Hydrological versus biogeochemical controls on catchment nitrate export: a test of the flushing mechanism

Deciphering the connection between streamflows and nitrate (NO − 3 ) discharge requires identification of the various water flow pathways within a catchment, and the different time‐scales at which hydrological and biogeochemical processes occur. Despite the complexity of the processes involved, many catchments around the world present a characteristic flushing response of NO − 3 export. Yet the controls on the flushing response, and how they vary across space and time, are still not clearly understood. In this paper, the ‘flushing response’ of NO − 3 export from a rural catchment in Western Australia was investigated using isotopic (deuterium), chemical (chloride, NO − 3 ), and hydrometric data across different antecedent conditions and time‐scales. The catchment streamflow was at all time‐scales dominated by a pre‐event water source, and the NO − 3 discharge was correlated with the magnitude of areas contributing to saturation overland flow. The NO − 3 discharge also appeared related to the shallow groundwater dynamics. Thus, the antecedent moisture condition of the catchment at seasonal and interannual time‐scales had a major impact on the NO − 3 flushing response. In particular, the dynamics of the shallow ephemeral perched aquifer drove a shift from hydrological controls on NO − 3 discharge during the ‘early flushing’ stage to an apparent biogeochemical control on NO − 3 discharge during the ‘steady decline’ stage of the flushing response. This temporally variable control hypothesis provides a new and alternative description of the mechanisms behind the commonly seen flushing response.