EVAPORATION AT THE SURFACE 141 



highest percentage of water in the first foot lost 

 13.30 pounds of water, while the other lost only 8.48 

 pounds per square foot. This great difference was 

 due no doubt to the fact that direct evaporation 

 takes place in considerable quantity only in the upper 

 twelve inches of soil, where the sun's heat has a 

 full chance to act. 



Any practice which enables the rains to sink 

 quickly to considerable depths should be adopted 

 by the dry-farmer. This is perhaps one of the great 

 reasons for advocating the expensive but usually 

 effective subsoil plowing on dry-farms. It is a very 

 common experience, in the arid region, that great, 

 deep cracks form during hot weather. From the 

 walls of these cracks evaporation goes on, as from 

 the topsoil, and the passing Avinds renew the air so 

 that the evaporation may go on rapidly. (See Fig. 

 33.) The dry-farmer must go over the land as often 

 as needs be with some implement that will destroy 

 and fill up the cracks that may have been formed. 

 In a field of growing crops this is often difficult to 

 do; but it is not impossible that hand hoeing, ex- 

 pensive as it is, would pay well in the saving of soil 

 moisture and the consequent increase in crop yield. 



How soil water reaches the surface 



It may be accepted as an established truth that 

 the direct evaporation of water from wet soils occurs 



