SOURCE OF GROUND WATER. 415 



This rule is well illustrated by the arid and semiarid regions, where a 

 very large portion of the run-off is that of the precipitation of the moun- 

 tain regions. Whitney estimates that in the level districts of the arid region ' 

 of the United States, even where the rainfall is as high as 50 cm., only 10 

 per cent, or 5 cm., of the rainfall passes into the run-off." 



The cultivation of the soil is another of the secondary factors which 

 has an important influence in the amount of run-off. Proper soil cultivation 

 produces a rough surface having many minor depressions arranged in such 

 a way as to have the channels follow the contours. Also, cultivation 

 produces numerous large openings in the upper 5 to 30 cm. of the soil. 

 As a result, when precipitation comes the depressions hold the water. 

 From these depressions it makes its way easily into the large openings. 

 The big- openings give a large surface from which the water can make its 

 way into the smaller openings of the soil. Therefore, in cultivated soil a 

 much larger portion of the water makes its way into the soil than under 

 natural conditions. The run-off is greatly decreased and the water for 

 circulation in the belt of weathering' is greatly increased. 



It will be seen later, in considering the belt of cementation, that the 

 run-off includes both the water which never goes below the surface of the 

 ground and that which issues from the belt of cementation. Much the 

 larger part of the water of the belt of cementation passes through the belt 

 of weathering on its way to the belt of cementation. All of the water not 

 included in the run-off has its entire circulation in the belt of weathering. 

 In the United States it follows, from the figures above given, that the 

 entire circulation of about 50 per cent of the precipitation in the more 

 humid regions is confined to the belt of weathering. In the less humid 

 regions from 53 to 75 per cent is confined to this belt. In the semiarid 

 regions from 75 to 90 per cent is confined to this belt. In the arid regions 

 from 90 to 100 per cent of the precipitation has its only ground circulation 

 in the belt of weathering. It is therefore clear that the proportion of the 

 water of precipitation concerned in the circulation of the belt of weathering 

 is much greater than that concerned in the circulation of the belt of cemen- 

 tation; for the belt of weathering has exclusive control of from 50 to 100 

 per cent of the total precipitation, and nearly all of the water of the belt 

 of cementation first circulates in the belt of weathering. 



« Whitney, Milton, Conditions in soils of the arid region: Yearbook of the Dept. of Agric, 1894, 

 pp. 157-159. 



