Snow Pack objects contain parameters that characterize the buildup, removal, and melting of snow over three types of sub-areas within a subcatchment:
- The Plowable snow pack area consists of a user-defined fraction of the total impervious area. It is meant to represent such areas as streets and parking lots where plowing and snow removal can be done.
- The Impervious snow pack area covers the remaining impervious area of a subcatchment.
- The Pervious snow pack area encompasses the entire pervious area of a subcatchment.
- Minimum and maximum snow melt coefficients
- Minimum air temperature for snow melt to occur
- Snow depth above which 100% areal coverage occurs
- Initial snow depth
- Initial and maximum free water content in the pack.
In addition, a set of snow removal parameters can be assigned to the Plowable area. These parameters consist of the depth at which snow removal begins and the fractions of snow moved onto various other areas.
Subcatchments are assigned a snow pack object through their Snow Pack property. A single snow pack object can be applied to any number of subcatchments. Assigning a snow pack to a subcatchment simply establishes the melt parameters and initial snow conditions for that subcatchment. Internally, SWMM creates a "physical" snow pack for each subcatchment, which tracks snow accumulation and melting for that particular subcatchment based on its snow pack parameters, its amount of pervious and impervious area, and the precipitation history it sees.
The Snow Pack Editor contains a data entry field for the snow pack's name and two tabbed pages,
one for Snow Pack Parameters and one for Snow Removal Parameters, click links below to see the parameter definations.
Snow Pack Parameters
Snow Removal Parameters
| Field | Description |
| Minimum Melt Coefficient | The degree-day snow melt coefficient that occurs on December 21. Units are either in/hr-deg F or mm/hr-deg C. |
| Maximum Melt Coefficient | The degree-day snow melt coefficient that occurs on June 21. Units are either in/hr-deg F or mm/hr-deg C. For a short term simulation of less than a week or so it is acceptable to use the same value for both the minimum and maximum melt coefficients. The minimum and maximum snow melt coefficients are used to estimate a melt coefficient that varies by day of the year. The latter is used in the following degree-day equation to compute the melt rate for any particular day: Melt Rate = (Melt Coefficient) * (Air Temperature - Base Temperature). |
| Base Temperature | Temperature at which snow begins to melt (degrees F or C). |
| Fraction Free Water Capacity | The volume of a snow pack's pore space which must fill with melted snow before liquid runoff from the pack begins, expressed as a fraction of snow pack depth. |
| Initial Snow Depth | Depth of snow at the start of the simulation (water equivalent depth in inches or millimeters). |
| Initial Free Water | Depth of melted water held within the pack at the start of the simulation (inches or mm). This number should be at or below the product of the initial snow depth and the fraction free water capacity. |
| Depth at 100% Cover | The depth of snow beyond which the entire area remains completely covered and is not subject to any areal depletion effect (inches or mm). |
| Fraction of Impervious Area That is Plowable | The fraction of impervious area that is plowable and therefore is not subject to areal depletion. |
| Field | Description |
| Depth at which snow removal begins (in or mm) | Depth which must be reached before any snow removal begins. |
| Fraction transferred out of the watershed | The fraction of snow depth that is removed from the system (and does not become runoff). |
| Fraction transferred to the impervious area | The fraction of snow depth that is added to snow accumulation on the pack's impervious area. |
| Fraction transferred to the pervious area | The fraction of snow depth that is added to snow accumulation on the pack's pervious area. |
| Fraction converted to immediate melt | The fraction of snow depth that becomes liquid water which runs onto any subcatchment associated with the snow pack. |
| Fraction moved to another subcatchment | The fraction of snow depth which is added to the snow accumulation on some other subcatchment. The name of the subcatchment must also be provided. |