Normalized difference water index
Normalized Difference Water Index (NDWI) may refer to one of at least two remote sensing-derived indexes related to liquid water:
Another is used to monitor changes related to water content in water bodies, using green and NIR wavelengths, defined by McFeeters (1996):
In remote sensing, ratio image or spectral rationing are enhancement techniques in which a raster pixel from one spectral band is divided by the corresponding value in another band. Both the indexes above share this same functional form; the choice of bands used is what makes them appropriate for a specific purpose.
If looking to monitor vegetation in drought affected areas, then it is advisable to use NIR and SWIR. The SWIR reflectance reflects changes in both the vegetation water content and the spongy mesophyll structure in vegetation canopies. The NIR reflectance is affected by leaf internal structure and leaf dry matter content, but not by water content. The combination of the NIR with the SWIR removes variations induced by leaf internal structure and leaf dry matter content, improving the accuracy in retrieving the vegetation water content.
If looking for water level or change in water level (e.g. flooding), then it is advisable to use the green and NIR spectral bands.
Visual or digital interpretation of the output image/raster created is similar to NDVI:
- -1 to 0 - Bright surface with no vegetation or water content
- +1 - represent water content
For the second variant of the NDWI, another threshold can also be found in  that avoids creating false alarms in urban areas:
- < 0.3 - Non-water
- >= 0.3 - Water.
- http://deltas.usgs.gov/fm/data/data_ndwi.aspx (regarding the McFeeters index for water bodies)
- Gao. "NDWI—A normalized difference water index for remote sensing of vegetation liquid water from space." 1996. http://ceeserver.cee.cornell.edu/wdp2/cee6150/Readings/Gao_1996_RSE_58_257-266_NDWI.pdf
- Lillisand & Kifer
- Ceccato et al. 2001