Hydrologic tracers and thresholds: A comparison of geochemical techniques for event-based stream hydrograph separation and flowpath interpretation across multiple land covers in the Panama Canal Watershed

- Tracers imply macropores drain storm-wetted soil-to-groundwater flowpaths.
- Multiple land covers share similar geochemical behavior.
- Dual-source tracers disagreed when a runoff efficiency threshold was exceeded.

Stream hydrograph separation using naturally occurring geochemical tracers holds great potential for elucidating mineral weathering and solute transport. This study addresses a critical need to characterize catchment runoff generation in the humid tropics using multiple natural tracers for hydrograph separation and concentration/discharge (C/Q) hysteresis analysis. We use hydrometric and geochemical data collected at the start of the wet season from three small, steep catchments located in the humid seasonal tropics of central Panama that differ primarily in land cover. We apply a dual source hydrograph separation model between two end-members: new event water precipitation and pre-event water stored in the catchment. We compare the effectiveness of electrical conductivity (EC) and stable water isotopes (δD and δ18O) tracers for identifying precipitation event water in stream runoff using across forested (1.43 km2), mixed land use 'mosaic' (1.82 km2) and pasture (0.42 km2) catchments. Hysteretic C/Q loops are analyzed for flowpath interpretation using δD, Ca2+, Mg2+, Na+, K+, Cl−, and SO42−. During a medium-large magnitude event on May 23, 2013, forest and mosaic stream δD, Ca2+, Mg2+, and Na+ exhibited clockwise hysteresis, SO42− exhibited anticlockwise hysteresis, and K+ and Cl− each showed no hysteresis. EC as a surrogate for total dissolved solids agrees acceptably with stable water isotope hydrograph separations during small peak runoff events (<3 mm/h). However, isotope and conductivity tracers strongly disagree during a large runoff event (>10 mm/h) in the mosaic catchment. Early wet-season events indicate lower event water fractions than events farther into the wet season. Despite previous work showing land cover strongly controls storm runoff efficiencies, hydrograph separation and hysteresis analyses only indicate weak event water delivery differences between the paired forest and mosaic catchments.