When the rain ceases, the visible saturation of the surface usually soon disappears in cultivated soils, and the zone of saturation begins to descend. The progress of this descent may be very strikingly observed in a series of holes (post-holes) dug or bored across a ridge; as indicated in the subjoined schematic diagram, in which the successive dotted lines represent the levels of the descending “bottom water” at successive intervals, as derived from the observation of the water levels in the several holes.[86]
Fig. 42.—Percolation in clay land after heavy rain.
It will be seen that while at first the upper surface of the zone of saturation coincides with the surface of the ground, in falling it descends most rapidly on the highest ground, while at the lower levels the holes may remain full or overflowing; the drainage taking place sideways as well as vertically. The curved surface connecting the levels in the several holes gradually flattens, rapidly at first, then progressively more slowly; the water disappearing entirely, first from the holes lying highest, then successively from those at lower levels; those located in valleys or drainage channels remaining full until surface-water ceases to run in such channels. But even after liquid water has ceased to be visible in the holes, the descent of the water continues within that portion of the soil, tending (unless more rain should come before that time), to establish the condition of equilibrium as existing in the soil columns shown in the diagram on [p. 205, chapt. 11]; such as results from the capillary ascent of water from below, but having above it a column of soil of minimum water-content, of greater or less height according to the length of time allowed for the water to descend. This is a very common state of things during the long summer droughts in the arid region, when neither rain nor irrigation has added to the water supply in the soil for many months, and yet ordinary deciduous fruit trees mature their normal crops. Frequently, however, before this state of equilibrium is reached, evaporation from the surface so draws upon the water supply within the first few feet, as to reduce the soil to undersaturation at the lowest point of the descending column, so stopping farther descent and soon reversing the direction of the movement. The latter is the usual condition of scantily irrigated ground.
Ground or Bottom Water, Water Table.—During and after long-continued and abundant rains, the zone of supersaturation continues to descend until it finally reaches a more or less permanent level, varying somewhat from season to season, but on the whole usually definable for each region and locality; being the depth to which wells must be sunk in order to secure a fairly permanent water supply. This is called the water table, ground water, bottom water, or “first water.”[87] The proportion of the rainfall that reaches the permanent water level varies enormously, of course, in different soils and at different times. With brief and moderate rains, in soils of high water-holding power and slow percolation, it may never reach the bottom-water level; this is very commonly the case in the arid regions. Where, as in the humid regions, rains are frequent or much prolonged, one half and even more may finally reach the permanent level; runoff and evaporation disposing of the balance.
Lysimeters.—For the determination of the amount of water percolating to given depths, water-tight receptacles called lysimeters are usually employed. The best way to establish such receptacles is to isolate a unit-area (usually a square meter) by digging all around it to the depth desired, then surrounding it with a metal sheet soldered tightly at the cut edges, and finally driving in a sharp-edged, stiff metal sheet so as to form the bottom when soldered to the upright walls; leaving on one side an outlet for the percolating water, which is then received into a measuring receptacle somewhat like a rain gauge.
Hall (The Soil, p. 75) states that at Rothamstead, where an average rainfall of 31.3 inches is distributed rather uniformly through the season, and where the soil is a moderately clayey loam, a little less than half percolates through 20 inches of soil, and about 45% through 60 inches.
Surface of Ground Water; Variations.—The surface of the water table, however, is rarely level except in level and very uniform ground, or after long periods of drought. The undulations of its surface conform, in general, to that of the ground surface, but are less abrupt; so that the water lies nearer to the surface in low than in high ground, as is indicated in the diagram above.
King[88] has shown, moreover, that the level of the ground water shows sensible variations due to increased or diminished barometric pressure, as well as to variations of temperature in the soil, which cause the air in the pores to expand or contract to a degree sufficient to bring about variations in the flow of springs and underdrains to the extent of 8 and 15% respectively, in conformity with the daily changes of temperature and pressure.
The Depth of the Ground Water most Favorable to Crops cannot be stated in a general manner, as it depends materially upon the nature of the crop, its root habit, and the nature of the soil. As has already been said, the amount of soil-moisture most favorable to plant growth is about half of the maximum it can hold; and this condition, as is shown in the table in [chapter 11, p. 208], is reached about the middle of the maximum height to which the water can rise by capillarity from the water level. Below this point the access of air to the roots becomes too limited, and in case of continuous rains the root-ends would soon begin to suffer from want of aeration. On “sub-irrigated” land, therefore, which is generally considered desirable, crops must be carefully selected with respect to their root habits. Thus while alfalfa needs considerable moisture to do its best, its deep-rooting habit renders it undesirable when the ground water is at less than five feet depth; but red clover may be grown even with the water level at three feet.