Technical note: Using distributed temperature sensing for Bowen ratio evaporation measurements
Rapid improvements in the precision and spatial resolution of distributed temperature sensing (DTS) technology now allow its use in hydrological and atmospheric sciences. Introduced by ) is the use of DTS for measuring the Bowen ratio (BR-DTS), to estimate the sensible and latent heat flux. The...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-01-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | https://www.hydrol-earth-syst-sci.net/22/819/2018/hess-22-819-2018.pdf |
| Summary: | Rapid improvements in the precision and spatial resolution of
distributed temperature sensing (DTS) technology now allow its use in
hydrological and atmospheric sciences. Introduced by ) is the
use of DTS for measuring the Bowen ratio (BR-DTS), to estimate the sensible
and latent heat flux. The Bowen ratio is derived from DTS-measured vertical
profiles of the air temperature and wet-bulb temperature. However, in
previous research the measured temperatures were not validated, and the
cables were not shielded from solar radiation. Additionally, the BR-DTS
method has not been tested above a forest before, where temperature gradients
are small and energy storage in the air column becomes important.
<br><br>
In this paper the accuracy of the wet-bulb and air temperature measurements
of the DTS are verified, and the resulting Bowen ratio and heat fluxes are
compared to eddy covariance data. The performance of BR-DTS was tested on a
46 m high tower in a mixed forest in the centre of the Netherlands in August
2016. The average tree height is 26 to 30 m, and the temperatures are
measured below, in, and above the canopy. Using the vertical temperature
profiles the storage of latent and sensible heat in the air column was
calculated.
<br><br>
We found a significant effect of solar radiation on the temperature
measurements, leading to a deviation of up to 3 K. By installing screens, the
error caused by sunlight is reduced to under 1 K. Wind speed seems to have a
minimal effect on the measured wet-bulb temperature, both below and above the
canopy. After a simple quality control, the Bowen ratio measured by DTS
correlates well with eddy covariance (EC) estimates (<i>r</i><sup>2</sup> = 0.59). The average
energy balance closure between BR-DTS and EC is good, with a mean
underestimation of 3.4 W m<sup>−2</sup> by the BR-DTS method. However,
during daytime the BR-DTS method overestimates the available energy, and
during night-time the BR-DTS method estimates the available energy to be more
negative. This difference could be related to the biomass heat storage, which
is neglected in this study.
<br><br>
The BR-DTS method overestimates the latent heat flux on average by 18.7 W m<sup>−2</sup>,
with RMSE = 90 W m<sup>−2</sup>. The
sensible heat flux is underestimated on average by 10.6 W m<sup>−2</sup>, with RMSE = 76 W m<sup>−2</sup>.
Estimates of the BR-DTS can be improved once the uncertainties in the energy
balance are reduced. However, applying, for example, Monin–Obukhov similarity theory
could provide independent estimates for the sensible heat flux. This would
make the determination of the highly uncertain and difficult to determine net
available energy redundant. |
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| ISSN: | 1027-5606 1607-7938 |