Assessment of the Melt Rate Function in a Temperature Index Snow Model Using Observed Data

Temperature index snowmelt models often require representation of a snowpack’s melt rate. Defining the melt rate as a function of an accumulated number of warming degree-days is achieved in some temperature index models through the antecedent temperature index–melt rate (ATIMR) function, and provides a reasonable method for adjusting the melt rate during the season. Modelers have typically relied on a default ATIMR function developed two decades ago. However, literature concerning the nature of this default function and how it was developed is largely nonexistent. This paper presents results of a comparison between the default ATIMR function and ATIMR functions computed from field data from two headwater stations within the American River watershed upstream of Folsom Lake, California. An error analysis was also performed using the site-specific data. Using the Hydrologic Engineering Center’s hydrologic modeling system (HEC-HMS), it was found that modeled results often varied dramatically between the default curve and the computed curves. In one case, modeled results showed significant improvement between the default method and that of the data-derived approach. This finding suggests that conventional use of the default 1991 curve, often employed in practice irrespective of season and/or location, should at the very least be reconsidered.