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.
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.
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.