NDWI - Water Content

NDWI — Normalized Difference Water Index

Overview

The Normalized Difference Water Index (NDWI) is a satellite spectral index that measures the water content of land surfaces. Values range from -1 to +1: positive values indicate water bodies or heavily saturated areas, negative values indicate dry land with sparse vegetation or bare soil.

NDWI is a versatile tool used for:

• Water body mapping — detecting lakes, rivers, ponds, and wetlands
• Flood monitoring — identifying flooded areas in near real time
• Water stress assessment — locating vegetation zones with water deficit
• Drought analysis — tracking the evolution of soil and vegetation dryness over time
• Water resource management — supporting irrigation planning and catchment management

The index exploits the contrast between the Green band (560 nm), abundantly reflected by water, and the NIR band (842 nm), strongly absorbed by water but reflected by healthy vegetation. The result is an index that emphasises water features while suppressing the terrestrial vegetation signal (McFeeters, 1996).

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Calculation Methodology

NDWI is calculated from ESA Sentinel-2 L2A imagery (surface reflectance) at 10 m resolution:

Formula: NDWI = (B3 - B8) / (B3 + B8) where B3 = Green band (560 nm, 10 m) and B8 = NIR band (842 nm, 10 m)

The calculation process follows the same pipeline as NDVI:

1. Image selection — acquisitions within the time interval of interest
2. Cloud masking — using the Scene Classification Layer (SCL) band to exclude cloudy pixels, shadows, and snow
3. Temporal composite — median of valid values over the interval to reduce atmospheric noise
4. Index calculation — application of the NDWI formula pixel by pixel
5. Aggregation — statistics (mean, median, percentiles) for each polygon of interest

Note: unlike NDMI, NDWI does not require prior masking of water bodies, since water itself produces positive values and constitutes the signal of interest.

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