Paddy ecosystems constitute a dominant source of greenhouse gases, particularly of methane (CH₄), due to the continuous flooding (CF) practiced under conventional paddy cultivation. A new management method, namely alternative wetting and draining (AWD) (i.e., flooding whenever surface water levels decline to 15 cm below the soil surface), is an emerging practice developed to mitigate CH₄ emissions while providing an optimal solution for freshwater scarcity. Despite extensive paddy cultivation in Sri Lanka, no systematic research study has been conducted to investigate CH₄ emissions under different water management practices. Thus, field experiments were conducted in Sri Lanka to investigate the feedback of controlled water management on seasonal and diel variation of CH₄ emission, water consumption, and crop productivity. Adopting the same rice variety, two water management methods, continuous flooding (CF) and alternative wetting and draining (AWD), were compared with plants (W/P) and without plants (N/P) present. The emission of CH₄ was measured using the static closed chamber method. The results show a 32% reduction in cumulative CH₄ emission, on average, under AWD when compared to CF. The yield under the AWD was slightly higher than that of CF. Although it was not statistically significant (p > 0.05) there was not any reduction in yield in AWD than in CF. The total water saving under AWD ranged between 27–35% when compared to CF. Thus, the results support (without considering the effect of nitrous oxide) AWD as a promising method for mitigating CH₄ emissions while preserving freshwater and maintaining grain yield in paddy systems.

Soil gas diffusivity ( D _p / D _o , gas diffusion coefficients in soil and in free air, respectively) and its relation to soil moisture is of great importance for describing and quantifying essential provisional and regulatory functions associated with terrestrial ecosystems such as soil aeration and greenhouse gas (GHG) emissions. Because gas migration in terrestrial soil systems is predominantly diffusion controlled, soil gas diffusivity becomes a fundamental prerequisite to quantify diffusive gas fluxes. Descriptive–predictive models are often used to estimate D _p / D _o from easily measurable soil physical properties. Most of the available models take the form of power‐law functions and often tend to mischaracterize soil moisture effects at high moisture regimes. Based on a wide range D _p / D _o data available in literature representing both intact and repacked soils, this study developed a novel air‐saturation‐dependent exponential (ASEX) gas diffusivity model to model D _p / D _o in relation to soil air saturation. The model variable α, which represents the diffusivity at half air saturation normalized by the same in complete soil air saturation, could potentially differentiate moisture effects on different soil structural states. For specific applications in intact soils, we propose corresponding α values for upper‐limit (α = .6) and lower‐limit (α = .05) estimates of diffusivity, while an average value (α = .3) for general applications in both intact and repacked soils. As expected, our model based on a few a priori measured supportive data showed a better performance over the classical predictive models that do not use such measurements. The new model was further used to derive useful implications to showcase soil density effects on D _p / D _o .

Soil-gas diffusivity and its variation with soil moisture plays a fundamental role in diffusion-controlled migration of climate-impact gases from different terrestrial agroecosystems including cultivated soils and managed pasture systems. The wide contrast in soil texture and structure (e.g., density, soil aggregation) in agriculture topsoils (0–10 cm) makes it challenging for soil-gas diffusivity predictive models to make accurate predictions across different moisture conditions. This study characterized gas diffusivity and gas-phase tortuosity in soils sampled from managed pasture and cultivated sites in Sri Lanka at 0–10 cm depth, together with selected soil-gas diffusivity data from the literature. Soil-gas diffusivity was measured using a one-chamber diffusion apparatus using N₂ and O₂ as experimental gases. The measured diffusivity, together with literature data representing both intact and repacked soils, were tested against five existing widely known gas diffusivity predictive models. The tested models tended to mischaracterize the two-region behavior in some of the aggregated soils, suggesting the need of soil-specific diffusivity models to better describe gas diffusivity in agricultural soils. We suggested a new parametric two-region model, developed in line with literature-based models, to represent both unimodal and bimodal/two-region behavior of selected soils. The new model statistically outperformed the existing predictive models for both intact and repacked soils and, hence, demonstrated its applicability to better characterize site-specific greenhouse gas emissions under different soil water regimes.

Solute migration through heterogeneous porous media remains a fundamental challenge in contaminant hydrology. This study investigates how systematic variation in the coarse-to-fine ratio of binary sand mixtures governs solute plume morphology, breakthrough behavior, and the predictive limits of the advection–dispersion equation (ADE). Eight coarse-to-fine sand mixtures (C100F0 to C0F100) were characterized for grain-size distribution, bulk density, porosity, and permeability. Solute transport experiments were conducted in a 2D transparent Plexiglass tank using 0.01 M NaCl as a non-reactive tracer under gravity-driven, fully saturated conditions. Plume migration monitored optically by color-dye tracing at successive time intervals. COMSOL Multiphysics was used to simulate the ADE framework for four representative mixtures. Physical characterization revealed non-monotonic variation in porosity (φ) and permeability (k) with fine fraction, with a minimum at C30F70 (φ = 0.247 cm3 cm-3, k = 9.23 × 10⁻¹² m²). Solutal Rayleigh numbers across all mixtures (0.013–1.19) confirmed that observed plume dynamics reflect texture-controlled pore geometry rather than density-driven convection. Fine-dominated mixtures (C0F100–C40F60) exhibited diffuse, near-symmetric plumes consistent with the ADE framework. The C30F70 mixture produced the most spatially constrained plume across the entire experimental series, consistent with its minimum porosity and permeability. Coarse-dominated compositions (C50F50–C100F0) displayed pronounced preferential fingering that the single-continuum ADE could not reproduce. A minor fine fraction (20%) acted as a horizontal flow mediator in C20F80, producing the broadest lateral plume spread observed. These results demonstrate that differences in grain-size composition and accompanying functional heterogeneity in pore structure, rather than the material homogeneity, is the primary control on solute transport behaviour in binary porous media, with direct implications for contaminant management, aquifer monitoring, and engineered filtration design.

Fractured porous media are characterised by the strong anisotropy of differently oriented fractures which are embedded within quasi-isotropic matrix porous structures. Existing empirical models for gas diffusivity therefore often fail to capture the directional anisotropy of gas diffusivity and the partitioning of diffusive gas transport between the fracture and matrix pores at steady state. We introduce a complex-plane framework that represents gas diffusivity as a vector-derived quantity, enabling simultaneous assessment/quantification of both fracture and matrix-controlled domains in a unified mathematical formulation. The framework generalizes single and multiple fractures of varying orientations, as well as dual fracture–matrix systems, by mapping porosity and orientation onto an Argand complex plane. In this representation, fractures impose a baseline anisotropic structure, while matrix pores contribute incremental, more omnidirectional transport component which can be relevant during e.g., water drainage. This produces distinctive trajectories of diffusivity in the complex plane. Validation against new and published fracture data showed excellent agreement between predicted and measured diffusivities (r² > 0.90), across fracture porosities ranging from ~0.2% to 22%. Application to a fractured limestone identified a transition of gas diffusion between diffusion regimes from a fracture-controlled regime characterized by mean fracture angles of ~58°, through an interactive fracture–matrix regime in which the angle increased up to ~85°, to a matrix-controlled regime where the angle decreased progressively to ~65°. These transitions quantitatively illustrate how the anisotropy imposed by fractures is modified by the evolving connectivity of matrix pores, thereby demonstrating the adaptability of our proposed framework.