=The ground-water surface. Water table.=—The water table has already been defined (p. 71) as the upper surface of the ground-water. In a flat region of uniform structure the ground-water surface is essentially level, but rises and falls with the rainfall. Where the topography of a region is not flat, the ground-water surface is not level. As a rule it is higher, though farther below the surface, under an elevation than under surrounding lowlands, as illustrated by Fig. 200. The explanation is not far to seek. If a hill of sand be exposed to rainfall, most of the water falling on its porous surface will sink into it. If the precipitation continues long enough, as in a protracted rain, the hill of sand will be filled with water, the water occupying the interstices between the grains. If the sand of the hill could be removed, leaving the water which it contains on the same area, it would constitute a mound perhaps a third or a fourth as high as the hill itself. If unsupported, this mound of water would spread promptly in all directions until its surface was level. While the sand remains, the water in it constitutes a mound, and has a tendency to spread. It does in fact spread, but since the process involves great friction the spreading is slow. With the spreading the surface of the water in the sand sinks, and sinks fastest at the center where it is highest (_b_, Fig. 201). If the process were not interrupted the surface of the water in the hill would, in time, sink approximately to the level of the water in the surrounding land (_d_, Fig. 201); but at every stage preceding the last, the surface of the water would be higher beneath the summit of the hill than elsewhere, though farther from the surface. In regions of even moderate precipitation the water surface beneath the hills rarely sinks to the level of that in the lowlands adjacent, before being raised by further rains. [Illustration: +Fig.+ 201.—Diagram to illustrate the relations of ground-water to the surface.] The water-level beneath the lowlands also sinks. Some of it finds its way into valleys, some of it sinks to greater depths, and some of it evaporates; but since the water surface beneath the elevation sinks more rapidly than that beneath the lowland, the two approach a common level. Their difference will be least at the end of a drought, and greatest just after heavy rains.