Quantitative Evaluation of Baseflow Separation Methods Using an Integrated Hydrologic Model: A Case Study in a Snow-Dominated Watershed

Baseflow constitutes over 50% of streamflow in mountainous regions of the Western United States, making its accurate quantification essential for water management and decision-making. Traditional automated baseflow separation methods are often arbitrary and ambiguous, complicating their validation. This study developed an integrated hydrologic model that integrated the exchange between surface and subsurface flows to physically quantify the baseflow component in a snow dominated catchment. Using the model's simulated baseflow and streamflow as a control, we evaluated four common baseflow separation methods: the Pettyjohn and Henning (PH) graphical, the United Kingdom Institute of Hydrology (UKIH) graphical, the Eckhardt digital filter, and conductance mass balance (CMB) methods. Both UKIH graphical and Eckhardt filter methods performed relatively well with high modified Kling-Gupta Efficiency (mKGE) (0.72 and 0.65, respectively) and Nash-Sutcliffe Efficiency (NSE) (0.58 and 0.62, respectively) values. However, the UKIH graphical method performed poorer than the Eckhardt filter method in average and dry years when stream hydrographs resemble unimodal peaks, common in snow-dominated catchments. Additionally, the Eckhardt digital filter showed better matching of the temporal dynamics. The PH graphical and CMB methods did not perform satisfactorily with low mKGE and NSE values. The PH graphical method has consistently overestimated baseflow with an average baseflow index BFI of 85%, whereas the CMB method has consistently underestimated baseflow with an average BFI of 24%. Our findings suggest that integrated hydrologic models, when calibrated, provide a quantitative way to evaluate and improve existing baseflow separation methods. Additionally, caution should be exercised when applying automated baseflow separation methods in snow-dominated catchments, and future work is needed to thoroughly evaluate these methods in catchments with diverse hydroclimate conditions.