ
is not
simply a constant
θ
s
as given in Equation (9.107), and the hourly changes of the surface water
vapor flux even from dry soil are not given by (9.108) (see Jackson
et al.
, 1973; Idso
et al.
,
1974). Hourly values of near-surface
θ
and evaporation depend markedly on net radiation
even after the soil surface has dried considerably. A simple explanation for this is that during
the night in the absence of the driving solar radiation, the soil moisture is able to redistribute
into some new equilibirum by early morning; this process involves hysteresis with drying
from above during the day, and wetting from below during the night, resulting in the distinct
diurnal pattern of the surface evaporation in the course of the following day. All this would
appear to suggest that, under conditions of a diurnally varying evaporative demand, the
two stages of bare soil evaporation and also the desorption approach may not be physically
relevant; even relatively dry soil surfaces continue to change from an energy-limiting state to
a soil-limiting state during the same day, and the transition is not instantaneous. Additional
