Geoscience Reference
In-Depth Information
Fig. 10.5 Time dependence of the infi ltration rate of rainwater when the rain intensity q R is con-
stant and greater than the value of the saturated conductivity K S . The relative infi ltration rate in the
graph q 0 = q R /K S . At the beginning of the rainfall event, the relative infi ltration rate q 0 is constant
and equal to the rain intensity q R until the soil starts to soak water more slowly than q R . At that
time, the ponding time t p , any water that does not infi ltrate starts to remain on the soil surface. If
there is a slope, runoff starts. For rains that continue to last longer than the ponding time, the infi l-
tration rate decreases and the shape of the time-dependent curve resembles that for a soil fl ooded
with water (see Fig. 10.1 ) shown here as a broken line
conductivity property of the soil surface is always jointly responsible for the value
of ponding time. Having already shown in Fig. 10.3 great differences in fl ood infi l-
tration rate between structured and structureless soils, ponding time is infl uenced
similarly by the quality of soil structure, especially at high rain intensity. The pond-
ing time for a structured soil may be up to 10-20 times higher than that for the same
loamy soil without any aggregate stability.
The real situation of rainfall and the consequent infi ltration are usually more
complicated with the determination of the ponding time and amount of infi ltrated
rainwater (“effective rain” in the language of hydrologists) being a matter of numer-
ical modeling. But the basic principles discussed above are valid. The most impor-
tant are two characteristics: (1) water infi ltrates into the soil in the form of a slightly
deformed piston, and (2) a specifi c time when the entire amount of rain no longer
infi ltrates into the soil and the non-infi ltrating water starts to pond on the soil sur-
face - a characteristic called ponding time. Its value is indicative of the beginning
of surface runoff and the potential onset of fl oods.
On several occasions while introducing the principal characteristics of infi ltra-
tion in relation to a water regime on a regional scale, we have mentioned the unique-
ness of soil structure. Because the role of soil structure and aggregate stability
cannot be replaced by any technological procedure, it is the moral obligation of
society to sustain and even improve well-developed soil structure for the benefi t of
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