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The Movement of Soil Moisture in Response to Temperature Gradients¹

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.

¹ Presented at the meetings of the Western Society of Soil Science, Santa Barbara, Calif., June 17, 1953.

² Associate Professor of Agronomy, Utah State Agricultural College, Logan; and formerly graduate student Utah State Agricultural College, now assistant in Agronomy, University of Bari, Italy, respectively.

Received for publication April 26, 1954.