CHAPTER XIII.. Water of Soils (_continued_).

WATER OF SOILS (Continued).

THE REGULATION AND CONSERVATION OF SOIL MOISTURE.

In view of the commanding importance of an adequate supply of water to vegetation, the possible and available means of assuring such supply by utilizing to the best advantage both rainfall and irrigation water, require the closest consideration.

Loosening of the Surface.—The first thing needful, of course, is to allow the water free opportunity to soak into the soil, so as to moisten the land as deeply as possible. That to this end the surface should be kept loose and pervious by tillage, breaking up crusts that may have been formed by the beating of rains, has already been discussed. In the case of heavy clay soils, however, this alone is not always sufficient. The most effectual way to loosen the land to greater depths than can be reached by tillage, is by means of underdrains laid at the greatest depth that is practically admissible.

Effects of Underdrains.—That drain tiles laid for the express purpose of carrying off surplus water should help to conserve soil moisture, seems at first sight to be a paradox. Yet the explanation of the fact, which has been demonstrated by long experience, is not difficult. The effect is most striking in clay soils, for sandy soils are commonly naturally underdrained already.

In discussing the changes of volume which soils undergo in wetting and drying, the fundamental points in the premises have already been mentioned (see chap. 7, p. 112). Clay soils in drying shrink considerably, and re-expand on wetting, but rather slowly; moreover, some clays crumble when wetted after drying, while others, very plastic when wet, crumble on drying (see chap. 7, p. 116).

It follows that while a clay subsoil when kept permanently wet, will form a uniform, pasty, difficultly penetrable mass: when subjected to frequent alternate wetting and drying, it becomes fissured and crumbly, so as to resemble in its texture a tilled soil. This frequent alternation of wetting and drying is precisely what, in the course of time, is brought about by underdrains; rendering clay subsoils pervious both to air and water. The consequence is that even heavy rains can be fully absorbed by the soil mass lying above the drains, the surplus draining off readily in a short time. Roots therefore can not only penetrate, but exercise their vegetative functions perfectly at the full depth of the drains. They are still at liberty to penetrate as much deeper as their demands for moisture may require; but the depth of four to four and a half feet is already so much greater than in the humid region would usually be reached by them in undrained clay soils, that commonly the moisture successively retained within that mass is as much as is required by them during the growing season. At the same time, their feeding roots are so far below the surface, that ordinary short droughts do not reach them at all; while the underdrains prevent any injurious stagnation of water around them. It need hardly be added that the entire task of cultivation is also greatly facilitated; not only because drained soils can be plowed within a few hours after the cessation of rains, as against the same number of days that would have to elapse in the undrained areas; but because tillage is easier, and less draft is required, even when it is carried to a much greater depth.

Underdrainage, then, must be counted as being among the most effective means both of utilizing the rainfall so as to prevent loss from runoff and injury from washing, and of creating a deep, loose, pervious soil mass, well adapted to root penetration as well as to the conservation of moisture; rendering possible timely tillage and cultivation, and early development of crops fully supplied with moisture and therefore secure against loss from drought. The safety and improvement of crops thus secured corresponds in the humid region to that brought about by the command of irrigation water in the arid countries. But it by no means follows that underdrainage can therefore be dispensed with in the latter, or irrigation in the former. Both have their proper place in both regions; but from special causes underdrainage, as has already been stated, should be widely used in irrigation countries to prevent the injuries otherwise but too likely to arise from over-irrigation (see chap. 12, p. 231).

Winter Irrigation.—In many regions where irrigation is desirable but not absolutely necessary in ordinary seasons, or where irrigation water is scarce in summer, much advantage is gained by insuring thorough saturation of the land during the latter part of winter, especially when spring or summer crops are to be sown. The not inconsiderable time required for water to reach its permanent level or the country drainage in most soils, often insures the retention of a certain surplus over what the soil can permanently hold, within the period when it can be utilized by growing crops; whose roots moreover are more likely to penetrate deeply in land where there is a steady increase of moisture as they descend, than when the contrary condition is encountered. The use of winter flood-waters to saturate the land is therefore in many cases the saving clause for a dry season.

METHODS OF IRRIGATION.

The manner in which irrigation water is supplied to land and especially to growing crops exerts such a potent influence not only upon the welfare of the plants but also upon the condition of the land, that a brief discussion of this topic seems necessary.

The following methods are in use to a greater or less extent:

1. Surface sprinkling. 2. Flooding. A. By lateral overflow from furrows or ditches. B. By the “check” system. 3. Furrow irrigation. 4. Lateral seepage from ditches. 5. Basin irrigation. 6. Irrigation from underground pipes.

Only a general outline of the principles of this subject is given in this volume; special works must be consulted for working details. Among these the volume by King on “Irrigation and Drainage” gives probably the most comprehensive presentation of the subject for both humid and arid climates. Also bulletins of the U. S. Dep’t of Agriculture.

Surface Sprinkling.—This method seems to be the closest imitation of the natural rainfall; and yet it is in practice about the most wasteful and least satisfactory of all. It is difficult of application on any large scale, from obvious causes; on the small scale, in gardens and on lawns, its disadvantages become amply apparent. As usually practiced, from a rose spout or spray nozzle, the water falls much more abundantly than in the case of any desirable rain, within the short time allowed by the patience of the operator. If continued for a sufficient length of time to soak the soil to the desirable depth, it compacts the surface of the ground so as to render subsequent tillage indispensable. To avoid this, amateur gardeners usually restrict the time of application, repeating the same at frequent intervals, sometimes daily. The result is that the very slight penetration of the water either fails to reach the absorbent roots, so that it is of little use to them, and is evaporated by the next day’s sun or wind; or else it tends to draw the roots close to the surface, where, unless the application of water is actually made daily, they are sure to suffer from the first intermission of the daily dose. In actual practice the sprinkling method is therefore both inefficient and wasteful of water, and exposes the plants to grave injury from any cessation of the water supply.

Flooding presupposes land either level or only slightly sloping naturally, or rendered so artificially; usually by means of the plow and horse scraper.

Flooding by lateral overflow from large furrows, or ditches, is very commonly practiced where the water supply is abundant and large areas, such as alfalfa or grain fields, are to be irrigated. The overflow is regulated by portable check-boards, proceeding from the highest points to the lowest, and leaving each temporary dike in place until the ground is adequately soaked or the water reaches the next furrow below. In heavy ground the operation may have to be repeated to insure proper depth of percolation.

Check flooding necessitates more careful leveling, and the throwing up of small dikes, either temporary or permanent. The costliness of the earth-work restricts the use of this method materially, and the inconvenience caused in tillage by the dikes is objectionable, especially in large-scale culture. For the case of alfalfa fields, which remain permanently set for a number of years, it is however the largely preferred method. In the case of field cultures, the consolidation of the surface that follows flooding on the heavier soils renders subsequent tillage necessary in all but very sandy soils; and hence it should always precede broadcast sowing.

One disadvantage of the surface flooding system is the slow penetration of the water caused by the resistance of the air in the soil to downward displacement; its buoyancy acting directly contrary to the percolation of the water. In close-grained, heavy soils this objection is very serious, on account of the loss of time involved when the irrigator’s time is limited. On sandy lands the air bubbles up quite livelily at first, but this soon ceases and the air is compelled to escape sideways as best it can.

Furrow Irrigation.—By this method it is intended to soak the land uniformly by allowing the water to flow through furrows drawn 3 to 8 feet apart, with a gentle slope from the supply or head ditch; the flow being continued until the water has reached the far end of the furrows, or longer according to the nature of the soil, especially if another ditch to receive the surplus flow lies below. The furrows should subsequently be closed by means of the plow or cultivator; but even if left open they are much less a source of waste by evaporation than would be a flooded surface. The water thus, in the main, soaks downward and only reaches the surface by capillary rise, so that the land between the furrows is not sensibly compacted when the furrows have been made deep enough. Evidently this is a much more rational procedure than surface flooding, as it tends to leave most of the surface in loose tilth, while penetrating to much greater advantage, because of the ready escape of the air from the soil. It is the system naturally and almost exclusively used in truck gardens and orchards, and generally where crops are grown in drills or rows sufficiently far apart to permit of cultivation.

Water running, in each, seven hours

FIG. 43.—Profiles of Water penetration in Furrow Irrigation.]

The figure annexed shows the manner in which water sinks and spreads from furrows of various depths and widths, as actually observed in the work of the Irrigation Division of the U. S. Dep’t of Agriculture, under the direct supervision of Prof. R. H. Loughridge of the California Station. The mode of percolation is shown for two soils, a heavy loam and a sandy one, both in the vicinity of Riverside, Cal.

Published by permission of the Department.

The upper section shows the variation in penetration in one and the same soil with the same kind of furrow, the broken line indicating the cessation of the flow in the furrows; after which there was a still farther penetration of the water to from 6 to 9 inches deeper.

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 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).

Such tests can be readily made by any one, by digging a pit to four or five feet depth, and supplying water to a shallow basin dug into the surface 8 to 12 inches distant from the vertical wall of the pit. The descent of the water is then readily observed on the vertical side of the pit nearest to the water basin. Preliminary tests with soil probe (see chap. 10, p. 177).

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.

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. 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.

See below, chap. 17.

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.

QUALITY OF THE IRRIGATION WATER.

Saline Waters.—Considering the large amount of water annually used in irrigation, among the most needful precautions to be observed by the irrigator is in the testing of the quality of his water-supply. First among the points to be noted is the possible content of soluble “alkali” salts. While in most cases what is called the “rise of the alkali” is due to the salts already contained in the soil and subsoil, in but too many the evil is either brought about, or greatly aggravated, by the excessive saline contents of the water used in irrigation. The effects of the use of saline irrigation water (containing in this case about 100 grains per gallon, or 1700 parts per million) are shown in the accompanying plate. The predominant ingredients of these alkali salts were common salt and carbonate of soda. In the lands near Corona, Cal., where this case was observed, the original alkali-content of the soil was about 2500 pounds per acre in four feet depth, and had been just quadrupled, with the results shown; viz., complete defoliation of the orange trees, while on the same land, where the trees had been irrigated with good artesian water, the orchard was in fine condition.

Limits of Salinity.—It is not easy to assign a definite limit of mineral content beyond which water should be considered unfit for irrigation purposes; partly because of the differences in the kind of the mineral salts, partly because the nature of the soil and the amount of water at command, materially influence its availability.

Forty grains per gallon is usually assigned as the limit for potable as well as irrigation waters. But if most or the whole of such mineral contents should consist of the carbonates and sulfates of lime and magnesia, the water while unsuitable for domestic use may be perfectly available for irrigation, since these salts are either beneficial or harmless in the amounts likely to be introduced by the water. But if most or the whole of such forty grains should consist of “alkali salts” proper, viz., the sulfates, chlorids and carbonates of potash and soda, or if they should contain even small amounts of the chlorid of magnesium, they might render the water either wholly unsuitable for irrigation, or if used it would be needful to take the mineral content into consideration, by regulating its application accordingly.

It has been found in California that practically the upper limit of mineral content for irrigation water under the ordinary practice lies below seventy grains per gallon in all cases; for when this strength is reached, even though such water may bathe the roots of almost any plant with impunity, yet accidental concentration by evaporation is so certain to happen, that injury to crops is practically almost unavoidable.

In South Dakota and other parts of the American semi-arid region, waters containing seventy grains and even more of alkali salts per gallon are annually used during the short irrigation season. This can be done harmlessly because the aggregate amount used is only small, and the more abundant rainfall of that region annually washes the salts out of the soil. But where almost the full amount of water required by crops must be supplied by irrigation, the total amount of salts thus introduced would speedily render the land uncultivable.

According to the observations of Means and other explorers of the U. S. Dep’t of Agriculture, waters of much higher mineral content are used for irrigation both in Egypt and in the Saharan region, some going as high as 8000 parts per million, or 214 grains per gallon. The cultivators are said to be very skilful in the use of these waters, applying them only to plants of known resistance, and in certain ways. These ways include doubtless a good deal more time and patience than American irrigators are ordinarily willing to bestow upon their work. Much depends of course not only upon the character of the salts in the water, but also upon the long experience had in the old irrigation regions.

Bull. No. 21, Bureau of Soils; also circular No. 10, ibid.

Mode of using Saline Irrigation Waters.—The fact that abundant growths of native as well as cultivated plants may sometimes be seen on the margins of “alkali lakes” where water of over a hundred grains of mineral salts per gallon continuously bathes the roots, while the same plants perish at some distance from the water’s edge, points the way to the utilization, in emergencies, of fairly strong saline waters; viz., by the prevention of their concentration to the point of injury by evaporation. It is clear that when such waters are used sparingly, so as to penetrate but a few feet underground, whence the moisture re-ascends for evaporation at the surface, a few repetitions of its use will accumulate so much alkali near the surface as to bring about serious injury. If, on the other hand, the water is used so abundantly that the roots may be considered as being, like the marginal vegetation of alkali lakes, bathed only by water of moderate strength, no such injury need occur; and what does accumulate in consequence of the inevitable measure of evaporation occurring in the course of a season, may be washed out of the land by copious winter irrigation.

This, of course, presupposes that the land, as is mostly the case in the arid region, is readily drained downwards when a sufficiency of water is used. When this is not the case, e. g., in clay or adobe soils, or in those underlaid by hardpan, waters which in sandy lands could have been used with impunity, may become inapplicable to irrigation use.

Apparent Paradox.—The prescription to use saline waters more abundantly than purer ones, in order to avoid injury from alkali, though paradoxical at first sight, is therefore plainly justified by common sense as well as by experience, in pervious (sandy) soils; while in difficultly permeable ones, their use may be either wholly impracticable, or subject to very close limitation.

Sometimes the alternate use of pure and salt-charged water serves to eke out a too scant supply of the former. But in all such cases, close attention to the measure of water that will wet the soil to a certain depth, and “eternal vigilance” with respect to the accumulation of alkali near the surface, must be the price of immunity from injury. In all cases the farmer should know how much of alkali salts he introduces into his land with the irrigation water, and watch that it does not approach too closely, or exceed, the tolerance of his crops for alkali salts, as given in chapter 26.

Use of Drainage Waters for Irrigation.—When lands charged with alkali salts are being reclaimed by drainage, the question sometimes arises whether the drainage-water may not be used for irrigation, lower down. This of course depends entirely upon the amount of alkali in the water, the nature of the lands to be irrigated, and the manner of applying it. In the Fresno drainage-district of California it has been shown that some of the drainage-water contains not more than 25 to 30 grains per gallon of objectionable salts, and such waters could of course be used on pervious lands with the precautions above noted.

“Black Alkali” Waters.—As regards, however, waters containing any large proportion of carbonate of soda, it must be remembered that even very dilute solutions of salsoda serve to puddle the soil and thus render it difficultly tillable. When such waters are used it is necessary to forestall injury either by the use of gypsum in the reservoir or ditch, or by annually using on the land a sufficient amount of gypsum to transform the carbonate of soda into the relatively innocuous sulfate.

Variations in the Saline Contents of Irrigation Waters.—When irrigation waters are derived from deep wells, there is little if any variation of their saline contents to be expected, and a single analysis will serve permanently. But in the case of relatively shallow wells, from which the water must be raised by pumping, it not unfrequently happens that after a series of seasons of short rainfall, saline waters are brought up by the pump and may seriously injure crops and orchards. Again, in the case of streams and rivers whose flow becomes very small in summer, the saline content may increase to several times the amount carried at the time of high water. Both kinds of cases occur in southern California, in Arizona, New Mexico and other states of the arid region. The Gila, Pecos and upper Rio Grande are cases in point, and to a certain extent the Colorado of the West.

Bull. Ariz. Exp’t Sta. No. 44.

Muddy Waters.—In the latter as well as other streams of Arizona, there is another point which sometimes creates difficulties to the irrigator, together with some current expense. It is the amount of silt or mud carried by the water, which while it is a benefit to the land over which it is spread, (“warping”) as in the classic case of the Nile, often clogs the irrigation ditches to such an extent as to cause considerable inconvenience and expense in cleaning them out. This is especially the case in the streams draining pasture lands that have been overstocked, and where the destruction of the natural herbage allows the rain water to run off rapidly, at first forming runlets and then gullies and ravines that originally were simply cow-paths leading toward the watering places. The devastation of lands thus caused in Arizona is almost as great as that which has occurred in the Cotton states, as mentioned above chap. 12. p. 217.

Bull. Arizona Exp’t Station Nos. 2, 38.

These variations in the character of the irrigation water must of course be watched by the farmer who does not receive directly from mountain streams, or from deep artesian wells water known to have a constant content of saline matter.

THE DUTY OF IRRIGATION WATER.—The amount of water thought to be needed for the production of satisfactory crops varies widely in different regions, ranging all the way from about two feet to as much as eight annually, within the United States; while in the sugar-cane fields of the Hawaiian Islands as much as three inches per week, or over twelve acre-feet in the course of the year, have been thought to be beneficial, if not absolutely required for the best crop results.

As has been stated above (chap. 12, p. 215), the rainfall limit below which irrigation becomes, if not absolutely essential, at least a highly desirable condition for the safety of crops, is usually assumed to lie at about 20 inches (500 milimeters). This general statement is, however, subject to material modification according to the manner in which the rainfall is distributed. Thus in central Montana with 24 inches of rainfall distributed throughout the year, irrigation is indispensable; while in the Santa Clara valley of central California, with an average rainfall of 15 inches falling through the winter and spring, the growth of all ordinary field crops has for fifty years not failed oftener than is commonly the case in the humid region of the North Central states. This is because in California the winter and spring are the growing seasons, while the rainless summers do not stand in the way, for crops are already harvested; and the deep rooting of trees and vines provides these with the needful moisture from the depths of the substrata (see chap. 10, pp. 163 to 173).

It would thus seem that twenty inches of irrigation water properly applied ought to be sufficient for all purposes, when added to the natural rainfall, which is rarely entirely absent. Yet in actual practice less than 24 acre-inches is rarely used, and much more is the rule; 72 to 96 ins. being sometimes used in Arizona. Evidently enormous losses occur in practice, and it is of the utmost importance to discover the causes of these.

Causes of Loss.—Since irrigation water is commonly measured at the distributing weirs, loss from seepage and evaporation on the way to the fields is an obvious source of an overestimate of the water actually supplied to the farmer. In sandy districts the loss thus incurred is reliably estimated at nearly 50% in many cases. The apparent duty of the water is thus at once reduced to half its effect, and four instead of two feet of water are supposed to have been used, and are charged for.

Evaporation resulting from surface flooding or use in shallow furrows may, again, cause the loss of from 30 to 50% of the water that actually reaches the land; so that in the latter case, between seepage and evaporation the irrigator may lose the effect of three-fourths of the water he pays for.

Loss by Percolation.—Finally, the water may be wasted on the land itself in leachy soils by over-use, i. e., it may percolate to a large extent beyond the reach of the roots when the flow is continued too long; as will always be the case when the head (supply) ditches are laid too far apart, so that the water may be wasting into the country drainage just below the upper ditch long before the water in the furrow reaches the lower one; as illustrated in the upper one of the subjoined diagrams. That this will not happen when the head ditches are nearer together, is shown in the lower diagram.

The means of avoiding the mechanical losses have already been discussed, and may be summarized thus: tightening of leaky ditches; use of water in deep furrows; and ascertaining the rapidity of percolation (see p. 242) so as to obtain a proper gauge for the time during which water should run, and for the distances at which head ditches or furrows should be placed.

The importance of thus diminishing the losses of water is obvious when it is considered that if the duty of water can be reduced to twenty instead of forty or fifty acre-inches, twice the area can be irrigated with the same amount of water, or the cost of water correspondingly reduced. It should be noted that when the land is leachy it may be pure waste to continue the flow beyond a few hours; but the irrigation must then be more frequently repeated.

EVAPORATION.

Alongside of and supplementary to the best possible utilization of the rainfall and irrigation water, the prevention of unnecessary evaporation has to be considered. Evaporation from the soil’s surface implies not only unnecessary loss of water that should have remained for the use of the crop, but also the depression of temperature which, as a rule, is unfavorable to the best development of vegetation. It is only in case of extreme stress from hot, drying wind that such evaporation and the consequent depression of the temperature of the surface soil can be of advantage to the farmer.

The amount of water evaporating either from a water-surface, or from a wet or moist soil, varies greatly according to the climatic conditions, and the state of the weather; also according to the condition of the soil-surface. There are damp climates, and days or periods when, the air being nearly saturated with moisture, evaporation even from a water-surface will be almost insensible. On the other hand, with dry air and a high temperature, enormous quantities of water may be evaporated in the course of a day. The evaporation from water-surfaces interests deeply those who supply, as well as those who are supplied with, water from storage reservoirs; evaporation from the soil-surface interests deeply all farmers, and more especially irrigators whose water-supply is scanty, or is paid for by them by measurement. Light rains, as well as light surface irrigations, may at times evaporate almost wholly without any effect save a lowering of the temperature of the soil. In the case of snow, it is a well-known fact in the northern arid regions that a light snowfall may in winter evaporate entirely without imparting any liquid moisture to the soil. A loss of 50% of the water actually brought upon land by surface irrigation is of common occurrence in some portions of the irrigated region.

The dependence of evaporation upon air-temperature under conditions otherwise identical, is well illustrated by the experiments made in 1904 by S. Fortier on the Experiment Station grounds at Berkeley, California, at a time when under the influence of the sea breeze the average saturation of the air might be assumed at about 70%. The tests were conducted in six tanks sunk into the ground so as to place the water-surfaces on a level with it, and the water-temperatures were maintained in four of the tanks by means of ice or heating lamps. The results are shown in the following table:

SUMMARY OF AVERAGE WEEKLY LOSSES BY EVAPORATION, WITH VARYING TEMPERATURES OF WATER, AT BERKELEY, CAL., IN JULY AND AUGUST, 1904. =======================+============ Temperature of water. | Weekly |evaporation. -----------------------+------------ Degrees Fahrenheit: | Inches. 55.5 | 0.42 62.0 | 0.77 69.2 | 1.54 80.1 | 3.08 89.2 | 3.92 -----------------------+------------

Progress Report on Coöperative Irrigations in Calif.; Cir. No. 56, Office Exp’t Stations.

A farther illustration is given in the subjoined table, showing maxima and minima of monthly evaporation, as well the totals of one (seasonal) year, in three California localities where the air-saturation is considerably below that at Berkeley, ranging in summer from 50% to 20% and even less (at Calexico in the Colorado desert):

SUMMARY OF EVAPORATION-LOSSES FROM WATER-SURFACES, AT POMONA, TULARE, AND CALEXICO, CAL., FROM JULY 1, 1903, TO JULY 31, 1904. ============+=================+=================+================= | Pomona. | Tulare. | Calexico. +---------+-------+---------+-------+---------+------- | Month. |Inches.| Month. |Inches.| Month. |Inches. ------------+---------+-------+---------+-------+---------+------- Maximum |Aug. 1903| 9.07 |July 1903| 12.34 |July 1903| 14.48 Minimum |Feb. 1904| 2.57 |Jan. 1904| 1.46 |Jan. 1904| 4.39 | | ----- | | ----- | | ------ Totals for | | 66.92 | | 74.68 | | 108.23 year | | | | | | ------------+---------+-------+---------+-------+---------+-------

Of these three stations, Pomona is located within reach of the ocean winds, but distant 25 to 30 miles from the shore. Tulare is situated in the upper San Joaquin valley, far in the interior; Calexico is in the southern part of the Colorado desert, with extremes of temperature ranging from 13° Fahr. in winter to 120° in summer.

Evaporation in Different Climates.—The following table conveys some general data regarding average evaporation from water-surfaces in different climates. Evaporation from the soil-surface depends largely, of course, upon the mechanical condition of the surface, the extent to which it is wetted, and the rapidity with which moisture will be supplied from the subsoil as the surface dries. A field plowed into rough furrows will evaporate more water than when harrowed, because of the larger surface exposed; and a harrowed field moderately compacted by rolling will lose less water by evaporation than when unrolled, other things being equal. On the other hand, a thoroughly compacted surface, even if suffering less loss at first than a plowed or harrowed field, will continue to lose moisture longer by withdrawing it from the substrata by its superior capillary suction; while a loose surface, once dried out, will prevent farther loss from the subsoil very effectually, as stated below.

TABLE SHOWING EVAPORATION, FROM WATER-SURFACE EXPOSED IN SHALLOW TANKS, NEAR WATER OR GROUND SURFACE. ===================================+========+==================== | Years. | Inches. -----------------------------------+--------+-------------------- Rothamsted, England | 9 | 17.80 (16.6 to 18.4) London, “ | 14 | 20.66 Oxford, “ | 5 | 31.04 Munich, Germany | ? | 24.00 Emdrup, Denmark | 10 | 27.09 Cambridge, Massachusetts | 1 | 56.00 Syracuse, New York | 1 | 50.20 Logan, Utah | 1 | 52.39 Tucson, Arizona | 1 | 75.80 Fort Collins, Colorado | 11 | 41.00 Fort Bliss, Texas | 1 | 82.70 San Francisco, California | | 45 to 50 Sweetwater Reservoir, | | San Diego, California | 1 | 57.6 Peking, China | ? | 38.80 Demerara, South America | 3 | 35.12 Bombay, East India | 5 | 82.28 Petro-Alexandrowsk, West Turkestan | ? | 96.40 Kimberley, South Africa | ? | 98.80 Alice Springs, South Australia | ? |103.50 -----------------------------------+--------+--------------------

This table, the data for which are taken from various sources, exhibits clearly the enormous variations in evaporation in different countries, and even in localities not very remote from each other. The low evaporation near London is doubtless due to its foggy and hazy atmosphere, but it is not clear why Rothamsted should show so low an evaporation compared with Oxford. Tropical Demerara stands nearest to Oxford in its evaporation; Bombay indicates its location on the hot and arid west coast of India, despite its nearness to the sea. The inland localities in the desert regions of South Africa, Australia and Western Turkestan, show how enormous may be the losses from evaporation of irrigation water, unless the latter is applied with special care for their prevention. Thus, with the wasteful methods of irrigation prevailing in portions of the American arid region, it is certain that in many cases 50% and more of the water evaporates before it reaches the crops.

Evaporation from Reservoirs and Ditches.—The evaporation from water-surfaces especially may, in many cases, exceed the rainfall of the year, so as to materially diminish the available water-supply in reservoirs. Thus the annual evaporation from the reservoir-lakes forming part of the water-supply of the city of San Francisco, ranges from 40 to 50 inches, while the rainfall averages less than 24 inches. Were it not, then, for the prevention of evaporation by a covering of dry earth during summer, no moisture would remain in the ground to sustain vegetation. In the cool coast climate of Berkeley, Cal., directly opposite the Golden Gate and subject to its summer fogs, evaporation from a water-surface maintaining the average climatic temperature of 60°, was found to be ¾ inch during the month from the middle of July to the middle of August, 1904. But at the high temperatures and low degree of air-saturation prevailing in the great interior valley, or in the Colorado desert, the evaporation from water-surfaces is enormously increased, exceeding even the figure given in the table for Bombay. Hence the great importance of preventing all avoidable evaporation, particularly in the use of irrigation water.

Prevention of Evaporation; Protective Surface Layer.—The loose tilth of the surface which is so conducive to the rapid absorption of surface-water, is also, broadly speaking, the best means of reducing evaporation to the lowest possible point. For while it is true that the floccules of well-tilled soil permit of the ready access of air, and therefore of evaporation, it is also true that these relatively coarse compound particles are incapable of withdrawing capillary moisture from the denser soil or subsoil underneath; just as a dry sponge is incapable of absorbing any moisture from a wet brick, while a dry brick will readily withdraw nearly all the water contained in the relatively large pores of the sponge (see chap. 11). A layer of loose, dry surface-soil is therefore an excellent preventive of evaporation of the moisture from soils, and may be regarded as the natural and most available means to be used by the farmer, both for the prevention of evaporation and to moderate the access of excessive heat and dryness to the active roots.

As regards the desirable thickness of this protective layer of tilled surface-soil, it should be kept in mind that in the humid region, where rain can be expected at intervals of from one to three weeks, the feeding roots may usually be found within a few inches of the surface; while in the arid region, where irrigation is practiced at long intervals or sometimes not at all, so that no water enters the soil oftener than from two to six months, the roots necessarily vegetate at lower depths, and hence the protective surface-layer can, and should be, of greater thickness, to prevent the penetration of excessive heat and dryness during the long interval.

The failure to appreciate this necessary difference often leads to heavy losses on the part of newcomers to the arid region, who in this as in other respects are apt to follow blindly the precepts familiar to them in the East, until taught better by sore experience. In the East and Middle West a depth of three inches is considered the proper one for the protective surface-layer; and in the case of maize even this is considered excessive in many cases. In the arid region this depth should be at least doubled where irrigation is not practiced at least every four to six weeks; and in some sandy soils even seven and eight inches is not too much for effective protection.

Illustrations of Effects of Surface Tillage.—The efficacy of loose surface tilth in preventing evaporation, as compared with mere superficial scratching or with the total omission of cultivation, is well exemplified in a series of investigations conducted on this subject during the extremely dry season of 1898, by the California Experiment Station; the seasonal rainfall having during that year been on an average from one-third to one-half only of the usual amount, so as to test to the utmost the endurance of all growing plants. Some of the details of this investigation have been given above (p. 214) in connection with the question of moisture requirements of crops. Loughridge also investigated the moisture conditions in adjacent orchards differently treated in cultivation. In one of these cases two orchards of apricots were separated only by a lane, and the soil identical; but one owner had omitted cultivation, while the other had cultivated to an extra depth in view of the dry season apparently impending. The results are best shown by the plates below, showing representative trees and the annual growth made by each. The table annexed shows the differences in the moisture-content of the two fields to the depth of six feet, in July:

MOISTURE IN CULTIVATED AND UNCULTIVATED LAND. ===================+=====================+================== | Cultivated. | Uncultivated. Depth in soil. +------------+--------+---------+-------- | Per cent. |Tons per|Per cent.|Tons per | | acre. | | acre. -------------------+------------+--------+---------+-------- First foot | 6.4 | 128 | 4.3 | 86 Second foot | 5.8 | 116 | 4.4 | 88 Third foot | 6.4 | 128 | 3.9 | 78 Fourth foot | 6.5 | 130 | 5.1 | 100 Fifth foot | 6.7 | 134 | 3.4 | 68 Sixth foot | 6.0 | 120 | 4.5 | 90 | --- | --- | --- | --- Total for six foot | 6.3 | 756 | 4.2 | 512 -------------------+------------+--------+---------+--------

Rep. Calif. Expt. Sta. for 1897-98, p. 65.

The difference of 244 tons per acre of ground shown by the analyses is quite sufficient to account for the observed difference in the cultural result. The cause of this difference was that in the uncultivated field there was a compacted surface-layer of several inches in thickness, which forcibly abstracted the moisture from the substrata and evaporated it from its surface; while the loose surface soil on the cultivated ground was unable to take any moisture from the denser subsoil.

FIGS. 48, 49.—Apricot Trees, Creek Bench Land, at Niles, Cal.]

The cultural results were that on the cultivated ground the trees made about three feet of annual growth, and the fruit was of good, normal size; while the trees in the uncultivated ground made barely three inches of growth, and the fruit was stunted and wholly unsaleable. It may be added that when, instructed by the season’s experience, the owner of the “uncultivated” orchard cultivated deeply the following season, his trees showed as good growth and fruit as his neighbor’s.

EVAPORATION THROUGH THE ROOTS AND LEAVES OF PLANTS.

Undesirable as is the evaporation from the surface of the soil, under all but exceptional conditions the evaporation from the leaves of plants is one of the essential functions of vegetable development. Not only because water serves as the vehicle of the plant-food absorbed by the roots and to be organized by and redistributed from the leaves, and the aeration occurring in the latter must of necessity result in a certain degree of evaporation; but largely because the conversion of liquid water into vapor serves to prevent an injurious rise of temperature in the leaves under the influence of hot sunshine and dry air. It is undoubtedly for the latter purpose that the greater part of the enormous amount of water required, as above stated (chap. 11) for the production of one part of dry substance, is actually used. When sufficient water to supply the required evaporation through the leaves cannot be brought up from the soil, the plant begins to wilt; or in the case of some plants with very thin and soft leaves the blade normally begins to droop during the hottest hours of the day; thus escaping excessive exposure to the sun’s rays, and recovering their turgor later in the afternoon.

The amount of water actually evaporated from orchard trees has unfortunately not been directly determined, the investigations made in this respect having borne mainly upon forest trees. The Austrian Forest Experiment Station made a series of elaborate investigations on this subject in 1878, and the following data (quoted from the Report of the U. S. Dep’t of Agriculture for 1889) convey some idea of the results.

It was found that the surface-areas of the leaves do not give reliable results, but that these depend very largely upon the thickness (mass) of the leaves. The dry weight of the latter was found, as in the case of field crops, to correspond most nearly to the observations made directly. It was thus found that e. g. birch and linden transpired during their annual period of vegetation from 600 to 700 pounds of water per pound of dry leaves; oaks 200 to 300, while the figures for ash, beech and maple were in between. On the other hand the conifers—spruce, fir and pine—ranged, under the same conditions, from 30 to 70 pounds of water only. In another year, these figures were increased for deciduous trees to from 500 to 1000, the conifers, 75 to 200 pounds. This great variability in different seasons, together with other elements of uncertainty, render these figures only roughly approximate; but it will be noted that the figures for deciduous trees are in general of the same order as those given above for field crops. Assuming the evaporation for citrus trees to be approximately the same as for the European evergreen oak (Q. cerris) viz. 500 pounds per pound of dry matter, and taking the weighings made by Loughridge of the leaves of a 15-year-old orange tree at Riverside as a basis (40 pounds of dry leaves), the water evaporated by each such tree would be about 20,000 pounds per year, or about 1000 tons per acre of 100 trees. This is equivalent to about 9 acre-inches of rainfall, out of the 35 inches commonly given.

Since different plants evaporate very different amounts of water during a given time, according to their leaf-surface and the number and size of their stomates, the maintenance of the equilibrium between the soil-supply and the evaporation of the leaf-surface requires correspondingly varying moisture-conditions in the soil. Therefore desert plants, with their elaborate structural provisions against leaf-evaporation, will develop normally, and without wilting, under conditions which in the case of most culture plants would result in severe injury or death. Since diminution of leaf-surface will in all cases diminish evaporation, the heroic measure of cutting back the twigs and branches of shrubs and trees in seasons of severe drought is sometimes resorted to in order to save their life. In Nature this diminution of leaf-surface may be observed in many cases of desert plants, whose “fugacious” leaves are developed during the rainy season, in winter and early spring; dropping off so soon as the dry season begins, and leaving only the green surface of twigs, stems or spines to perform the functions of the leaves.

The shading of the ground by leafy vegetation will, of course, greatly diminish and sometimes suppress evaporation from the soil-surface; thus very nearly fulfilling the same conditions referred to above (chap. 7, page 111) in discussing the effect of natural vegetation in rendering tillage unnecessary; the beating of rains, and the formation of surface crusts, being alike prevented. This fact is of essential importance in contributing to the welfare of crops sown broadcast, where subsequent cultivation is impracticable.

Weeds Waste Moisture.—The injurious effects of weedy growth among culture plants are in most cases due quite as much to the appropriation of moisture that should have gone to the crop, as to the abstraction of plant-food, to which the injury is generally attributed. This is much more obvious in the arid region, where during the dry summers every pound of moisture counts, than where summer rains obscure this influence. It has led orchardists in California almost to an excess of clean culture, resulting in the burning-out of the humus from the bare surface-soil during the long, hot summers, and an injurious compacting impossible to remedy by the most careful tillage. It thus happens that green-manuring, the natural remedy for this evil, cannot safely be done there with summer crops, but must be accomplished with winter crops, such as can be turned under before the dry season begins. The same objection holds against the growing of summer crops between the orchard-rows.

DISTRIBUTION OF MOISTURE IN THE SOIL AS AFFECTED BY VEGETATION.

The investigations of Wollny and others have long shown quantitatively what common experience has taught the farmer, viz., that a field in crops or grass is always drier within the soil-mass penetrated by the roots than is a cultivated field bare of crops, unless perhaps when heavily crusted on the surface. The depletion of moisture caused by grass sward is the most easily observed because of the shallowness of the root-system; and this is one cause at least why grass sward does not occur naturally in the arid region, and when planted cannot be maintained without irrigation repeated at short intervals. Deeper-rooted plants of course deplete the soil at different and varying levels; and where surface roots are few or absent it may readily happen that the surface soil is moister than the subsoil.

This was very strikingly shown by the investigations of Ototzky in the South-Russian steppes, in comparing both the moisture contents and the depth of bottom water as between forest land and the open plains. On the steppe near Chipoff, Government of Voronej, he found the ground water at from 3 to 5 meters (10-16 feet) depth; under the forest in the same region and in identical underground formations, the water level stood at 15 meters. In the Black Forest near Cherson, the water is found at about 15 feet beneath the surface; under the steppe and in cultivated ground it stood at 10 feet. At the same time the forest soil was moister in the upper two feet than the soil of the steppe, where surface evaporation (partly through shallow plant-roots, partly direct) was greater than under the shadow of the forest; under which, moreover, there were few shallow rooted plants to draw upon the moisture of the surface soil.

The great evaporation from forests is a matter demonstrated by actual measurement; hence it is not surprising that certain shallow-rooted trees should serve for the reclamation of wet ground, as has been demonstrated on the large scale, e. g., in the use of the eucalyptus in the Pontine Marshes of Italy, and of the maritime pine in the Landes of western France. Thus the sanitation of swampy districts through tree-planting has become one of the established measures in their settlement. But this refers only to the evaporation from the trees themselves; for in the shade of the forest, a free water-surface is found to evaporate on the average only one-third as much as in open ground. Of course there must be a correspondingly great difference in the amounts of evaporation from the soil-surfaces in the respective areas.

The great draft made by the Eucalyptus globulus upon soil-moisture has been also abundantly shown in California, where on account of its rapid growth this tree has been largely used for windbreaks. It was found that the trees deplete the fields of moisture for from twenty to thirty feet on either side, so as to materially reduce crops within that limit. For this reason the pine and cypress has of late found greater acceptance for this purpose.

Mulching.—Covering the soil with straw or similar loose materials to prevent waste of moisture is a common garden practice everywhere, although not usually applicable on the large scale. It may readily however, be carried to excess, in preventing not only evaporation but also the warming of the soil which is so needful to the thrifty growth of plants. It must not therefore be done too early in the season; and after cold rains it sometimes becomes necessary to remove the mulch in order to allow the ground to become properly warmed. Mulching in early spring is often used to retard blooming of trees where spring frosts are feared.

In the arid region, sanding of the surface is sometimes resorted to for the prevention of the evaporation which brings alkali salts to the surface. But the necessity of repeating this dressing annually unless cultivation can be omitted, restricts the use of this expedient to narrow limits.

The sanding of the surface of cranberry plantations in swamps or bogs in the northern parts of the humid region doubtless owes its efficacy largely, if not chiefly, to the retention of moisture, while at the same time it prevents the consolidation of the surface, so as to render tillage unnecessary.