Draining for Profit, and Draining for Health

by George E. Waring

In connection with the use of the word impervious, it should be understood that it is not used in its strict sense, for no substance which can be wetted by water is really impervious and the most retentive soil will become wet. Gisborne states the case clearly when he says: "Is your subsoil moister after the rains of mid-winter, than it is after the drought of mid-summer? If it is, it will drain."

The proportion of the rain-fall which will filtrate through the soil to the level of the drains, varies with the composition of the soil, and with the effect that the draining has had upon them.

In a very loose, gravelly, or sandy soil, which has a perfect outlet for water below, all but the heaviest falls of rain will sink at once, while on a heavy clay, no matter [pg 032] how well it is drained, the process of filtration will be much more slow, and if the land be steeply inclined, some of the water of ordinarily heavy rains must flow off over the surface, unless, by horizontal plowing, or catch drains on the surface, its flow be retarded until it has time to enter the soil.

The power of drained soils to hold water, by absorption, is very great. A cubic foot of very dry soil, of favorable character, has been estimated to absorb within its particles,—holding no free water, or water of drainage,—about one-half its bulk of water; if this is true, the amount required to moisten a dry soil, four feet deep, giving no excess to be drained away, would amount to a rain fall of from 20 to 30 inches in depth. If we consider, in addition to this, the amount of water drained away, we shall see that the soil has sufficient capacity for the reception of all the rain water that falls upon it.

In connection with the question of absorption and filtration, it is interesting to investigate the movements of water in the ground. The natural tendency of water, in the soil as well as out of it, is to descend perpendicularly toward the center of the earth. If it meet a flat layer of gravel lying upon clay, and having a free outlet, it will follow the course of the gravel,—laterally,—and find the outlet; if it meet water which is dammed up in the soil, and which has an outlet at a certain elevation, as at the floor of a drain, it will raise the general level of the water, and force it out through the drain; if it meet water which has no outlet, it will raise its level until the soil is filled, or until it accumulates sufficient pressure, (head,) to force its way through the adjoining lands, or until it finds an outlet at the surface.

The first two cases named represent the condition which it is desirable to obtain, by either natural or artificial drainage; the third case is the only one which makes[pg 033] drainage necessary. It is a fixed rule that water, descending in the soil, will find the lowest outlet to which there exists a channel through which it can flow, and that if, after heavy rains, it rise too near the surface of the ground, the proper remedy is to tap it at a lower level, and thus remove the water table to the proper distance from the surface. This subject will be more fully treated in a future chapter, in considering the question of the depth, and the intervals, at which drains should be placed.

Evaporation.—By evaporation is meant the process by which a liquid assumes the form of a gas or vapor, or "dries up." Water, exposed to the air, is constantly undergoing this change. It is changed from the liquid form, and becomes a vapor in the air. Water in the form of vapor occupies nearly 2000 times the space that it filled as a liquid. As the vapor at the time of its formation is of the same temperature with the water, and, from its highly expanded condition, requires a great amount of heat to maintain it as vapor, it follows that a given quantity of water contains, in the vapory form, many times as much heat as in the liquid form. This heat is taken from surrounding substances,—from the ground and from the air,—which are thereby made much cooler. For instance, if a shower moisten the ground, on a hot summer day, the drying up of the water will cool both the ground and the air. If we place a wet cloth on the head, and hasten the evaporation of the water by fanning, we cool the head; if we wrap a wet napkin around a pitcher of water, and place it in a current of air, the water in the pitcher is made cooler, by giving up its heat to the evaporating water of the napkin; when we sprinkle water on the floor of a room, its evaporation cools the air of the room.

So great is the effect of evaporation, on the temperature of the soil, that Dr. Madden found that the soil of a drained field, in which most of the water was removed[pg 034] from below, was 6-1/2° Far. warmer than a similar soil undrained, from which the water had to be removed by evaporation. This difference of 6-1/2° is equal to a difference of elevation of 1,950 feet.

It has been found, by experiments made in England, that the average evaporation of water from wet soils is equal to a depth of two inches per month, from May to August, inclusive; in America it must be very much greater than this in the summer months, but this is surely enough for the purposes of illustration, as two inches of water, over an acre of land, would weigh about two hundred tons. The amount of heat required to evaporate this is immense, and a very large part of it is taken from the soil, which, thereby, becomes cooler, and less favorable for a rapid growth. It is usual to speak of heavy, wet lands as being "cold," and it is now seen why they are so.

If none of the water which falls on a field is removed by drainage, (natural or artificial,) and if none runs off from the surface, the whole rain-fall of a year must be removed by evaporation, and the cooling of the soil will be proportionately great. The more completely we withdraw this water from the surface, and carry it off in underground drains, the more do we reduce the amount to be removed by evaporation. In land which is well drained, the amount evaporated, even in summer, will not be sufficient to so lower the temperature of the soil as to retard the growth of plants; the small amount dried out of the particles of the soil, (water of absorption,) will only keep it from being raised to too great a heat by the mid-summer sun.