Comparison of Baseflow Estimates from Hydrograph Separation Using End-Member Mixing Analysis and Analytical Techniques

Hydrograph separation is the process of separating stream flow into the various components contributing water to the stream. Each component is influenced by the variability of geology and geomorphology of a watershed; therefore contributions to stream flow should be analyzed using methods that inherently include such variables in their approach. Research presented uses the conductivity mass balance method (CMB) and analytical methods to conduct stream flow hydrograph separation. The CMB method assumes stream flow consists of two distinct flow components, runoff and baseflow, each having a unique specific conductance. Using end-member mixing analysis the ratio of groundwater to surface water contributions to stream flow can be estimated. The specific conductance of the stream integrates the hydrogeological and hydrological characteristics of the watershed above the gaging station, while the analytical methods take only watershed area into account. Hydrographs from 18 streams throughout the USA were analyzed over three year periods in which flow and specific conductance data are available. Baseflow estimates are compared with estimates made using standard flow separation analytical methods: fixed interval, sliding interval and local minima. For short duration events and little or no seasonal trends in flow, all methods yield similar baseflow contributions. However, for long duration events and streams with strong seasonal components to flow (e.g., snowmelt) the analytical methods estimate significantly greater baseflow than does the CMB method. Results suggest that when the CMB method can be used, it provides better estimates of baseflow over an annual hydrograph than do the analytical methods.