Rain-On-Snow Simulation Enhancements Within Hec-Hms

Rain-on-snow events are a common occurrence in the winter months throughout the United States including large portions of the Mid-Atlantic, Midwest, and Pacific Northwest regions. This is especially true for rivers in the mountainous West, which typically experience their largest floods when warm winter storms interact with extensive snow cover. In many of these watersheds, rain-on-snow events are the primary driver of flood hazards and numerous Federal projects have been constructed in order to mitigate flooding risks due to these events. The nonlinear effects of rain falling on snowmelt can be difficult to simulate accurately within hydrologic models.

The Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) has recently been modified to include three methods for accumulating and melting snow: Temperature Index (TI), Hybrid/Radiation-derived Temperature Index (RTI), and Energy Balance (EB). These three methods range from being relatively simple with only a few required boundary conditions (TI) to intricate with numerous required meteorological inputs (EB). All three methods include mechanisms to discriminate between precipitation falling as rain or snow, form a snowpack, melt the snowpack, reformulate the snowpack when temperatures drop, and eventually melt the entire snowpack. Users can employ these methods at either a subbasin scale (i.e., lumped) or within gridded (i.e., distributed) implementations.

HEC-HMS was used to compare these new snowmelt methods within the Truckee River watershed. This watershed offers an excellent opportunity to investigate snowmelt modeling techniques due to the relatively high density of meteorological, snow, and river observation stations. Each snowmelt method was tested using observed data. The discussion here also includes the amount of effort required for data preparation and computational time for each method.