Relationship of bidirectional reflectance of wheat with biophysical parameters and its radiative transfer modeling using PROSAIL

The algorithms for deriving vegetation biophysical parameters rely on the understanding of bi-directional interaction of radiation and its subsequent linkages with canopy radiative transfer models and their inversion. In this study, an attempt has been made to define the geometry of sensor and source position to best relate plant biophysical parameters with bidirectional reflectance of wheat varieties varying in canopy architecture and to validate the performance of PROSAIL (PROSPECT+SAIL) canopy radiative transfer model. A field experiment was conducted with two wheat cultivars varying in canopy geometry and phenology. The bidirectional measurements between 400nm–1100nm at 5nm interval were recorded every week at six view azimuth and four view zenith positions using spectro-radiometer. Canopy biophysical parameters were recorded synchronous to bi-directional reflectance measurements. The broadband reflectances were used to compute the NDVIs which were subsequently related to leaf area index and biomass. Results showed that the bidirectional reflectance increased with increase in view zenith from 200 to 600 irrespective of the sensor azimuth. For a given view zenith, the reflectance was observed to be maximum at 1500 azimuth where the difference between the sun and sensor azimuth was least. For sun azimuth of 1600 and zenith of 520, the view geometry defined by 1500 azimuth and 500 zenith corresponded to hotspot position. The measured bidirectional NDVI had significant logarithmic relationship with LAI and linear relationship with biomass for both the varieties of wheat and maximum correlation of NDVI with LAI and with biomass was obtained at the hotspot position. The PROSAIL validation results showed that the model simulated well the overall shape of spectra for all combination of view zenith and azimuth position for both wheat varieties with overall RMSE less than 5 per cent. The hotspot and dark spot positions were also well simulated and hence model performance may be suitable for deriving wheat biophysical parameters using satellite derived reflectances.

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