Geoscience Reference
In-Depth Information
Fig. 10.7
The actual
evaporation from a bare soil
with a groundwater level
(GWL) close to surface
depends upon the depth of
GWL and upon the potential
evaporation. The deeper is
GWL, the lower is actual
evaporation if we compare
values at a certain potential
evaporation. If the GWL
reaches to the soil surface,
then the actual evaporation
equals the potential
evaporation
very wet (at or near saturation), the evaporation rate
E
is initially determined by
potential evaporation. However, the rate is then later reduced due to two facts: there
is a reduction of the amount of water freely accessible to evaporation since a major
portion of the horizontal cross sections of the profi le is occupied by solid soil par-
ticles and aggregates. The second factor is the resistance opposing water vapor fl ux
as water molecules struggle through soil pores in their penetration upward to the
soil surface from positions below the soil surface. A simplifi ed process is thus com-
posed of two stages. The constant rate after the start of evaporation from saturated
soil is called the fi rst stage of evaporation. It is equal to potential evaporation. It
ends at a time when a very thin top layer is gradually dried out and reaches a certain
low value of water content. After that time the second stage starts with a steadily
decreasing rate of evaporation; see Fig.
10.8
. During this second stage, the depth of
the overlying dried layer increases.
The oversimplifi ed situation illustrated in Fig.
10.8
is never fully observed in
natural conditions. Nevertheless, the fi rst stage of evaporation is an approximate
reality just after the occurrence of long-lasting rains when the soil is saturated by
water to depths of about 30 cm and more. It is an approximate reality because
natural-occurring evaporation is never absolutely constant with time owing to the
fact that potential evaporation is never constant during day or night. The day/night
change of evaporation rate is high when the soil has a high water content (at fi eld
capacity and more) in the top layer reaching from the soil surface to the depth about
20-30 cm. The day/night change of evaporation rate is low when the soil water
content is low (close to wilting point - see Sect.
10.3.2
and Fig.
10.11
).
The evaporation rate is greatly infl uenced by the soil structure of the top horizon
especially in the second stage. If the topsoil has a stable well-developed structure,
