It is a critical input to many land-surface models such as crop growth models, net primary productivity models, and climate models. The objective of this research is to develop a new instantaneous quantitative FAPAR model based on the law of energy conservation and the concept of recollision probability p.
Using the ray-tracing method, the FAPAR-P model separates direct energy absorption by the canopy from energy absorption caused by multiple scattering between the soil and the canopy. Direct sunlight and diffuse skylight are also considered. This model has a clear physical meaning and can be applied to continuous and discrete vegetation. Few studies, however, have tested this relationship empirically based on in situ measurements throughout the entire growing season.
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Huete et al. All these knowledge gaps in the relationship between NDVI and fAPAR, and the uncertainties in the remote estimation methods of fAPAR influence the estimation of crop productivity, and make forecasts of agricultural yield less precise. The dataset stemmed from field spectral reflectance measurements and indirect fAPAR observations for a maize canopy in China from May to September during the growing season.
The study was carried out in a spring maize Zea mays L. This area belongs to the temperate continental monsoon climate zone, which is warm and moist in the summer and cold and dry in the winter. The selected crop type was rainfed spring maize and the maize hybrid was Nong Hua , which was the main crop type, sown in early May and harvested in late September. The soil type was a typical brown soil, with a pH value of 6.
Field reflectance spectra measurements were made biweekly from May to September in the growing season in total nine measuring campaigns. The observation dates were 24 May, 9 and 21 June, 5 and 29 July, 11 and 23 August, and 6 and 20 September. Spectral measurements were made on days with clear skies between 11 and 14h. At each site, we made 10 spectral measurements and replicated them three times.
Solar Radiation & Photosynthetically Active Radiation
During spectral measurements, standard white spectralon targets their reflectance is 1 were used as references against which the target objects could be calibrated. Thus, the reflectance values became dimensionless. Except for 24 May, the observation dates were the same as for the spectral reflectance measurements. Eight campaigns were carried out from the end of May to the end of September According to the agricultural and meteorological observation standards and guidelines of the China Meteorological Administration, the area-coefficient method was used to measure leaf area index LAI.
On each sampling date, five standard plants in the research area were harvested, and their individual lengths and widths were measured and multiplied by the calibration coefficient 0.
Observation dates were the same as for the spectral reflectance measurements, and nine campaigns were conducted in total. Nine field campaigns in this study were conducted for a maize canopy during the growing season, while there were only eight campaigns for fAPAR because of an equipment failure.
Thus, to obtain the observed fAPAR value with a wide range of variation we used this function.
The fAPAR and LAI increased rapidly during the vegetative stage, remained relatively constant at the stage of reproduction and finally decreased slowly during the senescence stage Fig. The reason for this was that LAI was measured only for green leaves, while indirectly observed values of fAPAR were influenced by senescent leaves containing less photosynthetic pigments. With increasing fAPAR, NDVI rapidly increased and then became saturated until maximum canopy development during the vegetative stage, and suddenly dropped from the beginning of the reproduction stage to the whole senescence stage Fig.
NDVI decreased with increased reflectance of the canopy in the visible spectrum, and this was especially pronounced in the red and the red-edge ranges when tassels i. Thus, appearance of tassels led to a decrease of VIs.
Solar Radiation & Photosynthetically Active Radiation
Results showed that satisfactory linear relationships existed between fAPAR green and five VIs derived from hyperspectral remote sensing data, with R 2 coefficients of 0. Remote estimation of fAPAR is seen as one of the greatest sources of uncertainty for the LUE model and as a key to accurate quantitative estimates of plant productivity at local to global scales Field et al.
Recently, hyperspectral remote sensing has provided new opportunities for accurate estimates of the structural and functional properties of the canopy e. Chen et al. The reason was that, although red-edge NDVI could effectively overcome the decreased sensitivity of NDVI to fAPAR at moderate-to-high biomass densities, spectral characteristics of the REP may be more important than VIs to describe green vegetation information within reflectance spectra in the —nm regions.
Mutanga and Skidmore showed that the REP, compared to the maximum first derivative value at the red-edge peak, the first derivative of the reflectance at and nm, is strongly correlated with chlorophyll concentration in plants. The other issue for fAPAR estimation stemmed from the effects of phenological stages on canopy spectral properties, i.
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For the former, VIs derived from the hyperspectral remote sensing data, which can overcome the losses of sensitivity to fAPAR beyond 0. The major mechanism was that an increase in chlorophyll concentration or biomass resulted in the broadening of the absorption feature centered around nm, causing the movement of the REP to longer wavelengths Dawson and Curran Gitelson et al. Spectral information around the red-edge region is more sensitive to variation in the canopy properties of vegetation, and can effectively address the saturation problem of NDVI vs.
The red-edge region has often been used for estimating chlorophyll and N contents Clevers and Gitelson The Sentinel-2 satellite, equipped with the Multi Spectral Instrument, also has red-edge bands, and incorporates two spectral bands in the red-edge region, which are centered at and nm at a band width of 15nm and a spatial resolution of 20 m Clevers and Gitelson To further illustrate the performance of the red-edge region, we used the definition of REP with the band setting of Sentinel-2 to retrieve fAPAR green.
In addition, Fig. Sign In. Access provided by: anon Sign Out. It is a critical input to many land-surface models such as crop growth models, net primary productivity models, and climate models. The objective of this research is to develop a new instantaneous quantitative FAPAR model based on the law of energy conservation and the concept of recollision probability p.
Using the ray-tracing method, the FAPAR-P model separates direct energy absorption by the canopy from energy absorption caused by multiple scattering between the soil and the canopy.