Inferring drivers of nitrate and sediment event dynamics from hysteresis metrics for two large agricultural watersheds

Excess nitrate and sediment, mobilized by precipitation events and transported into surface waters, is a global water quality challenge. Recent advances in high-frequency in situ water quality monitoring sensors have created opportunities to investigate constituent concentration dynamics during short-term hydrological changes. In this study, we characterized the event-scale variability of nitrate (NO3-) and turbidity (a surrogate for sediment transport) in two large agricultural watersheds of the Upper Mississippi River Basin using hysteresis loop characteristics to determine sources and dominant transport mechanisms. We then applied factor analysis to detect variable groupings and thus determine controls on nitrate and sediment dynamics. We observed consistent counterclockwise NO3- hysteresis patterns between the two watersheds. This was indicative of distal contributions and/or late-event mobilization and flushing, which was controlled by the characteristics of the event hydrology (such as, event duration and magnitude of event discharge). However, turbidity hys-teresis loops indicated different sediment delivery behaviours between the two watersheds. The smaller watershed with more diverse land use demonstrated consis-tent clockwise hysteresis patterns indicating early event flushing or rapidly responding pathways. The time lag between turbidity and discharge peaks was identified as a driver of hysteresis magnitude for the smaller site. In contrast, the larger and more agricultural watershed showed variability between dilution versus flushing as well as delivery pathways between events. The hysteresis magnitude was driven by the event peak discharge and discharge range demonstrating an increase in stream power with scale influenced sediment dynamics at the larger site that switched behaviour. This result is critical for watershed water quality management, especially in the con-text of a changing climate and further underscores the utility of high-frequency sen-sors monitoring data to offer deep insights into hydrological processes controls on contaminant transport and delivery.