26 



IRRIGATION PRACTICE 



contains more clay or other fine particles than the top 

 soil, and therefore has a higher water-holding power per 

 unit weight of soil. When the thickness of the soil-mois- 

 ture film is considered, there is, immediately after an 

 irrigation, a steady diminution with increasing depth. 

 In soils of approximately uniform structure, this is well 

 brought out, as in the following table, taken from the 

 work of the Utah Station. In this table are shown the 

 percentages of water in each foot to a depth of 8 feet, 

 one or two days after irrigations of various depths, and 

 in early spring after the winter precipitation has dis- 

 tributed itself. 



This distribution of soil moisture may be explained 

 as follows: Water added to a soil first saturates the upper 

 soil layers thoroughly, and there is a tendency to keep 

 the top soil as near as possible to this point of maximum 

 capillary saturation. Then, the lower drier layers begin 

 to draw water downward. The wettest particles are near 

 the top; the lower particles are all attracting the water. 

 As water moves downward through the thin capillary 

 film, friction has to be overcome. The farther the parti- 

 cle is away the more friction must be overcome. The 

 water above the point of lento-capillarity therefore 



