AWEI: Automating Water Extraction, Even in Shadow
Single-band-ratio water indices need a human to pick a threshold and still stumble over dark surfaces and mountain shadow. AWEI was engineered to be thresholded at zero and to survive exactly those hard cases.
The idea behind it
AWEI is a weighted linear combination of up to five bands, with coefficients tuned to maximise the separation between water and confounders such as shadow and dark built-up surfaces. It comes in two forms: AWEInsh for scenes without shadow, and AWEIsh, which adds a blue term to suppress shadow where terrain or tall buildings cast it.
The math behind it
AWEIsh = Blue + 2.5 × Green − 1.5 × (NIR + SWIR1) − 0.25 × SWIR2
where Blue is blue reflectance, Green is green reflectance, NIR is near-infrared reflectance, SWIR1 is shortwave-infrared 1 reflectance (~1.6 µm), SWIR2 is shortwave-infrared 2 reflectance (~2.2 µm).
How to interpret the output
Positive = water. Use AWEIsh where shadow or dark built-up surfaces are present, AWEInsh otherwise.
The bands it needs
| Sentinel-2 | Blue B2 · Green B3 · NIR B8 · SWIR1 B11 · SWIR2 B12 |
| Landsat 8/9 | Blue B2 · Green B3 · NIR B5 · SWIR1 B6 · SWIR2 B7 |
Real-world use
It is a strong choice for automated, large-area surface-water pipelines — national water inventories, multi-scene flood mapping — where hand-tuning a threshold for every scene is not feasible.
Watch out for
The coefficients were calibrated on particular Landsat scenes, so over snow, ice or very bright saline flats AWEI can misfire. Validate on your own region and pick the sh/nsh variant to match your terrain.
Compute AWEI on your own study area
Skip the code. Draw or upload a boundary and Spatial Research Suite runs AWEI on live Sentinel-2 or Landsat imagery — with cloud masking, exports and citations built in.
Run this analysis in GISforus →Frequently asked
What is the difference between AWEIsh and AWEInsh?
AWEIsh adds a blue-band term to suppress shadow and is used in scenes with terrain or building shadow; AWEInsh is for scenes without significant shadow.
Do I need to choose a threshold for AWEI?
It is designed to separate water at zero, which makes it well-suited to automated pipelines, though local validation is still wise.