Water Vapor Diffusion Through Soil as Affected by Temperature and Aggregate Size |
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Authors: | Jay D Jabro |
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Institution: | (1) Northern Plains Agricultural Research Laboratory, USDA-ARS, 1500 N. Central Avenue, 59270 Sidney, MT, USA |
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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 of
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/D
0 = (S – 0.1)/0.9]2 fit the D/D
0 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.
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An erratum to this article can be found at |
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Keywords: | Diffusion coefficient Absorption Thermal gradient Fick’ s law |
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