This article reviews prominent water detection indices - NDWI, MNDWI, NDMI, AWEI, and WRI - highlighting their distinct formulas, applications, and limitations in remote sensing analysis.

NDWI

Normalized Difference Water Index
Developed by McFeeters (1996) to identify open water bodies using spectral reflectance differences:

Formula:
NDWI = (NIR - Green) / (NIR + Green)

Principle:

  • High reflectance in Green band
  • Strong absorption in Near-Infrared (NIR) band
  • Maximizes contrast between water and vegetation/soil

MNDWI

Modified Normalized Difference Water Index
Proposed by Xu Hanqiu (2005) to address NDWI's limitations in urban areas:

Formula:
MNDWI = (Green - SWIR) / (Green + SWIR)

Advantages over NDWI:

  1. Suppresses urban building interference
  2. Enhances subtle water features (e.g., suspended sediments)
  3. Distinguishes water from shadows
  4. Higher accuracy in built-up areas

Limitations:

  • Less effective for shallow/turbid waters
  • Challenging in fragmented aquatic environments

NDMI

Normalized Difference Moisture Index
Created by Gao (1996) for vegetation water content monitoring:

Formula:
NDMI = (NIR - SWIR) / (NIR + SWIR)

Applications:

  1. Crop water stress detection
  2. Drought monitoring
  3. Wildfire risk assessment
  4. Vegetation health evaluation

Value Range:

  • High values (+1): Water-sufficient vegetation
  • Low values (-1): Water-stressed vegetation/non-vegetated areas

AWEI

Automated Water Extraction Index
Introduced by Feyisa et al. (2014) to overcome urban shadow and reflectance challenges:

Variations

AWEI_nsh (non-shadow suppression):
math
AWEI_{nsh} = 4 × (Green - SWIR1) - (0.25 × NIR + 2.75 × SWIR2)

Bands: Green (B3), SWIR1 (B6), NIR (B5), SWIR2 (B7)

AWEI_sh (shadow suppression):
math
AWEI_{sh} = Blue + 2.5 × Green - 1.5 × (NIR + SWIR1) - 0.25 × SWIR2

Bands: Blue (B2), Green (B3), NIR (B5), SWIR1 (B6), SWIR2 (B7)

Advantages:

  • Superior urban shadow resistance
  • Higher automation in water extraction
  • Robust cross-scenario performance

WRI

Water Ratio Index
Uses visible-NIR/SWIR ratios for water detection:

Formula:
WRI = (Green + Red) / (NIR + SWIR)

Interpretation:

  • High values (>1): Water bodies
  • Moderate values (~1): Moist vegetation/soil
  • Low values (<1): Dry land features

Applications:

  1. Water body mapping
  2. Vegetation moisture assessment
  3. Flood inundation delineation

Summary


Figure: Comparative characteristics and application scenarios of major water indices