330 Irrigation and Drainage 



PRINCIPLES GOVERNING THE WETTING OF SOILS 



When water is applied to a soil which becomes 

 more open in texture and coarser grained as the depth 

 below the surface increases, it will travel downward 

 in nearly straight lines, and will spread laterally but 

 very little except by the relatively slow process of 

 capillarity. This fact is forcibly illustrated in Fig. 

 82, where the experiment consisted in maintaining the 

 level of the water in a hole at the place designated by 

 the arrow until 200 cubic feet had percolated into the 

 soil. The heavily shaded area in the figure shows 

 the mass of soil completely filled with water on the 

 two dates, October 15 and 17, while the water was 

 running. It will be seen that although the hole was 

 kept full and the water-level within 8 inches of the 

 surface, the water did not spread sideways more than 

 2.5 feet until below a depth of 11 feet. 



If we imagine this to represent a cross -section of 

 the soil under a water -furrow extending across a 

 field, it will be readily seen how much water would 

 be lost by rapid percolation directly downward, and 

 how little, even after a long time, would have spread 

 laterally to wet the field. To irrigate such soils satis- 

 factorily and economically, the water must be spread 

 over the whole surface, or be led in furrows which 

 are near together across the field, so that the soil 

 between the furrows may quickly become wet. 



While the water is in the furrows, it will travel 

 sideways by capillarity fastest in those soils which are 

 coarsest, for the same reason that it flows downward 



