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2.7.4. Frozen Soils and Perched Water Table¶
When soils freeze, the power-law form of the ice impedance factor (section 2.7.3.1) can greatly decrease the hydraulic conductivity of the soil, leading to nearly impermeable soil layers. When unfrozen soil layers are present above relatively ice-rich frozen layers, the possibility exists for perched saturated zones. Lateral drainage from perched saturated regions is parameterized as a function of the thickness of the saturated zone
(2.7.106)¶\[q_{drai,perch} =k_{drai,\, perch} \left(z_{frost} -z_{\nabla ,perch} \right)\]
where \(k_{drai,\, perch}\) depends on topographic slope and soil hydraulic conductivity,
(2.7.107)¶\[k_{drai,\, perch} =10^{-5} \sin (\beta )\left(\frac{\sum _{i=N_{perch} }^{i=N_{frost} }\Theta_{ice,i} k_{sat} \left[z_{i} \right]\Delta z_{i} }{\sum _{i=N_{perch} }^{i=N_{frost} }\Delta z_{i} } \right)\]
where \(\Theta_{ice}\) is an ice impedance factor, \(\beta\) is the mean grid cell topographic slope in radians, \(z_{frost}\) is the depth to the frost table, and \(z_{\nabla,perch}\) is the depth to the perched saturated zone. The frost table \(z_{frost}\) is defined as the shallowest frozen layer having an unfrozen layer above it, while the perched water table \(z_{\nabla,perch}\) is defined as the depth at which the volumetric water content drops below a specified threshold. The default threshold is set to 0.9. Drainage from the perched saturated zone \(q_{drai,perch}\) is removed from layers \(N_{perch}\) through \(N_{frost}\), which are the layers containing \(z_{\nabla,perch}\) and, \(z_{frost}\) respectively.