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. 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.