Geoscience Reference
In-Depth Information
2
1 . 5
1
0 . 5
0
0
1
2
3
4
t
1/2
( d
1 / 2
)
Fig. 9.24 Cumulative evaporation from a bare sandy soil surface as a function of the square root of time
in days, obtained in the field by means of a weighing lysimeter in Wisconsin. The straight
line represents the integral of Equation (9.108) with a desorptivity
De
0
=
0
.
496 cm d
−
1
/
2
.
(Adapted from Black
et al
., 1969.)
effects come into play when the surface is not bare but, as is usually the case under natural
conditions, covered with vegetation. Nevertheless, as will be shown below, under certain
conditions the assumption that soil surface evaporation takes place in two stages, and that
the second stage can be described by a desorption formulation, is still a useful construct to
describe drying phenomena. But it should be kept in mind that, whenever (9.108) is applied
under conditions other than those specified by (9.6) and (9.107), it does not really represent
a capillary desorption phenomenon, in the strict sense of the word. Therefore, the value of
De
0
obtained this way is probably better referred to as “effective” desorptivity.
Daily evaporation from bare soil
In several field studies it was found that
E
=
E
(
t
) could be described reasonably well as a
desorption phenomenon by Equation (9.108), provided this was done with daily time steps.
In Wisconsin, Black
et al.
(1969) obtained good agreement between (9.108), in which
t
was set to zero after each heavy rainfall, and daily evaporation from a bare soil lysimeter
measured during an entire summer. The measurements illustrated in Figure 9.24 suggested
an effective desorptivity of around
De
0
=
496 cm d
−
1
/
2
. Interestingly, this value was also
not very different from the desorptivity
De
0
=
0.43 cm d
−
1
/
2
, calculated by means of Crank's
linearized solution. In light of the natural variability of the soil, and also of the likely errors
stemming from the problem formulation and its linearization, the difference is small. Black
et al.
(1969) suspected that after a rainfall the evaporation would eventually depart from the
t
−
1
/
2
relationship, because of the finite depth of wetting. Still, it was possible to simulate an
entire summer of evaporation this way. In a similar study in California, Parlange
et al.
(1992)
assumed that daily evaporation from a bare soil lysimeter followed the pattern of (9.108)
immediately after irrigation of the field; with this assumption their measurements suggested
an effective desorptivity
De
0
=
0
.
58 cm d
−
1
/
2
. In Arizona, Jackson
et al.
(1976) found that
Equation (9.108) could be used to describe the second stage of drying on a daily basis from
0
.
