A method to estimate the water vapour enhancement factor in soil

Abstract

Enhanced water vapour diffusion under temperature gradients has been proposed as a mechanism to explain the discrepancies between measured and predicted water fluxes in soils. Because of the difficulties in measuring soil vapour diffusion directly, modelling approaches have been used to estimate the vapour enhancement factor (η) by matching theory to measurements. In the method proposed by Hiraiwa & Kasubuchi (2000), soil thermal conductivity associated with conduction heat transfer (λ c ) is assumed to be equal to the apparent soil thermal conductivity (λ) measured at a low temperature, and η is significantly under-estimated. In the present study, an improved approach for estimating η is used, in which λ c is taken as the apparent soil thermal conductivity associated with infinite atmospheric pressure. The λ at infinite atmospheric pressure is estimated by extrapolating λ measurements made at finite air pressures. By subtracting λ c from measured λ values at a given atmospheric pressure, the contribution of thermal vapour diffusion to heat transfer (λ v ) is obtained and then used to estimate η. In the case of a lysimeter sand, λ v accounts for 4-25, 8-29 and 13-35% of λ at 3.5, 22.5 and 32.5°C, respectively, at soil water contents greater than 0.02 m 3 m -3 . Thus, the latent heat transfer through vapour diffusion is important even at temperatures as low as 3.5°C. The agreement between predictions from the new method and selected literature values suggests that the improved approach is able to provide accurate estimate of η. The results from this study show that the magnitude of latent heat transfer resulting from thermal vapour diffusion is strongly soil texture-dependent. Thus, it is important to estimate η on specific soils rather than assuming η from literature values.