Abstract
Concentration-discharge relationships are a key tool for understanding the sources and transport of material from watersheds to fluvial networks. Storm events in particular provide insight into variability in the sources of solutes and sediment within watersheds, and the hydrologic pathways that connect hillslope to stream channel. Here we examine high-frequency sensor-based specific conductance and turbidity data from multiple storm events across two watersheds (Quebrada Sonadora and Rio Icacos) with different lithology in the Luquillo Mountains of Puerto Rico, a forested tropical ecosystem. Our analyses include Hurricane Maria, a category 5 hurricane. To analyze hysteresis, we used a recently developed set of metrics to describe and quantify storm events including the hysteresis index (HI), which describes the directionality of hysteresis loops, and the flushing index (FI), which can be used to infer whether the mobilization of material is source or transport limited. We also examine the role of antecedent discharge to predict hysteretic behavior during storms. Overall, specific conductance and turbidity showed contrasting responses to storms. The hysteretic behavior of specific conductance was similar across sites, displaying clockwise hysteresis and a negative flushing index indicating proximal sources of solutes and consistent source limitation. In contrast, the directionality of turbidity hysteresis was significantly different between watersheds, although both had strong flushing behavior indicative of transport limitation. Overall, models that included antecedent discharge did not perform any better than models with peak discharge alone, suggesting that the magnitude and trajectory of an individual event was the strongest driver of material flux and hysteretic behavior. Hurricane Maria produced unique hysteresis metrics within both watersheds, indicating a distinctive response to this major hydrological event. The similarity in response of specific conductance to storms suggests that solute sources and pathways are similar in the two watersheds. The divergence in behavior for turbidity suggests that sources and pathways of particulate matter vary between the two watersheds. The use of high-frequency sensor data allows the quantification of storm events while index-based metrics of hysteresis allow for the direct comparison of complex storm events across a heterogeneous landscape and variable flow conditions.
Highlights
Concentration-discharge (C-Q) relationships provide fundamental insight into the mobilization and export of solutes and sediment from watersheds (Chorover et al, 2017)
Storms in the Luquillo Mountains reveal a high degree of complexity making the use of these metrics appropriate for this landscape (e.g., Figures 2A,D,G)
Our analysis showed that the mobilization and export of sediments are largely decoupled from that of solutes during storm events (Figure 3) and that solutes are generally source limited, while sediment is generally transport limited
Summary
Concentration-discharge (C-Q) relationships provide fundamental insight into the mobilization and export of solutes and sediment from watersheds (Chorover et al, 2017). Developing general models of solute and sediment behavior across a range of flow conditions would provide insights into watershed function as well as valuable guidance for sampling regimes that will most efficiently capture the impacts of large events. The study of C-Q relationships from individual events is challenging as individual events can be highly complex. Difficulty in interpreting C-Q patterns from individual storm events results from their spatial and temporal variability, their inherently ephemeral nature, and the challenges associated with traditional sampling approaches (e.g., grab samples) during short-term pulse events
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