Wadleigh and Richards 419 



moisture always streams through soil in the direction of the decrease in 

 hydraulic head. It is apparent that the hydraulic head at cup b is higher 

 than at cup a. Therefore, water is moving upward in the soil layer a— b 

 as indicated by the arrow. 



In a soil region where the hydraulic head is everywhere the same, 

 water will be at rest under gravity. The tension gradient will be of such 

 size and direction that it will just cancel gravity. The hydraulic head 

 at cups b and c is the same, so that on the average the hydraulic gradient 

 in this interval is zero. The hydraulic head at cup c is higher than at 

 cup d, so that in the interval c— d water is moving downward, as indi- 

 cated by the arrow. 



The hydraulic gradient is the change in head per unit distance along 

 a flow line. So in the interval a— b we have for the hydraulic gradient 

 /. lb — H. {h /L. th . In the soil interval b— c the average hydraulic gradient 

 is zero, because H bc is zero. This should not be interpreted to mean 

 that the transmission velocity at b and at c is zero, but rather that there 

 is a soil layer between a and b above which the moisture is moving up 

 and below which the moisture is moving down. In the lower soil 

 interval, i c& = H cd /L c6 and is in the opposite direction to /.,,,. Thus, the 

 direction in which water is streaming through soil can be predicted 

 from hydraulic head measurements. 



Darcy's law for water movement in soil has long been recognized 

 to hold for saturated soils. Its application to unsaturated soils is not so 

 familiar even though the definition of hydraulic head and hydraulic 

 gradient for the unsaturated case as just given are substantially identical 

 to the saturated case. The only difference lies in the fact that in unsatu- 

 rated soils some of the pores are filled with gas and are not available for 

 transmitting water. 



As the soil dries out and the pore spaces become filled with air, 

 moisture movement must take place in the water films over the sur- 

 face of the soil particles. Thus, the permeability changes with the 

 moisture content of the soil. For example, it is found that for a sandy 

 soil which is saturated with water, the soil transmits 8 cc. of water per 

 square centimeter per hour at unit hydraulic gradient. The permeabil- 

 ity is thus 8 cm. per hour. At a soil moisture tension of 100 cm. of water, 

 which for this soil corresponds approximately to the condition of field 



