• A piecewise D s / D 0 model is developed based on three distinct soil water regimes. • The model captures dynamic pore connectivity ignored by traditional methods. • Key parameters are estimated from soil texture and porosity. • Validation shows a 21.1–61.6% reduction in RMSE compared to existing models. • The model enables more accurate predictions of greenhouse gas fluxes. Soil gas diffusion coefficient ( D s ) is a critical parameter governing gas transport in unsaturated soils, affecting climate regulation, contaminant fate, and ecosystem functioning. Traditional models estimate the relative gas diffusion coefficient ( D s / D 0 , dimensionless, where D 0 is the gas diffusion coefficient in free air) as a function of air-filled porosity, neglecting changes in pore connectivity across moisture regimes. This study presents a novel regime-based model that partitions D s / D 0 into three domains derived from soil water retention characteristics: (1) a near-saturation regime, where gas diffusion is strongly inhibited due to disconnected air-filled pores; (2) a capillary-dominated regime where D s / D 0 decreases exponentially with decreasing air-filled saturation ( S a ); and (3) an adsorption-dominated regime where D s / D 0 decreases linearly with S a . The model integrates soil texture, porosity, and critical water thresholds, calibrated using D s / D 0 data from 48 soils (Soils 1–30, Table 1) spanning a wide range of textures and porosities. Validation with independent datasets (Soils 31-51c, Table 2) yielded a mean root-mean-square error (RMSE) of 0.019 for D s / D 0 estimates, representing a 21.1–61.6% improvement over established models. This improvement is particularly notable in capturing regime-dependent diffusion behavior. By advancing the predictive capability for gas transport in variably saturated soils, this framework facilitates more accurate modeling of greenhouse gas fluxes and contaminant volatilization.
Liu et al. (2026) studied this question.