Observing unseen flowlines and their contribution to near stream endmembers in forested headwater catchments.

The general scope of the PhD research project falls within the framework of developing integrated
catchment hydro-biogeochemical theories in the context of the Critical Zone (CZ).
Significant advances in the understanding of water transit time theory, subsurface structure
controls, and the quantification of catchment scale weathering rates have resulted in the
convergence of classical biogeochemical and hydrological theories. This will potentially
pave the way for a more mechanistic understanding of CZ because many challenges still
exist. Perhaps the most difficult of all is a unifying hydro-biogeochemical theory that can
compare catchments across gradients of climate, geology, and vegetation. Understanding
the processes driving the evolution of chemical tracers as they move through space and
time is of cardinal importance to validating mixing hypotheses and assisting in determining
the residence time of water in CZ.
The specific aim of the study is to investigate what physical and biogeochemical processes
are driving variations in observable endmembers in stream discharge as a function of the
hydrological state at headwater catchment scale. This requires looking beyond what can be
observed in the stream and what is called ”unseen flowlines” in this thesis. The Weierbach
Experimental Catchment (WEC) in Luxembourg provides a unique opportunity to study
these processes, with an extensive biweekly groundwater chemistry dataset spanning over
ten years. Additionally, WEC has been the subject of numerous published works in
the domain of CZ science, adding to an already detailed hydrological and geochemical
understanding of the system.
Multivariate analysis techniques were used to identify the unseen flowlines in the catchment.
Together with the excising hydrological perception model and a geochemical modelling
approach, these flowlines were rigorously investigated to understand what processes drive
their respective manifestations in the system. The existing perceptual model for WEC was
updated by the new findings and tested on 27 flood events to assess if it could adequately
explain the c − Q behaviour observed during these periods. The novelty of the study lies
in the fact that it uses both data-driven modelling approaches and geochemical processbased
modelling to look beyond what can be observed in the near-stream environment of
headwaters.