Why NDWI threshold varies in delineating water body from multitemporal images?

Water body can be extracted from a satellite image with normalized difference water index (NDWI). It is commonly known that the index varies if extracting water body using multi-temporal images. This study selected 33 cloud-free scenes of MODerate-resolution Imaging Spectroradiometer (MODIS) data over the Poyang Lake region located in a subtropical area of China. Both the data with and without radiometric correction were used to extract lake water surface. The NDWI thresholds varied from date to date for the cases using either corrected or uncorrected data. Comparison showed that there were large differences between the two kinds of thresholds. Approximate 90% of the difference could be explained by the temporal influences including sun-target-satellite geometry and atmospheric conditions.