THE WATER SUPPLIES IN THE ARID REGION. 



FIRST PAPER: "THE DUTY OF WATER AND THE SOURCES OF SUPPLY." 

 BY J. W. POWELL, DIRECTOR OF THE U. S. GEOLOGICAL SURVEY. 



IN the western half of the United States agriculture 

 is in part dependent upon irrigation, or the artifi- 

 cial supply of water. The extent to which such agri- 

 culture can be carried on depends, first, upon the 

 amount of water which a growing crop requires, and, 

 second, upon the amount of water which can be arti- 

 ficially supplied. In considering these questions it 

 is necessary to use some unit for the measurement of 

 water, and for this purpose one which is simple and 

 practical is readily found. Land is usually cultivated 

 by acres, and water can be measured in terms of 

 acres. An acre of water one inch deep may be known 

 as an acre-inch, and an acre of water one foot deep 

 may be known as an acre-foot. The first problem to 

 be solved is this: How many acre-inches of water 

 are necessary for the adequate supply of an acre of 

 growing crop for one year? There are two methods 

 by which this can be determined, and it is found that 

 substantially the same conclusion is reached by either 

 method. 



I. THE AMOUNT OF WATER 

 REQUIRED. 



1. For more than a century scientific men have been 

 engaged in determining the amount of water which 

 various plants will consume through their roots and 

 exhale through their leaves, the process being known 

 as transpiration. This investigation has been pur- 

 sued by various methods and by different men, and 

 a common general result has been obtained. It has 

 been found that different kinds of plants require vary- 

 ing amounts of water. Deciduous trees require more 

 than coniferous trees with needle-shaped leaves. In 

 general, grasses, vegetables, cereals, and fruits re- 

 quire a relatively large amount of water, as will be 

 seen by the following statements. 



Grass growing in turf will transpire in one day a 

 weight of water a little greater than the weight of the 

 dried grass. 



Many vegetables will exhale in one day an amount 

 of water as great or greater in weight than the dried 

 plants. 



Cereals, such as wheat, oats, barley and corn, will 

 exhale their dry weight in water every day. 



Perennial vines and trees that bear fruit, such as 

 the grape and the apple, will exhale every day a 



weight of water equaling the weight of the dry growth 

 of the year. 



Assembling these facts, the following general state- 

 ment can be made: All average cultivated plants 

 will daily exhale an amount of water equal to the 

 dry growth of the plant for the year. This growth is 

 effected in varying times with different plants. Some 

 plants continue their growth for 75 days, others for 

 150, or even longer; but in general the plant requires 

 for good growth water amounting to about one hun- 

 dred times the weight of its yearly growth when 

 dried. Thus a ton of hay requires 100 tons of water 

 for its growth. The hay is not perfectly dry, but the 

 loss by complete drying about equals the weight of the 

 dry stubble and roots. An acre-inch of water weighs 

 226,600 pounds, or about 11% short tons. Two tons 

 of hay require 200 tons of water, which is about 13 

 acre-inches. If the crop of hay on an acre is two tons 

 the acre of grass will transpire 18 acre-inches of 

 water. 



THE DUTY OF WATER. 



We thus turn proportions into measured quantities 

 by acres of crop and acre-inches of water, and we 

 have a statement of the acre-inches of water which it is 

 necessary to supply to an acre of growing crops for 

 one year, which may be called the duty of water. 

 This duty of water, then, as here defined, is the 

 amount of water in acre-inches which will be re- 

 quired by an acre of growing crop for one year. In 

 stating this duty of water it will be given for an av- 

 erage growth, not for the maximum growth, as will 

 afterward be explained. 



WHEN THERE IS VARIATION. 



There are variables to be considered in this prob- 

 lem; that is, the duty of water will depend upon 

 latitude, altitude, humidity of the air with clearness 

 of sky and kind of crop. In northern latitudes 

 plan s need less water than in southern; in higher 

 altitu les plants need less water than in lower; in 

 more humid conditions of the air plants exhale less 

 water than in arid conditions, though there seem to 

 be some curious exceptions to this; and, finally, some 

 plants require more water than others. But these 

 variations are not so great but that they may be 

 safely neglected for the general statements herein 

 proposed, and it may be stated that an acre of aver- 



