Influences of Macropores on Infiltration into Seasonally Frozen Soil

Abstract

Core Ideas Macropores allow water infiltration into a frozen soil close to 0°C and at high water saturation. Connected biopores channel water beneath the frozen layer. Frozen layer thickness in relation to connected macroporosity is a crucial factor. Infrared thermography can be used to measure frozen layer extent and variability. Water frozen in soil can reduce the soil infiltrability, depending on the water content. We hypothesize that air‐filled macropores control the infiltration of a seasonally frozen soil under high saturation degrees. Sprinkling experiments with different intensities on a seasonally frozen soil were conducted in two winters at high initial water contents. Brilliant Blue FCF (BB) was sprinkled on four plots equipped with soil moisture and temperature probes to mark flow paths. Frozen layer thickness was measured with infrared thermography of soil sections and overlaid with BB images. The frost depth of the experiments was 8 to 15 cm. Infiltration rates showed reduced infiltration compared with unfrozen conditions. By impeding refreezing of the infiltrating water with added NaCl, infiltration rates of 23 to 29 mm h−1 were measured. Without the addition of salt, the infiltration rates decreased to 5 to 10 mm h−1, attributed to pore blockage by refreezing water. Temperature measurements revealed that the frozen layer only thawed close to the soil surface during the experiments. Blue‐stained areas indicated that water was channeled through the frozen layer into the unfrozen soil. In addition, the soil moisture probes below the frozen layer measured an increase in unfrozen water content, whereas total water content in the frozen layer was constant. These observations were explained by a connected air‐filled porosity, such as biopores, which allowed water flow even under high initial water contents. These results illustrate the importance of macroporosity in relation to frost depth in controlling the infiltrability of seasonally frozen soils.

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

“…Finally, percolation recedes to the rainfall intensity of 1.5 mm h −1 . The results from the simulations with slow and intermediate energy transfer between pore domains are in line with dye tracer experiments conducted under winter conditions, which show that, depending on boundary conditions and soil properties, snowmelt infiltration may either be blocked by ice or bypass frozen layers through initially airfilled macropores (Stähli et al, 2004;Demand et al, 2019).…”

Effects of preferential flow on snowmelt partitioning and groundwater recharge in frozen soils

“…Dry frozen soils early in the winter would have a relatively high air-filled porosity and low ice content, and thus they would offer little obstruction to meltwater infiltration (Kane and Stein, 1983;Granger et al, 1984;. Macropores in this situation would be air filled and open for infiltration after freezing (Espeby, 1992). However, the refreezing of infiltrated water in the soil matrix and macropores near the infiltration source (i.e.…”

“…How this process affects the timing and magnitude of runoff generation compared to overland flow in these environments (e.g. Coles and McDonnell, 2018) is unclear and warrants further investigation.…”

“…Granger et al (1984) classified one of the modes of infiltration into frozen soils as "unlimited", where most available meltwater is infiltrated due to preferential flow through macropores. In unsaturated soils, macropores remain airfilled upon freezing, which can have a strong influence on the spatial and temporal characteristics of infiltration by enabling the transfer of water into and below the frost zone (Espeby, 1992;Ishikawa et al, 2006). Under these conditions, most infiltration occurs through macropores as freezing temperatures and pore ice greatly reduce the hydraulic conductivity of the soil matrix (Holten et al, 2018;Grant et al, 2019;Demand et al, 2019).…”

“…In unsaturated soils, macropores remain airfilled upon freezing, which can have a strong influence on the spatial and temporal characteristics of infiltration by enabling the transfer of water into and below the frost zone (Espeby, 1992;Ishikawa et al, 2006). Under these conditions, most infiltration occurs through macropores as freezing temperatures and pore ice greatly reduce the hydraulic conductivity of the soil matrix (Holten et al, 2018;Grant et al, 2019;Demand et al, 2019). However, depending on the soil temperature, infiltrated meltwater may be cooled by the surrounding frozen soil and refreeze along preferential pathways, blocking the flow of water to greater depths and reducing soil infiltrability (Stähli et al, 1996;Watanabe and Kugisaki, 2017).…”