Articles | Volume 20, issue 2
Research article
23 Feb 2016
Research article |  | 23 Feb 2016

Nitrate sinks and sources as controls of spatio-temporal water quality dynamics in an agricultural headwater catchment

Tobias Schuetz, Chantal Gascuel-Odoux, Patrick Durand, and Markus Weiler

Abstract. Several controls are known to affect water quality of stream networks during flow recession periods, such as solute leaching processes, surface water–groundwater interactions as well as biogeochemical in-stream turnover processes. Throughout the stream network, combinations of specific water and solute export rates and local in-stream conditions overlay the biogeochemical signals from upstream sections. Therefore, upstream sections can be considered functional units which could be distinguished and ordered regarding their relative contribution to nutrient dynamics at the catchment outlet. Based on snapshot sampling of flow and nitrate concentrations along the stream in an agricultural headwater during the summer flow recession period, we determined spatial and temporal patterns of water quality for the whole stream. A data-driven, in-stream-mixing-and-removal model was developed and applied for analysing the spatio-temporal in-stream retention processes and their effect on the spatio-temporal fluxes of nitrate from subcatchments. Thereby, we have been able to distinguish quantitatively between nitrate sinks, sources per stream reaches, and subcatchments, and thus we could disentangle the overlay of nitrate sink and source signals. For nitrate sources, we determined their permanent and temporal impact on stream water quality and for nitrate sinks, we found increasing nitrate removal efficiencies from upstream to downstream. Our results highlight the importance of distinct nitrate source locations within the watershed for in-stream concentrations and in-stream removal processes, respectively. Thus, our findings contribute to the development of a more dynamic perception of water quality in streams and rivers concerning ecological and sustainable water resource management.

Short summary
We quantify the spatio-temporal impact of distinct nitrate sinks and sources on stream network nitrate dynamics in an agricultural headwater. By applying a data-driven modelling approach, we are able to fully distinguish between mixing and dilution processes, and biogeochemical in-stream removal processes along the stream network. In-stream nitrate removal is estimated by applying a novel transfer coefficient based on energy availability.