Basin Irrigation.—In this method of irrigation, practiced only in the case of trees and sometimes vines, and when water is scarce, a wide circular furrow or basin is excavated around each trunk and water is run either from one to the other, or sideways from a furrow laid along the rows. The water thus applied of course percolates immediately around the trunk first, and in practice is found to follow also the large roots; so that it goes precisely where it is most wanted, besides forming a vertical body of moist soil reaching to considerable depth, where it is most desirable that the root system should follow. By this deep penetration to natural moisture in the depths of the soil, comparatively small quantities of water produce very marked effects.
On the same principle, the grape vines which bear some of the choicest raisins of Malaga on the arid coastward slopes, are made to supply themselves with moisture, without irrigation, by opening around them large, funnel-shaped pits, which remain open in winter so as to catch the rain, causing it to penetrate downward along the taproot of the vine, in clay shale quite similar to that of the California Coast Ranges, and like the latter almost vertically on edge. Yet on these same slopes scarcely any natural vegetation now finds a foothold.
Similarly the “ryats” of parts of India water their crops by applying to each plant immediately around the stem such scanty measure of the precious fluid as they have taken from wells, often of considerable depth, which form their only source of water-supply. Perhaps in imitation of these, an industrious farmer has practiced a similar system on the high benches of Kern River, California, and has successfully grown excellent fruit for years, on land that would originally grow nothing but cactus. Sub-irrigation from pipes has been applied in a similar manner.
A combination of the furrow- and basin-irrigation system is sometimes practiced in southern California by drawing the furrow so as to bring the tree within a square, one side of which is left closed. The same result may be accomplished by plowing cross furrows at right angles near the tree and then placing check-boards so as to force the water along the rows, zigzagging, on three sides.
The basin irrigation of orchards was originally largely practiced in California, but has now been mostly abandoned for furrow irrigation. The latter has been adopted partly because it requires a great deal less hand-labor, partly under the impression that the whole of the soil of the orchard is thus most thoroughly utilized; partly also because of the injurious effect upon trees produced at times by basin irrigation.
The explanation of such injurious effects is, essentially, that cold irrigation water depresses too much the temperature of the earth immediately around the roots, and thus hinders active vegetation to an injurious extent, sometimes so as to bring about the dropping of the fruit. This of course is a very serious objection, to obviate which it might be necessary to reservoir the water so as to allow it to warm before being applied to the trees.[92] In furrow-irrigation the amount of soil soaked with the water is so great that the latter is soon effectually warmed up, besides not coming in contact too intimately with the main roots of the tree; along which the water soaks very readily when applied to the trunk, thus affecting their temperature much more directly. It is for the farmer to determine which consideration should prevail in a given case. If the water-supply be scant and warm, the most effectual use that can be made of it is to apply it immediately around the tree, in a circular trench dug for the purpose. When on the contrary, irrigation water is abundant and its temperature low, it may be preferable to practice furrow irrigation, or possibly even flooding.
As to the supposed more complete use of the soil under the latter two methods, it must be remembered that while this is the case in a horizontal direction, if irrigation is practiced too copiously under the shallow-furrow system, it may easily happen that the gain made horizontally is more than offset by a corresponding loss in the vertical penetration of the root-system. This is amply apparent in some of the irrigated orange groves of southern California, where the fine roots of the trees fill the surface soil as do the roots of maize in a corn field of the Mississippi States; so that the plow can hardly be run without turning them up and under. In these same orchards it will often be observed, in digging down, that at a depth of a few feet the soil is too water-soaked to permit of the proper exercise of the root-functions, and that the roots existing there are either inactive or diseased. That in such cases frequent irrigation and abundant fertilization alone can maintain an orchard in bearing condition, is a matter of course; and there can be no question that a great deal of the constant cry for the fertilization of orchards in the irrigated sections is due quite as much to the shallowness of rooting induced by over-irrigation, as to any really necessary exhaustion of the land. When the roots are induced to come to and remain at the surface, within a surface layer of eighteen to twenty inches, it naturally becomes necessary to feed these roots abundantly, both with moisture and with plant-food. This has, as naturally, led to an overestimate of the requirements of the trees in both respects. Had deep-rooting been encouraged at first in the deep soils of the southern “citrus belt,” instead of over-stimulating the growth by surface fertilization and frequent irrigation, some delay in bearing would have been compensated for by less of current outlay for fertilizers, and less liability to injury from frequently unavoidable delay, or from inadequacy, of irrigation.
Irrigation by Underground Pipes.—Where economy in the use of irrigation water is a pressing requirement, its distribution through underground pipes affords the surest mode of accomplishing that end, in connection with the application of the water in accordance with the principles just discussed. The enormous saving of water effected by its conveyance in cement-lined ditches or concrete pipes, as compared with earth ditches, if additionally combined with its application to individual trees or vines, presents the maximum of economy that can be effected. The actual use of this method is unfortunately limited in practice by the high first cost of piping; but as its use renders unnecessary the digging of basins and plowing of furrows and their subsequent closing-up, it is when once established by far the cheapest system, both as to the use of water and of labor.
The best results of this system are undoubtedly achieved by the use of iron pipes for the distribution in field and orchard, whatever may be the material used for the main conduits. The use of concrete and tile in small sizes proves in the end very expensive, because of frequent breakage, and leakage due to varying pressure in the supply pipes or reservoirs; as well as from even slight earthquake tremors, undermining by water or by the burrowing of animals, and many other accidents which do not affect an iron pipe system. The pipes must in any case, of course, be laid deep enough to be out of reach of the deepest tillage; therefore not less than one foot, and preferably eighteen inches. A proper construction of the outlets, permitting of exact regulation of the flow and ready operation from above ground, as well as preventing their being clogged by earth, rust, roots or burrowing animals, insects etc., is of course of the greatest importance. A variety of devices for this purpose is already on the market.