4 BULLETIN 447, U. S. DEPARTMENT OF AGEICULTURE. 



deptli to which they extend depend somewhat upon the manner in which the 

 soil has been dried. Where a close^drilled crop has been grown, they are small 

 and numerous and break the soil into small lumps to a depth of about 15 inches. 

 Below this depth the soil is generally compact and practically free from cracks, 

 no matter how dry it may be. 



When the soil becomes wet it expands, and thus the cracks are closed. When 

 any excess of the expanding force in the cracked layer occurs, the whole force 

 of expansion in the uncracked area has the effect of crowding the soil particles 

 closer together. For this reason the wet soil is always compact throughout and 

 free from open spaces. 



Considering these structural differences between the wet and the 

 dry soil, it can readily be seen that the moisture content of the soil 

 may have a great influence upon water movement: but a study of 

 water movement in both the wet and the dry soil is necessary in 

 order to obtain information as to how water penetration takes place 

 under actual field conditions. 



On the Belle Fourche project it has been found in field practice 

 that the water content of the surface soil at the time the water is 

 applied determines to a great extent the quantity that will be ab- 

 sorbed, especially when the water is applied rapidly. When both 

 the surface soil and the subsoil are dry, over an inch of rain, even if 

 it comes in a torrential manner, will be absorbed with very little 

 loss, because much of the water makes its way into the soil through 

 the cracks. On the other hand, when the surface soil is wet these 

 cracks are closed and a rain of as little as one-fourth of an inch may 

 be largely lost by run-off. Any rain falling on a wet surface must 

 fall very slowly in order to be absorbed. • 



That the condition of the surface soil determines the amount of 

 water absorbed is especially true when irrigation water is applied. 

 A comparatively small rain, by wetting the surface and causing the 

 cracks to close, often stops irrigation. There are times when it is 

 possible to irrigate successfully in the afternoon, when an attempt 

 to irrigate during the forenoon of the same day has resulted in the 

 run-off of practically all the water applied. 



These facts indicate that water movement through this soil when 

 wet is very slow. 



RATE OF MOVEMENT OF WATER IN LOOSE, SATURATED SOIL. 



In order to make actual determinations of the rate at which water 

 moves in this soil when it is saturated, the following experiment was 

 performed : 



Sections of blotting paper were fitted, as bottoms, into a number of cans 

 that were open at both ends. Each can was filled with a composite sample 

 of a foot section of soil and then immersed in water. After the soil had 

 become thoroughly saturated the cans were removed from the water and placed 

 upon a screen. All the soil was then removed except a 3-inch layer in the bot- 

 tom of each can. 



