HGS RESEARCH HIGHLIGHT – Small-scale topography explains patterns and dynamics of dissolved organic carbon exports from the riparian zone of a temperate, forested catchment
Werner, B. J., Lechtenfeld, O. J., Musolff, A., de Rooij, G. H., Yang, J., Gründling, R., Werban, U., & 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. In Hydrology and Earth System Sciences (Vol. 25, Issue 12, pp. 6067–6086). Copernicus GmbH. https://doi.org/10.5194/hess-25-6067-2021
“[T]he physically based model [HydroGeoSphere] independently showed that the dominant runoff generating mechanism (and thus potential [dissolved organic carbon (DOC)] export pathway) at the study site is surface runoff from DOCI source zones. ”
Fig. 2. The boundary conditions of the study site (indicated by polygon A–D). Qup: groundwater influx (const.), : channel influx (dynamic), Qdown: groundwater leaving the model site (dynamic), and : surface water leaving the model site through channel outlet and through CD (dynamic), Qleft and Qright: groundwater rechargerate from side boundaries (const.). Red arrows indicate flow direction of the water. The 25 white points indicate wells which were used for model calibration. Labels of the piezometer run from a to e (lines) and 1 to 5 (columns).
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Examining the intricate dynamics of dissolved organic carbon (DOC) exports from riparian zones (RZs), a recent study conducted by a team of researchers highlights the predominant processes governing DOC export.
This research, conducted in a small forested catchment in central Germany from February 2017 to July 2019, unveils that DOC export is primarily influenced by the microtopography of the RZ (lateral variability) and riparian groundwater level dynamics (temporal variability). By employing Fourier transform ion cyclotron resonance mass spectrometry, the study classifies riparian groundwater into two distinct DOC clusters: plant-derived, aromatic, and oxygen-rich DOC with high concentrations (DOCI), and microbially processed, saturated, and heteroatom-enriched DOC with lower concentrations (DOCII).
Mapping the high-resolution topographic wetness index (TWIHR) within the study area, researchers observed a close association between DOC clusters and specific TWIHR values. Notably, despite covering only 15% of the RZ area, DOCI source zones contributed 1.5 times more to DOC export than DOCII source zones. This underscores the critical role of microtopography in dictating DOC export dynamics.
Furthermore, numerical water flow modelling using HydroGeoSphere (HGS) revealed that surface runoff from high-TWIHR zones, linked to the DOCI cluster, predominantly governs DOC export. These findings highlight the significance of surface runoff in DOC exports from RZs with low topographic relief, challenging previous conceptualizations that primarily focused on vertical distribution of DOC sources.
The study proposes that proxies of spatial heterogeneity, such as TWIHR, can delineate active source zones, offering a mechanistic basis for enhancing model conceptualization of DOC exports. By bridging small-scale mechanistic understanding with larger-scale model descriptions, this research paves the way for improved management of DOC exports and better preservation of water quality in temperate catchments.
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Fig. 5. High-resolution TWI (TWIHR) map of the modeled site (excluding hillslopes). White points indicate sampling locations; the Rappbode stream is indicated by the blue line. Black polygons are high-TWIHR zones, indicating DOCI source areas.
In this study, researchers aimed to better understand how dissolved organic carbon (DOC) moves from riparian zones (RZs) into streams by combining detailed field measurements, chemical analysis, and computer modeling. The findings reveal two distinct types of DOC, termed DOCI and DOCII, in the riparian zone. DOCI, originating from decomposing plant material, is found in wetter areas and is more easily removed during water treatment processes. On the other hand, DOCII, influenced by microbial processes, exhibits greater variability across seasons and is more persistent in surface water. By mapping high-resolution topographic wetness, researchers were able to identify and separate these two DOC sources. The modeling efforts showed that surface runoff, particularly from DOCI source zones, plays a significant role in DOC export from the riparian zone, contributing more during certain weather conditions. This challenges previous assumptions and underscores the importance of considering surface runoff in DOC export models. Understanding these mechanisms at a detailed level helps predict DOC export better under different climatic conditions. Additionally, the study highlights the potential for using microtopography and groundwater levels to manage DOC in streams effectively. By targeting specific source areas and flow paths, they can potentially reduce DOC levels in streams with minimal disturbance to the ecosystem, offering a cost-effective approach for stream management.
Abstract:
Export of dissolved organic carbon (DOC) from riparian zones (RZs) is an important component of temperate catchment carbon budgets, but export mechanisms are still poorly understood. Here we show that DOC export is predominantly controlled by the microtopography of the RZ (lateral variability) and by riparian groundwater level dynamics (temporal variability). From February 2017 until July 2019 we studied topography, DOC quality and water fluxes and pathways in the RZ of a small forested catchment and the receiving stream in central Germany. The chemical classification of the riparian groundwater and surface water samples (n=66) by Fourier transform ion cyclotron resonance mass spectrometry revealed a cluster of plant-derived, aromatic and oxygen-rich DOC with high concentrations (DOCI) and a cluster of microbially processed, saturated and heteroatom-enriched DOC with lower concentrations (DOCII). The two DOC clusters were connected to locations with distinctly different values of the high-resolution topographic wetness index (TWIHR; at 1 m resolution) within the study area. Numerical water flow modeling using the integrated surface–subsurface model HydroGeoSphere revealed that surface runoff from high-TWIHR zones associated with the DOCI cluster (DOCI source zones) dominated overall discharge generation and therefore DOC export. Although corresponding to only 15 % of the area in the studied RZ, the DOCI source zones contributed 1.5 times the DOC export of the remaining 85 % of the area associated with DOCII source zones. Accordingly, DOC quality in stream water sampled under five event flow conditions (n=73) was closely reflecting the DOCI quality. Our results suggest that DOC export by surface runoff along dynamically evolving surface flow networks can play a dominant role for DOC exports from RZs with overall low topographic relief and should consequently be considered in catchment-scale DOC export models. We propose that proxies of spatial heterogeneity such as the TWIHR can help to delineate the most active source zones and provide a mechanistic basis for improved model conceptualization of DOC exports.