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Abstract:
According to Dooge (1986) intermediate-scale
catchments are systems of organized complexity, being too
organized and yet too small to be characterized on a statistical/
conceptual basis, but too large and too heterogeneous to
be characterized in a deterministic manner. A key requirement
for building structurally adequate models precisely for
this intermediate scale is a better understanding of how different
forms of spatial organization affect storage and release
of water and energy. Here, we propose that a combination
of the concept of hydrological response units (HRUs) and
thermodynamics offers several helpful and partly novel perspectives
for gaining this improved understanding. Our key
idea is to define functional similarity based on similarity of
the terrestrial controls of gradients and resistance terms controlling
the land surface energy balance, rainfall runoff transformation,
and groundwater storage and release. This might
imply that functional similarity with respect to these specific
forms of water release emerges at different scales, namely
the small field scale, the hillslope, and the catchment scale.
We thus propose three different types of “functional units”
– specialized HRUs, so to speak – which behave similarly
with respect to one specific form of water release and with a
characteristic extent equal to one of those three scale levels.
We furthermore discuss an experimental strategy based on
exemplary learning and replicate experiments to identify and
delineate these functional units, and as a promising strategy
for characterizing the interplay and organization of water and
energy fluxes across scales. We believe the thermodynamic
perspective to be well suited to unmask equifinality as inherent
in the equations governing water, momentum, and energy
fluxes: this is because several combinations of gradients
and resistance terms yield the same mass or energy flux and
the terrestrial controls of gradients and resistance terms are
largely independent. We propose that structurally adequate
models at this scale should consequently disentangle driving gradients and resistance terms, because this optionally equifinality to be partly reduced by including available observations,
e.g., on driving gradients. Most importantly, the
thermodynamic perspective yields an energy-centered perspective
on rainfall-runoff transformation and evapotranspiration,
including fundamental limits for energy fluxes associated
with these processes. This might additionally reduce
equifinality and opens up opportunities for testing thermodynamic
optimality principles within independent predictions
of rainfall-runoff or land surface energy exchange. This is
pivotal to finding out whether or not spatial organization in catchments is in accordance with a fundamental organizing principle.
