DUTY AND MEASUREMENT OF WATER. 1 47 



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 evapora- 

 tion is but 20 inches a year, a very small amount of 

 water is thus lost, but if it be stored where the evapo- 

 ration reaches the amount of 100 inches a year the water 

 loss is very great. It is to be remembered that in the 

 a(5lual 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 gen- 

 eral 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 prac- 

 ticed. 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 



