## 2.6.4. Excess Ground Ice[¶](#excess-ground-ice "Permalink to this headline")
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An optional parameterization of excess ground ice melt and respective subsidence based on ([Lee et al., (2014)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#leeetal2014)). Initial excess ground ice concentrations for soil columns are derived from ([Brown et al., (1997)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#brownetal1997)). When the excess ground ice is present in the soil column, soil depth for a given layer (\\(z\_{i}\\)) is adjusted by the amount of excess ice in the column:

(2.6.92)[¶](#equation-6-94 "Permalink to this equation")\\\[z\_{i}^{'}=\\Sigma\_{j=1}^{i} \\ z\_{j}^{'}+\\frac{w\_{exice,\\, j}}{\\rho\_{ice} }\\\]

where \\(w\_{exice,\\,j}\\) is excess ground ice amount (kg m \-2) in layer \\(j\\) and \\(\\rho\_{ice}\\) is the density of ice (kg m \-3). After adjustment of layer depths have been made, all of the soil temperature equations (from [(2.6.78)](#equation-6-80) to [(2.6.87)](#equation-6-89)) are calculted based on the adjusted depths. Thermal properties are additionally adjusted ([(2.6.5)](#equation-6-8) and [(2.6.5)](#equation-6-8)) in the following way:

(2.6.93)[¶](#equation-6-95 "Permalink to this equation")\\\[\\begin{split}\\begin{array}{lr} \\theta\_{sat}^{'} =\\frac{\\theta \_{liq} }{\\theta \_{liq} +\\theta \_{ice} +\\theta\_{exice}}{\\theta\_{sat}} \\\\ \\lambda \_{sat}^{'} =\\lambda \_{s}^{1-\\theta \_{sat}^{'} } \\lambda \_{liq}^{\\frac{\\theta \_{liq} }{\\theta \_{liq} +\\theta \_{ice} +\\theta\_{exice}} \\theta \_{sat}^{'} } \\lambda \_{ice}^{\\theta \_{sat}^{'} \\left(1-\\frac{\\theta \_{liq} }{\\theta \_{liq} +\\theta \_{ice} +\\theta\_{exice}} \\right)} \\\\ c\_{i}^{'} =c\_{s,\\, i} \\left(1-\\theta \_{sat,\\, i}^{'} \\right)+\\frac{w\_{ice,\\, i} +w\_{exice,\\,j}}{\\Delta z\_{i}^{'} } C\_{ice} +\\frac{w\_{liq,\\, i} }{\\Delta z\_{i}^{'} } C\_{liq} \\end{array}\\end{split}\\\]

Soil subsidence at the timestep \\(n+1\\) (\\(z\_{exice}^{n+1}\\), m) is then calculated as:

(2.6.94)[¶](#equation-6-96 "Permalink to this equation")\\\[z\_{exice}^{n+1}=\\Sigma\_{i=1}^{N\_{levgrnd}} \\ z\_{j}^{',\\ ,n+1}-z\_{j}^{',\\ ,n }\\\]

With regards to hydraulic counductivity, excess ground ice is treated the same way normal soil ice is treated in [2.7.4](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#frozen-soils-and-perched-water-table). When a soil layer thaws, excess ground ice is only allowed to melt when no normals soil ice is present in the layer. When a soil layer refreezes, liquid soil water can only turn into normal soil ice, thus, no new of excess ice can be created but only melted. The excess liquid soil moisture from excess ice melt is distributed within the soil column according to [2.7.5](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#lateral-sub-surface-runoff).
