THE QUANTITATIVE VALUE OF WATER IN IRRIGATION.
In general, throughout the Arid Region the extent of the irrigable land is limited by the water supply; the arable lands are much greater than the irrigable. Hence it becomes necessary, in determining the amount of irrigable lands with reasonably approximate accuracy, to determine the value of water in irrigation; that is, the amount of land which a given amount of water will serve.
All questions of concrete or applied science are more or less complex by reason of the multifarious conditions found in nature, and this is eminently true of the problem we are now to solve, namely, how much water must an acre of land receive by irrigation to render agriculture thereon most successful; or, how much land will a given amount of water adequately supply. This will be affected by the following general conditions, namely, the amount of water that will be furnished by rainfall, for if there is rainfall in the season of growing crops, irrigation is necessary only to supply the deficiency; second, the character of the soil and subsoil. If the conditions of soil are unfavorable, the water supply may be speedily evaporated on the one hand, or quickly lost by subterranean drainage on the other; but if there be a soil permitting the proper permeation of water downward and upward, and an impervious subsoil, the amount furnished by artificial irrigation will be held in such a manner as to serve the soil bearing crops to the greatest extent; and, lastly, there is a great difference in the amount of water needed for different crops, some requiring less, others more.
Under these heads come the general complicating conditions. In the mountainous country the areal distribution of rainfall is preëminently variable, as the currents of air which carry the water are deflected in various ways by diverse topographic inequalities. The rainfall is also exceedingly irregular, varying from year to year, and again from season to season.
But in all these varying conditions of time and space there is one fact which must control our conclusions in considering most of the lands of the Arid Region, namely: any district of country which we may be studying is liable for many seasons in a long series to be without rainfall, when the whole supply must be received from irrigation. Safety in agricultural operations will be secured by neglecting the rainfall and considering only the supply of water to be furnished by artificial methods; the less favorable seasons must be considered; in the more favorable there will be a surplus. In general, this statement applies throughout the Arid Region, but there are some limited localities where a small amount of rainfall in the season of growing crops seems to be constant from year to year. In such districts irrigation will only be used to supply deficiencies.
The complicating conditions arising from soil and subsoil are many. Experience has already shown that there are occasional conditions of soil and subsoil so favorable that the water may be supplied before the growing season, and the subsoil will hold it for weeks, or even months, and gradually yield the moisture to the overlying soil by slow upward percolation or capillary attraction during the season when growing crops require its fertilizing effect. When such conditions of soil and subsoil obtain, the construction of reservoirs is unnecessary, and the whole annual supply of the streams may be utilized. On the other hand, there are extremely pervious soils underlaid by sands and gravels, which speedily carry away the water by a natural under drainage. Here a maximum supply by irrigation is necessary, as the soils must be kept moist by frequent flowing. Under such conditions the amount of water to be supplied is many fold greater than under the conditions previously mentioned, and between these extremes almost infinite variety prevails.
Practical agriculture by irrigation has also demonstrated the fact that the wants of different crops are exceedingly variable, some requiring many fold the amount of others. This is due in part to the length of time necessary to the maturing of the crops, in part to the amount of constant moisture necessary to their successful growth. But by excluding the variability due to rainfall, and considering only that due to differences of soils and crops, and by taking advantage of a wide experience, a general average may be obtained of sufficient accuracy for the purposes here in view.
In examining the literature of this subject it was found that the experience in other countries could not be used as a guide in considering our problems. In general, irrigation in Europe and Asia is practiced only to supply deficiencies, and the crops there raised are only in part the same as with us, and the variation on account of the crops is very great. Certain statements of Marsh in his “Man and Nature” have been copied into the journals and reports published in the United States, and made to do duty on many occasions; but these statements are rather misleading, as the experience of farmers in the Arid Region has abundantly demonstrated. The writers who have used them have in general overestimated the quantitative value of water in irrigation. The facts in Italy, in Spain, in Grenada, and India are valuable severally for discussion in the countries named, but must be used in a discussion of the arid lands of the United States with much care. It seemed better, under these circumstances, to determine the quantitative value of water in irrigation in Utah from the experience of the farmers of Utah. Irrigation has there been practiced for about thirty years, and gradually during that time the area of land thus redeemed has been increased, until at present about 325,000 acres of land are under cultivation. A great variety of crops have been cultivated—corn, wheat, oats, rye, garden vegetables, orchard trees, fruits, vines, etc., etc.; and even the fig tree and sugar cane are there raised.
During the past six or seven years I have from time to time, as occasion was afforded, directed my attention to this problem, but being exceedingly complex, a very wide range of facts must be considered in order to obtain a reasonably approximate average. During the past year the task of more thoroughly investigating this subject was delegated to Mr. Gilbert. The results of his studies appear in a foregoing chapter, written by him; but it may be stated here that he has reached the conclusion that a continuous flow of one cubic foot of water per second, i. e., a second-foot of water, will, in most of the lands of Utah, serve about 100 acres for the general average of crops cultivated in that country; but to secure that amount of service from the water very careful and economic methods of irrigation must be practiced. At present, there are few instances where such economic methods are used. In general, there is a great wastage, due to badly constructed canals, from which the water either percolates away or breaks away from time to time; due, also, to too rapid flow, and also to an excessive use of the water, as there is a tendency among the farmers to irrigate too frequently and too copiously, errors corrected only by long experience.
The studies of Mr. Gilbert, under the circumstances, were quite thorough, and his conclusions accord with my own, derived from a more desultory but longer study of the subject.