Using chemical fingerprints and process modeling to inform a parsimonious model for DOC export from a temperate headwater catchment

Dissolved organic carbon (DOC) concentrations in forested headwater streams have shown critical upward trends in the last decades with potentially harmful environmental consequences and potential impacts on drinking water production from downstream reservoirs. Using chemical fingerprints of DOC in the riparian zone (RZ) of a temperate headwater catchment in the Harz Mountains in Germany and a hydrologic process model for the riparian corridor, we could identify dominant stream flow generation processes and DOC source zones for a representative river reach (Werner et al. 2021). The gained local process understanding was used to adapt a parsimonious, box-type, hydrology-biogeochemistry model for the entire headwater catchment to reflect the dominant runoff generation and DOC mobilization processes using a threshold-controlled, surface flux mechanism for the RZ.  The model was used to simulate DOC export dynamics to a downstream drinking water reservoir. A multi-objective calibration on stream flow and instream DOC concentration (Kling-Gupta efficiencies of 0.79 and 0.73 for the hydrological and biogeochemical modules, respectively) yielded reasonable riparian zone water and DOC dynamics as well as stream DOC exports, which were in line with observations. Fast, surficial water flow components from the RZ accounted for the largest fraction of total DOC export.

Calibrating the hydrological module of the model to discharge first, followed by a consecutive calibration of the biogeochemical model to DOC flux, produced unrealistic groundwater (GW) dynamics and GW DOC concentrations, despite a reasonable match with observed discharge and stream DOC concentrations. In contrast, the multi-objective simultaneous calibration of both, the hydrologic and biogeochemical modules, yielded an internally consistent model with adequately simulated discharge and DOC at the catchment outlet. This highlights the strong coupling between catchment internal water partitioning and DOC mobilization and export, which cannot be captured, when calibrating water and solute fluxes separately.

References:

Werner, B.J., Musolff, A., Lechtenfeld, O.J., de Rooij, G.H., Oosterwoud, M.R., Fleckenstein, J.H. (2021) Small-scale topography explains patterns and dynamics of dissolved organic carbon exports from the riparian zone of a temperate, forested catchment, Hydrology and Earth System Sciences, 25, 6067–6086, https://doi.org/10.5194/hess-25-6067-2021