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| Titel |
A comparison between wet canopy evaporation estimated by stable isotope ratios of water and canopy interception measured by water balance |
| VerfasserIn |
Shigeki Murakami, Shohei Hattori, Ryu Uemura |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146873
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| Publikation (Nr.) |
 EGU/EGU2017-10933.pdf |
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| Zusammenfassung |
| Some papers proved that canopy interception is proportional to rainfall not only on a rain
event basis but also on an hourly basis (e.g. Murakami, 2006, J. Hydrol.; Saito
et al., 2013, J. Hydrol.). However, theoretically, evaporation does not depend on
rainfall amount. These results are enigmatic and we need to reevaluate wet canopy
evaporation.
We measured gross rainfall and net rainfall in a plastic Christmas tree stand with a height
of 165 cm placed on a 180-cm square tray as described in Murakami and Toba (2013, Hydrol.
Res. Lett.). The measurement was conducted outside under natural rainfall. We also estimated
wet canopy evaporation using stable isotope ratios of water. During a rain event, we manually
sampled gross and net rainwater on an hourly basis. Evaporation was calculated using the
difference between the δ18O (or δ2H) values in gross and net rainfall using isotope
fractionation factor.
Total gross rainfall in a target rain event in October, 2014, was 28.0 mm and net rainfall
(discharge from the tray) was 22.7 mm, i.e. canopy interception was 5.3 mm (18.9% of gross
rainfall). The δ18O (or δ2H) value in net rainfall was higher than that in gross rainfall
because of fractionation by evaporation on wet canopy surface. Hourly evaporation
calculated by the values of δ18O varied from 2% to 24% of gross rainfall, and the
weighted average by hourly gross rainfall was 5.2% of gross rainfall. Further, we
estimated rainfall interception using a tank model (Yoshida et al., 1993) assuming
constant evaporation rate, i.e. 20% of gross rainfall. Total net rainfall calculated by
the model was 23.1 mm, i.e. calculated canopy interception was 4.9 mm (17.5%
of gross rainfall). Then, keeping the parameters of the model, we simulated net
rainfall using hourly surface evaporation obtained by the δ18O values. Calculated net
rainfall was 25.6 mm, i.e. wet canopy evaporation was only 2.4 mm (8.6% of gross
rainfall).
So far, possible explanation of the discrepancy between the two methods is evaporation of
small droplets produced by raindrops splashed onto the canopy (Murakami, 2006). Assuming
that the canopy interception loss consists of the wet canopy evaporation and the splash
droplet evaporation, amount of the splash droplet evaporation would be equal to the
difference between the two methods. The result using δ2H will be shown at the time of
presentation.
This work was supported by JSPS KAKENHI Grant Number JP15H40250. |
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