Quantitative Evaluation of Baseflow Separation Methods Using an Integrated Hydrologic Model in a Snow‐Dominated Watershed

Abstract

Abstract Baseflow constitutes over half of streamflow in mountainous headwaters of the western United States (US), making accurate quantification critical for water resource management. However, traditional baseflow separation methods are often arbitrary and ambiguous, complicating validation. Here, we developed an integrated hydrologic model coupling surface and subsurface flow to quantify baseflow in the Coal Creek watershed, a snow‐dominated catchment in Colorado, US. A model‐agnostic approach was implemented to estimate baseflow from simulated hydrologic exchange fluxes and these estimates were used as a reference to evaluate four common baseflow separation methods: the Pettyjohn and Henning (PH) graphical, United Kingdom Institute of Hydrology (UKIH) graphical, Eckhardt digital filter, and conductance mass balance (CMB) methods. Both the UKIH and Eckhardt methods performed relatively well, with modified Kling‐Gupta Efficiency values of 0.73 and 0.68 and Nash‐Sutcliffe Efficiency values of 0.59 and 0.71, respectively. However, the UKIH method underperformed during average and dry years, when stream hydrographs exhibit unimodal peaks typical of snow‐dominated catchments. The PH and CMB methods showed poor performance overall: the PH method substantially overestimated baseflow (average baseflow index (BFI) of 85%) owing to its short moving window, while the CMB method underestimated it (average BFI of 38%), potentially due to the exclusion of interflow contributions, compared to a numerically derived BFI of 58%. These findings highlight the utility of integrated hydrologic models as a reference for evaluating baseflow separation methods, particularly in snow‐dominated catchments where graphical and tracer‐based approaches may misrepresent baseflow contributions.