Water Ratio Index

Water Ratio Index - A spectral index for water applications. Water indices exploit the strong absorption of shortwave infrared and near-infrared radiation by liquid water. They are critical for delineating water bodies, assessing moisture stress in vegetation, and monitoring hydrological changes over time. WRI is particularly suited for water. The general formula is (G + R)/(N + S1), which requires G and R and N and S1 spectral bands.

WRI is supported by 3 satellite sensors in our database, including Landsat 8/9, Sentinel-2, SuperView-2. Each sensor uses different band designations — for example, Landsat 8/9 uses the formula (B3 + B4)/(B5 + B6), while Sentinel-2 uses (B3 + B4)/(B8 + B11). Select a sensor above to see its specific band mapping.

WRI requires G (550), R (650), N (850), S1 (1610). Water absorbs strongly in the near-infrared and shortwave infrared portions of the spectrum, creating a measurable contrast with shorter wavelengths that penetrate the water surface.

Both Python and R code samples are provided above. In Python, use rasterio to load individual band GeoTIFF files and numpy for the arithmetic. In R, the terra package handles raster operations efficiently. The key is to load bands as floating-point arrays to avoid integer division, and to handle division-by-zero cases where the denominator equals zero. For production use, consider applying a valid data mask to exclude no-data pixels before calculation.

WRI is designed to enhance water features, but dark surfaces like shadows, asphalt, and dark soils can produce similar values. For reliable water mapping, consider combining WRI with a threshold analysis and, where possible, a secondary index to reduce false positives. Time-series analysis can also help distinguish permanent water bodies from temporary dark surfaces.