Combining Traditional Nutrient Load Analysis with Storm Hydrograph Separation Reveals Concealed Patterns in Event-Driven Nutrient Export in a Rural Headwater Catchment

Flushing and dilution are major phenomena of 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 constant 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 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 total phosphorus. The proposed method also highlights a shift from predominantly constant export behavior in the total discharge (coefficient of variation: 0.13) to more pronounced flushing or dilution patterns in the event water (coefficient of variation: 0.36). These findings indicate a fundamental difference between the hydrological processes governing the solute export dynamics of the catchment. While the signal of total event discharge indicates constant behavior, the separated event water exhibits strong flushing or dilution tendencies. The observed shifts in the export patterns, which are likely linked to the activation of drainage systems and depletion of NO3- legacy storages, raise the question if the event water fraction should be monitored more closely in terms of its potential for dynamic pollutant transport. 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.