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Titel |
Assessment of catchment-scale evapotranspiration via boundary condition switching versus root water uptake modeling |
VerfasserIn |
Matteo Camporese, Edoardo Daly, Claudio Paniconi |
Konferenz |
EGU General Assembly 2014
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 16 (2014) |
Datensatznummer |
250089083
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Publikation (Nr.) |
EGU/EGU2014-3272.pdf |
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Zusammenfassung |
Although being one of the fundamental terms of the hydrologic cycle at all scales,
evapotranspiration (ET ) is also one of the most difficult to model, because of its
dependency on many climatic and ecological factors. Therefore, practical applications
of hydrological models where ET plays a significant role are subjected to large
uncertainties.
Here we compare two methods to compute actual ET in CATHY (CATchment
HYdrology), a process-based coupled model of surface and subsurface flow that solves the
three-dimensional Richards equation for partially saturated porous media and a
one-dimensional diffusion wave approximation of the de Saint-Venant equation for overland
and channel routing.
The first method includes a sink term in the Richards equation to account for root water
uptake. The potential transpiration is distributed across the root depth as a function of the root
distribution and water stress is modeled using the reduction function suggested by Feddes.
Accordingly, in well-watered conditions the vegetation transpires at its potential rate,
while, when the soil dries below a certain value of soil moisture associated with
incipient water stress, transpiration reduces linearly until it reaches zero at the wilting
point.
The second method uses a switching procedure for the boundary conditions at the soil
surface relying on a pressure head, Ïmin. As long as the water potential at the soil
surface is larger than Ïmin, the boundary condition at the surface is a flux (Neumann
condition) that equals the potential evapotranspiration rate; when the water potential
reaches Ïmin, the boundary condition switches from a flux to a constant pressure
head (Dirichlet condition), and the evapotranspiration process becomes soil- and/or
vegetation-limited.
These two ET models are implemented in CATHY and applied to a paired catchment
experiment in southwestern Victoria, Australia, where two adjacent catchments with different
agricultural uses (grazing and blue gum plantation) provide an extensive hydrological data
set.
Our results show that the boundary condition-switching algorithm, with a proper choice
of Ïmin and for limited root depths, yields ET rates very similar to those computed by the
well established Feddes’ formulation and thus potentially represents a simple and effective
way to account for the impacts exerted on the catchment hydrological response by different
types of shallow-rooted vegetation. |
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