=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. [Illustration: 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). [Illustration: FIG. 189.—Diagram to show how an impervious layer conducts the descending water in a lateral direction to issue in surface springs.] =The trunk channels of descending water.=—While within the unconsolidated rock materials near the surface of the earth, it is clear that water can circulate in proportion as the materials are porous and so relatively pervious. As the pore spaces become minute and capillary, the difficulty of permeation through the materials becomes very great. Thus in the noncoherent rocks it is the coarse gravel and the layers of sand which serve as the underground channels, while the fine clays have the effect of an impervious wall upon the circulating waters. In coarse sand as much as a third of the volume of the material is pore space for the absorption and transmission of water. Even under these favorable conditions the movement of the water is exceedingly slow and usually less than a fifth of a mile a year. [Illustration: