CONDITIONS WHICH PERMIT EVAPORATION 197 



from the soil, is only a small proportion of the annual 

 rainfall of most of the cultivated sections. These losses 

 are therefore preventable; and that this is true is 

 exemplified by the large difference in average crop 

 yield on those lands where the best conservation prac- 

 tices are in vogue over those where they are neglected. 

 It should be remembered that over the vastly larger 

 proportion of cultivated land area the crop yields are 

 controlled more directly by the lack of water than by 

 the excess of water. It is a common observation that 

 soils which ordinarily give a low yield in seasons of. 

 normal or low rainfall give good yields in wet season, 

 indicating how large a dominating factor is the moisture 

 supply. For the moisture concerns not only its direct 

 use as a food and carrier for the plant, but by its influence 

 on solution, and other essential conditions of plant 

 growth, its is a chief dominating factor in growth. 



Soil evaporation occurs almost entirely at the surface. 

 Exception may be made where evaporation occurs into 

 large, deep cracks in heavy clay soil, which is the primary 

 source of subsoil loss in such cases. If this be prevented, 

 as it may be, the loss will be very small. Since evapora- 

 tion is chiefly at the surface, the nearer the available 

 store of moisture is held to the surface, the larger pro- 

 portionate loss will occur. This principle has its appli- 

 cation in the amount and distribution of the rainfall 

 or irrigation. Frequent small rainfalls are much less 

 effective than less frequent rains in larger amounts. 

 For if the rainfall or irrigation produces only shallow 

 percolation before the water assumes capillary forms, 

 it may be quickly returned to the surface, and lost. 



