DUTY AND MEASUREMENT OF WATER. 113 



Evaporation. Throughout the arid region of the 

 United States the conditions which determine the amount 

 of evaporation are exceedingly variable, and it ranges 

 from a probable minimum of 20 inches to a probable 

 maximum of 105 inches per annum. If water, therefore, 

 be stored in artificial lakes, where evaporation is but 20 

 inches a year, a very small amount of water is thus lost, 

 .but if it be stored where the evaporation reaches the 

 amount of 100 inches a year the water loss is very great. 

 It is to be remembered that in the actual application of 

 water unnecessary slowness of flow induces increased 

 evaporation and absorption, and hence it is that in the 

 flooding system the quick head sharply applied gives the 

 best results. 



On the average all cultivated plants will exhale each 

 day a quantity of water equal to the dry growth of the 

 plant for the .year. The time of growth varies from 

 seventy-five to one hundred and fifty days, but in general 

 the plant requires for good growth about one hundred 

 times as much water as the yearly growth when dried. 

 This is equal to eighteen inches in depth, which therefore 

 may be called the absolute duty of water. To this must 

 be added one-third for seepage and evaporation. But to 

 calculate more readily the Colorado irrigators will usually 

 estimate that twenty-one total acre inches are sufficient 

 for a season's water supply for ordinary crops, and the 

 writer is inclined to favor this estimate as being about 

 right. 



The duty of water is constantly increasing in nearly 

 every portion of the country where irrigation is practiced. 

 To-day in Colorado some engineers and canal companies 

 are making the standard of duty nearly double what it 

 was formerly. But the crops grown, the system used 

 and the means of applying water, all cut a very impor- 

 tant figure. As we have already indicated, where flooding 

 takes thousands of gallons the furrow system only re- 

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