NDWI (Normalized Difference Water Index) in agricultural applications is typically derived from the NIR and SWIR bands (near-infrared and shortwave infrared radiation) and expresses the water content in biomass — in leaves, stems, and ears — not directly grain moisture. Dry plant material reflects more SWIR, moist biomass reflects more NIR — this ratio produces the index value.
How to read the blue scale
Dark blue = high water content in biomass → the crop is still green/moist, physiologically immature, or these are areas with higher moisture (terrain depressions, denser stand, lodging, weed infestation, later sowing).
Transitional shades of blue = progressing dehydration of biomass, the crop is approaching maturity but hasn't yet dropped to harvest moisture.
Light blue to white = low water content, biomass is dry, the crop is physiologically mature and closest to optimal harvest moisture.
It's important to look at the development over time (multiple images in sequence), not a single image — you're interested in the drying trend across the field, not the absolute value on one given day.
Interpretation for maturity zoning
The map usually shows a mosaic: white/light spots mature first, dark blue ones later, as they have higher residual moisture. However, before marking an area as "immature," verify whether the darker spot actually corresponds to later maturity, or whether it's a different phenomenon with the same optical signature — a weedy/lodged section, a wetter soil type, cloud shadow, or a wet spot in the terrain. White spots on turning strips or near roads can also distort the picture (sparse stand, bare soil).
Recommendations for combine harvesting
Plan the harvest in order from the lightest (driest) areas to the darkest — for cereals, this usually also means from the edges/elevated spots toward the depressions.
Treat NDWI as a navigation layer for combine routing and prioritizing areas, not as a substitute for measuring grain moisture — verify actual moisture with the combine's moisture meter before and during harvest (optimum for grain is typically 15–20%, though harvesting is technically possible in the range of roughly 12–20%).
Let dark blue zones mature further and harvest them later — harvesting at high biomass moisture means higher drying costs and a risk of grain damage.
Conversely, waiting too long on very light/white zones doesn't pay off either — risk of shattering, stalk brittleness, and losses from overripeness.
For large blocks, consider harvesting by zones (splitting the area according to NDWI) if the combine and logistics allow a variable-rate approach.
Choose the most recent image possible — NDWI after rain or morning dew will appear misleadingly "wetter"; ideally compare images taken under similar conditions.