NDWI Range Delta

Land Usev06.00 credits/per_km2

Responsible:Niccolò Calandri

NDWI Range Delta

Overview

The NDWI Range Delta compares the mean water index between two consecutive time periods. The annual NDWI mean represents the average water index value over the period. By computing the difference between the means of two distinct periods, we obtain a measure of how surface hydrological dynamics have changed over time.

Formula:

Delta = Mean_NDWI(period 2) - Mean_NDWI(period 1)
Mean_NDWI(period) = mean(NDWI, period)
NDWI = (GREEN - NIR) / (GREEN + NIR)

Positive delta: seasonal water body variability has increased — potentially indicating increased stream intermittency, creation or expansion of artificial reservoirs with varying levels, or more pronounced seasonal drought effects.
Negative delta: water body variability has decreased, suggesting water presence stabilization — possibly associated with regulated water flows, creation of permanent retention basins, or disappearance of seasonal water bodies.
Near-zero delta: hydrological dynamics have remained stable between the two periods.

NDWI is fundamental for monitoring water resources and surface water availability changes. Seasonal range variations provide crucial insights into water resource management, climate change impacts on water availability, and effectiveness of water conservation interventions.

Calculation Methodology

Data acquisition: Sentinel-2 images for both reference periods with cloud masks (SCL band).
NDWI calculation: computed using GREEN (B3, 10m) and NIR (B8, 10m) bands, resampled to 15m.
Mean calculation for period 1: mean NDWI computed pixel-wise across all valid scenes of the first period.
Range calculation for period 2: same for year N+1.
Delta calculation: pixel-wise subtraction: Delta = Mean(period 2) - Mean(period 1).
Validation: pixels lacking valid data in either period masked as NoData.

Data Sources

Source Code	Name	Provider	Resolution	Availability
WRD_S2XXX_99	Sentinel-2 MSI L2A	ESA/Copernicus	15m (10m native GREEN/NIR)	From 2015

Technical Notes

NDWI is particularly sensitive to surface water presence but can be influenced by soil moisture and aquatic vegetation.
Natural seasonal variations of water bodies (glacial lakes, alpine rivers) should be considered in interpretation.

References

McFeeters, S.K. (1996). "The use of the Normalized Difference Water Index (NDWI) in the delineation of open water features." International Journal of Remote Sensing, 17(7), 1425-1432.
Xu, H. (2006). "Modification of normalised difference water index (NDWI) to enhance open water features." IJRS, 27(14), 3025-3033.

Data Sources & References

Sources

WRD_S2XXX_99

Methodology

Il delta del range dell'NDWI viene calcolato confrontando i range annuali dell'indice di acqua tra due periodi consecutivi. Per ciascun periodo si calcola la media dell'NDWI. Il delta è ottenuto tramite sottrazione pixel per pixel: Delta = Media(periodo 2) - Media(periodo 1). I dati provengono da Sentinel-2 (bande GREEN B3 e NIR B8) con risoluzione 15m.