MEASUREMENT OF SUN INDUCED CHLOROPHYLL FLUORESCENCE USING HYPERSPECTRAL SATELLITE IMAGERY

Solar Induced Chlorophyll Fluorescence (SIF), can be used as an indicator of stress in vegetation. Several scientific approaches have been made and there is considerable evidence that steady state Chlorophyll fluorescence is an accurate indicator of plant stress hence a reliable tool to monitor vegetation health status. Retrieval of Chlorophyll fluorescence provides an insight into photochemical and carbon sequestration processes within vegetation. Detection of Chlorophyll fluorescence has been well understood in the laboratory and field measurement. Fluorescence retrieval methods were applied in and around the atmospheric absorption bands 02B (Red wavelength) approximately 690 nm and 02A (Far red wavelengths) 740 nm. Hyperion satellite images were acquired for the years 2012 to 2015 in different seasons. Atmospheric corrections were applied using the 6S Model. The Fraunhofer Line Discrimanator (FLD) method was applied for retrieval of SIF from the Hyperion images by measuring the signal around the absorption bands in both vegetated and non vegetated land cover types. Absorption values were extracted in all the selected bands and the fluorescence signal was detected. The relationships between NDVI and Fluorescence derived from the satellite images are investigated to understand vegetation response within the absorption bands.


INTRODUCTION
Retrieval of fluorescence signal provides a unique understanding of global vegetation photosynthesis cycle as there is no specific space based observation platform measuring vegetation fluorescence (Moreno et al., 2006).Particularly, SIF is the direct indicator of canopy photosynthesis and gives quantitative measure of carbon sinks (Coops, 2015).SIF technique consider to be very fast and sensitive approach for the detection of physiological response of vegetation cover (Guanter et al., 2014).Red and far red fluorescence bands are more suitable to measure vegetation health.In Frauhonfer Line Depth (FLD) method, SIF is calculated in radiance unit by considering the Sun irradiance.Radiative properties of the leaves of the plant affect the reflectance from canopies of vegetation.Fluorescence signal can be detected in the atmospheric absorption bands (Guanter et al., 2007).
Healthy green vegetation emits chlorophyll fluorescence signal maxima in the red and far red around 690 nm and 740 nm, respectively.Fruanhofer lines or oxygen absorption lines such as O 2 -A and O 2 -B bands provide the main fluorescence bands.Recent studies (Goetz, 2009, Raychaudhuri, 2014a) have shown that retrieval of SIF is possible using hyperspectral images such as from the Hyperion satellite.We applied the modified FLD method using O 2 -A and O 2 -B band of Hyperion satellite images.In this study only comparisons of O 2 -A with the NDVI values are presented.

Satellite Data
For this project, a request for cloud free Hyperion images was submitted and eight Hyperion images were obtained (     The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLI-B8, 2016

Satellite Data Pre-processing
To investigate the fluorescence, L1Gst Product of converted into surface reflectance using 6S method.Atmospheric correction model 6S proved to give most suitable results over all surfaces in the Hong Kong region 2014).For SIF based fluorescence retrieval, we selected 6 bands.For O 2 -fluorescence retrieval B33, B34, B35 were used as shown in Table 2.
Table 2: Hyperion Selected Band Central Wavelength

Retrieval of Sun Induced Fluorescence (SIF)
SIF in the O 2 -A and O 2 -B spectral wavelength retrieved using the FLD method after applying preprocessing tasks.For this we selected 10 sites based on canopy cover (CC) of forest (CC > 75%).A total of 10 non vegetated sites near to the vegetated sites were also selected.
The fluorescence signal was retrieved from both vegetated and non vegetated sites by calculating difference at sensor radiance inside and outside for the O 2 -A and O 2 -B absorption waveband and which can be expressed as three wavelengths inside and outside the band (Equation 1) (Raychaudhuri, 2014)  spectral wavelength regions was retrieved using the FLD method after applying preprocessing tasks.For this we selected 10 sites based on canopy cover (CC) of forest (CC > 75%).A total of 10 non vegetated sites near to e signal was retrieved from both vegetated and non vegetated sites by calculating difference at sensor radiance B absorption waveband and which can be expressed as three wavelengths inside and Raychaudhuri, 2014) (3) pixels were generated Fluorescence values were retrieved the fluorescence emission contribution of estimated as a percentage of the average reflected radiance for vegetation outside the absorption band.All the reported results are normalized for comparison purposes.By fluorescence retrieved value with the NDVI value in different seasons and in different years, an understanding of fluorescence retrieval method at local scale has been presented.

NDVI Extraction
We summarize our results according to four seasons which are;   decreased fluorescence response is observed sites 2 and 6, where a sudden increase in fluorescence signal is detected.In Spring shows similar a trend again at A except at the site 9 where an .For Summer (JJA), an at site 9 was detected.In at site 3 there is an increase in : Chlorophyll fluorescence intensity for different Hyperion.

Relationship Between NDVI and SIF for O 2 A Band
For the purpose of the validation of the fluorescence value and to observe the trend of both NDVI and fluorescence, a comparison between NDVI and Fluorescence were also performed as shown in Figure 4.It showed that at site 3 and site 9 correlation between both NDVI and fluorescence is around 0.03 and 0.06 respectively which is very low despite of the fact that the highest NDVI value is detected on both sites.

Figure 1 :
Figure 1: Hyperion Image of Hong Kong Region 1) Whereas, D = Difference in radiance and has unit (Wm µm −1 ) λ F = Fluorescence wavelength L L = Radiance values at wavelengths λL L R = Radiance values at wavelengths λR L F = Radiance corresponding to the wavelength (λ and are the relative weights of the absorption bands and represented by the following equations (2) and equation (3) and On each sample site windows of 5 X 5 pixels and the average was taken.Fluorescence values were retrieved as a percentage i.e the fluorescence emission vegetation, estimated as a percentage of the average reflected radiance for vegetation outside the absorption band.All the reported results are normalized for comparison comparing the fluorescence retrieved value with the NDVI value in different seasons and in different years, an

F
= Difference in radiance and has unit (Wm −2 sr −1 adiance corresponding to the wavelength (λF) are the relative weights of the absorption bands and(2) and equation (3) , November (Autumn, SON) All ten vegetated sites were selected in each image which showed NDVI values greater than 0.5.Figure winter and spring season, NDVI value was equal to or greater than 0.6 in most of sites.

Figure 2 :
Figure 2: Normalized Difference Vegetation Index (NDVI) calculated for the Hyperion images acquired over different seasons (a) -Winter (DJF), (b) -Spring (MAM) 4.2 O 2 A Fluorescence The pattern of fluorescence signal is shown in Figure A band for all the months at 10 different sites.For Winter (DJF) decreased fluorescence response is throughout the O 2 -A band except at site sudden increase in fluorescence signal is detected.In (MAM) the fluorescence signal shows similar all the selected sites in O 2 -A except at the site 9 increase in O 2 -A band was detected.For increase in fluorescence for O 2 -A band at site 9 was detected Autumn season (SON) at site 3 there is a fluorescence signal in O 2 -A Band.

Figure 3 :
Figure 3: Chlorophyll fluorescence intensity for different seasons obtained from O2-A bands of Hyperion For site 5 and site 8 highest correlations were detected between NDVI and fluorescence.Some other factors might affect the retrieved fluorescence value.Those reasons might include AOD high values, soil contamination and any other variable effecting the retrieval of fluorescence value.

Figure 4 :
Figure 4: Relationship between NDVI and Fluorescence derived using O2A bands of Hyperion from 2012 to 2015 for sites ten vegetation sites

Table 1 : Hyperion Satellite images acquired for the retrieval of fluorescence signal. Figure 1 shows the Hyperion image over Hong Kong region. 10 sample
sites were also shown over the image at different sites in the Hong Kong country parks.

Table 2 :
Hyperion Selected Band Central Wavelength