Geoscience Reference
In-Depth Information
within and across large land areas. As a remote sensing technique, the radiation is
detected and measured either from a balloon or, more frequently, from an airplane
or satellite.
8.2
How Water Is Kept in Soil Pores
When it rains on a rock, and if it is a heavy rain, little or no water is retained and it
fl ows away except from depressions in the rock surface where small isolated water
puddles could appear. On the other hand, the majority of rains change their appear-
ance on the rock surface from individual drops to a continuous surface fl ow just at
the beginning of rainfall and briefl y after it.
When it rains on a soil surface, either all of the water or at least a portion of it
infi ltrates and is captured and held within soil pores. Why does the infi ltrated water
not fl ow through the soil to the depths where it would not be of any use for plants?
Except within a few centimeters from its surface, why does the soil remain wet even
for a couple of weeks after the rain? Retained rainwater evaporates fi rst from the
surface, while the soil below the thin surface layer remains nearly at the same water
content as it was when the rain stopped. We recognize it when we remove a lump of
soil from below the dried-out surface layer and subsequently squeeze it in the palm
of our hand. Opening our palm, we observe that the skin of our palm is very wet and
perhaps even partially covered by small drops of water. With such an experience we
realize that some of the rainwater was kept in the soil while other portions of it were
pushed out. The mechanism is similar to what we know from our household activi-
ties. After we take a wet sponge from a pan of water, squeeze it with all of our
might, and wipe a table, the squeezed sponge leaves a wet trail across the surface of
the table. It is similar with soil. Although we pushed a small amount of water into
our palm by squeezing the lump of soil, some water remained strongly bound within
its fi ne soil pores. Water is kept in fi ne pores mainly by
capillary forces
.
In reality, soil water is never immobile. It continually wanders, migrates, and
fl ows at different rates from one location to another. But in some instances its
instantaneous fl ow rate is so slow that we can simplify the situation with a hydro-
static model where all forces acting upon water are in equilibrium and where only
the laws of hydrostatics are valid. After discussing wetting angles between water
and solid surfaces, we shall explain the principles of capillarity and those of
adsorption.
When water meets or contacts a solid surface, it has three potential impacts: (a)
to completely wet the surface, (b) to incompletely wet the surface, and (c) not to wet
the surface. An example of such behavior is shown in the top half of Fig.
8.4
when
water rests on top of three horizontal solid plates forming different contact angles
ʳ
between the water-solid interface and the water surface. If the solid surface is com-
pletely wet, the interface and the water surface coincide and manifest a value
ʳ
= 0°.
When the solid surface is incompletely wetted, values of
ʳ
range between 0° and
90°. A non-wetting surface is indicated when values of
ʳ
are greater than 90°.
