Explaining the Variability in High-Frequency Nitrate Export Patterns Using Long-Term Hydrological Event Classification

Runoff events play an important role in nitrate export from catchments, but the variability of export patterns between events and catchments is high and the dominant drivers remain difficult to disentangle. Here, we rigorously asses if detailed knowledge on runoff event characteristics can help to explain this variability. To this end, we conducted a long-term (1955-2018) event classification using hydro-meteorological data, including rainfall characteristics, soil moisture and snowmelt, in six neighboring mesoscale catchments with contrasting land use. We related these event characteristics to nitrate export patterns from high-frequency nitrate concentration monitoring (2013-2017) using concentration-discharge (CQ) relationships. Our results show that low-magnitude rainfall-induced events with dry antecedent conditions exported lowest nitrate concentrations and loads but exhibited highly variable CQ relationships. We explain this by a low fraction of active flow paths, revealing the spatial heterogeneity of nitrate sources within the catchments and by an increased impact of biogeochemical retention processes. In contrast, high-magnitude rainfall or snowmelt-induced events exported highest nitrate concentrations and loads and converged to similar chemostatic export patterns across all catchments, without exhibiting source limitation. We explain these homogeneous export patterns by high catchment wetness that activated a high number of flow paths and by higher nitrate availability during high-flow seasons. Long-term hydro-meteorological data indicated an increased number of events with dry antecedent conditions in summer and a decreased number of snow-influenced events. These trends will likely continue and cause increased nitrate concentration variability during low-flow seasons and changes in the timing of nitrate export peaks during high-flow seasons.

Automated summary

Runoff events are important for nitrate export, but there is high variability between events and catchments, making it difficult to identify the dominant drivers. This study investigates the relationship between event characteristics and nitrate export patterns in six neighboring catchments. The results show that low-magnitude rainfall events with dry antecedent conditions have the lowest nitrate concentrations and loads, while high-magnitude rainfall or snowmelt events have the highest concentrations and loads. These findings suggest that understanding event characteristics can help explain variability in nitrate export.