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| Titel |
A thermodynamic approach to link self-organization, preferential flow and rainfall-runoff behaviour |
| VerfasserIn |
E. Zehe, U. Ehret, T. Blume, A. Kleidon, U. Scherer, M. Westhoff |
| 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 ; 17, no. 11 ; Nr. 17, no. 11 (2013-11-01), S.4297-4322 |
| Datensatznummer |
250085980
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| Publikation (Nr.) |
 copernicus.org/hess-17-4297-2013.pdf |
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| Zusammenfassung |
| This study investigates whether a thermodynamically optimal hillslope
structure can, if existent, serve as a first guess for uncalibrated
predictions of rainfall–runoff. To this end we propose a thermodynamic
framework to link rainfall–runoff processes and dynamics of potential
energy, kinetic energy and capillary binding energy in catchments and
hillslopes. The starting point is that hydraulic equilibrium in soil corresponds
to local thermodynamic equilibrium (LTE), characterized by a local maximum
entropy/minimum of free energy of soil water. Deviations from LTE occur
either due to evaporative losses, which increase absolute values of negative
capillary binding energy of soil water and reduce its potential energy, or
due to infiltration of rainfall, which increases potential energy of soil
water and reduces the strength of capillary binding energy. The amplitude
and relaxation time of these deviations depend on climate, vegetation, soil
hydraulic functions, topography and density of macropores. Based on this
framework we analysed the free energy balance of hillslopes within numerical
experiments that perturbed model structures with respect to the surface
density of macropores. These model structures have been previously shown to
allow successful long-term simulations of the water balances of the
Weiherbach and the Malalcahuello catchments, which are located in distinctly
different pedological and climatic settings. Our findings offer a new
perspective on different functions of preferential flow paths depending on
the pedological setting. Free energy dynamics of soil water in the cohesive
soils of the Weiherbach is dominated by dynamics of capillary binding
energy. Macropores act as dissipative wetting structures by enlarging water
flows against steep gradients in soil water potential after long dry spells.
This implies accelerated depletion of these gradients and faster relaxation
back towards LTE. We found two local optima in macropore density that
maximize reduction rates of free energy of soil water during rainfall-driven
conditions. These two optima exist because reduction rates of free energy
are, in this case, a second-order polynomial of the wetting rate, which
implicitly depends on macroporosity. An uncalibrated long-term simulation of
the water balance of the Weiherbach catchment based on the first optimum
macroporosity performed almost as well as the best fit when macroporosity
was calibrated to match rainfall–runoff. In the Malalcahuello catchment we
did not find an apparent optimum density of macropores, because free energy
dynamics of soil water during rainfall-driven conditions is dominated by increases of
potential energy. Macropores act as dissipative drainage structures by
enhancing export of potential energy. No optimum macropore density exists in
this case because potential energy change rates scale linearly with the
wetting rate. We found, however, a distinguished macroporosity that assures
steady-state conditions of the potential energy balance of the soil, in the
sense that average storage of potential energy is compensated by average
potential energy export. This distinguished macroporosity was close to the value
that yielded the best fit of rainfall–runoff behaviour during a calibration
exercise and allowed a robust estimate of the annual runoff coefficient. Our
findings are promising for predictions in ungauged catchments (PUB) as the
optimal/distinguished model structures can serve as a first guess for
uncalibrated predictions of rainfall–runoff. They also offer an alternative
for classifying catchments according to their similarity of the free energy
balance components. |
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