Geoscience Reference
In-Depth Information
relationships on one conical pore. It is drawn in the cross section as a triangle. When
the pore is fully separated from the outside environment and when there is no water,
the air is absolutely dry. Then we allow air with a humidity of about 20-30 % to
enter the pore at a constant temperature, let us say at 20 °C. The individual water
molecules are attracted to the solid surface of the pore forming a fi lm of the average
thickness of one molecule as a direct result of water vapor adsorption. We continue
in increasing the air humidity up to about 50-60 % and the adsorption continues; the
fi lm thickness of adsorbed water makes fi rst two, then three molecules. Now, the
fi lm starts to form a curved horn at the contact of fi lms and the curvature is depen-
dent upon the air humidity. The process is called capillary condensation and the
situation is depicted in B section of Fig.
8.8
.
The higher is air humidity, the smaller is the curvature in the horn. Now we add
a little bit of liquid water. The meniscus still exists but it is less curved, the surface
pressure is increased, and the air humidity has increased, too; see the situation in
case C. We continue in adding liquid water until the water level reaches the top of
the cone where the capillarity does not exist anymore. The water level is a fl at plane.
The content of the number of water molecules as vapor in the air is the maximum
possible at the given temperature. Since the air is fully saturated by water vapor, the
air humidity is 100 %.
However, we have not reached the end of our description of the various kinds of
forces acting upon soil water. The air within air bubbles completely enclosed and
surrounded by the continuous phase of water within the soil cannot readily escape
from the soil. When this enclosed air is compressed by infi ltrating water or the
weight of agricultural machinery, a soil hydrologist must consider this mechanical
component of force acting on the soil water. Moreover, whenever the content and
kinds of soluble salts within a soil fl uctuates, changes of osmotic pressure also com-
plicate our simple way of studying force fi elds acting upon soil water. The consid-
eration of osmotic behavior within soil profi les is especially important in arid and
semiarid zones where salinity dynamics frequently prevail and shifts of salt content
are ubiquitous.
8.3
Does the Soil Like Its Water Every Time?
The friendship between soil and water, i.e., the degree of soil hydrophilicity, differs
according to each kind of soil as well as to the predisposing conditions at any given
time. If a soil is fi rst saturated by water, then partly dried, then wetted again, etc.,
each change of soil water content regardless of its frequency alters the friendship or
the degree of hydrophilicity. The simplest demonstration of hydrophilicity is using
a capillary tube to observe an effect called capillary
hysteresis
. Let us assume that
we have two capillary tubes of the same material and of the same radius. We insert
the bottom end of one capillary in Fig.
8.9a
into water and allow the water to rise
due to capillary forces. Next, after totally submerging the other capillary in water,
we subsequently pull it partly out keeping its bottom end still below the free water
