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| Titel |
Simple physics-based models of compensatory plant water uptake: concepts and eco-hydrological consequences |
| VerfasserIn |
N. J. Jarvis |
| 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 ; 15, no. 11 ; Nr. 15, no. 11 (2011-11-16), S.3431-3446 |
| Datensatznummer |
250013023
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| Publikation (Nr.) |
 copernicus.org/hess-15-3431-2011.pdf |
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| Zusammenfassung |
| Many land surface schemes and simulation models of plant growth designed for
practical use employ simple empirical sub-models of root water uptake that
cannot adequately reflect the critical role water uptake from sparsely
rooted deep subsoil plays in meeting atmospheric transpiration demand in
water-limited environments, especially in the presence of shallow
groundwater. A failure to account for this so-called "compensatory" water
uptake may have serious consequences for both local and global modeling of
water and energy fluxes, carbon balances and climate. Some purely empirical
compensatory root water uptake models have been proposed, but they are of
limited use in global modeling exercises since their parameters cannot be
related to measurable soil and vegetation properties. A parsimonious
physics-based model of uptake compensation has been developed that requires
no more parameters than empirical approaches. This model is described and
some aspects of its behavior are illustrated with the help of example
simulations. These analyses demonstrate that hydraulic lift can be
considered as an extreme form of compensation and that the degree of
compensation is principally a function of soil capillarity and the ratio of
total effective root length to potential transpiration. Thus, uptake
compensation increases as root to leaf area ratios increase, since potential
transpiration depends on leaf area. Results of "scenario" simulations for
two case studies, one at the local scale (riparian vegetation growing above
shallow water tables in seasonally dry or arid climates) and one at a global
scale (water balances across an aridity gradient in the continental USA),
are presented to illustrate biases in model predictions that arise when
water uptake compensation is neglected. In the first case, it is shown that
only a compensated model can match the strong relationships between water
table depth and leaf area and transpiration observed in riparian forest
ecosystems, where sparse roots in the capillary fringe contribute a
significant proportion of the water uptake during extended dry periods. The
results of the second case study suggest that uncompensated models may give
biased estimates of long-term evapotranspiration at the continental scale.
In the example presented here, the uncompensated model underestimated total
evapotranspiration by 5–7% in climates of intermediate aridity, while the
ratio of transpiration to evaporation was also smaller than for the
compensated model, especially in arid climates. It is concluded that the
parsimonious physics-based model concepts described here may be useful in
the context of eco-hydrological modeling at local, regional and global
scales. |
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