This tutorial demonstrates how to calculate MNDWI using ERDAS Imagine Model Maker or Spatial Modeller

The Modified Normalized Difference Water Index (MNDWI) is is an index of open water features and produce much better result in comparison to NDWI Indices.

MNDWI = (GREEN— SWIR)/(GREEN + SWIR)

This index maximizes reflectance of water by using green light wavelengths and minimizes low reflectance of SWIR by water features while taking advantage of the high reflectance of SWIR by vegetation and soil features. As a result, water features are enhanced owing to having positive values and vegetation and soil are suppressed due to having zero or negative values.