NDWI

The Normalized Difference Water Index algorithm was developed by Gao (19964), being a measure of liquid water molecules in vegetation canopies that interacted with the incoming solar radiation.

NDWI is sensitive to changes in liquid water content of vegetation canopies. It is less sensitive to atmospheric effects than NDVI.
NDWI does not remove completely the background soil reflectance effects, therefore it should be considered as an independent vegetation index.
It is complementary to, not a substitute for NDVI.

The NDWI results from the following equation:

NDWI = (IR_factor * near_IR - mir_factor * middle_IR) / (IR_factor * near_IR + mir_factor * middle_IR)

NDWI-Flags

Also the processor computes an additional flags band called 'ndwi_flags' with the following bit coding:

Bit Position	Description
Bit 0	The computed value for NDWI is NAN or is Infinite
Bit 1	The computed value for NDWI is less than -1 (minus one)
Bit 2	The computed value for NDWI is greater than 1 (one)