252 SOILS AND SOIL WATER RELATIONS 



but to the greatest height in cla}' soils, and most rapidlj' but to lesser heights 

 in sandy soils, loam soils occupying an intermediate position. 



In many soils the water table lies so far below the soil surface that it has 

 little or no effect on the soil moisture conditions in the soil layers which are 

 penetrated by the roots of most plants. Generally speaking even in loam or 

 clay loam soils, a capillary rise of water from a zone of complete saturation 

 is probably ineffective in providing the roots of most species of plants with 

 any considerable part of the water which they absorb unless the water table 

 is within about 15 feet of the surface. Some of the more deeply rooted species 

 may obtain some water from a water table located at depths as great as about 

 30 feet, but the presence of a water table at greater depths than this is a 

 negligible factor in supplying the roots of any species of plants with water. 

 A capillary rise of water from a water table is usually an important source of 

 water for plants in such locations as river bottomlands or in fields or forests 

 in close proximity to ponds or lakes. In many and perhaps most agricultural 

 soils in moist climate regions plants obtain some water from the water table 

 at least during the earlier part of the growing season. However the wide- 

 spread practice of drainage of agricultural regions has often resulted in such 

 a marked lowering of the water table as to greatly reduce the possibility that 

 capillary rise of water from below will supply crop plants with any significant 

 part of the water which they absorb. 



The other important source of soil water is that part of the precipitation, 

 usually rain, although often melting snow or ice, which percolates downward 

 into the soil. In dry regions this is the only source of water available to 

 plants. Even in more humid regions this is the principal or only source of 

 available soil water in most soils during the dry season of the year. Not all 

 of the rain which falls on the surface of a soil penetrates into it. Some may 

 be lost by surface run-off and a portion usually evaporates before it can sink 

 into the soil. The proportion of the precipitation which is lost in the run-off is 

 in general much less with open, porous soils such as those found in most forests 

 than with those of a lower porosity. A smaller proportion of the water is 

 lost in the run-off when it falls as a gentle rain than when it comes as sudden 

 downpour. The proportion of the precipitation which is lost by evaporation 

 from the surface layers of the soil depends in part on the color, texture, 

 porosity, and other properties of the soil, and in part on the temperature and 

 vapor pressure of the atmosphere, as well as upon the impinging solar 

 radiation. 



Let us now consider how percolating rainwater becomes distributed in a 

 soil. We will assume that the soil under consideration is approximately air 

 dry, that it is essentially homogeneous to a depth of several feet, and that 



