The infiltrability of frozen soils modulates the partitioning of snowmelt between infiltration and runoff in cold regions. Preferential flow in macropores may enhance infiltration, but flow dynamics in frozen soil are complicated by soil heat transfer processes. We developed a dual-permeability model that considers the interacting effects of freeze-thaw and preferential flow on infiltration and runoff generation in structured soils. This formulation was incorporated into the fully integrated groundwatersurface water model HydroGeoSphere, to represent water-ice phase change in macropores such that porewater freezing is governed by macropore-matrix heat exchange. Model performance was evaluated against laboratory experiments and synthetic test cases designed to examine the effects of preferential flow on snowmelt partitioning between infiltration, runoff, and drainage. Simulations were able to reproduce experimental observations of rapid infiltration and drainage behavior due to macropores very well, and approximated soil thaw to an acceptable degree. Simulation of measured data highlighted the importance of macropore hydraulic conductivity, as well as macropore-matrix heat and water transfer, on controlling preferential flow dynamics. Test cases replicated a range of snowmelt partitioning behavior commonly observed in frozen soils, including subsurface conditions that produce rapid infiltration and deeper drainage, the contrast between limited vs. unlimited infiltration responses to snowmelt, and the temporal evolution of runoff generation. This study demonstrates the important influence that water freezing along preferential flowpaths can have on infiltrability and runoff characteristics in frozen soils and provides a physically based description of this mechanism that links infiltration behavior to hydraulic and thermal properties of structured soils.
Multiyear monitoring and simulation of a conservative tracer was used in this study to investigate preferential flow and macropore-matrix interactions in low permeability, macroporous soil. 2,6-Difluorobenzoic acid (DFBA) tracer was applied to a 20 3 20 m drip irrigated test plot situated over two tile drains. Tracer movement over the 2009 and 2010 field seasons was monitored using tile drain effluent, suction lysimeters, monitoring wells, and soil cores. Despite similar volumes of water application to the plot in each season, 10 times more water and 14 times more DFBA were captured by the drains in 2010 due to wetter regional hydrologic conditions. The importance of preferential flow along macropores was shown by rapid DFBA breakthrough to the tile (<47 h), and DFBA detections in sand units below the tile drains. Preferential flow resulted in less than 8% of the DFBA mass being captured by the tiles over both years. With much of the DFBA mass (75%) retained in the upper 0.25 m of the soil at the end of 2009, numerical simulations were used to quantify the migration of this in situ tracer during the subsequent 2010 field season. Dual permeability and dual porosity models produced similar matches to measured tile drain flows and concentrations, but solute leaching was captured more effectively by the dual permeability formulation. The simulations highlighted limitations in current descriptions for small-scale mass transfer between matrix and macropore domains, which do not consider time-dependent transfer coefficients or nonuniform distributions of solute mass within soil matrix blocks.
The infiltrability of frozen soils strongly influences snowmelt partitioning and redistribution in cold regions. Preferential flow in frozen soil can enhance infiltration, but dynamics are complicated by coupled water and heat transfer processes as well as landscape conditions prior to and during snowmelt. Hypothetical model simulations based on hydrological functioning and landscape properties of the Canadian Prairies were used to evaluate a dual-domain (matrix and macropore) formulation of variablysaturated flow in frozen soils, with distinct water and heat transport regimes in each domain. The description was incorporated into a fully-integrated groundwatersurface water model. Two-dimensional hillslope simulations were able to capture the landscape hydrologic response to snowmelt fluxes observed in the prairies and similar landscapes, specifically: (1) enhanced infiltration into frozen soil due to preferential flow, (2) refreezing of infiltrated water and its effect on the evolution of runoff generation in frozen soils, and (3) groundwater recharge prior to ground thaw.Results showed that multiple meltwater input events progressively decreased frozen soil infiltrability and increased runoff generation. Simulations demonstrated that refreezing of infiltrated water along preferential flowpaths is an important process governing the timing and magnitude of both runoff generation and groundwater recharge in frozen soils, but that this behaviour can be highly counterintuitive and depends on soil structure. The modeling framework provides a physically-based approach for describing these interacting preferential flow and soil freezing processes at the hillslope scale needed to simulate the hydrologic functioning of seasonally frozen landscapes.
Leaching of salts in the presence of elevated soil sodicity can result in reduced hydraulic conductivity due to clay swelling or dispersion. This study examines the dynamics and mechanisms of hydraulic conductivity decrease induced during salt leaching of a structured, smectite‐bearing subsoil. Intact soil cores were initially equilibrated with aqueous solutions of elevated salinity and sodicity. Results of leaching with a solution of fixed sodicity demonstrated that, in this structured soil, salinity levels were successfully leached before reaching a stable hydraulic conductivity. This disequilibrium between decreases in hydraulic conductivity and permeameter effluent salinity appear to be the result of diffusion‐controlled swelling. Diffusion of salts out of low permeability soil aggregates may be slowed by diffuse double layer swelling trapping salts within the smallest pores. A higher degree of sensitivity to saline–sodic swelling effects was observed for the structured soil in this study compared to repacked soils of higher swelling clay content from other studies. Higher sensitivity of structured subsoils to saline–sodic swelling effects may be attributed to aggregate breakdown and plugging of macropores, heterogeneity in swelling‐clay distribution resulting in a locally higher degree of swelling, and/or increased internal swelling due to physical restriction caused by overburden loading. Potential irreversibility of pore network damage caused by soil structure breakdown motivates mitigating saline–sodic disruption of field hydraulic conductivity before it occurs.
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