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,[94] 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.
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.[95] 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].
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.