Development of a snowfall–snowmelt routine for mountainous terrain for the soil water assessment tool (SWAT)

The soil water assessment tool (SWAT) is a hydrologic model originally developed to evaluate water resources in large agricultural basins. SWAT was not designed to model heterogeneous mountain basins typical of the western United States, and as a result, has performed poorly when applied to mountainous locations. The intent of this study was to increase the versatility of SWAT by developing the capability to simulate hydrology of a non-agricultural mountainous region with a large snowmelt component. A western Wyoming basin, representative of Rocky Mountain basins, was selected to evaluate model performance, identify governing hydrologic processes, and improve the snowmelt routine. An initial evaluation of SWAT performance indicated an inability of the model to represent snowmelt processes. Based on simulation results and field observations, algorithms were developed which use elevation bands to distribute temperature and precipitation with elevation. Additional routines which control snowpack temperature, meltwater production, and areal snow coverage were designed to simulate the influence of season and elevation on the evolution of basin snowpack. The development of the new snowmelt algorithms improved the average annual Nash–Sutcliffe R2 correlation between simulated and observed Wind River streamflow from an initial value of −0.70 to +0.86.