EXPERIMENT STATION BULLETINS. 



490 



EFFECT OF TEMPERATURE ON THE MOVEMENT OF WATER 

 VAPOR AND CAPILLARY MOISTURE IN SOILS. 



Technical Bulletin No. 22. 



MOVEMENT OF MOISTURE FROM WARM TO COLD COLUMN OF SOIL OF UNIFORM 



MOISTURE CONTENT. 



The surface tension and viscosity of water decrease with rise in tem- 

 perature to the extent shown by the data in table 1. 



TABLE I.— RELATION OF TEMPERATURE TO THE SURFACE TENSION AND VISCOSITY 



OF WATER. 



It will be noled that the degree of diminntion with rise in temperature 

 is considerably greater in the case of viscosity than in surface tension. 



During the warm part of the year the soil at the upper depths main- 

 tains a rather marked temperature gradient which reverses itself between 

 day and night to the depth that the diurnal amplitude of temperature 

 oscillation extends. This diurnal change of temperature gradient occa- 

 sions an alteration in surface tension and viscosity of the soil moisture, 

 (he amount depending- upon its variation at the different depths. Since 

 capillary action is said to depend upon surface tension, and facility of 

 movement upon viscosity, then there should occur an upward and down- 

 ward movement of moisture as the temperature gradient changes diurnal- 

 ly. During the day, for example, the temperature of the soil is highest 

 at the surface and diminishes Avith depth; the surface tension and the 

 viscosity of soil moisture, are lowest at the surface and rise with depth; 

 consequently the movement of moisture should be downward. During the 

 night, the reverse is true; the soil temperature is lowest at the surface 

 and increases with depth; the surface tension and the viscosity of the soil 

 water are greatest at the top and diminish doAvnward with increase of 

 temperature ; hence, the w^ater translocation should be upward. 



These considerations are a priori deductions from the laws of surface 

 tension and viscosity in their relation to temperature. Whether or not 

 they are valid, however, is heretofore not knowm; since there appear to be 

 no experimental data bearing directly upon the subject. 



With the object in view of ascertaining this important and much de- 

 sired information an investigation of the problem was undertaken. The 

 general method of procedure consisted of placing soil of different but 



