Simulating the evolution of seasonal snowcover and snowmelt runoff using a distributed energy balance model: Application to an alpine watershed in the Tobacco Root Mountains, Montana
A distributed energy-balance model was developed for simulating the seasonal snowmelt in rugged alpine terrain and applied to a 45 km2 headwater catchment in the Tobacco Root Mountains, Montana. Micrometeorological data that were collected over the 1997–1998 snow-accumulation and snowmelt seasons were used as boundary conditions for estimating initial snow distributions as well as calculating the timing and distribution of snowmelt from the basin. Simulated snowmelt volumes compare favorably with measured outflow from a stream gage located just downstream of the study area. Results of this study demonstrate that the representation of heterogeneity in complex terrain produces realistic energy and resultant snow-cover distributions throughout the snowmelt season. As in many other studies, the patterns of snowmelt were highly correlated to the distribution of net radiation in the watershed. Although the primary control on snowmelt patterns was net radiation, the initial distribution of snow-water equivalent was also a dominant factor. Snowmelt from individual subcatchments in the study area produces inflow hydrographs that reflect the heterogeneous attributes of terrain and ground cover that characterize each sub-basin. Season-long simulations were also conducted using simplifying assumptions of isotropy in calculating the diffuse irradiance components in the energy-balance snowmelt model. Results from the simulations using isotropic assumptions were compared to the energy-balance snowmelt model, which assumes that diffuse irradiances are anisotropic. The amount of calculated snowmelt varied between the two methods, suggesting that isotropic assumptions could account for underestimation of as much as 106 cubic meters of snowmelt over a single season. These results indicate that the assumption of isotropic conditions appears to be suitable for representation of areas with little surrounding topographic influence but not for areas of rugged terrain that constitute most snowmelt-driven watersheds of the western United States.