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| Titel |
Hyperspectral measurements for estimating biophysical parameters and CO2 exchanges in a rice field |
| VerfasserIn |
M. Rossini, M. Migliavacca, M. Meroni, G. Manca, S. Cogliati, L. Busetto, V. Picchi, M. Galvagno, R. Colombo, G. Seufert |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030440
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| Zusammenfassung |
| The objective of this work was to monitor the main biophysical and structural parameters as
well as the CO2 exchanges between atmosphere and a terrestrial ecosystem from remote and
high spectral resolution spectroradiometric measurements. Estimation of photosynthetic rate
or gross primary productivity from remotely sensed data is based on the light use efficiency
model (LUE), which states that carbon exchange is a function of the photosynthetically active
radiation absorbed by vegetation (APAR) and the radiation use efficiency (É) which
represents the conversion efficiency of energy to fixed carbon. Hyperspectral data were
used in this study in order to derived both the APAR of green vegetation and the É
term.
The experimental site was a rice paddy field in North Italy equipped with an Eddy
Covariance (EC) flux measurement tower (Castellaro IES-JRC site). Intensive field
campaigns were conducted during summer 2007 and 2008. In each sampling day, canopy
optical properties, canopy structure, biophysical and ecophysiological parameters were
measured.
EC fluxes were calculated with a time step of 30 minutes according to EUROFLUX
methodology. Measured half-hourly net ecosystem exchange (NEE) was partitioned to derive
half hourly gross ecosystem production (GEP). Canopy reflectance spectra were collected
under clear sky conditions using two portable spectrometers (HR4000, OceanOptics, USA)
characterised by different spectral resolutions. A spectrometer characterised by a Full Width
at Half Maximum (FWHM) of 0.13 nm was used to estimate steady-state fluorescence (F)
and a second one with a FWHM of 2.8 nm was used for the computation of traditional
vegetation indices (e.g. NVDI, Normalized Difference Vegetation Index and SAVI, Soil
Adjusted Vegetation Index) and PRI (Photochemical Reflectance Index, Gamon et al. 1992).
F was estimated by exploiting a variation of the Fraunhofer Line Depth (FLD) principle
(Plascyk 1975): the spectral fitting method described in Meroni and Colombo (2006) applied
at the 760 nm atmospheric oxygen absorption band. An index of F efficiency, the apparent
fluorescence yield (NF760) was also computed as the ratio between F760 and the incident
radiation.
Results show that Leaf Area Index (LAI), the fraction of absorbed photosynthetically
active radiation (fAPAR) and plant height values were well correlated with SAVI (R2
from 0.68 to 0.83) while NDVI was poorly or not correlated. The NDVI-fAPAR
relationship, as well as the relationships NDVI-LAI and NDVI-plant height, is
very different in the vegetative and ripening stages. The lower correlation with
NDVI in this analysis could be explained by the dependence of the relationship on
phenology. In contrast, other indices adjusting for background effects (like SAVI) showed
highly linear relationships with fAPAR, LAI and plant height for the entire growing
period.
Furthermore, the use of innovative spectral indices related to physiological processes such
as the activation of photoprotective mechanisms and excess energy dissipation via
sun-induced passive fluorescence allowed the development of semi-empirical models
between radiometric measurements and GEP. The average of half-hourly GEP acquired
between 11 a.m. and 1 p.m. (solar time) was related with hyperspectral indices and
fluorescence parameters acquired at the same time with the spectrometers. Different LUE
models were tested. SAVI was selected to estimate fAPAR because it showed higher
correlation than NDVI. Results showed very high coefficient of determination (i.e. R2from
0.88 to 0.98) between GEP and F760 and the product (NF760 x PARi x SAVI) for both years.
The regression between GEP and the product (PRI x PARi x SAVI) was instead not
significant. The semi-empirical models show high correlation between GEP and chlorophyll
fluorescence parameters throughout the two years study period. This result opens
up new possibilities for the application of semi-empirical model for the spatial
estimation of biophysical parameters and carbon GEP based on aerial and satellite
images.
References
Gamon J.A., Penuelas J., Field C.B. 1992. A Narrow-Waveband Spectral Index That
Tracks Diurnal Changes in Photosynthetic Efficiency. Remote Sensing of Environment,
41:35-44.
Meroni M., Colombo R. 2006. Leaf level detection of solar induced chlorophyll
fluorescence by means of a subnanometer resolution spectroradiometer. Remote Sensing of
Environment, 103:438-448.
Plascyk J.A. 1975. The MK II Fraunhofer line discriminator (FLD-II) for airborne and
orbital remote sensing of solar-stimulated luminescence. Optical Engineering, 14:339-346. |
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