Leaf and canopy water content estimation in cotton using hyperspectral indices and radiative transfer models

In present study some vegetation indices for estimating leaf EWT and EWTcanopy were investigated using simulations and field measurements. Leaf and canopy spectral reflectance as well as leaf EWT and EWTcanopy were measured in cotton during the growing seasons of 2010 and 2011. The PROSPECT-5 model was coupled with the SAILH model to explore the performance of water-related vegetation indices for leaf EWT and EWTcanopy estimation. The vegetation indices evaluated were published formulations and new simple ratio vegetation indices formulated with wavebands at 1060 nm and 1640 nm. The sensitivities of these indices to leaf internal structural N and LAI effects were assessed. Simulation results indicated that all of the water-related vegetation indices were insensitive to leaf internal structural N, with the highest coefficient of determination R2 < 0.15 and the proposed index SR1640 (R1060/R1640) and published index SR2 (R1070/R1340) showed the lowest relationships (R2 < 0.35) with LAI of all the vegetation indices. Furthermore, coefficients of determination between simulated leaf EWT as well as EWTcanopy and vegetation indices tested revealed that the new simple-ratio vegetation indices proposed in this study (SR1060: R1640/R1060 and SR1640) were found to be significantly related with leaf EWT (R2 > 0.9; P < 0.001) and EWTcanopy (R2 > 0.8; P < 0.001). Results obtained with field measurements were in agreement with simulation results, with the coefficient of determination R2 = 0.5 (P < 0.001) for leaf EWT and R2 = 0.57 (P < 0.001) for EWTcanopy by the new simple ratio indices. This study provides a new candidate for leaf EWT and EWTcanopy estimation using hyperspectral vegetation indices.