1,721,022 research outputs found
Spectroradiometer characterization for continuous operation in an Eddy Covariance flux tower
No full textHyperspectral sensors are increasingly being used to continuously collect optical data that can be
related to carbon and water ecosystem exchanges. Automated proximal sensing can solve the temporal
mismatch existing between the periodic remote observations and the continuous acquisition of the Eddy
Covariance systems, and can provide also tools for the up-scaling. However, characterization of
spectroradiometers used continuously outdoors is necessary to assure data quality; because environmental
conditions can influence the instrumentation performance, but also are drivers of the vegetation physiology
estimated through the optical measurements. We describe the laboratory characterization previous to field
deployment of a Unispec-DC, a dual channel spectroradiometer integrated in an automated multi-angular
system (AMSPEC-MED). The instrument operates in a savanna ecosystem in Majadas del Tiétar, Cáceres,
Spain, under a wide range of temperatures, radiation levels, illumination angles and internal settings.Laboratory experiments were conducted in order to characterize several features of the
spectroradiometer and to estimate correction models. Dark current, the signal produced by thermally
generated electrons, was modelled as a function of the temperature and the integration time set. Thermal
sensitivity, the sensor’s responsivity dependence on temperature, and spectral calibration were also modeled
as a function of temperature. Moreover, non-linearities, deviations from a linear relationship between the
input radiance and the output signal, were characterized as a function of the grey level and (innovatively) of
the integration time. Since a cosine receptor is used to sample irradiance; we also modeled the diffuser
directional response deviations from the ideal response, the cosine of the incident angle of illumination.Calibrated models are used to correct the Hemispherical-Conical Reflectance Factors (HCRF)
measured in a continuous mode in the field, and the influence of each model is discussed. Results suggest that
effects of thermal sensitivity and non-linearity partially cancel out when reflectance is computed using
channel A (irradiance) and B (radiance) ratio, however, in the case of non-linearities, this may not occur
when signals are very different in each channel. Dark measurements showed a bias inversely dependent on
temperature that was added to dark current. Wavelength calibration showed a dependency on temperature;
however, this was small considering the spectral features of the instrument (Full Width at Half Maximum~10nm, interval sampling ~3.3nm). Finally, the cosine directional response correction model produced the
largest differences between he corrected and the non-corrected HCRF. This correction requires accounting for
the diffuse-to-global radiation ratios. Differences between corrected and non-corrected reflectances were
larger in the near infrared region than in the visible. We conclude that characterization of spectroradiometers
installed outdoors in automated continuous systems is necessary to ensure comparability and quality of data.
Thermal insulation of the instruments could reduce errors related with dark current, thermal sensitivity and
wavelength calibration; however these still should have to be known in order to compare with data from other
instruments. Moreover, non-linearities and directional response of the cosine receptors would have to still be
corrected in order to achieve reliable measurements under different ranges of irradiance and sun elevation.Peer reviewe
Variación temporal del comportamiento espectral y la composición química en el dosel arbóreo de una dehesa
No full textEn el contexto de los proyectos BIOSPEC y FLUXPEC (http://www.lineas.cchs.csic.es/fluxpec/), se han
realizado mediciones espectrales y de variables biofísicas a nivel de hoja en el dosel arbóreo de una dehesa de encina
(Quercus ilex) durante cuatro períodos vegetativos. Se han llevado a cabo mediciones de reflectividad bi-cónica de hoja
intacta, LMA, contenido de agua, nutrientes (N, P, K, Ca, Mg, Mn, Fe, y Zn) y pigmentos. Las mediciones espectrales se
han relacionado con las variables biofísicas mediante análisis de regresión múltiple stepwise y regresión de mínimos
cuadrados parciales. Estos análisis han permitido identificar las bandas espectrales que explican la evolución de las
variables biofísicas y estimar los contenidos de nutrientes a lo largo del proceso de maduración de las hojas en la copa.
Se han obtenido estimaciones significativas para la mayoría de las variables foliares estudiadas. Las longitudes de onda
con mayor contribución aparecen en la zona del eje rojo, el SWIR y la región verde del visible.Peer reviewe
Comparing the quantum use efficiency of red and far-red sun-induced fluorescence at leaf and canopy under heat-drought stress
No full textSun-Induced chlorophyll Fluorescence (SIF) is the most promising remote sensing signal to monitor photosynthesis in space and time. However, under stress conditions its interpretation is often complicated by factors such as light absorption and plant morphological and physiological adaptations. To ultimately derive the quantum yield of fluorescence (ΦF) at the photosystem from canopy measurements, the so-called escape probability (fesc) needs to be accounted for.
In this study, we aim to compare ΦF measured at leaf- and canopy-scale to evaluate the influence of stress responses on the two signals based on a potato mesocosm heat-drought experiment. First, we compared the performance of recently proposed reflectance-based approaches to estimate leaf and canopy red fesc using data-supported simulations of the radiative transfer model SCOPE. While the leaf red fesc showed a strong correlation (r2 ≥ 0.76), the canopy red fesc exhibited no relationship with the SCOPE retrieved red fesc in our experiment. We therefore propose modifications to the canopy model to address this limitation.
We then used the modified models of red fesc, along with an existing model for far-red fesc to analyse the dynamics of leaf and canopy red and far-red fluorescence under increasing drought and heat stress conditions. By incorporating fesc, we obtained a closer agreement between leaf and canopy measurements. Specifically, for red fesc, the r2 of the two variables increased from 0.3 to 0.50, and for far-red fesc, from 0.36 to 0.48.
When comparing the dynamics of the quantum yield of red and far-red fluorescence (ΦF,687 and ΦF,760) under increasing stress, we observed a statistically significant decrease of both leaf and canopy ΦF,687 as well as leaf ΦF,760, as drought and heat conditions intensified. Canopy ΦF,760, on the contrary, did not exhibit the same trend, since measurements under low stress conditions showed a wider spread and lower median than under high stress conditions. Finally, we analysed the sensitivity of ΦF,687 and ΦF,760 to changing solar incidence angle, by comparing the variability of the measurements without and with mesocosm rotation. Our results suggest that the variation in ΦF,760 strongly increased with changing solar incidence angle. These findings highlight the need for further research to understand the causes of discrepancies between leaf and canopy scale ΦF,760. On the contrary, the underutilised and understudied ΦF,687 showed great potential in assessing plant responses to drought and heat stress.This research was supported by the Research Foundation— Flanders (FWO), and by The Research Council of the University of Antwerp. Sebastian Wieneke has received funding from the European Union Horizon 2020 Research and Innovation program under the Marie Skłodowska-Curie grant (ReSPEc, grant no 795299). Sílvia Poblador was financially supported by a Postdoctoral Marie Skłodowska-Curie actions - Seal of Excellence Fellowship of the Research Foundation – Flanders (MSCA SoE FWO, 12ZZ521N).Peer reviewe
Sun-Induced Chlorophyll Fluorescence I: Instrumental Considerations for Proximal Spectroradiometers
Get PDFGrowing interest in the proximal sensing of sun-induced chlorophyll fluorescence (SIF) has been boosted by space-based retrievals and up-coming missions such as the FLuorescence EXplorer (FLEX). The European COST Action ES1309 “Innovative optical tools for proximal sensing of ecophysiological processes„ (OPTIMISE, ES1309; https://optimise.dcs.aber.ac.uk/) has produced three manuscripts addressing the main current challenges in this field. This article provides a framework to model the impact of different instrument noise and bias on the retrieval of SIF; and to assess uncertainty requirements for the calibration and characterization of state-of-the-art SIF-oriented spectroradiometers. We developed a sensor simulator capable of reproducing biases and noises usually found in field spectroradiometers. First the sensor simulator was calibrated and characterized using synthetic datasets of known uncertainties defined from laboratory measurements and literature. Secondly, we used the sensor simulator and the characterized sensor models to simulate the acquisition of atmospheric and vegetation radiances from a synthetic dataset. Each of the sensor models predicted biases with propagated uncertainties that modified the simulated measurements as a function of different factors. Finally, the impact of each sensor model on SIF retrieval was analyzed. Results show that SIF retrieval can be significantly affected in situations where reflectance factors are barely modified. SIF errors were found to correlate with drivers of instrumental-induced biases which are as also drivers of plant physiology. This jeopardizes not only the retrieval of SIF, but also the understanding of its relationship with vegetation function, the study of diel and seasonal cycles and the validation of remote sensing SIF products. Further work is needed to determine the optimal requirements in terms of sensor design, characterization and signal correction for SIF retrieval by proximal sensing. In addition, evaluation/validation methods to characterize and correct instrumental responses should be developed and used to test sensors performance in operational conditions
Widespread and complex drought effects on vegetation physiology inferred from space
Get PDFAbstract The response of vegetation physiology to drought at large spatial scales is poorly understood due to a lack of direct observations. Here, we study vegetation drought responses related to photosynthesis, evaporation, and vegetation water content using remotely sensed data, and we isolate physiological responses using a machine learning technique. We find that vegetation functional decreases are largely driven by the downregulation of vegetation physiology such as stomatal conductance and light use efficiency, with the strongest downregulation in water-limited regions. Vegetation physiological decreases in wet regions also result in a discrepancy between functional and structural changes under severe drought. We find similar patterns of physiological drought response using simulations from a soil–plant–atmosphere continuum model coupled with a radiative transfer model. Observation-derived vegetation physiological responses to drought across space are mainly controlled by aridity and additionally modulated by abnormal hydro-meteorological conditions and vegetation types. Hence, isolating and quantifying vegetation physiological responses to drought enables a better understanding of ecosystem biogeochemical and biophysical feedback in modulating climate change
Inter-comparison of hemispherical conical reflectance factors (HCRF) measured with four fibre-based spectrometers
No full text©2013 Optical Society of America (Open Access)We describe the results of an experiment designed to compare the radiometric performance of four different spectroradiometers in ideal field conditions. A carefully designed experiment where instruments were simultaneously triggered was used to measure the Hemispherical Conical Reflectance Factors (HCRF) of four targets of varying reflectance. The experiment was in two parts. Stage 1 covered a 2 hour period finishing at solar noon, where 50 measurements of the targets were collected in sequence. Stage 2 comprised 10 rapid sequential measurements over each target. We applied a method for normalising full width half maximum (FWHM) differences between the instruments, which was a source of variability in the raw data. The work allowed us to determine data reproducibility, and we found that lower-cost instruments (Ocean Optics and PP Systems) produced data of similar radiometric quality to those manufactured by Analytical Spectral Devices (ASD -here we used the ASD FieldSpec Pro) in the spectral range 400-850 nm, which is the most significant region for research communities interested in measuring vegetation dynamics. Over the longer time-series there were changes in HCRF caused by the structural and spectral characteristics of some targets. © 2013 Optical Society of America.The field experiment described in this paper was undertaken by participants in the EU funded COST Action (ES0903 “Eurospec”; http://cost-es0903.fem-environment.eu/) as part of a Summer School exercise in July 2011. Travel to the field site in Monte Bondone was part-funded by the EU COST action (PI: Loris Vescovo) through travel reimbursement to the Summer School instructors who are the authors of this paper. We are grateful to the participants of the Summer School for their help in setting up the experiment. We also wish to thank PP Systems, who lent Javier Pacheco-Labrador the 2m fibre optic cables used in the experiment and we are grateful to the BIOSPEC project (CGL2008-02301/CLI; http://www.lineas.cchs.csic.es/biospec) funded by the Spanish Ministry of Science and Innovation which has provided funding for Javier Pacheco-Labrador’s researchPeer reviewe
Challenging the link between functional and spectral diversity with radiative transfer modeling and data
Get PDFIn a context of accelerated human-induced biodiversity loss, remote sensing (RS) is emerging as a promising tool to map plant biodiversity from space. Proposed approaches often rely on the Spectral Variation Hypothesis (SVH), linking the heterogeneity of terrestrial vegetation to the variability of the spectroradiometric signals. Yet, due to observational limitations, the SVH has been insufficiently tested, remaining unclear which metrics, methods, and sensors could provide the most reliable estimates of plant biodiversity. Here we assessed the potential of RS to infer plant biodiversity using radiative transfer simulations and inversion. We focused specifically on “functional diversity,” which represents the spatial variability in plant functional traits. First, we simulated vegetation communities and evaluated the information content of different functional diversity metrics (FDMs) derived from their optical reflectance factors (R) or the corresponding vegetation “optical traits,” estimated via radiative transfer model inversion. Second, we assessed the effect of the spatial resolution, the spectral characteristics of the sensor, and signal noise on the relationships between FDMs derived from field and remote sensing datasets. Finally, we evaluated the plausibility of the simulations using Sentinel-2 (multispectral, 10 m pixel) and DESIS (hyperspectral, 30 m pixel) imagery acquired over sites of the Functional Significance of Forest Biodiversity in Europe (FunDivEUROPE) network. We demonstrate that functional diversity can be inferred both by reflectance and optical traits. However, not all the FDMs tested were suited for assessing plant functional diversity from RS. Rao's Q index, functional dispersion, and functional richness were the best-performing metrics. Furthermore, we demonstrated that spatial resolution is the most limiting RS feature. In agreement with simulations, Sentinel-2 imagery provided better estimates of plant diversity than DESIS, despite the coarser spectral resolution. However, Sentinel-2 offered inaccurate results at DESIS spatial resolution. Overall, our results identify the strengths and weaknesses of optical RS to monitor plant functional diversity. Future missions and biodiversity products should consider and benefit from the identified potentials and limitations of the SVH
Comparación y validación local de los productos BioPar (Geoland 2) y MODIS en una dehesa de Extremadura
No full text[ES] En este trabajo hemos comparado y validado dos productos obtenidos a escala global a partir de los sensores MODIS y
VEGETATION, el índice de área foliar (LAI) y el índice de vegetación de diferencia normalizada (NDVI). La
validación se ha realizado a partir de datos de campo recogidos en una dehesa situada al NE de la provincia de Cáceres.
Los resultados obtenidos muestran que los productos VEGETATION desarrollados en el ámbito del proyecto
GEOLAND 2-BioPar se ajustan mejor a los datos de campo para la variable LAI (r2=0,81, RMSE=0,26), mientras que
MODIS captura mejor el NDVI medido en terreno (r2=0,91 y 0,89, RMSE=0,02 y 0,03, para Terra y Aqua
respectivamente).[EN] In this paper we compared and validated two global products obtained from MODIS and VEGETATION sensors, leaf
area index (LAI) and the index of normalized difference vegetation index (NDVI). The validation was carried out using
field data collected in a pasture located NE of the province of Cáceres. The results obtained show that the
VEGETATION products developed within the project GEOLAND 2-BioPar best fit the data for the variable field LAI
(r2 = 0.81, RMSE = 0.26), whereas MODIS better capture field measured NDVI (r2 = 0,91 and 0,89, RMSE = 0,02 and
0,03 for Terra and Aqua, respectively)Peer reviewe
Relaciones espacio-temporales entre datos ópticos adquiridos por el sensor hiperespectral CASI y flujos de carbono en un ecosistema de dehesa
No full textSe analizan los efectos de la agregación espacial de variables espectrales y temporal de flujos de carbono sobre
la estimación de la productividad primaria bruta en un ecosistema de dehesa. Para ello se hace uso de una serie temporal
de imágenes hiperespectrales de alta resolución espacial sobre el área de influencia de varias torres de flujo. Los diferentes
modelos empleados revelan diferencias relacionadas con la agregación temporal de los datos de flujos, mientras que las
relacionadas con la agregación espacial de los datos espectrales son menos relevantes.Este trabajo ha sido financiado por los proyectos
BIOSPEC (CGL2008-02301/CLI, Ministerio de Ciencia
en Innovación) y FLUXPEC (CGL2012-34383,
Ministerio de Economía y Competitividad).
Agradecemos a la COST Action ES1309 OPTIMISE la
financiación de una STMS. Agradecemos también la
colaboración del personal de SpecLab-CSIC, CEAM,
Universidad de Alcalá, Instituto Nacional de
Investigación y Tecnología Agraria y Alimentaria,
Universidad de Zaragoza, Universidad de Milano-
Bicocca y Max Planck Institute for Biogeochemistry.Peer reviewe
New approaches in multi-angular proximal sensing of vegetation: Accounting for spatial heterogeneity and diffuse radiation in directional reflectance distribution models
No full textThe development of tower-mounted automated multi-angular hyperspectral systems has brought new opportunities and challenges for the characterization of the Bidirectional Reflectance Distribution Function (BRDF) on a continuous basis. This study describes the deployment of one of these systems in a Mediterranean savanna ecosystem (AMSPEC-MED), and proposes new approaches for modeling of directional effects. In this study, a Hemispherical-Directional Reflectance Distribution Function (HDRDF) was introduced in order to quantify the effect of diffuse radiation on the estimation of BRDF. The HDRDFs of the two covers of the ecosystem - trees and grasses - were un-mixed using a 3-Dimensional (3-D) model of the observed scene. Up-scaling HDRDF estimates to MODIS BRDF product.
Despite the uncertainties in the estimation of diffuse irradiance and the 3-D representation of the scene, HDRDF un-mixing demonstrates the potential of automated multi-angular proximal sensing to study vegetation properties in heterogeneous ecosystems and the correction of directional effects of different sources.Peer reviewe
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