Estimation and validation of LAI using physical and semi-empirical BRDF models

This letter deals with estimation of LAI for a wheat crop using physical and semi-empirical BRDF models and IRS-1D LISS-III sensor data. NDVI was computed for both the models with LAI as a free parameter. The model-computed NDVI was compared with corresponding atmospherically corrected LISS-III NDVI. The estimation of LAI was carried out on the basis of a look-up table approach and minimum root mean squared deviation between model computed and observed NDVI. The estimated LAI was validated against field measurements carried out during the months of February and March 2003, at the Central State Farm, Rajasthan, India. It was found that LAI was underestimated in both physical and semi-empirical models. Results show that inclusion of multiple scattering in physical models may not always lead to a more accurate estimation of LAI and that it may be possible to estimate LAI at early stages of crop growth using semi-empirical models. The coefficient of determination (R 2) between model estimated and measured LAI was 0.57 (standard error of estimate (SE) 0.156) and 0.63 (SE 0.187) for semi-empirical and physical models, respectively, in the single scattering approximation, for February data. The corresponding values for March data were 0.57 (SE 0.206) and 0.51 (SE 0.216), respectively.