Melt rates for the snowpack require calibration. It can be increased or reduced by the segment aspect. Long wave radiation is emitted by the snow, forests and cloud cover in the atmosphere at a temperature dependent, near constant rate. The wind velocity on or at the snowpack is seldom known. Wind velocity measurements may be made many miles from the watershed, and wind is altered by elevation and forest cover.
Air temperature can also be uncertain.
It will be zero if the soil beneath the snowpack is frozen. Frost penetration into the soil occurs when there is no snowpack, and when air temperatures are below freezing. The frozen ground algorithms in HSPF need more development. The HSPF parameters affecting Snow Accumulation and Heat Exchange and Melt are the only parameters that are typically calibrated. The Snow Aging and Transformation parameters and the Snow and Soil parameters are run using either physically defined or default parameter levels.