NDMI

  Normalized Difference Moisture Index (NDMI) detects moisture levels in vegetation using a combination of near-infrared (NIR) and short-wave infrared (SWIR) spectral bands. SWIR band reflects changes in both the vegetation water content and the spongy mesophyll structure in vegetation canopies, while the NIR reflectance is affected by leaf internal structure and leaf dry matter content but not by water content. The combination of the NIR with the SWIR removes variations induced by leaf internal structure and leaf dry matter content, improving the accuracy in retrieving the vegetation water content. The amount of water available in the internal leaf structure largely controls the spectral reflectance in the SWIR interval of the electromagnetic spectrum. SWIR reflectance is therefore negatively related to leaf water content. 

It is a reliable indicator of water stress in crops. The data is available at 20 metre resolution.

The NDMI values have been calculated using the following equation:

• NDMI = (NIR – SWIR) / (NIR + SWIR) 

The NDMI range is -1 to +1 and we use the NDMI colour map which is shown on the right side of the image.