[Sidenote: Strata above level of subsoil water.]
Two main strata may here be distinguished in subsoil, one above the
level of the subsoil water and one below this level. The latter strata
do not interest us, and those above the level of the subsoil[7] water
may again be subdivided into three zones, which in descending order
are as follows:—
The evaporation zone;
The passage zone; and
The capillary zone.
[Sidenote: One-third of the rain-water evaporates. One-third flows off
the surface. One-third percolates.]
All these three zones must be passed by the water in its descent from
the surface of the ground to the subsoil water level, and the quantity
of water retained by them will depend on their state of dryness.
Speaking quite generally and within wide limits, one-third of the
rain-water flows off the surface, one-third evaporates, and one-third
percolates into the subsoil.
[Sidenote: Evaporation zone.]
The evaporation zone reaches from the surface of the soil to that
point below, which marks the extent of the drying influence of the
atmospheric air. In the same the quantity of water stored in the pores
may at times sink below the retentive power of the soil, i.e. below
that quantity which can be retained in the pores owing to the
mechanical powers of adhesion, etc. When it has become very dry
through evaporation and other causes the zone, especially when it
extends some way down, may retain large quantities of water. In a
depth of 10 inches, 1 square yard of soil, with fine pores, may retain
about 10 gallons of water, and as a rainfall of ½ inch produces only
2·3 gallons per square yard, it is clear that subsoil of this nature
may retain a number of successive showers. During the height of summer
fine porous soil may become so dry that practically no water finds its
way into deeper zones; in this state the evaporation zone can be
compared to a large sponge.
[Sidenote: Passage zone.]
The next zone traversed by the water in its downward movement is the
passage zone, which lies beyond the drying influence of atmospheric
air. When too far removed from the level of the subsoil water, its
pores will not be completely filled with water, but will only contain
that amount which is due to the retentive powers of the soil. By
direct measurement it has been found that on an average a cubic yard
of fine porous soil will retain from 30 to 80 gallons of water, and it
can easily be calculated that in a layer from 1 to 2 yards in
thickness the rainfall of a whole year may be retained. The passage
zone, especially if it is of considerable thickness, represents a very
large storage reservoir.
[Sidenote: Capillary zone.]
The last zone before the level of the subsoil water is reached is the
capillary zone, in which the pores are partially or wholly filled by
the upward movement—due to capillary attraction—from the subsoil
water. The extent of this filling will depend on the size of the pores.
[Sidenote: Springs.]
When the descending water has finally reached the subsoil water it
either comes to a standstill altogether on the impervious layer or
moves along the same, if the latter is not horizontal, until it may
eventually leave the subsoil again by issuing therefrom in the form of
visible or invisible springs.
[Sidenote: Rate of downward movement governed by pores.]
The rate of movement of any liquid—rain-water, sewage or other
polluting liquid—is largely governed by the size of the pores. Where
these are large, as for instance in coarse gravel, the descent of the
water will be comparatively rapid, but when they are small it may take
a very long time before the water reaches the level of the subsoil
water, and in that case it will have undergone material changes as
regards its chemical or bacterial composition.
[Sidenote: With a high level of subsoil water zones become
indistinguishable.]
With a high level of subsoil water the zones may become
indistinguishable, one zone reaching into the other, with the result
that the whole of the soil becomes very wet.
When subsoil has been artificially drained the amount of water
reaching the subsoil water below the general level of the drains will
depend on the size of the latter and the distance between them. In
such a case the downward movement of the water through undrained soil,
previously described, may be further interfered with through the
ventilation of the subsoil by drains, and the drying up action caused
thereby.
[7] The term subsoil water is here used to denote that
portion of the water in the pores of the soil, which is
either at rest on or moves along the inclined plane of an
impervious layer.