Geoscience Reference
In-Depth Information
Fig. 10.3
Infi ltration rates
q
0
from fl ooded land into a
well-aggregated loamy soil
decrease more slowly than
those into another loamy soil
that lacks structural
properties. The infi ltration
rate
q
0
into the structured soil
is up to ten times larger
during the fi rst 30 min. The
aggregated soil maintains its
substantially higher
infi ltration rate even after
more than 1 h in our graph.
The terms structured and
structureless soil denote the
properties of the top A
horizon, only
somewhat higher than it was before the rain started. At times
t
2
and
t
3
as the rainfall
continues, the soil water content at the soil surface continues to increase. In a little
more time, it reaches
θ
S
- the saturated soil water content - that is at the top of the
curve labeled
t
4
in Fig.
10.4
. Because this time is very signifi cant and linked with
other events during and after infi ltration, it is equated to
t
p
and called the ponding
time - the instant that free water starts to exist and form a pool on the soil surface or
starts to fl ow across the topsoil.
We have compared soil water profi les during fl ood infi ltration to a piston system-
atically penetrating into the soil profi le with time from the very beginning of the
process (Fig.
10.2
). If we plot the soil water profi les during rain infi ltration, we can
use the comparison to a piston only with the provision that the piston gradually
increases its horizontal diameter until the surface soil water content reaches satura-
tion at the ponding time
t
p
; see Fig.
10.4
. For times larger than
t
p
, the profi les during
rainfall resemble those fl ooded profi les in Fig.
10.2
- both look like pistons of con-
stant diameter penetrating deeper and deeper into the soil profi le.
The term ponding time and its decisive role follow from Fig.
10.5
, where the
infi ltration rate is plotted as dependent upon the time. Infi ltration rate is constant in
the fi rst part since the source, the rain, has a constant intensity that is smaller than
the ability of soil to swallow water from the surface. This situation lasts up to the
ponding time. This is the moment when the ability of soil to soak water equals the
rate of incoming water, i.e., the rain intensity. After this ponding time is reached, the
ability of soil to consume all incoming water is smaller than is the rain intensity
causing some of the rainwater to remain on the soil surface. Hence, the soil surface
is ponded by water and the longer is the ponding time, the greater is the difference
between rain intensity and the rate of actual infi ltration. During this ponding time,
the height of water remaining on the soil surface continues to increase provided that
