Vegetation cover on the Earth's surface undergoes severe stress during a drought. If affected areas are not identified in time, entire crops may be damaged. The early detection of water stress can prevent many of the negative impacts on crops. Remote sensing of land and the NDWI index can control irrigation in real time, significantly improving agriculture, especially in areas where meeting the need for water is difficult.
Normalized Difference Water Index (NDWI), introduced for the first time in 1996 in Gao (Gao), reflects moisture content in plants and soil and is determined by analogy with NDVI as:
NIR - near infrared range with wavelengths in the range of 0.841 - 0.876 nm
SWIR - a part of the range with wavelengths in the range of 1.628-1.652 nm
The functionality of the formula is explained by the following considerations: instead of using the red range, the reflection intensity in which is determined by the presence of chlorophyll, a short-wave near-infrared (SWIR) is used in which high absorption of light by water occurs. A wider range of 1500-1750 nm is possible. The use of the same near infrared (NIR) as in the case of NDVI is due to the fact that water does not absorb this part of the electromagnetic spectrum, thus the index is resistant to atmospheric effects, distinguishing it from NDVI. Note that when observing forests, the NDWI index is characterized by a more stable decrease in values upon reaching critical anthropogenic load, which can serve as an indicator of the ecological state of forests more sensitive than NDVI.
The results of NDWI can be presented in the form of maps and graphs, providing information on both the spatial distribution of water stress on vegetation and its temporal evolution over longer periods of time.
The NDWI product is dimensionless and varies from -1 to +1, depending on the hardwood content, as well as the type of vegetation and cover. The high NDWI values (in blue) correspond to high plant water content and coating of high plant fraction. Low NDWI values (in red) correspond to low vegetation content and cover with low vegetation. During periods of water stress the NDWI rate will decrease.
The NDWI index for assessing risk of fire is used to determine the presence of moisture in vegetation cover. Higher NDWI values indicate sufficient moisture, while a low value indicates water stress.