SWMM - Computational Methods - Snow Melt

The snow melt routine in SWMM is a part of the flow modeling process. It updates the status of snow packs associated with each subcatchment, accounting for Snow Pack, snow redistribution by area depletion, and snow removal and melting operations through heat budget accounting. Any snow melt that exits the pack is treated as an additional rainfall input to the subcatchment.

At each step of the flow time, the following calculations are made:

1. The air temperature and melting coefficients are updated according to the calendar date.
2. Any precipitation that falls as snow is added to the snow pack.
3. Any excess snow depth in the washable area of the package is redistributed according to the removal parameters established for the package.
4. Snow covers in the impermeable and permeable areas of the package are reduced according to the area depletion curves defined for the study area.
5. The amount of snow in the pack that melts into liquid water is found using:
   • A heat budget equation for wet seasons, where melt rate increases with increasing air temperature, wind speed, and rainfall intensity
   • A degree-day equation for periods without rain, where the melt rate is equal to the product of a melt coefficient and the difference between the air temperature and the base melt temperature of the package.
6. If no melting occurs, the package temperature is adjusted up or down based on the product of the difference between the current and previous air temperatures and an adjusted melting coefficient. If melting occurs, the pack temperature is increased by the equivalent heat content of the melted snow, up to the base melting temperature. Any remaining molten liquid beyond this is available for disposal from the pack.
7. The available snow melt is then reduced by the amount of water holding capacity remaining in the package. The remaining melt is treated in the same way as an additional rainfall input to the subcatchment.