Disentangling the Variability in High-Frequency Nitrate Export Patterns Using Long-term Hydrological Event Classification

Runoff events play a dominant role for the mobilization and transport of nitrate from catchments to the downstream receiving water resources. However, the variability of nutrient export patterns among different 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 an event classification from long-term daily hydro-meteorological data (1955 2018), including antecedent wetness, snowmelt and the temporal organization of rainfall, in six neighboring mesoscale catchments with contrasting land use settings. We related these runoff event characteristics to nitrate export patterns from a shorter period of high-frequency (hourly) nitrate concentration monitoring from 2013 to 2017, using concentration-discharge relationships. Our results show that small rainfall-induced events with dry antecedent conditions exported lowest nitrate concentrations and loads but exhibited a high variability in their concentration-discharge relationships. We explain this by a low fraction of active flow path, revealing the spatial heterogeneity of nitrate sources within the catchments, and by an increased impact of biogeochemical retention processes. In contrast, large rainfall or snowmelt-induced events exported highest nitrate concentrations and loads, and converged to similar chemostatic export patterns across all catchments, showing no signs of source limitation even within forest-dominated catchments. This indicates that event size and catchment saturation rather than the meltwater fraction are the main drivers of nitrate export during large runoff events. The homogeneous export patterns can be explained by high catchment wetness that activated a high number of flow paths within the catchments. Long-term hydro-meteorological data indicated an increase of dry antecedent conditions in summer and a decrease of snow-influenced events. In view of the different export patterns for different event types, these trends will likely lead to an increasing variability in nitrate concentrations during summer and autumn and to changes in the timing of largest nitrate export peaks during winter and spring.