The Normalized Difference Water Index, a spectral index designed to highlight open water features and assess vegetation water content from satellite imagery. NDWI is essential for mapping surface water extent and monitoring flood inundation.
NDWI (Normalized Difference Water Index) is a spectral index with two common formulations depending on the application. McFeeters' NDWI uses green and near-infrared bands (NDWI = (Green - NIR) / (Green + NIR)) to delineate open water bodies, while Gao's NDWI uses near-infrared and shortwave infrared bands (NDWI = (NIR - SWIR) / (NIR + SWIR)) to assess vegetation moisture content. The McFeeters formulation exploits the strong absorption of NIR radiation by water, producing positive values for water and negative values for vegetation and soil. This index is widely used for mapping lakes, reservoirs, rivers, and flood inundation extent from Landsat and Sentinel-2 imagery. A related index, MNDWI (Modified NDWI), substitutes the green band with a modified formulation to reduce noise from built-up areas. Time-series NDWI analysis enables monitoring of reservoir storage changes, wetland hydroperiod dynamics, and long-term surface water area trends. The Global Surface Water dataset by the European Commission's Joint Research Centre used NDWI-based algorithms on the entire Landsat archive to map 35 years of surface water dynamics globally.