Surface Water Routing:Overland Routing with Snow

Overland Routing with Snow
Anytime GSSHA is used in continuous simulation mode, Chapter 9, snow will accumulate in cells with precipitation and below freezing air temperatures, as described in Section 9.4. For overland flow routing, GSSHA ignores snow in an overland flow cell unless the user specfies to route the flow through the snow using Darcy's law with the project card ROUTE_LAT_SNOW. Routing overland flow through the snow as free surface flow may cause simulated flows to be higher and arrive earlier than measured flows strongly influenced by the snowpack. When this card is specifed, the flow in cells with a snowpack will be computed using Darcy's law. If this card is specified the default is to use the SNAP calculated vertical hydrualic conductivity (K) for computation of flow through the snow in the lateral flow computations. The alternative is to specify the lateral hydraulic conductivity with the SNOW_DARCY card, which is followed with a value of hydraulic conductivity (m/s). References report hydraulic conductivities of snow on the order of 1 cm/s (0.01 m/s) (Colbeck and Anderson,1982 for example). The current implementation of the SNAP model produces simliar values but as of v6.2 the implementation of SNAP in GSSHA is considered experimental and is not currently reccomended for general use. It is reccomended that the user specify the lateral hydrualic conductivity of the snow using the SNOW_DARCY card.