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

Authors: Jabro, Jalal - Jay

Submitted to: Transport in Porous Media
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2008
Publication Date: 4/1/2009
Citation: Jabro, J.D. 2009. Water Vapor Diffusion through Soil as Affected by Temperature and Aggregate Size. Transport in Porous Media. 77: 417-428.

Interpretive Summary: Water vapor diffusivity through dry soil in response to a concentration gradient increases with increasing soil porosity and rising temperature. The dry soil in this experiment absorbed from 1/8 to 2/3 of the diffusing water. Maximum absorption rates occurred with the most compact soil samples at the highest temperature, though the maximum absorption as a percentage of the diffusing water was in the compact samples at the lowest temperature. Equations derived to determine diffusion coefficients from these data include a sink term to account for the absorbed water and for the variations in the amount of water diffusing through different positions within the soil samples. Even the water that diffused all the way through the soil samples must have been slowed by temporary sorption on soil particles. It may be inferred from the data that the diffusing water molecules in this experiment spent 2 to 2.5 times as long in sorbed positions as they did in actual diffusion. The diffusivity equation D/D0 = [(S -0.1)/0.9]2 fit the D/D0 values obtained from these data if a coefficient of 1/3 or 1/3.5 is added to correct for the time delays caused by temporary sorption of the diffusing water vapor. The results are influenced by the interaction of water vapor with the soil materials so that the data from this study represent an apparent diffusion rate that is slower than the rate that would have occurred without such interaction. The movement of water vapor in the soil plays an important and critical role in the overall water and energy balance of the surface environment of arid and semiarid regions in many agricultural, meteorological and engineering applications.

Technical Abstract: Water vapor diffusion through the soil is an important part in the total water flux in the unsaturated zone of arid or semiarid regions and has several significant agricultural and engineering applications because soil moisture contents near the surface are relatively low. Water vapor diffusing through dry soil is absorbed for both long and short terms. Long-term absorption allows more water to enter than exit the soil, as reflected in the concentration gradient. Short-term absorption leads to an apparent reduction in the diffusion rate, as reflected in the diffusion coefficient. This investigation studied the effects of soil temperature and porosity on the isothermal diffusion of water vapor through soil. The diffusion model consisted 25.4 cm ' 8.9 cm ' 20.3 cm Plexiglas box divided into two compartments by a partition holding a soil reservoir. Water vapor moved from a container suspended by a spring in one compartment, through the porous medium in the center of the model, to calcium chloride in a container suspended by a spring in the other compartment. The porous materials consisted of aggregates of varying size (2 - 2.8, 1 - 2, and 0.5 - 1 mm) of a Fayatte silty clay loam (a fine-silty, mixed mesic Typic Hapludalf). The flow rates of water vapor were measured at temperatures of 10, 20, 30, and 40' C. Warmer temperatures increased the rate of diffusion through dry soil while reduced the amount of water absorbed by that soil. Reducing porosity slowed the rate of diffusion and increased the amount of water absorbed. The dry soil in this study absorbed from 1/8 to 2/3 of the diffusing water. Maximum absorption rates occurred with the most compact soil samples at the highest temperature, though the maximum absorption as a percentage of the diffusing water was in the compact samples at the lowest temperature. The diffusivity equation D/D0 = [(S - 0.1)/0.9]2 fit the D/D0 values obtained from these data if a coefficient of 1/3 or 1/3.5 is added to correct for the time delays caused by temporary sorption of the diffusing water vapor. The data, influenced by the interaction of water vapor and soil materials, represent a diffusion rate lower than the diffusion rate that would have resulted without this interaction.