THE WATER OF SOILS. 



257 



the hot and arid west coast of India, despite its nearness to 

 the sea. The inland localities in the desert regions of South 

 Africa, Australia and Western Turkestan, show how enormous 

 may be the losses from evaporation of irrigation water, unless 

 the latter is applied with special care for their prevention. 

 Thus, with the wasteful methods of irrigation prevailing in 

 portions of the American arid region, it is certain that in 

 many cases 50% and more of the water evaporates before it 

 reaches the crops. 



Evaporation from Reservoirs and Ditches. The evapora- 

 tion from water-surfaces especially may, in many cases, ex- 

 ceed the rainfall of the year, so as to materially diminish the 

 available water-supply in reservoirs. Thus the annual evap- 

 oration from the reservoir-lakes forming part of the water- 

 supply of the city of San Francisco, ranges from 40 to 50 

 inches, while the rainfall averages less than 24 inches. Were 

 it not, then, for the prevention of evaporation by a covering of 

 dry earth during summer, no moisture would remain in the 

 ground to sustain vegetation. In the cool coast climate of 

 Berkeley, Cal., directly opposite the Golden Gate and subject 

 to its summer fogs, evaporation from a water-surface main- 

 taining the average climatic temperature of 60, was found 

 to be */4 inch during the month from the middle of July to the 

 middle of August, 1904. But at the high temperatures and 

 low degree of air-saturation prevailing in the great interior 

 valley, or in the Colorado desert, the evaporation from water- 

 surfaces is enormously increased, exceeding even the figure 

 given in the table for Bombay. Hence the great importance of 

 preventing all avoidable evaporation, particularly in the use of 

 irrigation water. 



Prevention of Evaporation; Protective Surface Layer. The 

 loose tilth of the surface which is so conducive to the rapid 

 absorption of surface-water, is also, broadly speaking, the best 

 means of reducing evaporation to the lowest possible point. 

 For while it is true that the floccules of well-tilled soil permit 

 of the ready access of air, and therefore of evaporation, it is 

 also true that these relatively coarse compound particles are 

 incapable of withdrawing capillary moisture from the denser 

 soil or subsoil underneath; just as a dry sponge is incapable 



