If looking to monitor vegetation in drought affected areas, then it is advisable to use NDWI index proposed by Gao utilizing NIR and SWIR.
[3] NDWI concept as formulated by Gao combining reflectance of NIR and SWIR is more common and has wider range of application.
[6][7][8][9][10] The range of application of NDWI (Gao, 1996) spreads from agricultural monitoring for crop irrigation[11] and pasture management [12] to forest monitoring for assessing fire risk and live fuel moisture [13][14][15] particularly relevant in the context of climate change.
Different SWIR bands can be used to characterize the water absorption in generalized form of NDWI as shown in eq.
Sentinel-2 NDWI for agricultural monitoring of drought and irrigation management can be constructed using either combinations: Both formulations are suitable.