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| Titel |
Verifying the distributed temperature sensing Bowen ratio method for
measuring evaporation |
| VerfasserIn |
Bart Schilperoort, Miriam Coenders-Gerrits, Willem Luxemburg, Cesar Cisneros Vaca, Murat Ucer |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128495
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| Publikation (Nr.) |
 EGU/EGU2016-8486.pdf |
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| Zusammenfassung |
| Evaporation is an important process in the hydrological cycle, therefore measuring
evaporation accurately is essential for water resource management, hydrological management
and climate change models. Current techniques to measure evaporation, like eddy covariance
systems, scintillometers, or lysimeters, have their limitations and therefore cannot always be
used to estimate evaporation correctly. Also the conventional Bowen ratio surface energy
balance method has as drawback that two sensors are used, which results in large measuring
errors. In Euser et al. (2014) a new method was introduced, the DTS-based Bowen ratio
(BR-DTS), that overcomes this drawback. It uses a distributed temperature sensing technique
(DTS) whereby a fibre optic cable is placed vertically, going up and down along a
measurement tower. One stretch of the cable is dry, the other wrapped with cloth and
kept wet, akin to a psychrometer. Using this, the wet and dry bulb temperatures are
determined every 12.5 cm over the height, from which the Bowen ratio can be
determined.
As radiation and wind have an effect on the cooling and heating of the cable’s sheath as
well, the DTS cables do not necessarily always measure dry and wet bulb temperature
of the air accurately. In this study the accuracy in representing the dry and wet
bulb temperatures of the cable are verified, and evaporation observations of the
BR-DTS method are compared to Eddy Covariance (EC) measurements. Two ways to
correct for errors due to wind and solar radiation warming up the DTS cables are
presented: one for the dry cable and one for the wet cable. The measurements were
carried out in a pine forest near Garderen (The Netherlands), along a 46-meter tall
scaffold tower (15 meters above the canopy). Both the wet (Twet) and dry (Tdry)
temperature of the DTS cable were compared to temperature and humidity (from
which Twet is derived) observations from sensors placed along the height of the
tower.
Underneath the canopy, where there was barely any direct sunlight, the non-corrected
temperatures correlated well for both Tdry (R2=0.998) and Twet (R2=0.995). Above the
canopy the two temperature corrections worked well Tdry (R2=0.978) and Twet
(R2=0.979).
The comparison of the latent and sensible heat flux from the BR-DTS and the EC-system
was often not possible, due to large energy balance residuals estimated during north-eastern
winds (using an averaging interval of 30 minutes). For the limited days with other wind
directions the BR-DTS overestimated the latent and sensible heat flux. Additionally, we even
found that the applied temperature corrections resulted in a lower performance due
to increased uncertainties in the applied corrections. Furthermore, we found that
both the corrected and uncorrected DTS-temperatures resulted in rather similar
latent and sensible heat fluxes, due to the fact that BR-DTS applies gradients of
temperatures over the height, rather than absolute values. Hence, based on our limited
data, the correction methods are not recommended if one is interested in the fluxes. |
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