Determining Watershed Flow Pathways Using Geochemistry and Timing

Investigating storm runoff generation in watersheds is an area of ongoing hydrologic research. Geochemical tracer studies, such as static end-member mixing analysis (EMMA) and hysteresis loop analysis, have been used to evaluate these processes. While EMMA can assess the relative input of flow pathways for individual stream water quality samples collected during a storm, it cannot quantify their contributions continuously. Hysteresis loops of stream discharge versus geochemical tracer concentration can be used to estimate relative inputs of basic end-member pathways, but this approach only suggests the timing and dominance of flow pathways and these patterns alone cannot quantify their contributions. We propose a new method that incorporates both hysteresis loops and geochemical tracer studies to quantify runoff contributions from watershed flow pathways during a storm. The approach involves estimating relative tracer concentrations of four end-members, along with estimating the percentage of total stream discharge from each end-member. The method has been applied to a 22 year dataset from Panola Mountain Research Watershed, Georgia and has identified two distinct watershed responses to rain events. The responses appear be related to a threshold of 50-60mm of total rain.