CHAPTER VII. 



MOVEMENTS OF SOIL MOISTURE. 



186. Types of Soil Moisture Movement. The moisture 

 which is found in the soil above the surface of the ground 

 water is continually subjected to three types of movement : 

 (1) Gravitational/ (2) Capillary and (3) Thermal; the 

 first due to the action of gravity^ the second to surface ten- 

 sion and the third to heat. 



When rain falls upon the soil one portion of it begins 

 to flow vertically downward through the pore spaces, urged 

 to do so by the pull of gravity; a second portion increases 

 the thickness of the water film surrounding the soil grains 

 and root hairs and is made to do so by surface tension; 

 while a third portion is returned to the atmosphere through 

 evaporation, caused by heat. 



GRAVITATIONAL MOVEMENTS. 



187. Percolation of Soil Moisture The direct gravita- 

 tional flow of soil moisture, which occurs during and after 

 rains, is nearly always vertically downward until the 

 ground-water surface is reached. The movement takes 

 place chiefly through the shrinkage cracks and passage- 

 ways left by the decay of roots and the burrowing of ani- 

 mals, but also through the capillary pores formed by the 

 grains of the coarser soils and by the granules of the finer 

 types. 



The rate of movement is most rapid following heavy 

 rains when the soil is already well saturated. After pro- 

 longed periods of drought, when the soil has become very 

 dry, there is so much air in the pore spaces that it greatly 



