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Normalized difference water index

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The Normalized Difference Water Index (NDWI) uses two near-infrared channels; one centered at a wavelength of approximately 0.86 micrometres (μm), and the other at 1.24 μm. NDWI is a remote sensing based spectral rationing technique which will help to monitor changes in water content of leaves using NIR (Near Infra Red) and SWIF (Short Wave Infra Red).(Gao, 1996)

[NDWI = (Xnir – Xswif)/(Xnir + Xswif)]

Though, we have another NDWI defined by McFeeters (1996), in which Green and NIR bands are used to monitor changes related to water content in water bodies.

[NDWI = (Xgreen – Xnir)/(Xgreen + Xnir)]

Overview

In remote sensing, Ratio Image or Spectral rationing are the enhancement techniques in which, pixel or cell in raster is from one spectral band is divided by corresponding value in another band.(by Lillisand & Kifer). Hence if user wants to use this technique in his/her study, they must be aware of the purpose of the study and which of the above two NDWI will help them in extracting useful information.

In case use is looking to monitor drought affected area, then it is advisable to use  :

[NDWI = (Xnir – Xswif)/(Xnir + Xswif)]

The SWIR reflectance reflects changes in both the vegetation water content and the spongy mesophyll structure in vegetation canopies, while 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 (Ceccato et al. 2001).

If user is looking for regional level or change in water content like flooding, then it is advisable to use NDWI proposed by McFeeters :

[NDWI = (Xgreen – Xnir)/(Xgreen + Xnir)]

Interpretation

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

References

[1] [2]

http://deltas.usgs.gov/fm/data/data_ndwi.aspx

Notes

  1. ^ http://ceeserver.cee.cornell.edu/wdp2/cee6150/Readings/Gao_1996_RSE_58_257-266_NDWI.pdf
  2. ^ http://deltas.usgs.gov/fm/data/data_ndwi.aspx
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