To assess the canopy water content of summer barley from spectroradiometer and HyMap image data, spectral indices and a canopy reflectance model (PROSPECT + SAIL) were applied. In addition to traditional indices (WI, NDWI), a Simple Ratio index (SR*) combining SWIR (1355 nm) and red edge (710 nm) reflectances was found to provide highly reliable estimates for the calibration data. After the adaption to HyMap resolution, the calibrated WI and SR* estimation models were applied to the image data. Here, limitations of the purely empirical index approach became apparent as the retrieved spatial patterns differed distinctly. The physically based approach of PROSPECT + SAIL provided estimates clearly too low but also highly correlated with the measured canopy water contents; the underestimation is due to the fact that some of the canopy water is partly unseeable for the remote sensor which can be traced back to erectophile canopy elements and thick or clumped tissues. Thus, PROSPECT + SAIL estimates were much better in line with the water content of the leaf fraction. In total, the PROSPECT + SAIL results kept stable when applied to synthetic and real HyMap data. This point gives confidence to the spatial pattern of water content as retrieved by PROSPECT + SAIL that differed clearly from the results obtained from the spectral indices.
Michael Vohland, Michael Vohland,
"Using imaging and non-imaging spectroradiometer data for the remote detection of vegetation water content," Journal of Applied Remote Sensing 2(1), 023520 (1 May 2008). https://doi.org/10.1117/1.2937937