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.
AREA OF IRRIGABLE LAND SOMETIMES NOT LIMITED BY WATER SUPPLY.
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.
METHOD OF DETERMINING THE SUPPLY OF WATER.
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.
METHODS OF DETERMINING THE EXTENT OF IRRIGABLE LAND UNLIMITED BY WATER SUPPLY.
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.
THE SELECTION OF IRRIGABLE LANDS.
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.