Evaluation of PROSAIL inversion for retrieval of chlorophyll, leaf dry matter, leaf angle, and leaf area index of wheat using spectrodirectional measurements

Bidirectional reflectance measurements of wheat crop acquired with field goniometer system were used to evaluate PROSAIL (PROSPECT and SAIL (Scattering by Arbitrary Inclined Leaves)) inversion for retrieval of biophysical parameters. The measurements on wheat involved 54 view angles (6 azimuthal planes x9 view zenith angles) to cover the hemispheric span up to 60 degrees view zenith acquired at 7-8 days interval during early growth stages to physiological maturity. Parallel observations on crop, soil, and atmosphere were made for input parameters of radiative transfer model PROSAIL. The spectral sensitivity of the model indicated that the reflectance in visible was relatively more important for chlorophyll retrieval and reflectance in near infrared has more importance for retrieval for leaf inclination angle, dry matter and leaf area index. The coupling of spectrodirectional reflectance with the model using simplex algorithm was found successful in precise retrieval of chlorophyll (root mean square error (RMSE) = 15.62 mu g cm(-2)), leaf inclination angle (RMSE = 2.14), and dry matter (RMSE = 0.00053 g cm(-2)). LAI retrieval was poor (RMSE = 1.05) because of saturation effect at LAI >4. Limited angular sampling close to hotspot region was found sufficient for precise retrieval of chlorophyll (RMSE = 10.50 mu g cm(-2)) and leaf inclination angle (RMSE = 2.14). This signifies the importance of off-nadir observations in biophysical parameters' retrieval.