## 2.7.6. Runoff from glaciers and snow-capped surfaces[¶](#runoff-from-glaciers-and-snow-capped-surfaces "Permalink to this headline")
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All surfaces are constrained to have a snow water equivalent \\(W\_{sno} \\le W\_{cap} = 10,000\\) kg m\-2. For snow-capped columns, any addition of mass at the top (precipitation, dew/riping) is balanced by an equally large mass flux at the bottom of the snow column. This so-called capping flux is separated into solid \\(q\_{snwcp,ice}\\) and liquid \\(q\_{snwcp,liq}\\) runoff terms. The partitioning of these phases is based on the phase ratio in the bottom snow layer at the time of the capping, such that phase ratio in this layer is unaltered.

The \\(q\_{snwcp,ice}\\) runoff is sent to the River Transport Model (RTM) (Chapter 11) where it is routed to the ocean as an ice stream and, if applicable, the ice is melted there.

For snow-capped surfaces other than glaciers and lakes the \\(q\_{snwcp,liq}\\) runoff is assigned to the glaciers and lakes runoff term \\(q\_{rgwl}\\) (e.g. \\(q\_{rgwl} =q\_{snwcp,liq}\\) ). For glacier surfaces the runoff term \\(q\_{rgwl}\\) is calculated from the residual of the water balance

(2.7.114)[¶](#equation-7-180 "Permalink to this equation")\\\[q\_{rgwl} =q\_{grnd,ice} +q\_{grnd,liq} -E\_{g} -E\_{v} -\\frac{\\left(W\_{b}^{n+1} -W\_{b}^{n} \\right)}{\\Delta t} -q\_{snwcp,ice}\\\]

where \\(W\_{b}^{n}\\) and \\(W\_{b}^{n+1}\\) are the water balances at the beginning and ending of the time step defined as

(2.7.115)[¶](#equation-7-181 "Permalink to this equation")\\\[W\_{b} =W\_{can} +W\_{sno} +\\sum \_{i=1}^{N}\\left(w\_{ice,i} +w\_{liq,i} \\right) .\\\]

Currently, glaciers are non-vegetated and \\(E\_{v} =W\_{can} =0\\). The contribution of lake runoff to \\(q\_{rgwl}\\) is described in section [2.12.6.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Lake/CLM50_Tech_Note_Lake.html#precipitation-evaporation-and-runoff-lake). The runoff term \\(q\_{rgwl}\\) may be negative for glaciers and lakes, which reduces the total amount of runoff available to the river routing model (Chapter [2.14](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/MOSART/CLM50_Tech_Note_MOSART.html#rst-river-transport-model-rtm)).
