Combination of traditional nutrient load analysis and storm hydrograph separation unveil unexpected patterns in event-driven nutrient export dynamics in a rural headwater catchment

Abstract. First flush and dilution are major effects on solute export dynamics during precipitation events in headwater catchments but are hard to predict, even if catchment properties are well known. Normalized cumulative load (NCL) functions have been used to visualize and classify event-based discharge–load relationships, distinguishing between dilution, flushing, and linear export behavior. This study presents an enhanced version of the classical NCL function approach by combining it with hydrograph separation. Over an 18-month period, discharge and solute concentrations were monitored in an agriculturally influenced headwater catchment in the German low mountain ranges, with a focus on nitrate (NO₃⁻) and total phosphorus, and a complementary dataset of major ions. Discharge was separated by using stable water isotope signals into event water and total discharge. Both discharge components were then analyzed for solute loads (NO₃⁻, total phosphorus, and major ions). The results reveal significant differences in solute export dynamics between event water and total discharge, including unexpected similarities in the export patterns of nitrate and phosphorus. The proposed method also highlights a shift from predominantly linear export behavior in the total discharge (coefficient of variation = 0.13) to more pronounced first flush or dilution patterns in the event water (coefficient of variation = 0.36). These findings indicate a fundamental difference between the discharge processes governing the solute export dynamics of the catchment. While the signal of total event discharge indicates linear behavior, the separated event water exhibits strong flushing or dilution tendencies, likely linked to the activation of drainage systems and draining of NO₃⁻ legacy storages. The proposed method is straightforward to implement, yields statistically robust results for the dataset and provides new insights into solute input pathways in headwater catchments.