WATER CIRCTLATinN AND ITS CONTROL 



34 T 



not too open. The films of water with- 

 in it are then of such size and are so 

 distributed that they support the open 

 texture. When rain falls it enters each 

 little capillary tube between the grains 

 and penetrates rapidly in the many 

 minute channels. Thus the soil, though 

 moist, absorbs more water than if it had 

 been drier to begin with. 



When soil is very dry and powdery 

 rain does not penetrate far into it. 

 There are several conditions that pro- 

 duce this well-known result. Much 

 water is used in filling the large volume 

 of the small spaces between the soil 

 grains ; the capillary films are still 

 separate at the bottom of the sinking 

 moisture and they sink deeper only as 

 they join, and the air in the soil, being 

 imprisoned by the water that fills the 

 upper layer, offers effective resistance 

 to percolation. 



The absorbent character of a soil 

 determines the amount which the im- 

 mediate surface will take in, but it does 

 not determine the rate at which water 

 will soak into the lower layers of the 

 soil and subsoil; if the latter is imper- 

 meable, the pores of the surface soil 

 may become so filled that no more 

 water can enter, and the rest of the rain 

 runs off. This happens when a heavy 

 rain falls on a field that has been 

 plowed only three or four inches deep, 

 leaving an unbroken, clayey subsoil. 

 If the field is level, pools of water will 

 stand over it till they dry up. If the 

 field be on a slope, the gathering water 

 runs off along and across the shallow 

 furrows and takes more or less soil 

 with it. Where the subsoil is broken to 

 twelve or fourteen inches by deep disk 

 plowing the same heavy rain sinks in 

 completely and is stored for the crops 

 when needed. The soil also stays. 



Thus on plains the conditions which 

 affect percolation of rain into the soil 

 and subsoil and which are consequently 

 to the advantage of ground-water stor- 



age are 



(a) That the cultivated surface 

 shall be level or but gently sloping; 



(6) That the surface soil shall be 

 in that condition which is best described 

 by the gardener's term of "live soil ;" 



(c) That the lower soil and subsoil 

 should afford the largest possible 

 amount of space into which the water 

 from the surface may run. 



Another means which is at the fann- 

 er's disposal is that of crop rotation. 

 Clean cropping year after year exhausts 

 the plant food and hunnis of the soil, 

 affects unfavorably the mechanical con- 

 dition of the soil, and brings it into a 

 state in which its fertility is greatly re- 

 duced and it is most easily eroded. In 

 order that these effects should be pre- 

 vented it is important to introduce a 

 rotation of crops, including grass or 

 clover, whose deep and thickly matted 

 roots help to open up the texture of the 

 soil and to restore that essential constit- 

 uent, humus. 



We have thus far considered only 

 how the farmer can produce a condition 

 of the soil and subsoil which will reduce 

 run-off and increase ground storage. 

 It is also important when the water is 

 in the ground to keep it there, except 

 as crops need it. It should not be 

 wasted by useless plants nor by evap- 

 oration. 



The waste by weeds is obvious. A 

 growing weed takes water which the 

 useful crop might have. The remedy- 

 keeping out weeds is equally obvious 

 and is limited simply by cost. 



Evaporation exhausts ground water 

 most effectively when there is free cir- 

 culation from the ground water to the 

 surface. This happens when the films 

 of water about the soil grains are in 

 contact and there is ready movement in 

 them. As the water immediately at the 

 surface is vaporized an additional sup- 

 ply is drawn to the surface through the 

 capillary films from below. This is 

 evaporated in turn and more is drawn 

 up, and so through evaporation and 

 capillary attraction the soil moisture is 

 gradually dissipated into the air. 



That condition of dryness which 

 stops the upward movement of ground 

 water by breaking the continuous water 

 films is not readily established in humid 

 regions, where the ground water lies 

 not far below the surface and rains are 

 not infrequent. Thus it appears that 

 since in humid regions the farmer can- 



