STREAM FLOW HYDROGRAPH SEPARATION USING END-MEMBER MIXING ANALYSIS ON STREAMS THROUGHOUT THE USA

Hydrograph separation is the process of separating stream flow into two components: baseflow and runoff. Baseflow is considered to be groundwater contribution to stream flow, and runoff includes overland flow and interflow contributions. Each stream flow contribution component is highly affected by the variability of geology and geomorphology of a watershed; therefore contributions to stream flow should be analyzed using methods that inherently include these variables in their approach. Hydrograph separation can be performed using chemical end-member mixing or analytical methods (e.g., HYSEP). This research 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, each having a unique specific conductance, which can yield an approximate ratio of groundwater to surface water contributions. Analytical methods use the moving time window algorithms described by Pettyjohn and Henning (1979) based on the duration of runoff N derived from the area of a watershed described by Linsley et al. (1982) to find successive discharge minima on a hydrograph; these include fixed interval, sliding interval, and local minima methods. Each method uses a calculated 2N value as the length of time window used to select discharge minima from a hydrograph, and uses a 2N* value (the nearest odd integer between 3 and 11 closest to 2N; Pettyjohn and Henning, 1979) for a moving time window. Two-component hydrograph separation of 29 US streams using the CMB method and analytical methods was conducted to determine baseflow. Comparison of CMB baseflow values to those derived by analytical methods will yield the most appropriate analytical method for determining baseflow and 2N* value for a given stream. Presented here are preliminary results from streams analyzed thus far. Results will yield new information about runoff cessation times, resulting in more accurate determination of baseflow contributions to stream flow and ultimately leading to examination of duration of runoff equation proposed by Linsley et al. (1982). Such work has implications in increasing accuracy of water balances, estimations of groundwater recharge, and water resource management.