A Dual‐Permeability Approach for Modeling Soil Water Flow and Heat Transport during Freezing and Thawing

Abstract

Core Ideas Macropore flow may be enhanced in partly frozen soil. We developed a dual‐permeability modeling approach for frozen soils. Four test cases of increasing complexity were evaluated. Modeling results are in agreement with current process understanding. Measured data are lacking for a more quantitative evaluation of the model. Preferential flow may become significant in partially frozen soils because infiltration can occur through large, initially air‐filled pores surrounded by a soil matrix with limited infiltration capacity. The objectives of this study were to develop and evaluate a dual‐permeability approach for simulating water flow and heat transport in macroporous soils undergoing freezing and thawing. This was achieved by introducing physically based equations for soil freezing and thawing into the dual‐permeability model MACRO. Richards' equation and the heat flow equation were loosely coupled using the generalized Clapeyron equation for the soil micropore domain. Freezing and thawing of macropore water is governed by a first‐order equation for energy transfer between the micropore and macropore domains. We assumed that macropore water was unaffected by capillary forces, so that water in macropores freezes at 0°C. The performance of the model was evaluated for four test cases: (i) redistribution of water in the micropore domain during freezing, (ii) a comparison between the first‐order energy transfer approach and the heat conduction equation, (iii) infiltration and water flow in frozen soil with an initially air‐filled macropore domain, and (iv) thawing from the soil surface during constant‐rate rainfall. Results show that the model behaves in accordance with the current understanding of water flow and heat transport in frozen macroporous soil. To improve modeling of water and heat flow in frozen soils, attention should now be focused on providing experimental data suitable for evaluating models that account for macropore flow.

Dual‐permeability modeling of preferential flow and snowmelt partitioning in frozen soils

“…All test cases are simulated with the same 1D vertical soil domain (Table 1). These cases are similar to test cases presented in Larsbo et al (2019), but with a larger domain and greater amount of water input, in order to illustrate the effects of preferential flow on infiltration in addition to runoff generation and deeper drainage in frozen soil.…”

“…However, in certain environments, these assumptions will not be acceptable and transient heat transfer relations explicitly considering latent heat effects and advection by flowing water in the soil matrix would be needed to adequately assess ground thaw mechanisms (Kane et al, 2001;Kurylyk & Watanabe, 2013). A significant, but computationally demanding, improvement to the heat transfer and freeze-thaw relations here would be to include fully coupled water and thermal transport processes in the bulk soil matrix (Larsbo et al, 2019). Overall, results show that using measured soil matrix properties (porosity, soil moisture characteristic, and K s ) and estimated macropore (macroporosity and macropore aperture) properties is effective for simulating preferential flow in frozen soils with the dual-permeability approach presented.…”

Effects of antecedent moisture and macroporosity on infiltration and water flow in frozen soil

“…Calculations for exfiltration out the base of soil columns result in similar macropore density estimates. Physically based models that consider these effects have only recently began to be developed and there is a corresponding need for data sets to test and improve such efforts (Larsbo, Holten, Stenrod, Eklo, & Jarvis, 2019). In fact, given the ponded infiltration conditions at the top of the columns, it is possible that both modes of preferential flow may have occurred at different times and locations within the columns.…”

“…As limited data exist on flow through macroporous frozen soils, these results combined with an appropriate model could help interpret these observations and improve understanding of water flow in frozen structured soils (Mohammed et al, 2018). Physically based models that consider these effects have only recently began to be developed and there is a corresponding need for data sets to test and improve such efforts (Larsbo, Holten, Stenrod, Eklo, & Jarvis, 2019).…”

Variability of Soil Water Heat and Energy Transfer Under Different Cover Conditions in a Seasonally Frozen Soil Area

“…During the soil freezing process, energy in the soil is gradually transferred to the atmospheric environment, and the soil temperature is lowered accordingly; as a result, the liquid water in the soil is transformed into solid ice, and a frozen front is formed at the ground surface. Driven by the temperature gradient, the unfrozen water gradually migrates to the frozen front and accumulates at the ground surface [31]. Therefore, during the freezing period, the liquid water content in the frozen soil layer decreased significantly, while the total water content showed an upward trend, and the hydrological environment in the frozen soil region fluctuated to a certain extent.…”