What is NDWI? (Normalized Difference Water Index)
What is NDWI?
NDWI, the Normalized Difference Water Index, is a satellite index that highlights open water and maps surface wetness. Where NDVI tells you how green a field is, NDWI tells you where water is sitting — whether that is an irrigation reservoir, a flooded rice paddy, or a waterlogged patch after heavy rain. It is the index you reach for when you want to see water: rivers, ponds, flooded fields, saturated soil, or the boundaries of an irrigation dam.
Water absorbs near-infrared light strongly and reflects a little green, while soil and vegetation do the opposite. By exploiting this contrast, NDWI turns the presence of surface water into a single number.
How it’s calculated
NDWI uses the contrast between green and near-infrared reflectance:
NDWI = (Green − NIR) / (Green + NIR)On Sentinel-2 these are Band 3 (Green) and Band 8 (NIR). Where open water dominates the pixel, green reflectance exceeds NIR and NDWI goes positive. Where vegetation or dry soil dominates, NIR stays high and NDWI goes negative. As with NDVI, the ratio cancels out raw brightness so values are comparable across scenes and dates.
Typical value ranges
| NDWI range | Meaning | Typical for |
|---|---|---|
| 0.2 – 1.0 | Open water | Lakes, rivers, reservoirs, flooded fields |
| 0.0 – 0.2 | Wet or saturated surface | Shallow water, wet soil, marsh, snowmelt |
| −0.1 – 0.0 | Moist vegetation / damp soil | Well-watered crops, dewy mornings |
| −0.3 – −0.1 | Dry vegetation | Stressed or senescent crops, dry pasture |
| Below −0.3 | Dry soil / built-up | Bare dry ground, pavement, dense dry vegetation |
The 0.0 line is the practical water/non-water boundary; values above ~0.2 are confidently open water.
When to use it
Use NDWI when your question is about surface water: flooded rice paddies, drainage after heavy rain, reservoir levels, or waterlogging in low-lying fields. Over a season, NDWI on an irrigated field rises after watering or rainfall and falls steadily as the surface dries. Flooded rice paddies show a sharp spike to positive values during the flooded phase, then collapse toward zero at drainage. Drought shows up as a sustained decline into negative territory.
NDWI works best on rural and agricultural land. In built-up areas, dark rooftops and asphalt can mimic water and push NDWI slightly positive — interpret with care in mixed land-use settings. Small water bodies below the 10 m pixel size (narrow ditches, small ponds) may not fully fill a pixel and read as a partial value.
Comparison with other indices
The most common confusion is NDWI vs NDMI. NDWI (McFeeters, Green/NIR) detects open surface water — is there water sitting on the ground? NDMI (Gao, NIR/SWIR) measures water inside leaves — how much moisture is in the vegetation? A drought-stressed crop can have very low NDMI (dry leaves) while its NDWI reads near zero (no standing water). A flooded field has high NDWI but its NDMI depends on the crop’s health, not the floodwater. Pick the index that answers the question you are actually asking.
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Frequently asked questions
What is a good NDWI value?
For crops, the right NDWI depends on what you’re monitoring. Slightly negative values (−0.1 to −0.3) are normal for healthy vegetated land — you are looking for stability, not high values. Positive values indicate surface water, which may be desirable (irrigation, rice) or a problem (waterlogging). Watch for sudden drops, which indicate drying.
What is the difference between NDWI and NDMI?
NDWI (McFeeters, Green/NIR) detects open water on the ground. NDMI (Gao, NIR/SWIR) measures water inside leaves. A flooded field has high NDWI; a healthy well-watered crop has high NDMI. They answer different questions — use NDWI for flooding and drainage, NDMI for crop moisture stress.
Can NDWI detect drought?
Indirectly. NDWI shows surface drying, which often accompanies drought, but it does not measure the water inside the plant. For drought stress in vegetation, NDMI is more reliable because it responds to leaf water content, not just surface wetness.
Why is my field's NDWI negative?
Negative NDWI on a field simply means there is no open water — which is normal for dry land and most crops. It does not mean your crop is thirsty. Only positive NDWI (above zero) indicates standing water; for plant moisture, check NDMI instead.