Will there be a larger proportion of run-off after long rains or after a season of drought? after long and gentle rains, or after the same amount of precipitation in a violent rain? during the months of growing vegetation, from June to August, or during the autumn months? DESERT STREAMS. In arid regions the ground-water surface lies so low that for the most part stream ways do not intersect it.
As a river system advances toward maturity the deepening and extending valleys of the tributaries lower the ground-water surface and invade the undrained depressions of the region. Lakes having outlets are drained away as their basin rims are cut down by the outflowing streams, a slow process where the rim is of hard rock, but a rapid one where it is of soft material such as glacial drift.
The Bulletin particularly speaks of one of these areas: "In an investigation recently made by O. E. Meinzer, of the United States Geological Survey of the Department of the Interior, in Big Smokey Valley and adjacent area near Tonopah, Nev., the character of the vegetation and other surface criteria show that the ground-water stands within ten feet of the surface over an area of 130,000 acres.
It may be possible to get data from different wells and to draw a diagram showing the ground-water surface as compared with the surface of the ground. FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in pervious strata which are overlain by some impervious stratum.
We could never understand why people do not grow more of it on shallow land over hardpan which is free from alkali and not irrigated too much at a time. It is good on shallow land over water, where alfalfa roots decay, etc. Though we have no exact figures, we should expect to get about two-thirds as much weight from it as from an equally good stand of alfalfa. Clovers for High Ground-Water.
So well is this water hidden that its existence was not suspected by many of the early travelers, and even today long desert roads on which there are no watering places, lead over areas where ground-water could easily be obtained.
In a humid climate the larger ravines through which the run-off flows soon descend below the ground-water surface. Here springs discharge along the sides of the little valleys and permanent streams begin. The water supplied by the run-off here joins that part of the rainfall which had soaked into the soil, and both now proceed together by way of the stream to the sea.
In wet seasons these constant losses are more than made good by fresh supplies from that part of the rainfall which soaks into the ground, and the ground-water surface rises. In dry climates permanent ground water may be found only at depths of hundreds of feet. Ground water is held at its height by the fact that its circulation is constantly impeded by capillarity and friction.
If it were as free to drain away as are surface streams, it would sink soon after a rain to the level of the deepest valleys of the region. WELLS AND SPRINGS. Excavations made in permeable rocks below the ground-water surface fill to its level and are known as wells.
Does California experience show that citrus trees can be grown upon land successfully where the water-level is 6 feet from the surface; that is, where water is found at that level at all seasons and does not appear to rise higher during the rainy season? We do not know of citrus trees in California with ground-water permanently at six feet below the surface.