CHAPTER XIV
THE TRAVELS OF THE UNDERGROUND WATER
The descent within the unsaturated zone.—Of the moisture precipitated from the atmosphere, that portion which neither evaporates into the air nor runs off upon the surface, sinks into the ground and is described as the ground water. Here it descends by gravity through the pores and open spaces, and at a quite moderate depth arrives at a zone which is completely saturated with water. The depth of the upper surface of this saturated zone varies with the humidity of the climate, with the altitude of the earth’s surface, and with many other similarly varying factors. Within humid regions its depth may vary from a few feet to a few hundred feet, while in desert areas the surface may lie as low as a thousand feet or more.
The surface of the zone of the lithosphere that is saturated with water is called the water table, and though less accentuated it conforms in general to the relief of the country ([Fig. 188]). Its depth at any point is found from the levels of all perennial streams and from the levels at which water stands in wells.
Fig. 188.—Diagram to show the seasonal range in the position of the water table and the cause of intermittent streams.
During the season of small precipitation the water table is lowered, and if at such times it falls below the bed of a valley, the surface stream within the valley dries up, to be revived when, after heavier precipitation, the water table has in turn been raised. Such streams are said to be intermittent, and are especially characteristic of semiarid regions ([Fig. 188]).
Wherever in descending from the surface an impervious layer, such as clay, is encountered, the further downward progress of the water is arrested. Now conducted in a lateral direction it issues at the surface as a spring at the line of emergence of the upper surface of the impervious layer ([Fig. 189]).
