Hydrologic pathways and baseflow contributions, and not the proximity of sediment sources, determine the shape of sediment hysteresis curves: Theory development and application in a karst basin in Kentucky USA

Researchers use sediment hysteresis in watershed sedimentation studies, however underlying processes controlling sediment hysteresis observations remain an open topic of investigation. We investigate the hypothesis that baseflow water and sediment can control sediment hysteresis in some cases by: (i) modelling water–sediment mixing permutations that considers baseflow and runoff with their own sediment concentration distributions; (ii) analyzing sediment hysteresis for a karst basin with high baseflow contributions in Kentucky, USA by decoupling baseflow and runoff hysteresis via sensor data, a mixing model, and sediment transport modelling; and (iii) analyzing the alternative hypothesis of sediment origin controlling hysteresis for this system using modelling and tracing of sediment origin with stable isotopes. Results from mixing model permutations show that changes to the timing and magnitude of baseflow water and its sediment concentration can shift hysteresis looping from clockwise (HI > 0.1) to counterclockwise (HI < -0.1). Varying the baseflow contribution can reproduce most of the sediment hysteresis results reported in the literature, such as single loops, double loops, figure eights and complex loops. Results from the Kentucky basin where baseflow contributions are high show the dominance of a new taxonomy of sediment hysteresis loops called a ‘J-loop’. 71 % of the loops observed were J-loops while the remaining 29 % were complex loops. The J-loop occurs when baseflow dominates over runoff for a hydrologic event and the baseflow to runoff volume ratio falls between 1.5 and 3. Analyses of the alternative hypothesis show that looping patterns do not depend on sediment origin for the events studied. Sediment origin varied by dominance of the distal sediment source (26 % of events), proximal source (55 %), and nearly equal mixture of the two sources (18 %). J-loops and complex loop occurrence was not consistent with any sediment origin dominance. We analyzed 43 sediment hysteresis studies reported in hydrology journals and found many studies show hysteresis that resembles J-loops. These occur during events with low antecedent moisture, low-intensity rain, and low amounts of runoff—all of which point towards baseflow dominance. The results herein suggest the importance of the J-loops in systems with high baseflow contributions as well as the overall influence of baseflow to impact loop interpretations. This result is relevant because recent findings show that the majority of hydrologic events in many regions are dominated by baseflow. In such systems, the baseflow contribution and it's control on sediment hysteresis looping challenges the common interpretation that hysteresis loops reflect proximal and distal sediment sources.