Thermal Movements of Soil Moisture. 179 



208. Internal Evaporation of Soil Moisture. It is likely 

 that under certain conditions the thermal movements of 

 soil moisture may be considerable and perhaps of sufficient 

 importance to materially influence vegetation, directly or 

 indirectly. When the per cent, of unoccupied pore space 

 in a soil has been materially increased by the loss of wa- 

 ter and when the moisture films have become so thin that 

 capillarity is much enfeebled it is possible that internal 

 evaporation of soil moisture may result in a considerable 

 change of its position. If, for example, when the soil has 

 become quite dry, to considerable depths, the surface six 

 inches should become cooler than that below, the tendency 

 to continual diffusion of water vapor under the impulse 

 of heat would produce more internal evaporation of moist- 

 ure where the soil is warmest and most moist, and a larger 

 condensation of moisture where the soil is dryer and cool- 

 er. Even where there is little difference in temperature be- 

 tween adjacent layers of soil there must be, if they are not 

 equally saturated, a tendency for diffusion to take place 

 more rapidly from the wettest layer of soil toward that 

 which is least moist. It is possible that during dry times 

 and in dry climates during the dry season some moisture, 

 too far below the root zone to be made available through 

 capillarity, may be carried upward by these thermal or 

 evaporation movements so as to become helpful to crops in 

 a measure. We are yet lacking in experimental data to 

 form any just conception as to the magnitude of such a 

 movement. 



209. Temperature Influence of Hygroscopic Moisture. It 



Is Hilgard's view that, in dry climates and during droughty 

 periods in humid climates, the moisture still retained by 

 soils when capillarity has become very feeble may exert 

 an important influence in preventing the soil from becom- 

 ing overheated during dry soil conditions, by the cooling 

 effect of internal evaporation. It must be observed, how- 

 ever, that in order that this influence may become effective 

 the moisture evaporated must have left the soil and not 



