Geoscience Reference
In-Depth Information
Fig. 9.9
The unsaturated conductivity of sandy soil is steeply falling down with small decrease of
soil water content, compared to loam
contents, fl ow is maintained primarily by small pores (micropores) and the unsatu-
rated conductivity sinks by an order of magnitude; see Fig.
9.10
.
When a dissolved substance is transported with the fl owing water and the soil
water content is high, the solute is carried deep into the profi le by fast-fl owing water
between the aggregates, while it is only negligibly transported into aggregates, since
the conductivity within aggregates is many times, even a hundred times, smaller
than that between the aggregates. This kind of fl ow in coarse pores is called “pref-
erential fl ow” or “preferential transport,” since it is preferentially restricted to coarse
pores conducting water and dissolved matter at high transport rates through soils at
or close to water saturation.
From our experience observing the behavior of soil water in natural outdoor
environments, we are inclined to say that water fl ows in unsaturated soils from a
place of high water content to one of low water content and additionally comment
that the driving force of the process is the difference in soil water content. We often
further state that the water content gradient determines the direction of fl ow and that
its magnitude controls the rate of fl ow. However, the above statements are valid only
for horizontal fl ow in the same type of soil with the same hydraulic characteristics,
like saturated hydraulic conductivity, soil water retention curve, pore-size
distribution, and compaction, and, moreover, with the same concentration of dis-
solved chemicals in soil pores. When these highly unlikely circumstances occur, the
basic equations for this exceptional situation are similar to diffusion equations. For
all other situations, we must adopt the concept of soil water potential as explained
in Chap.
8
.
When we observe the vertical fl ow of water in a soil that is homogeneous through-
out its profi le, at a specifi c time the soil water content together with its soil water
potential may not vary within the upper 10, 20, or even more centimeters of topsoil -
