CIRCULATION OF GROUND WATER, 417 



The influence of molecular attraction is to draw the water from areas 

 of more moisture to areas of less moisture. Since the water falls iipon the 

 surface, the first tendency of this force is, like that of gravity, to draw the 

 water from the surface into the soil and downward. However, there are 

 many modifications of this general tendency. Immediately after precipi- 

 tation molecular attraction works very effectively with gravity in carrying 

 the water downward, but after rainfall has ceased, evaporation near the 

 surface very frequently results in the surface soil containing less moisture 

 than the subsoil or the deeper parts of the belt of weathering. Under these 

 circumstances the force of molecular attraction draws the water from below 

 the surface toward the surface, in opposition to gravity. Also, whenever 

 the belt of weathering is somewhat deficient in moisture, molecular attrac- 

 tion tends to draw water from below the level of ground water into the belt 

 of weathering. These movements are either upward or downward; but 

 whenever local conditions produce lateral variations in moisture, molecular 

 attraction tends to give the water a horizontal component from places of 

 more moisture to places of less moisture. 



Wherever roots are present they absorb water from the immediately 

 adjacent soils, and carry it surfaceward. The roots extend both vertically 

 and laterally, and hence the movement of water due to roots is both lateral 

 and vertical. Where roots are present, on account of the absorption of 

 water the soil adjacent to the roots is deficient in amount. As soon as this 

 deficiency appears the force of molecixlar attraction carries the water 

 from places of more moisture to those places of less moisture, and thus 

 replenishes the supply. 



The downward and upward movements of water controlled by the 

 above forces are of such consequence that they are entitled to separate 

 consideration. 



DOWNWARD MOVEMENTS OF WATER. 



As has been seen, the general forces producing downward movement 

 of water are gravity and molecular attraction; with these forces mechanical 

 movement may cooperate. The results which these forces accomplish are 

 very different under different circumstances. Some of the most important 

 of the variable circumstances affecting the downward movement of water 

 are the irregularities in precipitation and the amount of water in the soil 

 and subsoil. 



3ION xlvii — 04 27 



