What is Normalized Difference RedEdge (NDRE)?
NDRE is a spectral index that is built as a blend of several bands: Nea-InfraRed (NIR) spectrum and a band that uses a narrow spectral range between visible Red and NIR. The index is very similar to the more common and powerful analog with a greater history – NDVI. However, NDRE is more sensitive than NDVI for a certain period of crop maturation. NDRE is a better marker of plant conditions than NDVI for middle and late season crops that have already accumulated a large amount of chlorophyll. The reason is that red-edge light (the one used in NDRE) can pass through the leaves far deeper than red light (the one used in NDVI). It’s more relevant than NDVI for intensive use during the entire cultivation season, as NDVI often becomes inaccurate after plants accumulate a maximum amount of chlorophyll content.
How can NDRE be used and interpreted?
NDRE can be used as a single monitoring point, as well as in combination with NDVI over the entire canopy aging period from the date of seeding to harvest. The index gives a full picture of chlorophyll content which is one of the main Nitrogen indicators and helps with fertilizer application mapping. While analyzing NDRE images, the color palette is typically red to green, where red is bare soil, pale yellow to pale green are unhealthy plants or vegetation that is early in its maturation, and lastly, green is a healthy canopy. Let’s consider the sample data below:
The field has recently been sown with winter crops and as you can see from the NDRE image, it already has a pale green color. This color directly indicates a low chlorophyll content in plants, most likely due to improper Nitrogen-rich fertilizer (N-fertilizer) management.