Patterns and dynamics of dissolved organic carbon exports from a riparian zone of a temperate, forested catchment

Abstract. Export of dissolved organic carbon (DOC) from riparian zones (RZs) is an important, but poorly understood component of temperate catchment carbon budgets. This paper delineates explicit DOC source zones within the RZ of a small forested catchment in central Germany, and identifies and quantifies their dominant DOC export mechanism at high spatio-temporal resolution. Stream water DOC samples from differing hydrological situations were compared to riparian DOC groundwater and surface water samples and classified chemically (via Fourier-transform ion cyclotron resonance mass spectrometry) and spatially via a small-scale topographic analysis of the RZ at a resolution of 1 m. Explicit water fluxes from the resulting riparian DOC source zones were then simulated by a physically-based, fully-integrated numerical flow model (HydroGeoSphere). Chemical classification revealed two distinct DOC pools (DOCI and DOCII) in the RZ. The comparison of stream and riparian water samples indicated a predominant export of DOCI during wet conditions and high groundwater levels. The two DOC pools were spatially separated and mapped using a threshold value in high-resolution topographical wetness index (TWIHR). Hydrological modelling revealed that surface runoff from DOCI source zones with high TWIHR values dominated overall discharge generation and therefore DOC export. Although corresponding to only 15 % of the area in the studied RZ, the high TWIHR zones provided in total 1.5 times the load of DOC from the remaining 85 % of the area associated with the DOCII pool. Our results suggest that surface DOC export can play a dominant role for DOC export in RZs with overall low topographic relief and should be considered in DOC export models. We propose that proxies of spatial heterogeneity (here: TWIHR) can delineate the most active riparian source zones and provide a meaningful basis for improved model conceptualization of surficial DOC export.