### 2.12.6.3. Precipitation, Evaporation, and Runoff[¶](#precipitation-evaporation-and-runoff "Permalink to this headline")

All precipitation reaches the ground, as there is no vegetated fraction. As for other land types, incident snowfall accumulates (with ice mass \\(W\_{sno}\\) and thickness \\(z\_{sno}\\) ) until its thickness exceeds a minimum thickness \\(s\_{\\min }\\), at which point a resolved snow layer is initiated, with water, ice, dissolved aerosol, snow-grain radius, etc., state variables tracked by the Snow Hydrology submodel (Chapter [2.8](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Snow_Hydrology/CLM50_Tech_Note_Snow_Hydrology.html#rst-snow-hydrology)). The density of fresh snow is assigned as for other land types (Chapter [2.8](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Snow_Hydrology/CLM50_Tech_Note_Snow_Hydrology.html#rst-snow-hydrology)). Solid precipitation is added immediately to the snow, while liquid precipitation is added to snow layers, if they exist, after accounting for dew, frost, and sublimation (below). If \\(z\_{sno}\\) exceeds \\(s\_{\\min }\\) after solid precipitation is added but no snow layers are present, a new snow layer is initiated immediately, and then dew, frost, and sublimation are accounted for. Snow-capping is invoked if the snow depth \\(z\_{sno} >1000{\\rm m}\\), in which case additional precipitation and frost deposition is added to \\(q\_{snwcp,\\, ice}\\).

If there are resolved snow layers, the generalized “evaporation” \\(E\_{g}\\) (i.e., evaporation, dew, frost, and sublimation) is treated as over other land units, except that the allowed evaporation from the ground is unlimited (though the top snow layer cannot lose more water mass than it contains). If there are no resolved snow layers but \\(W\_{sno} >0\\) and \\(E\_{g} >0\\), sublimation \\(q\_{sub,sno}\\) (kg m\-2 s\-1) will be given by

(2.12.65)[¶](#equation-12-60 "Permalink to this equation")\\\[q\_{sub,sno} =\\min \\left\\{E\_{g} ,\\frac{W\_{sno} }{\\Delta t} \\right\\} .\\\]

If \\(E\_{g} <0,T\_{g} \\le T\_{f}\\), and there are no resolved snow layers or the top snow layer is not unfrozen, then the rate of frost production \\(q\_{frost} =\\left|E\_{g} \\right|\\). If \\(E\_{g} <0\\) but the top snow layer has completely thawed during the Phase Change step of the Lake Temperature solution (section [2.12.5.8](#phase-change-lake)), then frost (or dew) is not allowed to accumulate (\\(q\_{frost} =0\\)), to insure that the layer is eliminated by the Snow Hydrology (Chapter [2.8](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Snow_Hydrology/CLM50_Tech_Note_Snow_Hydrology.html#rst-snow-hydrology)) code. (If \\(T\_{g} >T\_{f}\\), then no snow is present (section [2.12.4.2](#surface-flux-solution-lake)), and evaporation or dew deposition is balanced by \\(q\_{rgwl}\\).) The snowpack is updated for frost and sublimation:

(2.12.66)[¶](#equation-12-61 "Permalink to this equation")\\\[W\_{sno} =W\_{sno} +\\Delta t\\left(q\_{frost} -q\_{sub,sno} \\right) .\\\]

If there are resolved snow layers, then this update occurs using the Snow Hydrology submodel (Chapter [2.8](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Snow_Hydrology/CLM50_Tech_Note_Snow_Hydrology.html#rst-snow-hydrology)). Otherwise, the snow ice mass is updated directly, and \\(z\_{sno}\\) is adjusted by the same proportion as the snow ice (i.e., maintaining the same density), unless there was no snow before adding the frost, in which case the density is assumed to be 250 kg m\-3.

