NDWI (Gao, vegetation water)
The normalized difference water index of Gao 1996, from near-infrared and shortwave-infrared reflectance: the one that measures liquid water in vegetation.
Experimental: not yet fully verified. How results are checked
NDWI is 0.2667.
Provenance
- Computed by
- raster.index.ndwi-gao 1.0.0, core 0.1.0
- Model
- NDWI = (NIR - SWIR1) / (NIR + SWIR1) on surface reflectance (Gao 1996)
- Accuracy
- Exact arithmetic on the reflectance given.
- Notes
- 1 shown with the answer
- Cites
- Gao, B.-C., Remote Sensing of Environment, NDWI - A normalized difference water index for remote sensing of vegetation liquid water from space
Something look off?
How we got thisFormula, worked example, sources, and proof
Model: NDWI = (NIR - SWIR1) / (NIR + SWIR1) on surface reflectance (Gao 1996)
Accuracy: Exact arithmetic on the reflectance given.
When to use this: Use this for how much water the vegetation itself holds: shortwave-infrared is absorbed by liquid water in leaves, so the index falls as a canopy dries. It is used for drought stress, fuel moisture, and irrigation scheduling, and is the same quantity often published as NDMI.
Limitations: It is not an open-water index, despite the shared name: for mapping lakes use the McFeeters or modified index. It responds to canopy structure as well as water, and the shortwave band it needs is missing from sensors that carry only visible and near-infrared bands.
Worked example: NDWI (Gao, vegetation water). Source: Gao 1996: NIR 0.38 and SWIR1 0.22 give 0.2667. It is golden test vector v002, and every build checks the tool still gives its answer within its tolerance.
You enter
- Near-infrared reflectance
- 0.38
- Shortwave-infrared reflectance
- 0.22
You get
- NDWI
- 0.2667
Review: Not yet independently reviewed by a GIS professional.
Last verified: 2026-09-22, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.
Status: version 1.0.0, core 0.1.0. See this tool in the verification report.
Checked against: 10 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.
Sources
- NDWI - A normalized difference water index for remote sensing of vegetation liquid water from space, Gao, B.-C., Remote Sensing of Environment, Volume 58, issue 3, pages 257-266. NDWI = (NIR - SWIR) / (NIR + SWIR), for water in vegetation.
Terms
- NDWI: normalized difference water index
- Two different indices share this name: McFeeters 1996 uses green and near-infrared to map open water, and Gao 1996 uses near-infrared and shortwave-infrared to measure water inside vegetation. Source: The use of the Normalized Difference Water Index (NDWI) in the delineation of open water features
Experimental means this tool has not yet met the stable bar: at least 20 golden vectors, differential tests, and an independent worked example.