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| Titel |
Evapotranspiration of alfalfa: comparison between eddy covariance measurements and the FAO-56 approach estimates in Central Italy |
| VerfasserIn |
Alessandra Vinci, Lorenzo Vergni, Francesca Todisco |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047960
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| Zusammenfassung |
| The objective of this study was the comparison between evapotranspiration measured by eddy
covariance (ETec) and evapotranspiration estimated by the FAO-56 approach (ETc). In
particular the tabulated alfalfa crop coefficients (Kc) have been compared with Kc computed
as the ratio of ETec to reference evapotranspiration (ET0) during the growing stages
characterized by standard conditions (no water stress). An open patch eddy covariance (EC)
system has been installed in the middle of an alfalfa farmland in Central Italy. The EC system
consisted of a 3D sonic anemometer/thermometer (CSAT3) and a gas-analyzer (Li-7500).
CSAT3 and Li-7500 measured three-directions fluctuations of wind, sonic temperature, and
concentrations of H2O and CO2 at 20Hz. These instruments allowed to measure
independently latent heat flux (LE) and sensible heat flux (H). Soil heat flux (G) and net
radiation (Rn) were measured using soil heat flux plates (HFP01) and a net radiometer
respectively, in order to check energy balance closure. All the sensors were connected to a
datalogger (CR3000) and the 10-min statistics were computed. Daily precipitation and air
temperature were also recorded. The sensors were placed at 1.8m height over the soil
surface.
The available energy (Rn-G) was balanced by the measured fluxes (LE+H) on a daily
time scale.
The evapotranspiration was measured by the EC system during different growing stages
of the years 2009 and 2010. For some days data are missing due to the EC system
malfunctioning.
For the same periods ETc was also calculated as the product between ET0 estimated by
the FAO Penman-Monteith equation and the factor Kc-
Ks (where Ks is a water stress
coefficient). Tabulated Kc values, adjusted for the local climatic conditions, were 0.4
(Kc-ini), 1.14 (Kc-mid), 1.08 (Kc-end), immediately following cutting, at full cover,
and immediately before cutting respectively. The lengths of the growing stages
were calibrated according to local conditions. Ks values were computed simulating
the daily soil water balance. Ks varied between 0 (full stress condition) and 1 (no
stress).
Two cutting cycles (2nd and 3rd) were analyzed in the year 2009. For the 2nd cutting
cycle the cumulated ETec is 60mm, ETc is 71mm and RMSE=0.69. During this cycle, being
Ks always equal to 1, it has been possible to estimate the crop coefficients Kc. Kc-ini and
Kc-mid were about 0.25 and 0.93 respectively, whereas Kc-end was not evaluated due to the
presence of missing data. For the 3rd cutting cycle the cumulated ETec is 145mm and the
ETcis 143.1mm with RMSE=0.70. The presence of water stress conditions didn’t allow the
evaluation of Kc.
Three cutting cycles were analyzed in the year 2010. For the 1st cutting cycle the
cumulated ETec is 76.31mm, ETc is 99.3mm and RMSE=1.13. The Ks is always equal to 1
and the Kc-mid value was about 0.99, Kc-ini and Kc-end were not evaluated for missing
data.
For the 2nd cutting cycle the cumulated ETec is 87.8mm, ETC is 101.88mm and
RMSE=1.22. Kc-ini and Kc-mid were 0.29, 1.10 whereas Kc-end was not computed due the
presence of water stress conditions.
For the 3rd cutting cycle the cumulated ETec is 62.53mm and the ETCis 43.23mm with
RMSE=0.82. Kc were not quantified due to the presence of water stress conditions.
The comparison between ETec and ETc showed that the performance of FAO-56
approach can be improved with the determination of appropriate Kc values. In particular it
was observed that the FAO-56 method overestimates the actual crop evapotranspiration. The
computed Kc values were lower (by about 10%) than the corresponding tabulated values.
This difference could reflect the local climate and cropping conditions that are included
implicitly in the single crop coefficient. |
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