82 LANDS OF THE ARID REGION OF THE UNITED STATES. 
value of water in irrigation; that is, the amount of land which a given 
amount ot 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 giyen amount of water 
adequately supply. This will be affected by the following general condi- 
tions, 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 preéminently 
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 opera- 
tions 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 
