Geoscience Reference
In-Depth Information
established in the laboratory to use the equilibrated soil water content as the basis
for computing the specifi c soil particle surface. When the air humidity is increased
up to about 60 %, a value occurring frequently in the mild climatic zone, the water
layer covering the solid soil surfaces is a fi lm having a thickness of 4-6 water mol-
ecules. The thickness of these fi lms is the result of the combined simultaneous
forces of both adsorption and capillary condensation. Computed values of the pres-
sure potential of these fi lms range from about 100 to 150 MPa. When the air humid-
ity rises to values in the vicinity of 100 %, water continues to accumulate in soil
pores, but the mechanism is only that of capillary condensation with the pressure
potential reaching just to about 5 MPa and less.
Mitscherlich, a German soil scientist, proposed in 1901 that the hydroscopic
properties of a soil be described and quantifi ed by a term named the hydroscopic
coeffi cient that equaled the soil water content in equilibrium with 95 % air humidity
verifi ed with laboratory equipment. Because two phenomena, adsorption and capil-
lary condensation, contribute to the magnitude of the proposed term, its magnitude
does not actually isolate, characterize, nor quantify the true hydroscopic property of
a soil. Hence, after more than a century, the term with its method of approximation
used for many decades is now obsolete.
There is another consequence of the curved water level. The lower is the surface
pressure, the less water molecules escape into the atmosphere above the curved
water level. Or, in other words, the narrower is the cylindrical capillary and the
greater is the curvature of water level, the lower is the partial pressure of water vapor
in the atmosphere above the water level; see Fig.
8.8
. We model the mentioned
Fig. 8.8
The greater the curvature of a liquid water surface, the fewer are the number of neighbor-
ing water vapor molecules - or more precisely, the greater the curvature, the lower is the partial
pressure of water vapor in the air next to a water surface. Here, we illustrate this relationship by the
density of dots (water molecules in the air) and the amount of water within a triangle-like soil pore:
(
a
) No water molecules (
dots
) are present within the completely dried-out soil pore (
triangle
). (
b
)
Within the triangle, a very small amount of liquid water resulting from capillary condensation
manifests a strongly curved surface in the vicinity of a very low air humidity (a small number of
dots
). (
c
) Capillarity has caused a low curvature of the liquid water surface and a high number of
water molecules in the air, i.e., a high air humidity. (
d
) The triangular pore completely fi lled with
liquid water manifests a fl at water plain covered by a maximum possible number of water mole-
cules in the air having a humidity of 100 %
