The process of separating a stream's total discharge hydrograph into baseflow (groundwater contribution) and quickflow (surface runoff) components. Baseflow separation reveals the relative contributions of different water sources to streamflow.
Baseflow separation is an analytical technique used to partition a stream's total discharge hydrograph into its component parts: baseflow (the sustained contribution from groundwater discharge and other delayed sources) and quickflow or direct runoff (the rapid response to precipitation events via surface runoff, interflow, and precipitation directly on the channel). Accurate baseflow separation is important for understanding watershed hydrology, estimating groundwater recharge, calibrating hydrological models, and assessing water supply reliability during dry periods. Common separation methods include graphical techniques (straight-line method, fixed-interval method, sliding-interval method), recursive digital filters (Lyne-Hollick, Eckhardt), and chemical hydrograph separation using natural tracers (stable isotopes, silica, chloride, specific conductance). The USGS HYSEP program and more recently the Web-based Hydrograph Analysis Tool (WHAT) provide standardized baseflow separation for gauged watersheds. The baseflow index (BFI), defined as the ratio of baseflow volume to total streamflow volume, is a useful watershed characteristic that reflects geology, soils, land use, and climate. BFI values range from near 0 in arid, impervious watersheds to over 0.9 in permeable karst or sand aquifer systems. Chemical separation techniques, while more complex and data-intensive, provide physically based estimates that can distinguish multiple source waters beyond the simple two-component separation.