PHYSIOGRAPHIC FACTORS 165 



new root hairs brings them in contact with new films and helps 

 to keep up the supply of necessary available water. 



Movement of water vapor is along vapor pressure gradients, 

 which are affected by temperatures and vapor pressures of the air 

 and the different soil horizons. There must, therefore, be some 

 movement in all soils, but its effects are most noticeable in semi- 

 arid regions where there is no connection between the water table 

 and capillary water near the surface. In winter or in any cool pe- 

 riod, water vapor moves upward from the warmer subsoil and 

 cools and condenses in the surface layers. When temperatures rise 

 at the surface, evaporation takes place into the air, and the total 

 ground water is reduced. Usually the surface soil is warmest in 

 summer and results in downward movement of vapor with con- 

 densation at lower levels. If the surface soil is cooler than the air 

 above it, water vapor may move into the soil and condense there 

 in quantities sufficient to be of significance under semiarid condi- 

 tions. 



Water Lost to the Atmosphere.— The loss of water from soil to 

 the air by evaporation varies with the factors affecting the steep- 

 ness of the vapor pressure gradient. Temperature, humidity, and 

 movement of the air, as well as temperature and moisture content 

 of the soil, are factors, which in turn are modified by exposure, 

 cover, and color of the soil. Probably the loss of water by evapora- 

 tion is much less than is commonly supposed, for numerous studies 

 indicate that there is little capillary rise to replace water lost by 

 evaporation unless the water table is within a few feet of the sur- 

 face. In those areas where water lost by evaporation might be 

 critical, the water table lies so deep that precipitation rarely wets 

 the soil down to it and, consequently, the upward rise is of no 

 consequence. In general, the loss of water by evaporation seems 

 mostly to be from the top foot of soil. Under natural conditions, 

 this probably affects few species and is rarely of significance. 



In agriculture, water lost by evaporation has been the subject of 

 much argument, particularly with regard to the effects of cultiva- 

 tion. Evaporation from a dry soil surface is much less than from a 

 moist one because diffusion through soil is very slow. Since a dry 

 soil surface can be moistened only by an upward capillary move- 

 ment of water if no rain falls, it has been maintained that cultiva- 



