Disentangling Scatter in Long-Term Concentration-Discharge Relationships: the Role of Event Types

Abstract. Relationships between nitrate concentrations and discharge rates (C-Q) at the catchment outlet can provide insights into sources, mobilization and biogeochemical transformations of nitrate within the catchment. Nitrate C-Q relationships often exhibit considerable scatter that might be related to variable hydrologic conditions during runoff events at sampling time, corresponding to variable sources and flow paths despite similar discharge rates. Although the origins of this scatter was investigated in individual catchments, the role of different runoff event types on the C-Q relationships across a large dataset of catchments was not yet evaluated.

In order to better understand the role of different runoff events in shaping long-term C-Q relationships, we analyzed low-frequency nitrate data from 184 German catchments, and quantified the deviation of samples collected during different types of events from the long-term power-law C-Q relationships. In most of the catchments, snow-impacted events produce positive deviations of concentrations, indicating an increased nitrate mobilization compared to the long-term pattern. In contrast, negative deviations occur mostly for rainfall-induced events with dry antecedent conditions, indicating lower nitrate concentrations. Pronounced differences in event runoff coefficients among different event types indicate their contrasting levels of hydrologic connectivity that in turn might play a key role controlling nitrate transport due to the activation of faster flow paths between sources and streams. Our study demonstrates using long-term, low-frequency nitrate data that runoff event types shape observed scatter in long-term C-Q relationships according to their level of hydrologic connectivity.