The Delineation of the Rain/Snow Line on the Side of a Mountain and Its Effects on Runoff

For the purposes of hydrological modeling, the rain/snow line in complex terrain is often assumed to be a hard transition.  Many agencies use a simple temperature-based threshold to differentiate between the two regions.  In reality, there is a melting layer (referred to as Dmelt) of finite depth separating frozen from liquid precipitation.  Using explicit equations for melting, it is found that the Dmelt is functionally dependent on the stability, hydrometeor size, and amount of riming on hydrometeors.  Furthermore, it is found that the altitude at which hydrometeors are at least 70% liquid (a reasonable threshold to assume for the bottom of Dmelt), does not align with any particular temperature threshold for the majority of heavy precipitation events.  The effect of this on basin-total precipitation for the American River basin is quantified.  The basin-total liquid precipitation can be in error by as much as XX mm.  A similar exercise is conducted for the Salt and Colorado river basins.  In these basins, the total liquid precipitation can be in error by as much as XX and XX mm.  The effects of these errors on streamflow predictions are quantified using hydrological prediction models.