64 



THE IRRIGATION AGE. 



The practical catch and the practical duty of water 

 set these limits. 



HOW MOUNTAINS EFFECT RAINFALL. 



Before proceeding to apply these laws in estimat- 

 ing the amount of land which can be irrigated, an- 

 other important fact with regard to the water supply 

 should be pointed out. Mountains concentrate the 

 rainfall. Both maps serve to bring out this fact. 

 Thus, in the great system of the Park mountains of 

 Colorado, in the Wasatch and Uinta mountains, of 

 Utah and Colorado, in the Black Hills and the geyser 

 mountains of Wyoming, stretching up into Montana, 

 and in the great mountain systems of Montana and 

 Idaho, large areas of increased rainfall are found. 

 Again, in central Washington, Oregon, and eastern 

 California, the Cascade mountains and Sierra Nevada 

 furnish another example of increased rainfall. This 

 gives to the arid lands of these states where irrigation 

 is necessary, large streams of water having their 

 sources in the mountains where the rainfall is great and 

 the runoff is also great. In the upper regions little 

 or no irrigation is necessary, and only small areas 

 can be cultivated because of the mountainous char- 

 acter of the country. These mountain-born waters, 

 therefore, may be used upon the mesas, plains and 

 valleys below. This gives to all of these districts a 

 large source of water supply, which is often limited 

 only by the distance to which it can be practically 

 carried in canals. In making a general statement of 

 the amount of land which can be irrigated in the Uni- 

 ted States it is necessary to consider these facts also. 



AMOUNT OF LAND THAT CAN BE IRRIGATED. 



It is proposed now to consider those areas of coun- 

 try in the western half of the United States where 

 the rainfall is 20 inches or less. There are large 

 districts of country which can be profitably irriga- 

 ted where the rainfall is more than 20 inches, but for 

 present purposes these are neglected. 



The total area where the mean annual rainfall does 

 not exceed 20 inches is about 750 million acres. 

 Where this is situated is shown on the map of Mean 

 Annual Rainfall. The water to be used in irrigation 

 in all of this country, with some slight exceptions 

 hereafter to be noted, is the natural runoff from the 

 same areas, to which must be added the amount of 

 water caught in the mountain regions where the 

 rainfall is more than 20 inches, but which runs down 

 where it can be taken upon the lands having 20 

 inches of rainfall or less. When we compute the 

 absolute runoff of all of this region, it is found that if 

 it could be all caught and all distributed upon irriga- 

 ble lands at the rate of 24 acre-inches to every acre 

 of crop, the amount which could thus be irrigated 

 would be about one-tenth of the whole, or 75 million 

 acres. But this supposes an absolute catch, which is 



impossible. Reduced to the possible catch (which is 

 not practical) the amount is a little over 52 million 

 acres. This reduced again to the practical catch 

 gives a little over 36^j million acres. This, then, is 

 the amount of land in the arid region where the 

 rainfall is 20 inches or less which can under practical 

 conditions be redeemed for agriculture by irrigation 

 through the use of the natural runoff. But to re- 

 deem it, all the practical catch from the arid region 

 and from the mountains which deliver their water 

 into the arid region must be utilized. 



In the above estimate only the natural runoff has 

 been considered. We must now turn attention to- 

 other sources of water to which reference has been 

 made from time to time. 



It will be seen that in making this estimate it is 

 proposed to transform runoff water into flyoff water 

 through the agency of growing crops and the evapo- 

 ration consequent on the processes of irrigation. 

 Now a portion of the natural flyoff can be caught be- 

 fore it is evaporated and can be used in irrigation. 



SUPPLIES BY PUMPING. 



First. Some of the rainfall sinks away into the 

 earth, where it in part runs off by springs, but in chief 

 part by slow evaporation, coming to the surface by 

 capillary attraction. This water which sinks into the 

 soil can be utilized as an important supply, adding 

 to the total only a small percentage, it is true, but 

 when measured in acres the amount is worthy of con- 

 sideration. Into the surface soils and rocks wells 

 may be sunk and the water may be pumped upon the 

 land for irrigation. The actual experience of man- 

 kind throughout the world exhibits the fact that mill- 

 ions of acres are thus cultivated. The tracts re- 

 deemed by single wells may be small, but such pump 

 wells in the aggregate furnish considerable quantities 

 of water, even in measures which irrigation requires, 

 by acre-inches and acre-feet. 



ARTESIAN WATERS. 



Second. Waters which sink away underground 

 are often carried to considerable depth, and may be 

 returned to the surface by hydraulic pressure under 

 proper geological conditions. These supplies are 

 known as artesian waters. In the practical opera- 

 tions of irrigation throughout the world, it is found 

 that artesian wells may be made to supply consider- 

 able quantities of water for irrigation. Single artesian 

 wells furnish on the average much greater quantities 

 than single pump wells; but artesian wells are suc- 

 cessful only under greater intervals of areal space. 



FLOODPLAIN WATERS. 



Third. Along the course of storm-water and pe- 

 rennial streams there is usually found a floodplain 

 a belt of country on either side of the stream which 

 receives the overflow from the stream when great 



