Geoscience Reference
In-Depth Information
Fig. 9.8
Pore-size distribution in an ideal homogeneous soil with pore radius
r
1
<
r
2
<
r
3
… (see the
rectangles
) and the fl uxes in individual pore-size categories (see the
arrows
). The most frequent
are pores here denoted by
r
3
, but the maximum fl ux
q
j
is in the group of pores of maximum size,
radius
r
j
, in spite of their relatively low frequency
fl uxes
q
i
within each pore-size category, and percentages of each size category
n
i
.
The smallest and largest radii are
r
1
and
r
j
, respectively. Assuming the water fl ux
q
i
for each category is a function of
r
i
4
and
n
i
, it is obvious that a nonsymmetric water
fl ux distribution exists inasmuch as the water fl ux at
r
j
manifests a maximum value
compared with all of those at smaller radii. As in many structureless soils, the pore-
size distribution is approximately bell shaped with a peak of
n
3
at radius
r
3
.
In the vicinity of water saturation, values of hydraulic conductivity of sandy soils
are generally much higher than those of fi ner-textured soils. In contrast, as their soil
water contents progressively decrease, values of hydraulic conductivity of fi ner
soils are greater than those of sands. Consequently, a curve of hydraulic conductiv-
ity versus soil water content of a sand is not parallel to that of a loam. Indeed, as
shown in Fig.
9.9a
, their curves cross each other at a common value of soil water
content
ʸ
c
. Similarly, as shown in Fig.
9.9b
, their curves of hydraulic conductivity
versus pressure head cross at a common value of pressure head
h
c
.
Let us now consider the majority of structured soils. We fi nd that curves of their
pore-size distribution do not have just one peak but usually at least two peaks as
demonstrated by Fig.
7.7
in Sect.
7.3
.
For such soils having a bimodal porosity, one
portion of coarse pores occurring between aggregates (interpedal pores) has a peak
for a radius somewhere between 20 and 60
m and another group of fi ne pores
occurring inside of aggregates (intrapedal pores) has a peak for a radius between 0.5
and 5
ʼ
m. When the soil is close to saturation, the fl ow of water is dominantly infl u-
enced by most of the coarse interpedal pores remaining fi lled with water and sus-
taining a high value of the unsaturated conductivity. When the soil is at lower water
ʼ
