THE WATER SUPPLIES IN THE ARID REGION. 



55 



age growing crop will require a mean supply of water 

 of 18 acre-inches. This will be called the absolute 

 duty of water. 



ABSOLUTE AND POSSIBLE DUTY. 



When water is applied to the land by pipes and 

 all possible precautions against evaporation are 

 taken the ground well prepared and the water ap- 

 plied in such a manner that there is no loss by over- 

 flow, no loss by seepage, and a minimum loss by 

 evaporation either by reason of mulching or shallow 

 surface cultivation, at least 2 acre-inches of water 

 will be lost by evaporation from the soil. We have, 

 then, the absolute duty of water as 18 acre-inches and 

 the possible duty of water as 20 acre-inches. 



THE PRACTICAL DUTY. 



For average crops, all water given to the land in 

 excess of this amount evaporates from the surface of 

 the land or runs away over the surface and under- 

 ground, and is therefore wasted. But all such waste 

 of water cannot be avoided except at an impractic- 

 able cost. In putting the water on the land some 

 amount must necessarily be evaporated. Under good 

 conditions of cultivation, therefore.it is believed that 

 6 acre-inches of water must be added to the 18 inches; 

 so that, in the western half of the United States, the 

 mean absolute duty of water, plus the practically un- 

 avoidable evaporation, is 24 acre-inches for every 

 average acre of crop. Otherwise stated, an acre of 

 growing crop will drink up by its roots and exhale by 

 its leaves an acre of water 18 inches deep during one 

 season, and in applying this water under economic 

 conditions an acre of water 6 inches deep must be 

 wasted by evaporation. The absolute duty of water 

 is 18 inches; the possible duty, 20 inches; the prac- 

 tical duty, 24 inches. 



EXPERIENCE SUSTAINS SCIENTIFIC INDUCTION. 



2. When the last census was taken it was found 

 that an area of about four million acres of land was 

 cultivated by irrigation in the western half of the 

 United States. These lands were scattered widely 

 over the whole region in small tracts; so that the ex- 

 perience of irrigatorsin this country is already exten- 

 sive and highly diversified in relation to latitude, 

 altitude, atmospheric humidity and crop. In taking 

 the census many other facts were collected, and 

 among these were the amounts of water actually sup- 

 plied to the lands by the farmers themselves. It was 

 generally found that the farmers are putting more 

 water on the land than is really necessary, and that 

 this extra amount is usually recognized by irrigators 

 as excessive. The excess occurs in part through 

 evaporation and in part by overflow onto other lands 

 not irrigated, and still another part is lost under- 

 ground by seepage. The general average in practice 

 was found to be about 30 inches. It was further 



found, however, that in a few places, where for eco- 

 nomic considerations great care was demanded, the 

 duty of water was actually brought down to nearly 20 

 inches. Thus the widely diversified experience of 

 the farmer is a reasonably approximate confirmation 

 of scientific induction. But this 20 acre-inches is 

 sufficient only when the water is carried in water- 

 tight canals or in pipes, and the ground is thoroughly 

 prepared for its reception, and the most advantageous 

 methods of applying the water are used. Excluding 

 the very exceptional cases, good practical irrigation 

 requires 24 acre-inches of water. Any amount addi- 

 tional to 24 inches put upon the land is not only 

 wasteful of water, but injurious to the crop, as will 

 hereafter be shown. 



The conclusion is thus reached that the mean abso- 

 lute duty of water for the arid region of the United 

 States is 18 inches, the mean possible duty 20 inches, 

 and the mean practical duty 24 inches; and all water 

 put upon the land in excess of 24 inches is an injuri- 

 ous duty. 



IMPORTANT QUALIFICATIONS. 



With regard to the duty of water, as heretofore 

 set forth, some very interesting and important quali- 

 fications should be made. The quantity of water 

 which has been given as necessary for growing 

 crops is for average crops, not for maximum crops. 

 It has been found by numerous experiments that the 

 growth of most crops can be greatly increased by 

 providing conditions for the utilization of a greater 

 amount of water. If the soil is underdrained and 

 properly prepared and the water supplied with the 

 proper fertilizers, a much larger production can be 

 realized than that which our farming usually secures. 

 The water supplied to the plant is the vehicle of the 

 materials wrought into the plant. Those who have 

 studied this subject with great care state that the 

 crop can be doubled, or even multiplied five-fold, by 

 properly supplying it with water and plant food. 

 But under these circumstances water supply must be 

 increased proportionately. Suppose the farmer 

 could control all the conditions, as the gardener can in 

 his hothouse covered with glass; then 150 or more 

 bushels of wheat, barley or corn could be raised on 

 an acre of land. But 60 to 100 inches or more of 

 water would be necessary. Again, it is found that in 

 order to secure the maximum growth of plants cer- 

 tain careful conditions of supply must be observed. 

 Some plants will grow in water; a few cultivated 

 plants will grow in marshes; but dry land crops will 

 be injured by an excess of water. To such plants 

 water must be supplied in minutely divided quanti- 

 ties and thoroughly aired. Soil will often contain 30 

 per cent, or more of its weight if completely satu- 

 rated, but in this condition dry-land plants do not 

 flourish. It is found that about 60 per cent, of satu- 



