The second section from above shows the percolation of the water respectively in wide and narrow furrows of the same depth. It is evident at a glance how much more effective is the wide furrow in utilizing the limited time during which the irrigator usually has the flow at his command.
The third section shows several practically important points in favor of the wide and deep instead of narrow and shallow furrow. It is seen that in doubling the width and depth, the penetration has also nearly doubled. Moreover, it is seen that in the deep furrow the water has not in the course of seven hours reached the surface at all, being still six inches away; so that in view of the diminishing ratio of capillary ascent, it probably would not have reached the edge of the furrow, at the surface, in less than thirty hours. Thus all surface evaporation, which oftentimes causes the loss of 50 % of the water entering the shallow furrows, would be prevented; and a dry furrow-slice might be turned into the furrow immediately after the cessation of the water-flow, effectually obviating the need of subsequent tillage also. The cost of the latter, together with the saving in water, and increased efficiency of the water by deeper penetration, will much more than offset the additional cost and trouble of plowing deeper furrows.
There is therefore every reason for doing away with the wasteful, easy-going practice of irrigating in numerous shallow furrows, by which the irrigator loses up to half of the water paid for, by evaporation; is compelled to wait for the soaked surface to dry before being able to turn back a furrow-slice into the furrows to prevent the drying-out of their moisture; and by losing penetration of the water, is obliged to irrigate again within a much shorter time than will be necessary if deep-furrow irrigation be used.
A similar experiment with deep and shallow furrows was made at the Southern California station near Pomona in 1901, as reported in Bulletin 138 of the California Station. The results as far as they went were precisely similar, and upon the basis of these the writer earnestly advocated deep-furrow irrigation, and had the satisfaction of seeing it strongly approved by orange-growers at Riverside and elsewhere, by putting it into practice.
In addition to the saving and better utilization of the water used, this mode of application has the advantage of preventing the roots from coming too near the surface; it will also largely eliminate “irrigation hardpan” or plowsole.
The results produced by long-continued shallow plowing and irrigation in shallow furrows is well illustrated in the last of the irrigation profiles, which shows the observations made on the same land as the others, but where rational cultivation and deep-furrow irrigation had not yet been introduced. It will be seen that after applying, and of course paying for, the water for three days, its average penetration was only about eighteen inches; so that the trees of the orchard received very little benefit, and were supposed to be needing fertilization when in fact they were simply suffering from lack of water at the lower roots.
One somewhat unexpected point is shown by these diagrams, viz., the slight sidewise penetration of the water; the wetted areas having a nearly vertical lateral outline. This means, of course, that unless the furrows run very near the trees of an orchard, the soil immediately beneath the trees will remain dry; thus inducing the roots to spread sideways and losing depth of penetration and soil. It will be noted especially in the lower figure that here again the deep furrow offers a material advantage over the shallow, the sidewise spread being much more pronounced than in the shallow furrow alongside.
Distance Between Furrows and Ditches.—The distance between the furrows must, of course, be proportioned to the readiness with which the water penetrates, being less as the land is of closer texture. The distance between head ditches must, on the contrary, vary in the opposite sense, since if these are too far apart, the water near the head ditch will in sandy lands be wasting into the subdrainage before the end of the furrows is reached; so that the distribution will be very uneven. The great differences observed between crops, and especially trees, below and above the head ditches, are mainly due to this unevenness in water distribution, caused by too great distance between successive head ditches. Each farmer must himself, however, determine by actual trial the proper distances between ditches as well as furrows, for his particular case; since everything depends upon the rapidity with which water will penetrate the soil and subsoil. Actual tests to determine this point[91] should be the first step, before laying off the system of ditches as well as furrows. It not uncommonly happens that the failure to do this at first, compels a subsequent total change of arrangements in this respect. (See page 253 below).
Thus while in some very pervious land furrows may be six or even eight feet apart, in other cases, in certain finely pulverulent or silty soils such as the “dust soils” described in a former chapter; ([see chapter 6, p. 104]), furrows drawn three feet apart may fail to allow the water to penetrate so as to prevent grain on the middle foot from suffering from drought after the water has run for twenty-four hours.
Irrigation by lateral Seepage.—Is in reality a mere modification of furrow irrigation, practiced in the case of lands very readily permeable, and where water is abundant. The fields are laid off in “lands” twelve to twenty-five feet wide, with a deep furrow or narrow ditch between, from which water percolates in a short time so as to overlap from the two sides. In this case sometimes the water does not reach the surface visibly at all; a very great advantage where alkali exists, as surface evaporation, and the consequent accumulation of alkali, is thus effectually prevented; while deep-rooting is favored to the utmost.