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Summary
The article discusses the impact of frozen soils on the water table and drainage in a land surface model. When soils freeze, the hydraulic conductivity of the soil can decrease dramatically, leading to the formation of nearly impermeable soil layers. This can result in the development of perched saturated zones above the frozen layers.
The lateral drainage from these perched saturated regions is parameterized as a function of the thickness of the saturated zone. The drainage rate is calculated using the equation:
q_drai,perch = k_drai,perch (z_frost - z_∇,perch)
where k_drai,perch depends on the topographic slope and the soil hydraulic conductivity, as shown in the equation:
k_drai,perch = 10^-5 sin(β) (∑_i=N_perch^i=N_frost Θ_ice,i k_sat[z_i] Δz_i / ∑_i=N_perch^i=N_frost Δz_i)
The frost table (z_frost) is defined as the shallowest frozen layer with an unfrozen layer above it, while the perched water table (z_∇,perch) is the depth at which the volumetric water content drops below a specified threshold (default is 0.9). The drainage from the perched saturated zone is removed from the layers containing the perched water table and the frost table.