Geoscience Reference
In-Depth Information
Fig. 9.7
A sim ple laboratory
equipment for measuring
saturated hydraulic
conductivity
K
S
9.2.2
Unsaturated Flow
Although equations for saturated fl ow are relatively simple and straightforward,
computing water movement in unsaturated soils is a complicated task even for a
seemingly very simple situation. One of the several diffi culties encountered is the
fact that the hydraulic conductivity of an unsaturated soil does not have a unique
value. Its value sinks signifi cantly whenever the soil water content decreases by
only a fraction of a percentage point. When the water content of an initially water-
saturated soil starts to decrease, the biggest pores are the fi rst to empty and cause an
abrupt reduction of the hydraulic conductivity. We have already learned that big
water-fi lled pores cause high values of saturated conductivity. However, when the
soil water content drops below saturation, the majority of those big pores no longer
conduct any water. And the lower the soil water content, the smaller is the size and
total volume of water-fi lled pores that conduct water through the unsaturated soil. It
is a common occurrence that the conductivity decreases by tenfold or even more
when the soil water content decreases from saturation by only a very few percentage
points. In other words, we would measure a value of
K
in a very wet, almost water-
saturated soil to be only 10 cm/day compared with a value of 100 cm/day when it is
fully saturated.
Complications between unsaturated conductivity and its relationship to soil tex-
ture never completely disappear as implicitly implied in Chap.
7
where the graph in
Fig.
7.6
of the shape of an idealized log-normal soil pore-size distribution was pre-
sented while judiciously ignoring soil structure. Here in this and the next paragraph
we continue to ignore complications of soil structure. Figure
9.8
illustrates relation-
ships between the water-fi lled cylindrical pores of different sizes of radii
r
i
, water
