Geoscience Reference
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steadily. So it is with infi ltration rate - its extremely rapid value quickly decreases
with time of infi ltration to eventually approach the value of the saturated conductiv-
ity. Similarly, the increase of the cumulative infi ltration
I
is high at the start, but after
several hours it slows down and reaches a constant value of increase.
During infi ltration into homogeneous soils, those without layers or horizons, if
we measure the distribution of soil water content
θ
within the profi le at several dif-
ferent times and draw a smooth curve through the measured data, we discover that
the water penetrates into the soil like a piston. The bottom of this “piston” is the
wetting front of infi ltrating water. From the soil surface down to just above the wet-
ting front, saturated values of soil water content prevail with all pores being fi lled
with water. Only within a very narrow range of depths immediately above the front
does the water content sink below saturation. Although such profi les are commonly
observed in sandy soils, infi ltration profi les within loamy and clay soils do not man-
ifest such a vivid resemblance to a piston; see Fig.
10.2
. Profi les in this fi gure
illustrate soils with simple pore-size distributions when the role of soil structure is
of minor consequences.
Soil structure alters the size and distribution of pores in the majority of loamy
and clayey soils. Their soil water profi les during infi ltration are less regular than
those illustrated in Fig.
10.2
. Simply saying, deviations occur due to the more com-
plex, irregular nature of their pores; see Sect.
7.3
.
Water penetrates these structured
soils mainly and preferentially through sequences of their big pores that we previ-
ously described as preferential fl ow. The fl ow rate in these big pores is even more
than ten times faster than that in the majority of fi ne pores existing mainly inside
aggregates. This great difference of infi ltration rate cannot disappear when the wet-
ting front penetrates to a deeper soil horizon (below A horizon) even if there the
conductivity of the “structureless” soil does not differ from the “structured” soil.
The terms structured and structureless soil are used as a concise description of the
topsoil (A horizon). Let us recall our description of fl uxes in layered soils in
Chap.
9
,
Figs.
9.3
and
9.4
.
If a soil has a well-developed structure in the top horizon with stable aggregates
keeping their shape even after being soaked by water, the infi ltration rate slowly and
steadily decreases with time. On the other hand, if the soil contains only quasi-stable
aggregates that slake abruptly into slushy mass just after wetting in the top horizon,
the infi ltration rate sharply decreases. The differences between
q
0
values for struc-
tured and structureless soils are about tenfold during initial stages of infi ltration; see
the Fig.
10.3
. The slaking of crumbs is more drastically increased when a nonsaline
water infi ltrates a saline soil with an abundance of monovalent Na
+
cations.
10.1.2
Rain Infi ltration
Initially, infi ltration from rainfall depends upon the rain intensity (velocity)
expressed usually in millimeters per minute. Let us fi rst assume that the soil is
homogeneous without any distinct layering. The infi ltration rate is limited by the
