The Movement of Soil Moisture in Response to Temperature Gradients

Abstract

Using an air gap technique, it has been shown that the flow of moisture in the soil from warm to cool regions might occur largely in the vapor phase. This is accompanied by a return flow of liquid water in response to an induced flow potential gradient. The use of wire screens to make air gaps interferes with this process. The measured diffusion coefficient for vapor flow was higher than had been expected from diffusion data, but was substantially in agreement with that of other workers. The reasons for this high diffusion coefficient might be that the diffusion of moisture and heat is coupled in soils; the phenomena of surface migration and molecular hopping of adsorbed water might also cause this increase. The factors having greatest effect on the moisture distribution in continuous soil columns in a closed system are: (1) the diffusion and the convection of vapor or some related processes; (2) the evaporation and condensation occurring in the two end regions of the cylinders; (3) the hydraulic conductivity of the soil. The thickness of the air gap in broken columns influences the amount of water that will be transferred in the vapor phase and must be considered whenever an air gap is used to prevent fluid flow. The existence of two distinct drying and wetting moisture characteristic curves has been verified, and they play an important part in determining the liquid flow of moisture in the soil. The influence of this factor as well as the reason for the large vapor transfer need further study in relation to the transfer of soil moisture.

Heat and Moisture Transfer in Closed Systems of Two Granular Materials

“…A moisture content gradient is therefore set up in the direction of cold to hot, and liquid moisture may be returned to the hot side by capillary forces. Under steady-state conditions, a dynamic equilibrium exists between the two opposing processes, and the moisture is no longer uniformly distributed in the material (4,7,9).…”

“…2), the average ~noisture content in the test region is seen to be less than 2 per cent. Under the action of a temperature gradient, moisture moves from hot to cool regions (4,7,9). Thus the nloisture content at the cold side probably attained a value large enough to permit gravity drainage out of the test region, leaving an average moisture content in the test region of approximately 1.5 per cent.…”

Modelling of coupled heat and moisture flows around a buried electrical cable

“…The water vapour flux under thermal gradients is an important part of water flow in soil. Philip and De Vries [11] also added a mechanistic enhancement factor () into the equation of thermally induced water vapour flow to consider the discrepancy between the predicted transfers using Fick's law and the measured water fluxes [12,13]. Despite the importance of  in modelling of coupled heat and moisture flow, few works have been focused on evaluating it experimentally and quantitatively [14][15][16][17].…”

Water Vapor Diffusion Through Soil as Affected by Temperature and Aggregate Size

“…Early pioneering studies, including Taylor and Cavazza (1954), Rollins et al (1954), Kuzmak and Sereda (1957), and Matthes and Bowen (1963), have demonstrated that moisture movement in response to a thermal gradient through an unsaturated soil occurs mainly in the vapor phase. In fact, experimentally observed values for water vapor diffusion under thermal gradients are several times higher than those predicted by a simple diffusion equation (Philip and DeVries 1957).…”