Evaluating Snow-depletion Curve Refinements in Selected Watersheds Across the Continental United States

Snow depletion curves (SDC) are used in continuous watershed hydrologic models to simulate snowmelt processes for the period between maximum snowpack accumulation and total melt. The SDCs are defined as the relationship between snow-water equivalence (SWE) and the percent snow cover area (SCA) during the snowmelt period. The shape of the SDC can vary depending on local watershed, topographic, regional, and climatic conditions. SDCs slopes for windy sparsely vegetated high-elevation watersheds, for example, are often less steep than those for lower-elevation forested watersheds. To improve model performance new SDCs were derived for over 110,000 hydrologic response units (HRU) of the National Hydrologic Model (NHM) across the continental United States (CONUS) using Snow Data Assimilation System (SNODAS) remote sensing data. The spatially varying set of SDCs was integrated into the Precipitation Runoff Modeling System (PRMS), a deterministic daily time-step, rainfall-runoff model, which is installed in the NHM discretization. In addition to the SNODAS-derived SDCs, seven synthetic SDCs representing convex and concave shapes were created. PRMS was run on the NHM to test the sensitivity of melt characteristics and the response of different water budget components to the different synthetic SDCs. Correlations between SDC shape and landscape variables such as region, topography, elevation, aspect, and vegetation cover were evaluated. From this analysis a subset of PRMS watershed models, from a pool of 873 unregulated PRMS watershed models within CONUS and which included a representation of the SDC shape categories, were selected. These selected watershed models were calibrated using both the default PRMS and the new NHM SDCs. Model performance statistics were used to evaluate improvements in streamflow predication using an improved SDC.