Missing connectivity during summer drought controls DOC mobilization and export in a small, forested catchment

Abstract

Abstract. Understanding the controls on event-driven DOC export is crucial, as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesize that topography is a key driver of DOC export because it influences hydrologic connectivity, which can inhibit or facilitate DOC mobilization. To test this we studied the mechanisms controlling DOC mobilization and export in the Große Ohe catchment, a forested headwater in a mid-elevation mountainous region in Southeastern Germany. Discharge and stream DOC concentrations were continuously measured using in-situ UV-Vis spectrometry from June 2018 until October 2020 at two topographically contrasting sub-catchments of the same stream: at a steep hillslope (888 m.a.s.l.) and in a flat and wide riparian zone (771 m.a.s.l). We focus on four events with contrasting antecedent hydrological conditions and event size. During events, in-stream DOC concentrations increased up to 19 mg L−1 in comparison to 2–3 mg L−1 during baseflow. The concentration-discharge relationships exhibited pronounced but almost exclusively anti-clockwise hysteresis loops, which were generally wider in the lower catchment than in the upper catchment due to a delayed DOC mobilization in the flat riparian zone. The riparian zone released considerable amounts of DOC, which led to a total DOC load up to 522 kg per event. The total DOC load increased with the total catchment wetness. We found a disproportionally high contribution to the total DOC export of the upper catchment during events following a long dry period. We attribute this to the lack of hydrological connectivity in the lower catchment during drought, which inhibited DOC mobilization, especially at the beginning of the events. Our data show that not only event size but also antecedent hydrological conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of different catchment parts depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different catchment parts to DOC export may change in the future, when hydrological connectivity will occur less often.

Delineating Source Contributions to Stream Dissolved Organic Matter Composition Under Baseflow Conditions in Forested Headwater Catchments

“…Rainfall and snowmelt events also occurred during the study period. Their effect on DOC mobilization is discussed in a separate paper by Blaurock et al (2021).…”

“…The objective of this study is to characterize the spatial changes in DOC concentration and DOM composition along a low‐order stream in a temperate, mountainous forested catchment in southern Germany with a focus on baseflow conditions. Previous work has shown that DOC export during rain events reflects varying contribution of different parts of the catchment (upper and lower) triggered by antecedent hydrological conditions and hydrological connectivity (Blaurock et al., 2021). Due to the strong gradient in catchment properties between upper and lower part (mean slope, proportion of hydromorphic soils, see below), we hypothesize that also during baseflow, the DOM composition in the steep (upper) part of the catchment is mainly driven by groundwater input, while in the flat (lower) part, also varying contribution from riparian topsoil shape the DOM composition in the stream.…”

Hydrology and riparian forests drive carbon and nitrogen supply and DOC:NO3− stoichiometry along a headwater Mediterranean stream

“…Each event included all days classified as storm flow for the corresponding time period, plus the day classified as base flow prior the days classified as storm flow, which was considered as the starting date of the event. This strategy followed Blaurock et al (2021) and ensured that the relevant magnitude and range of the associated hydroclimatic event https://doi.org/10.5194/hess-2021-401 Preprint. Discussion started: 12 August 2021 c Author(s) 2021.…”

Seasonal and storm event-based dynamics of dissolved organic carbon (DOC) concentration in a Mediterranean headwater catchment

“…Precipitation activates direct wet deposition and indirect dry deposition deriving from vegetation canopy and stem (Song et al, 2021) and soil erosion (Chaplot and Mutema, 2021). This contribution to total DOC export is further emphasized if the wet event occurs at the end of prolonged dry periods (Blaurock et al, 2021). Fazekas et al (2020) highlighted that anomalous events lasting overall less than 20 days in a year could define the annual behaviour of the relationships between streamflow and organic matter concentration.…”

“…Within the same catchment, significant differences can arise, which cannot be fully captured by a single downstream monitoring section that integrates heterogeneous upstream biogeochemical signals. It is a typical problem of scale (Lowe et al, 2006;McGuire et al, 2014), which also affects streamwater chemistry and needs to be addressed with innovative theoretical concepts and technical approaches, including intensive spatially distributed monitoring campaigns in nested sections of the same catchment (McGuire et al, 2014;Blaurock et al, 2021). DOC dynamics monitoring across different spatial and temporal scales is possible thanks to the advancements in optical aquatic sensors technology.…”