Application of the water-related spectral reflectance indices: A review

Water status is an important indicator of ecosystem health. Based on the spectral signatures of liquid water and other land surface features (e.g., soil and vegetation), many water-related spectral reflectance indices (WIs) have been developed to characterize the presence of liquid water. These WIs are widely used in agricultural and ecological applications including surface water body characterization, vegetation water status estimation, soil water content assessment, and wetland (e.g., paddy rice) monitoring. However, choosing the correct WIs for specific studies is confusing due to arbitrary selections of WIs in respective applications and the mixing of WIs names in different studies. To increase the clarity of appropriate usage of WIs in specific conditions, this study reviewed the principles, developments, and applications of various WIs to identify the suitability of WIs for different applications. We started with the discussion of the underlying principles and developments (e.g., from ratio to normalized difference of two bands) of different WIs, based on the spectrum features of water, vegetation, and soil from widely used spectral libraries. Applications from different studies, in each category, were then compared to show the performances of different Wls. This is followed by a brief summary about the terminology of WIs in different studies to further reduce the confusion caused by the mixed use of WIs terms. A field experiment was designed to investigate the dynamics of various WIs in response to different mixing ratios of water and soil or plant. The effectiveness of combining WIs and greenness indices for detecting surface water body and wetland were evaluated using the field experiment data. Finally, we identified the major gaps and pointed out the potential improvements for the future.