The unit of water employed in mining as well as manufacturing enterprises in the west is usually the inch, meaning thereby the amount of water which will flow through an orifice one inch square. But in practice this quantity is very indefinite, due to the “head” or amount of pressure from above. In some districts this latter is taken at six inches. Another source of uncertainty exists in the fact that increase in the size of the orifice and increase in the amount of flow do not progress in the same ratio. An orifice of one square inch will not admit of a discharge one-tenth as great as an orifice of ten square inches. An inch of water, therefore, is variable with the size of the stream as well as with the head or pressure. For these reasons it seemed better to take a more definite quantity of water, and for this purpose the second-foot has been adopted. By its use the volume of a stream will be given by stating the number of cubic feet which the stream will deliver per second.
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.
While, as a general fact, the area of arable land is greater than the area of irrigable land, by reason of the insufficient supply of water, yet in considering limited tracts it may often be found that the supply of water is so great that only a part of it can be used thereon. In such cases the area of irrigable land is limited by the extent to which the water can be used by proper engineering skill. This is true in considering some portions of Utah, where the waters of the Green and Colorado cannot all be used within that territory. Eventually these surplus waters will be used in southern California.
To determine the amount of irrigable land in Utah, it was necessary to consider the supply; that is, to determine the amount of water flowing in the several streams. Again, this quantity is variable in each stream from season to season and from year to year. The irrigable season is but a small portion of the year. To utilize the entire annual discharge of the water, it would be necessary to hold the surplus flowing in the non-growing season in reservoirs, and even by this method the whole amount could not be utilized, as a great quantity would be lost by evaporation. As the utilization of the water by reservoirs will be to a great extent postponed for many years, the question of immediate practical importance is resolved into a consideration of the amount of water that the streams will afford during the irrigating season. But in the earlier part of the season the flow in most of the streams in this western region is great, and it steadily diminishes to the end of the summer. Earlier in the season there is more water, while for the average of crops the greater amount is needed later.
The practical capacity of a stream will then be determined by its flow at the time when that is least in comparison with the demands of the growing crops. This will be called the critical period, and the volume of water of the critical period will determine the capacity of the stream. The critical period will vary in different parts of the region from the latter part of June until the first part of August. For the purposes of this discussion it was only necessary to determine the flow of the water during the critical period. This has been done by very simple methods. Usually in each case a section of the stream has been selected having the least possible variation of outline and flow. A cross-section of the stream has been measured, and the velocity of flow determined. With these factors the capacity of the streams has been obtained. In some cases single measurements have been made; in others several at different seasons, rarely in different years. The determination of the available volume of the several streams by such methods is necessarily uncertain, especially from the fact that it has not always been possible to gauge the streams exactly at the critical period; and, again, the flow in one season may differ materially from that in another. But as the capacity of a stream should never be rated by its volume in seasons of abundant flow, we have endeavored as far as possible to determine the capacity of the streams in low water years. Altogether the amount of water in the several streams has been determined crudely, and at best the data given must be considered tolerable approximations. In considering the several streams experience may hereafter discover many errors, but as the number of determinations is great, the average may be considered good.
In the few cases where the water supply is more than sufficient to serve the arable lands, the character of the problem is entirely changed, and it becomes necessary then to determine the area to which the waters can be carried. These problems are hypsometric; relative altitudes are the governing conditions. The hypsometric methods were barometric and angular; that is, from the barometric stations vertical angles were taken and recorded to all the principal points in the topography of the country; mercurial and aneroid barometers were used, chiefly the former; the latter to a limited extent, for subsidiary work. Angular measurements were made with gradientors to a slight extent, but chiefly with the orograph, an instrument by which a great multiplicity of angles are observed and recorded by mechanical methods. This instrument was devised by Professor Thompson for the use of the survey, and has been fully described in the reports on the geographical operations. To run hypsometric lines with spirit levels would have involved a great amount of labor and been exceedingly expensive, and such a method was entirely impracticable with the means at command, but the methods used give fairly approximate results, and perhaps all that is necessary for the purposes to be subserved.
From the fact that the area of arable lands greatly exceeds the irrigable, or the amount which the waters of the streams will serve, a wide choice in the selection of the latter is permitted. The considerations affecting the choice are diverse, but fall readily into two classes, viz: physical conditions and artificial conditions. The mountains and high plateaus are the great aqueous condensers; the mountains and high plateaus are also the reservoirs that hold the water fed to the streams in the irrigating season, for the fountains from which the rivers flow are the snow fields of the highlands. After the streams leave the highlands they steadily diminish in volume, the loss being due in part to direct evaporation, and in part to percolation in the sands from which the waters are eventually evaporated. In like manner irrigating canals starting near the mountains and running far out into the valleys and plains rapidly diminish in the volume of flowing water. Looking to the conservation of water, it is best to select lands as high along the streams as possible. But this consideration is directly opposed by considerations relating to temperature; the higher the land the colder the climate. Where the great majority of streams have their sources, agriculture is impossible on account of prevailing summer frosts; the lower the altitude the more genial the temperature; the lower the land the greater the variety of crops which can be cultivated; and to the extent that the variety of crops is multiplied the irrigating season is lengthened, until the maximum is reached in low altitudes and low latitudes where two crops can be raised annually on the same land. In the selection of lands, as governed by these conditions, the higher lands will be avoided on the one hand because of the rigor of the climate; if these conditions alone governed, no settlement should be made in Utah above 6,500 feet above the level of the sea, and in general still lower lands should be used; on the other hand the irrigable lands should not be selected at such a distance from the source of the stream as to be the occasion of a great loss of water by direct and indirect evaporation. For general climatic reasons, the lands should be selected as low as possible; for economy of water as high as possible; and these conditions in the main will cause the selections to be made along the middle courses of the streams. But this general rule will be modified by minor physical conditions relating to soil and slope—soils that will best conserve the water will be selected, and land with the gentlest slopes will be taken.
In general, the descent of the streams in the arid land is very great; for this reason the flood plains are small, that is, the extent of the lands adjacent to the streams which are subject to overflow at high water is limited. In general, these flood-plain lands should not be chosen for irrigation, from the fact that the irrigating canals are liable to be destroyed during flood seasons. Where the plan of irrigation includes the storage of the water of the non-growing season, by which all the waters of the year are held under control, the flood-plain lands can be used to advantage, from the fact that they lie in such a way as to be easily irrigated and their soils possess elements and conditions of great fertility.
Other locally controlling conditions are found in selecting the most advantageous sites for the necessary water works.
These are the chief physical factors which enter into the problem, and in general it will be solved by considering these factors only; but occasionally artificial conditions will control.
The mining industries of the Arid Region are proportionately greater than in the more humid country. Where valuable mines are discovered towns spring up in their immediate vicinity, and they must be served with water for domestic purposes and for garden culture. When possible, agriculture will be practiced in the immediate vicinity for the purpose of taking advantage of the local market. In like manner towns spring up along the railroads, and agriculture will be carried on in their vicinity. For this and like reasons the streams of the Arid Region will often be used on lands where they cannot be made the most available under physical conditions, and yet under such circumstances artificial conditions must prevail.
In the indication of specific areas as irrigable on the accompanying map of Utah, it must be considered that the selections made are but tentative; the areas chosen are supposed to be, under all the circumstances, the most available; but each community will settle this problem for itself, and the circumstances which will control any particular selection cannot be foretold. It is believed that the selections made will be advantageous to the settler, by giving him the opinions of men who have made the subject a study, and will save many mistakes.
The history of this subject in Utah is very instructive. The greater number of people in the territory who engage in agriculture are organized into ecclesiastical bodies, trying the experiment of communal institutions. In this way the communal towns are mobile. This mobility is increased by the fact that the towns are usually laid out on Government lands, and for a long time titles to the land in severalty are not obtained by the people. It has been the custom of the church to send a number of people, organized as a community, to a town site on some stream to be used in the cultivation of the lands, and rarely has the first selection made been final. Luxuriant vegetation has often tempted the settlers to select lands at too great an altitude, and many towns have been moved down stream. Sometimes selections have been made too far away from the sources of the streams, and to increase the supply of water, towns have been moved up stream. Sometimes lands of too great slope have been chosen, and here the waters have rapidly cut deep channels and destroyed the fields. Sometimes alkaline lands are selected and abandoned, and sometimes excessively sandy lands have caused a change to be made; but the question of the best sites for the construction of works for controlling and distributing the water has usually determined the selection of lands within restricted limits.
To a very slight extent indeed have artificial conditions controlled in Utah; the several problems have generally been solved by the consideration of physical facts.
Irrigation has been practiced in different portions of the Arid Region for the last twenty-five or thirty years, and the area cultivated by this means has been steadily increasing during that time. In California and New Mexico irrigation has been practiced to a limited extent for a much longer time at the several Catholic missions under the old Spanish regime. In the history of the settlement of the several districts an important fact has been uniformly observed—in the first years of settlement the streams have steadily increased in volume. This fact has been observed alike in California, Utah, Colorado, and wherever irrigation has been practiced. As the chief development of this industry has been within the last fifteen years, it has been a fact especially observed during that time. An increase in the water supply, so universal of late years, has led to many conjectures and hypotheses as to its origin. It has generally been supposed to result from increased rainfall, and this increased rainfall now from this, now from that, condition of affairs. Many have attributed the change to the laying of railroad tracks and construction of telegraph lines; others to the cultivation of the soil, and not a few to the interposition of Divine Providence in behalf of the Latter Day Saints.
If each physical cause was indeed a vera causa, their inability to produce the results is quite manifest. A single railroad line has been built across the Arid Region from east to west, and a short north and south line has been constructed in Colorado, another in Utah, and several in California. But an exceedingly small portion of the country where increase of water supply has been noticed has been reached by the railroads, and but a small fraction of one per cent. of the lands of the Arid Region have been redeemed by irrigation. This fully demonstrates their inadequacy. In what manner rainfall could be affected through the cultivation of the land, building of railroads, telegraph lines, etc., has not been shown. Of course such hypotheses obtain credence because of a lack of information relating to the laws which govern aqueous precipitation. The motions of the earth on its axis and about the sun; the unequal heating of the atmosphere, which decreases steadily from equator to poles; the great ocean currents and air currents; the distribution of land and water over the earth; the mountain systems—these are all grand conditions affecting the distribution of rainfall. Many minor conditions also prevail in topographic reliefs, and surfaces favorable to the absorption or reflection of the sun’s heat, etc., etc., affecting in a slight degree the general results. But the operations of man on the surface of the earth are so trivial that the conditions which they produce are of minute effect, and in presence of the grand effects of nature escape discernment. Thus the alleged causes for the increase of rainfall fail. The rain gauge records of the country have been made but for a brief period, and the stations have been widely scattered, so that no very definite conclusions can be drawn from them, but so far as they are of value they fail to show any increase. But if it be true that increase of the water supply is due to increase in precipitation, as many have supposed, the fact is not cheering to the agriculturist of the Arid Region. The permanent changes of nature are secular; any great sudden change is ephemeral, and usually such changes go in cycles, and the opposite or compensating conditions may reasonably be anticipated.
For the reasons so briefly stated, the question of the origin and permanence of the increase of the water supply is one of prime importance to the people of the country. If it is due to a temporary increase of rainfall, or any briefly cyclic cause, we shall have to expect a speedy return to extreme aridity, in which case a large portion of the agricultural industries of the country now growing up would be destroyed.
The increase is abundantly proved; it is a matter of universal experience. The observations of the writer thereon have been widely extended. Having examined as far as possible all the facts seeming to bear on the subject, the theory of the increase of rainfall was rejected, and another explanation more flattering to the future of agriculture accepted.
The amount of water flowing in the streams is but a very small part of that which falls from the heavens. The greater part of the rainfall evaporates from the surfaces which immediately receive it. The exceedingly dry atmosphere quickly reabsorbs the moisture occasionally thrown down by a conjunction of favoring conditions. Any changes in the surfaces which receive the precipitation favorable to the rapid gathering of the rain into rills and brooks and creeks, while taking to the streams but a small amount of that precipitated, will greatly increase the volume of the streams themselves, because the water in the streams bears so small a proportion to the amount discharged from the clouds. The artificial changes wrought by man on the surface of the earth appear to be adequate to the production of the observed effects. The destruction of forests, which has been immense in this country for the past fifteen years; the cropping of the grasses, and the treading of the soil by cattle; the destruction of the beaver dams, causing a drainage of the ponds; the clearing of drift wood from stream channels; the draining of upland meadows, and many other slight modifications, all conspire to increase the accumulation of water in the streams, and all this is added to the supply of water to be used in irrigation.
Students of geology and physical geography have long been aware of these facts. It is well known that, under the modifying influences of man, the streams of any region redeemed from the wilderness are changed in many important characteristics. In flood times their volumes are excessively increased and their powers of destruction multiplied. In seasons of drought, some streams that were perennial before man modified the surface of the country become entirely dry; the smaller navigable streams have their periods of navigation shortened, and the great rivers run so low at times that navigation becomes more and more difficult during dry seasons; in multiplied ways these effects are demonstrated. While in the main the artificial changes wrought by man on the surface are productive of bad results in humid regions, the changes are chiefly advantageous to man in arid regions where agriculture is dependent upon irrigation, for here the result is to increase the supply of water. Mr. Gilbert, while engaged during the past season in studying the lands of Utah, paid especial attention to this subject, and in his chapter has more thoroughly discussed the diverse special methods by which increase in the flow of the streams is caused by the changes wrought by man upon the surface of the earth. His statement of facts is clear, and his conclusions are deemed valid.