Low hydrological connectivity after summer drought inhibits DOC export in a forested headwater catchment

Abstract

Abstract. Understanding the controls on event-driven dissolved organic carbon (DOC) export is crucial as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesized that topography is a key driver of DOC export in headwater catchments because it influences hydrological connectivity, which can inhibit or facilitate DOC mobilization. To test this hypothesis, 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 measured at an interval of 15 min using in situ UV-Vis (ultraviolet–visible) spectrometry from June 2018 until October 2020 at two topographically contrasting subcatchments of the same stream. At the upper location (888 m above sea level, a.s.l.), the stream drains steep hillslopes, whereas, at the lower location (771 m a.s.l.), it drains a larger area, including a flat and wide riparian zone. We focus on four events with contrasting antecedent wetness conditions and event size. During the 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 counterclockwise 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 DOC load up to 7.4 kg h−1. The 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 low 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 wetness conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of subcatchments, depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different subcatchments to DOC export may change in the future when hydrological connectivity will be reduced more often.

Anthropogenically driven climate and landscape change effects on inland water carbon dynamics: What have we learned and where are we going?

“…In contrast, drought conditions disconnect surface and subsurface flowpaths (Kleine et al, 2021; Lake, 2003; Murphy et al, 2018) and have the opposite effects on C inputs. Lower precipitation and discharge reduce OC inputs (Blaurock et al, 2021; Dahm et al, 2003), while higher contributions from CO 2 ‐rich groundwater during periods with low discharge result in higher in‐situ CO 2 concentrations (Dinsmore & Billett, 2008; Öquist et al, 2009). Hence, both the intensity and frequency of precipitation events are important determinants of OC vs. IC inputs to inland waters.…”

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

“…no gaps in chemical data for the event dates were allowed because we were ultimately interested in stream chemical responses). Each event included all days classified as storm flow for the corresponding time period, plus the last day classified as base flow prior to the days classified as storm flow, which was considered to be the starting date of the event (Blaurock et al, 2021). This strategy ensured that the relevant magnitude and range of the hydroclimatic descriptors associated with each event were covered.…”

“…However, riparian zones that experience frequent dryness, such as those in Mediterranean catch-ments, can manifest nitrification and be net sources of NO − 3 to streams (Lupon et al, 2016a). As a result of these riparian zone characteristics, increased DOC concentrations during storm events in forest headwaters are an almost universal phenomenon (Hinton et al, 1997;Boy et al, 2008;Yang et al, 2015;Musolff et al, 2018), whereas stream NO − 3 concentrations can increase, decrease, or remain unchanged during storms (Àvila et al, 1992;Bernal et al, 2002;Inamdar and Mitchell, 2006;Dupas et al, 2017).…”

Riparian Microtopography Affects Event‐Driven Stream DOC Concentrations and DOM Quality in a Forested Headwater Catchment

“…Indeed, the riparian zone is generally regarded to be of great importance for DOC export due to the accumulation of soil organic matter and the often high groundwater level (Grabs et al., 2012; Inamdar & Mitchell, 2007; Ledesma et al., 2015; Ledesma, Kothawala, et al., 2018; Mei et al., 2014; Musolff et al., 2018; Ploum et al., 2020; Strohmeier et al., 2013). Therefore, hydrological connectivity between the riparian zone and streams has important implications for DOC export (Birkel et al., 2017; Bishop et al., 2004; Blaurock et al., 2021; Broder et al., 2017; Werner et al., 2021). Hydrological connectivity and, therefore, runoff and solute response to precipitation are dependent on the antecedent hydrological conditions in a catchment as well as individual event characteristics (Blaurock et al., 2021; Detty & McGuire, 2010; McGuire & McDonnell, 2010; Penna et al., 2015).…”