Normalized Difference Water Index (NDWI)

Normalized Difference Water Index (NDWI) can refer to one of two remote sensing-derived indexes related to liquid water: to monitor changes in water content of leaves, and to monitor changes related to water content in water bodies. 

"Known to be strongly related to the plant water content, NDWI is a very good proxy for plant water stress. It is a satellite-derived index from the Near-Infrared (NIR) and Short Wave Infrared (SWIR) channels. The SWIR reflectance 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. " (European Commission, 2011)

"The NDWI index is most appropriate for water body mapping. The water body has strong absorbability and low radiation in the range from visible to infrared wavelengths. The index uses the green and Near Infra-red bands of remote sensing images based on this phenomenon. The NDWI can enhance the water information effectively in most cases. It is sensitive to built-up land and often results in over-estimated water bodies.

Values description: Values of water bodies are larger than 0.5. Vegetation has much smaller values, which results in distinguishing vegetation from water bodies easier. Built-up features have positive values between zero and 0.2." (Sinergise Ltd., n.y.)

NDWI = (Red - NIR) / (Red + NIR)
(Sinergise Ltd., n.d.)

Sources

European Commission. NDWI: Normalized Difference Water Index. 2011. Version 1. DESERT Action - LMNH Unit. Accessed March 13, 2019. Available at: http://edo.jrc.ec.europa.eu/documents/factsheets/factsheet_ndwi.pdf

NDWI (Normalized Difference Water Index)". Seninel Hub,  Sinergise Ltd. n.d. https://www.sentinel-hub.com/eoproducts/ndwi-normalized-difference-wate…, Accessed Mar 1, 2019.