DISTANCES DEPEND UPON THE DEPTH OF THE DRAINS.
The relations of the depth and distance of drains will be more fully considered, in treating of the depth of drains. The idea that depth will compensate for frequency, in all cases, seems now to be abandoned. It is conceded that clay-soils, which readily absorb moisture, and yet are strongly retentive, cannot be drained with sufficient rapidity, or even thoroughness, by drains at any depth, unless they are also within certain distances.
In a porous soil, as a general rule, the deeper the drain, the further it will draw. The tendency of water is to lie level in the soil; but capillary attraction and mechanical obstructions offer constant resistance to this tendency. The farther water has to pass in the soil, the longer time, other things being equal, will be required for the passage. Therefore, although a single deep drain might, in ten days lower the water-line as much as two drains of the same depth, or, in other words, might draw the water all down to its own level, yet, it is quite evident that the two drains might do the work in less time—possibly, in five days. We have seen already the necessity of laying drains deep enough to be below the reach of the subsoil plow and below frost, so that, in the Northern States, the question of shallow drainage seems hardly debatable. Yet, if we adopt the conclusion that four feet is the least allowable depth, where an outfall can be found, there may be the question still, whether, in very open soils, a still greater depth may not be expedient, to be compensated by increased distance.
DISTANCES DEPEND UPON CLIMATE.
Climate includes the conditions of temperature and moisture, and so, necessarily, the seasons. In the chapter which treats of Rain, it will be seen that the quantity of rain which falls in the year is singularly various in different places. Even, in England, "the annual average rain-fall of the wettest place in Cumberland is stated to be 141 inches, while 19½ inches may be taken as the average fall in Essex. In Cumberland, there are 210 days in the year in which rain falls, and in Chiswick, near London, but 124."
A reference to the tables in another place, will show us an infinite variety in the rain-fall at different points of our own country.
If we expect, therefore, to furnish passage for but two feet of water in the year, our drains need not be so numerous as would be necessary to accommodate twice that quantity, unless, indeed, the time for its passage may be different; and this leads us to another point which should ever be kept in mind in New England—the necessity of quick drainage. The more violent storms and showers of our country, as compared with England, have been spoken of when considering The Size of Tiles. The sudden transition from Winter to Summer, from the breaking up of deep snows with the heavy falls of rain, to our brief and hasty planting time, requires that our system of drainage should be efficient, not only to take off large quantities of water, but to take them off in a very short time. How rapidly water may be expected to pass off by drainage, is not made clear by writers on the subject.
"One inch in depth," says an English writer, "is a very heavy fall of rain in a day, and it generally takes two days for the water to drain fully from deep drained land." One inch of water over an acre is calculated to be something more than one hundred tons. This seems, in gross, to be a large amount, but we should expect that an inch, or even two inches of water, spread evenly over a field, would soon disappear from the surface; and if not prevented by some impervious obstruction, it must continue downward.
It is said, on good authority, that, in England, the smallest sized pipes, if the fall be good, will be sufficiently large, at ordinary distances, to carry off all the surplus water. In the author's own fields, where two-inch tiles are laid at four feet depth and fifty feet apart, in an open soil, they seem amply sufficient to relieve the ground of all surplus water from rain, in a very few days. Most of them have never ceased to run every day in the year, but as they are carried up into an undrained plain, they probably convey much more water than falls upon the land in which they lie.
So far as our own observation goes, their flow increases almost as soon as rain begins to fall, and subsides, after it ceases, about as soon as the water in the little river into which they lead, sinks back into its ordinary channel, the freshet in the drains and in the stream being nearly simultaneous. Probably, two-inch pipes, at fifty feet distances, will carry off, with all desirable rapidity, any quantity of water that will ever fall, if the soil be such that the water can pass through it to the distance necessary to find the drains; but it is equally probable that, in a compact clay soil, fifty feet distance is quite too great for sufficiently rapid drainage, because the water cannot get to the drains with sufficient rapidity.