Density‐Corrected Models for Gas Diffusivity and Air Permeability in Unsaturated Soil

Abstract

Accurate predic on of gas diff usivity (D p /D o ) and air permeability (k a ) and their variaons with air-fi lled porosity (ε) in soil is cri cal for simula ng subsurface migra on and emission of climate gases and organic vapors. Gas diff usivity and air permeability measurements from Danish soil profi le data (total of 150 undisturbed soil samples) were used to inves gate soil type and density eff ects on the gas transport parameters and for model development. The measurements were within a given range of matric poten als (−10 to −500 cm H 2 O) typically represen ng natural fi eld condi ons in subsurface soil. The data were regrouped into four categories based on compac on (total porosity Φ <0.4 or >0.4 m 3 m −3 ) and soil texture (volume-based content of clay, silt, and organic ma er <15 or >15%). The results suggested that soil compac on more than soil type was the major control on gas diff usivity and to some extent also on air permeability. We developed a density-corrected (D-C) D p (ε)/D o model as a generalized form of a previous model for D p / D o at −100 cm H 2 O of matric poten al (D p , 100 /D o ). The D-C model performed well across soil types and density levels compared with exis ng models. Also, a power-law k a model with exponent 1.5 (derived from analogy with a previous gas diff usivity model) used in combina on with the D-C approach for k a,100 (reference point) seemed promising for k a (ε) predic ons, with good accuracy and minimum parameter requirements. Finally, the new D-C model concept for gas diff usivity was extended to bimodal (aggregated) media and performed well against data for uncompacted and compacted volcanic ash soil.Abbrevia ons: D-C, density-corrected; GMP, generalized macroporosity; MQ, Millington and Quirk; OM, organic ma er; WLR, water-induced linear reduc on.

A new mechanistic framework to predict OCS fluxes from soils

“…For undisturbed soils with high porosity such as volcanic ash, the expression proposed by Moldrup et al (2003; τ a = ε 1+3/b a /ϕ 3/b , where b is the pore-size distribution parameter) seems a better predictor (Moldrup et al, 2003). Recently a new density-corrected expression for undisturbed soils has also been proposed by Deepagoda et al ((2011); τ a = [0.2(ε a /ϕ) 2 +0.004]/ϕ) that seems to be superior to previous formulations and has the advantage of not requiring knowledge of the pore-size distribution parameter b. A summary of these different formulations of the tortuosity factor and their range of application is given in Table 1.…”

“…3). In highly compacted, highly aggregated soils the gas diffusivity response to soil moisture content can even become bimodal (Deepagoda et al, 2011), which would certainly have a strong impact on the V d −θ relationship. Even without such a complication, our results suggest that deposition rate measurements on repacked soils may not be representative of field conditions because the soil treatment would modify the diffusivity properties of the soil and alter the soil moisture response of the OCS deposition rate.…”

Physical-Chemical Quality of a Latossol Under Direct Seeding and Soybean-Corn Succession in the Cerrado-Amazonian Ecotone

“…However, the amplitude of Ma varied between 0.04 and 0.15 m 3 m -3 , considering the two layers of the soil. Therefore, there are sites in the area with macroporosity values well below 10%, possibly restricting soil air diffusion, respiration and root growth (DEEPAGODA et al, 2011), being one of the possible soil factors that is restrictive to productivity.…”

A modeling approach of the relationship between nitrous oxide fluxes from soils and the water-filled pore space

“…Generalized density-corrected model of Deepagoda et al (2011) Ds D0 ¼ 0:1 2 1 À WFPS ð Þ 3 þ0:04 1 À WFPS ð Þ h i a D 0 is the gas diffusion coefficient in free air (D 0 = 1.42 9 10 -5 m 2 s -1 at 20°C and 1 atm, Pritchard and Currie 1982), WFPS is the water-filled pore space, P is the soil total porosity (cm 3 cm -3 ), and b is the Campbell pore size distribution index Model parameterization…”