Geoscience Reference
In-Depth Information
Chapter 9
How Water Flows in Soil
9.1
The First Observations
From the very beginning of their studies, soil scientists noticed two characteristic
values of soil water content. One was when water moving down from the nearly
saturated topsoil starts to drastically slow down even though the soil contains still a
substantial amount of stored water. If evaporation was prevented, the change of rate
of drainage was refl ected by the soil water content, which did not appear to change
for several days after the initial decrease. Today, it is a common practice to observe
such a characteristic by fi rst surrounding a small, fl at soil surface of about 1 m
2
with
a small dike having a height of about 10 cm. Water is next steadily ponded on the
area isolated from its surroundings until we guess that the soil is water saturated to
the depth we wish to study. With the surface no longer ponded with water and cov-
ered by a folio in order to prevent evaporation, a limited portion of the soil water
fl ows quickly and readily from the topsoil into the subsoil. The soil water content in
the top layers decreases, while the deeper layers are gaining water. In a few days, the
soil water content in the topsoil looks as though it is not changing - indeed the
downward fl ux is ten to hundred times smaller than it was when water fi rst started
to drain immediately after its surface was not ponded with water. This soil water
characteristic was originally called
capillary capacity
and later renamed to
fi eld
capacity
. It can be estimated using undisturbed soil samples within metallic cylin-
ders of 100 cm
3
volume maintained at approximately 10
4
Pa negative pres-
sure = 100 cm of water column. A rough value of capillary capacity could be read
from the soil water retention curve (Fig.
8.11
)
.
The second characteristic value was when the roots of a plant were unable to
extract enough soil water for the plant to survive and function - a condition known
as permanent wilting that prevented the revival of the plant. At the end of the nine-
teenth century and during the fi rst part of the twentieth century, it was apparent to
all soil scientists that each soil type as well as each textural class was easily associ-
ated with specifi c values for the abovementioned two characteristics. Therefore,
