## 2.7.4. Frozen Soils and Perched Water Table[¶](#frozen-soils-and-perched-water-table "Permalink to this headline")
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When soils freeze, the power-law form of the ice impedance factor (section [2.7.3.1](#hydraulic-properties)) 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)[¶](#equation-7-166 "Permalink to this equation")\\\[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)[¶](#equation-7-167 "Permalink to this equation")\\\[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.

