Patterns of long-term variations of nitrate concentration – Stream discharge relationships for a drained agricultural watershed in Mid-western USA

•We analyzed event C-Q hysteresis for a 43-yr dataset from a drained watershed.•Event-scale nitrate flushing patterns were controlled by antecedent conditions.•Most events had anti-clockwise hysteresis highlighting the role of near-drain zone.•Fertilization rates did not affect the direction of hysteresis patterns.•Intensive drainage may dampen N legacy in historically agricultural landscapes. Nitrate Concentration–discharge (C-Q) relationshipshave been used to infer nitrate sources, storage, reactions, and transport in watersheds, and to reveal key processes that control runoff chemistry. Yet, studies on long-term nitrate C-Q relationships are limited due to scarce high frequency (e.g., daily) concentration data. In this paper, using a long-term high-frequency dataset (1976–2019) comprising stream flow and nitrate concentrations, we quantitatively analyzed the long-term variations of event-scale hysteresis patterns (quantified by hysteresis index, HI, and flushing index, FI) to infer the leaching mechanisms of nitrate in an artificially drained agricultural watershed in Mid-western U.S. Our results revealed that most events exhibited anti-clockwise behaviors (HI