Surprising consistency in event-scale nitrate export patterns across catchments

High nitrate concentrations in groundwater and surface water threaten drinking water quality and the integrity of aquatic ecosystems. Discharge events can play a disproportionate role in nitrate mobilization and transport from source to stream, while observed inter-event variability in export patterns is often high. One approach for analyzing the variability of nitrate export is the relationship between nitrate concentrations and discharge. Such C-Q relationships applied across different time scales can inform about source availability (or limitation) of the specific solute and hydrological connectivity to the stream network. Recent studies revealed striking differences between long-term and event-scale C-Q relationships for nitrate, and further that inter-event variability in C-Q relationships decreases with event magnitude (Knapp et al., 2020; Musolff et al., 2021; Winter et al., 2022). This suggests that an integrated measure for nitrate export from long-term data may be insufficient to understand these mechanisms. Here, we hypothesize that event-specific nitrate export patterns systematically diverge from long-term patterns and converge towards chemostatic or dilution patterns at high-magnitude events, depending on the availability and hydrological connectivity of nitrate sources within the catchment. To verify this hypothesis, we analyzed C-Q relationships across 41 catchments in the U.S., using daily discharge and nitrate concentration data. We compared long-term and event-specific C-Q relationships for 5067 discharge events and described inter-event variability in relation to event magnitude. We found that the long-term C-Q relationship was more dynamic than the one averaged for individual events and that the variability of event-specific C-Q slopes significantly decreased with event magnitude, indicating that different mechanisms of source mobilization and transport operate at different time scales and event magnitudes. Notably, high-magnitude events converged towards chemostatic patterns and rarely showed evidence of dilution and thus source limitation, which might hint at substantial nitrogen legacies. The divergence between long-term and event-specific C-Q slopes increased with the share of agricultural area and fertilizer application in the catchment. The consistent patterns in long-term and event-scale nitrate export patterns across a large number of catchments allow us to relate these patterns with the availability, spatial distribution and hydrological connectivity of nitrate sources within the catchments. As such, our study is an important step towards understanding the relevant mechanisms for nitrate mobilization and transport during runoff events.

References

Knapp, J. L., Freyberg, J. von, Studer, B., Kiewiet, L., and Kirchner, J. W.: Concentration-discharge relationships vary among hydrological events, reflecting differences in event characteristics, Hydrol. Earth Syst. Sci. Discuss., 1–27, https://doi.org/10.5194/hess-24-2561-2020, 2020.

Musolff, A., Zhan, Q., Dupas, R., Minaudo, C., Fleckenstein, J. H., Rode, M., Dehaspe, J., and Rinke, K.: Spatial and Temporal Variability in Concentration-Discharge Relationships at the Event Scale, Water Resour. Res., n/a, e2020WR029442, https://doi.org/10.1029/2020WR029442, 2021.

Winter, C., Tarasova, L., Lutz, S. R., Musolff, A., Kumar, R., and Fleckenstein, J. H.: Explaining the Variability in High-Frequency Nitrate Export Patterns Using Long-Term Hydrological Event Classification, Water Resour. Res., 58, e2021WR030938, https://doi.org/10.1029/2021WR030938, 2022.