PHYSIOGRAPHIC FACTORS 163 



snow is the ultimate source of water found in the soil, but not all 

 precipitation becomes soil water. The steeper a slope, the more 

 water will run off from its surface before it can enter the soil. 

 Excessive precipitation in a short period of time results in greater 

 runoff than that following a gentle rain, since infiltration cannot 

 keep pace with the rate of fall. If soil becomes saturated and pre- 

 cipitation continues, little, if any, will enter the soil. A larger pro- 

 portion of water from slowly melting snow is apt to enter the soil 

 than from an equal amount of rain. Infiltration into a fine-textured, 

 clayey soil is slower than into a coarse-textured, sandy soil, and a 

 compact mineral soil absorbs water more slowly than a loose soil 

 or one with a high organic content or heavy litter. The particles 

 of a bare mineral soil tend to pack at the surface when rained upon 

 for only a few minutes and thus reduce the rate of infiltration (see 

 Fig. 39). Variation of local conditions may, therefore, modify the 

 effectiveness of a given amount of precipitation. 



Movement of Soil Water.— Water moves downward in quantity 

 during and immediately after rain or irrigation. Later it may move 

 upward or laterally to some extent when evaporation and use by 

 plants reduces the amount near the surface. Its principal movement 

 occurs as a liquid in capillary films or through noncapillary pores, 

 but some movement also occurs in the form of vapor. Gravity, 

 hydrostatic pressure, and capillary action are the forces involved, 

 and movement may be the result of interaction of all three. 



The rate at which infiltration takes place is at first determined 

 by surface conditions. When they are favorable, practically all of 

 a light rain is absorbed. Within a half hour or less, however, ab- 

 sorption declines and is then controlled by conditions in the lower 

 horizons, where percolation may be very slow. Movement of 

 gravitational water through the soif is controlled by the number, 

 size, and continuity of the noncapillary pores through which it 

 percolates. Drainage is rapid in coarse-textured soils, but in clays 

 movement is slow since the pores are small and may be blocked by 

 the swelling of colloidal gels or by trapped air. Channels left by 

 earthworms or other animals and those left by dead roots greatly 

 facilitate downward movement. If there is no impermeable hard- 

 pan layer and if the water table has not been raised too near the 

 surface, all gravitational water drains from surface strata within 



