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:

One is used to monitor changes in water content of leaves, using near-infrared (NIR) and short-wave infrared (SWIR) wavelengths, proposed by Gao in 1996:

${\mbox{NDWI}}={\frac {(Xnir-Xswir)}{(Xnir+Xswir)}}$

Another is used to monitor changes related to water content in water bodies, using green and NIR wavelengths, defined by McFeeters (1996):

${\mbox{NDWI}}={\frac {(Xgreen-Xnir)}{(Xgreen+Xnir)}}$

Overview

The functionality of the formula is explained by the following considerations: instead of using the red range, the reflection intensity in which is determined by the presence of chlorophyll, a short-wave near-infrared (SWIR) is used in which high absorption of light by water occurs. A wider range of 1500-1750 nm is possible. The use of the same near infrared (NIR) as in the case of NDVI is due to the fact that water does not absorb this part of the electromagnetic spectrum, thus the index is resistant to atmospheric effects, distinguishing it from NDVI. Note that when observing forests, the NDWI index is characterized by a more stable decrease in values upon reaching critical anthropogenic load, which can serve as an indicator of the ecological state of forests more sensitive than NDVI. The results of NDWI can be presented in the form of maps and graphs, providing information on both the spatial distribution of water stress on vegetation and its temporal evolution over longer periods of time.

Satellite image taken over Amsterdam with an applied NDWI index compared to regular satellite image.

Interpretation

The NDWI product is dimensionless and varies from -1 to +1, depending on the hardwood content, as well as the type of vegetation and cover. The high NDWI values (in blue) correspond to high plant water content and coating of high plant fraction. Low NDWI values (in red) correspond to low vegetation content and cover with low vegetation. During periods of water stress the NDWI rate will decrease.

External links

• http://deltas.usgs.gov/fm/data/data_ndwi.aspx (regarding the McFeeters index for water bodies)
• https://eos.com/ndwi/