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| Titel |
HESS Opinions: From response units to functional units: a thermodynamic reinterpretation of the HRU concept to link spatial organization and functioning of intermediate scale catchments |
| VerfasserIn |
E. Zehe, U. Ehret, L. Pfister, T. Blume, B. Schröder, M. Westhoff, C. Jackisch, S. J. Schymanski, M. Weiler, K. Schulz, N. Allroggen, J. Tronicke, L. van Schaik, P. Dietrich, U. Scherer, J. Eccard, V. Wulfmeyer, A. Kleidon |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 18, no. 11 ; Nr. 18, no. 11 (2014-11-26), S.4635-4655 |
| Datensatznummer |
250120532
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| Publikation (Nr.) |
 copernicus.org/hess-18-4635-2014.pdf |
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| Zusammenfassung |
| 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 allows 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. |
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