## 2.8.6. Snow Compaction[¶](#snow-compaction "Permalink to this headline")
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Snow compaction is initiated after the soil hydrology calculations \[surface runoff (section [2.7.2.1](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#surface-runoff)), infiltration (section [2.7.2.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#infiltration)), soil water (section [2.7.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#soil-water))\] are complete. Currently, there are four processes included that lead to snow compaction:

> 1.  destructive metamorphism of new snow (crystal breakdown due to wind or thermodynamic stress)
>     
> 2.  snow load or compaction by overburden pressure
>     
> 3.  melting (changes in snow structure due to melt-freeze cycles plus changes in crystals due to liquid water)
>     
> 4.  drifting snow compaction.
>     

The total fractional compaction rate for each snow layer \\(C\_{R,\\, i}\\) (s\-1) is the sum of multiple compaction processes

(2.8.30)[¶](#equation-8-41 "Permalink to this equation")\\\[C\_{R,\\, i} =\\frac{1}{\\Delta z\_{i} } \\frac{\\partial \\Delta z\_{i} }{\\partial t} =C\_{R1,\\, i} +C\_{R2,\\, i} +C\_{R3,\\, i} +C\_{R4,\\, i} +C\_{R5,\\, i} .\\\]

Compaction is not allowed if the layer is saturated

(2.8.31)[¶](#equation-8-42 "Permalink to this equation")\\\[1-\\left(\\frac{w\_{ice,\\, i} }{f\_{sno} \\Delta z\_{i} \\rho \_{ice} } +\\frac{w\_{liq,\\, i} }{f\_{sno} \\Delta z\_{i} \\rho \_{liq} } \\right)\\le 0.001\\\]

or if the ice content is below a minimum value (\\(w\_{ice,\\, i} \\le 0.1\\)).

The snow layer thickness after compaction is

(2.8.32)[¶](#equation-8-42b "Permalink to this equation")\\\[\\Delta z\_{i}^{n+1} =\\Delta z\_{i}^{n} \\left(1+C\_{R,\\, i} \\Delta t\\right).\\\]

