The Azimuth Project
Normalized difference water index

Idea

The Normalized Difference Water Index (NDWI) is a satellite index derived from Near-Infrared (NIR) and Short Wave Infrared (SWIR) channels:

NDWI=1SWIR/NIR1+SWIR/NIR NDWI=\frac{1-SWIR/NIR}{1+SWIR/NIR}

The amount of water present in leaf internal structure mainly affects the spectral reflectance in the SWIR interval (ca. 1.2-1.7 μ\mu m). The SWIR reflectance is also sensitive to leaf internal structure. Because the NIR is affected by leaf internal structure and leaf dry matter, but not by the water content, the combination of both into NDWI “removes” leaf dry matter and internal structure.

NDWI is less susceptible to atmospheric scattering than NDVI, but does not remove completely the background soil reflectance effects, similar to NDVI. Because the information about vegetation canopies contained in the SWIR channel is very different from that contained in the VIS channel, NDWI should be considered as an independent vegetation index.

NDWI defined as above is also called NDWI 1NDWI_1 because there are a number of alternative spectral indices for water detection:

  • NDWI 2NDWI_2 : uses a combination of the NIR and the Green spectral bands
NDWI 2=1NIR/G1+NIR/G NDWI_2 = \frac{1-NIR/G}{1+NIR/G}
  • MNDWI: (‘modified NDWI’)
MNDWI=1SWIR/G1+SWIR/G MNDWI = \frac{1-SWIR/G}{1+SWIR/G}
  • NHI: (‘normalized humididy index’, also called NDPI ‘normalized difference pond index’)
NHI=1G/SWIR1+G/SWIR=MNDWI NHI = \frac{1-G/SWIR}{1+G/SWIR} = -MNDWI

Sometimes the NDVI is also used for water detection, though it’s primary application is the detection of vegetation.

References

category: earth science