Temperature controls production but hydrology regulates export of dissolved organic carbon at the catchment scale
Authors: Wen Hang , Perdrial Julia , Abbott Benjamin W. , Bernal Susana , Dupas Rémi , Godsey Sarah E. , Harpold Adrian , Rizzo Donna , Underwood Kristen , Adler Thomas , Sterle Gary , Li Li 
Published: February 2020

Abstract. Lateral carbon flux through river networks is an
important and poorly understood component of the global carbon budget. This
work investigates how temperature and hydrology control the production and
export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills
Critical Zone Observatory in Pennsylvania, USA. Using field measurements of
daily stream discharge, evapotranspiration, and stream DOC concentration, we
calibrated the catchment-scale biogeochemical reactive transport model
BioRT-Flux-PIHM (Biogeochemical Reactive
Transport–Flux–Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash–Sutcliffe
efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate
and compare the daily DOC production rates (Rp; the sum of the local DOC
production rates in individual grid cells) and export rate (Re; the
product of the concentration and discharge at the stream outlet, or load). Results showed that daily Rp varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily Re
varied by more than 3 orders of magnitude and was strongly associated with
variation in discharge and hydrological connectivity. In summer, high
temperature and evapotranspiration dried and disconnected hillslopes from
the stream, driving Rp to its maximum but Re to its minimum. During
this period, the stream only exported DOC from the organic-poor groundwater
and from organic-rich soil water in the swales bordering the stream.
The DOC produced accumulated in hillslopes and was later flushed out during the
wet and cold period (winter and spring) when Re peaked as the stream
reconnected with uphill and Rp reached its minimum. The model reproduced the observed concentration–discharge (C–Q) relationship
characterized by an unusual flushing–dilution pattern with maximum
concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated
that this nonlinearity was caused by shifts in the relative contribution of
different source waters to the stream under different flow conditions. At low
discharge, stream water reflected the chemistry of organic-poor groundwater;
at intermediate discharge, stream water was dominated by the organic-rich
soil water from swales; at high discharge, the stream reflected uphill soil
water with an intermediate DOC concentration. This pattern persisted regardless
of the DOC production rate as long as the contribution of deeper groundwater
flow remained low (<18 % of the streamflow). When groundwater
flow increased above 18 %, comparable amounts of
groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C–Q patterns switched to a
flushing-only pattern with increasing DOC concentration at high discharge.
These results depict a conceptual model that the catchment serves as a
producer and storage reservoir for DOC under hot and dry conditions and
transitions into a DOC exporter under wet and cold conditions. This study
also illustrates how different controls on DOC production and export –
temperature and hydrological flow paths, respectively – can create temporal
asynchrony at the catchment scale. Future warming and increasing
hydrological extremes could accentuate this asynchrony, with DOC production
occurring primarily during dry periods and lateral export of DOC dominating
in major storm events.