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| Titel |
Measuring Evapotranspiration of five Alley Cropping systems in Germany using the Eddy-Covariance- and Bowen-Ratio Energy-Balance methods |
| VerfasserIn |
Christian Markwitz, Alexander Knohl, Lukas Siebicke |
| 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 |
250151298
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| Publikation (Nr.) |
 EGU/EGU2017-15864.pdf |
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| Zusammenfassung |
| The inclusion of trees into the agricultural landscape of Europe is gaining popularity as a
source for energy production. Fast growing tree species such as poplar or willow are included
as short rotation coppice or alley cropping systems, which consist of tree alleys interleaved by
annual rotating crops or perennial grasslands. Estimating turbulent fluxes of those systems
using the eddy-covariance- (ECEB) and bowen-ratio energy-balance (BREB) method is
challenging due to the methods limitation to horizontally homogeneous terrain and steady
state conditions. As the conditions are not fulfilled for those systems the energy-balance is
commonly not fully closed, with the non-closure being site specific. An underestimation of
measured heat fluxes leads to an overestimation of the latent heat fluxes inferred from the
ECEB method.
The aim of our study is to 1) quantify the site specific non-closure of the energy-balance
and 2) characterize the performance of both methods, compared to direct eddy-covariance
measurements using a high frequency infra-red gas analyzer (LI-7200, Licor Inc.). To assess
continuous evapotranspiration (ET) rates on a 30-minute time scale we installed a
combined ECEB and BREB system at five alley cropping and five agricultural reference
sites across Germany. For time periods of four weeks we performed direct eddy
covariance flux measurements for H2O and CO2 over one crop- and one grassland
alley cropping- and their respective reference systems during the growing season of
2016.
We found a non-closure between 21 and 26 % for all sites, considering all day- and
night-time data. The residual energy was highest during the morning and lowest in the
afternoon. Related to that the energy-balance ratio (EBR), i.e. the ratio between the turbulent
heat fluxes and available energy, was below one in the morning hours and increased slightly
during the day up to 1.8, until the EBR decreased sharply after sunset. The EBR correlated to
the daily cycle of solar radiation, the main driver of turbulent fluxes. Corresponding, we
found an increasing EBR with increasing friction velocity, indicating, that under
turbulent condition the energy-balance closure improves. Further, we found that
the turbulent fluxes estimated by the BREB method compared well with direct
eddy-covariance measurements and that the accuracy improved with increasing sensor
distance.
We conclude, when calculating ET rates on a 30-minute time scale using the ECEB
method the site specific non-closure should be assessed beforehand by eddy-covariance. In
the current study, ignoring the non-closure would have lead to an overestimation of the latent
heat flux of about 25 % for the ECEB method. For a longer averaging period of one day the
overestimation was reduced to less than 5 %. |
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