NDRE: Normalized Difference Red Edge Index

Normalized difference red edge index (NDRE for short) is a method of measuring the amount of chlorophyll in the plants. The best timing to apply NDRE is mid-to-late growing season when the plants are mature and ready to be harvested. At this point, other indices would be less effective to use.

Let’s dive into details and see how to use normalized difference red edge vegetation index for monitoring crops on our Platform. You will find out why this index is more effective at later stages of plant development.

NDRE Vegetation Index: How It Monitors The Health Of Crops

NDRE is a vegetation index used in remote sensing for measuring the chlorophyll content in plants (read more about precision agriculture). It is represented by a certain value calculated using a combination of a Near-InfraRed (NIR) band and the RedEdge range between visible Red and NIR. The NDRE formula is:

NDRE = (NIR – RedEdge)/(NIR + RedEdge)

Such a band combination makes NDRE extremely sensitive to those wavelengths of light that are reflected by the chlorophyll contained within the plants.

The normalized difference red edge index can, therefore, be applied to confirm whether a growing plant is healthy or not. Low amounts of chlorophyll may indicate the problem crops:

  • sick plants,
  • plants infested with pests,
  • plants with nutrient deficiencies,
  • damaged plants.
NDRE will point you to the problem crop, so you can further investigate the cause of the low chlorophyll content therein.

How To Read NDRE Imagery On EOSDA Crop Monitoring

Since chlorophyll is crucial to the process of photosynthesis, its quantity is an important indicator of the plant’s health. On EOSDA Crop Monitoring, we’re using a standard scale of -1 to 1 for the NDRE index. It’s important to know how to correctly interpret the values:

  • -1 to 0.2 indicate bare soil or a developing crop;
  • 0.2 to 0.6 can be interpreted as either an unhealthy plant or a crop that is not mature yet;
  • 0.6 to 1 are good values indicating healthy, mature, ripening crops.
colors and corresponding index values
How NDRE map looks on EOSDA Crop Monitoring.

To make things a little easier for the user, we’ve tied colors to each possible value. The scale ranges from saturated red for -1 to saturated green at +1. In between there are different hues of red, orange, yellow, and green, according to the standard “rainbow” palette.

So, as we can see, on EOSDA Crop Monitoring, values of NDRE lower than 0.3 may be a tell-tale sign of an unhealthy or stressed plant. On the flip side, low values could also stand for bare soil and young plants. Which is why, at early growth stages, we recommend using MSAVI.

NDRE In Action: Practical Applications On EOSDA Crop Monitoring

When To Use NDRE Index

The best time to apply the normalized difference red edge index is when the crop is mature and ripening. That usually occurs towards the end of the growing season. During the mid-to-late season, values below 0.6 will almost definitely suggest some form of damage to the crop.

At this point, NDRE is a much better indicator of crop health or lack thereof than NDVI. The latter will focus on the density of the plant canopy and the greenness of its upper layer. That is why using a NDRE map for variable-rate fertilization, spraying, precision irrigation, fertigation, and other field activities will be more effective at later growth stages.

Precision Fertilizer Mapping

When it comes to nitrogen fertilizer application, NDRE imagery on EOSDA Crop Monitoring provides the most accurate picture. This is due to the correlation between nitrogen and chlorophyll content in plants.

In fact, NDRE maps can be successfully used for the differential application of other fertilizers as well. Especially at later growth stages, when NDVI vegetation maps are not as accurate. For example, these two images below show the same field with its vegetation measured by NDVI (on the left) and NDRE (on the right), respectively. Notice how the top corner of the field looks healthy (dark green) according to NDRE, while NDVI measures lower density and/or greenness.

zoning tool
Differences in vegetation maps based on NDRE and NDVI.

The crop in the image was in its latest stages of development. Its upper layer of canopy started to turn yellow and/or shrink, which is a normal indicator of readiness for harvesting. NDVI naturally dropped detecting lower density and less greenness.

Nevertheless, NDRE easily identified the presence of large quantities of chlorophyll within multiple canopy layers. It meant that beneath the upper layer of canopy, the plant was healthy and ripe for harvest.

If the owner’s decisions had been based solely on NDVI measurements, they might have opted for fertilizing a large portion of the field that appeared stressed. Yet this would have been a very expensive mistake. Luckily, the farmer applied NDRE available on EOSDA Crop Monitoring and avoided wasting as much as 500 units of fertilizer.

NDRE For Deep Canopy Penetration

Normalized difference red-edge index is designed to “see” deep through the dense vegetation canopy. This makes it an ideal tool for monitoring densely planted fruit crops, like:

  • apple,
  • banana,
  • citrus,
  • mango,
  • pineapple.

As well as some other crops, like: coffee, corn, cotton, pepper vine, sunflower, among others.

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NDRE vs NDVI: What’s The Difference?

Despite the fact that both vegetation indices are effective remote sensing tools (if you know how and when to use them), there is a difference between NDRE and NDVI that must be taken into account.

Why Is NDVI Not Enough?

Let’s first look at why NDVI is a “classic” vegetation index. To begin with, this index has been in use for much longer and is definitely more commonly used.

In simplest terms, NDVI is capable of measuring the density and greenness of the vegetation in the field. In the right circumstances and at a certain time of the season, dense green vegetation is a good general indicator of crop health.

However, NDVI was not designed to measure density past the canopy cover. As the plant keeps growing, and more layers of leaves form beneath the canopy, measuring excessive greenness for NDVI becomes more challenging. This is exactly the reason why, for the crops in mid-to-late stages of development, NDVI should be replaced with NDRE.

NDRE Comes To Aid

Later on in the growing season, it is much harder for NDVI to register subtle variations in the state of certain crops. For example, to grow sugar cane, cereal crops, trees with a lot of foliage, etc., you would better swap to another index in the late season.

NDRE, on the other hand, will pick up variations in the amount of chlorophyll not just in the upper canopy, but all through the foliage. The plant may remain visibly green on the surface “puzzling” the NDVI, but experience stress in the lower layers at the same time. In this case, focusing specifically on the chlorophyll content would be a better indicator of crop health.

To illustrate this difference, here are NDVI and NDRE images from EOSDA Crop Monitoring put side by side. The crop here is at its late growth stage, so it is green and dense.

How NDRE and NDVI index maps differ.

The NDVI image on the right does not show much variety in vegetation across the field. Meanwhile, the NDRE image on the left indicates diverse levels of chlorophyll from area to area. Thus, applying NDRE in this case is more effective. Areas with lower chlorophyll content might indicate plants that are:

  • stressed (dehydrated, affected by pests, or
  • otherwise damaged);
  • less mature than other plants.
We recommend sending scouts to check what the problem really is and make decisions based on all the available data.

NDRE – A Powerful Remote Sensing Tool

The choice of a vegetation index largely depends on the needs of a farmer, type of crop, and growth stage. NDRE can be used as a stand-alone index for monitoring the health of crops like coffee, corn, and banana, among others, throughout most of the growing season. Yet it can also be used in combination with NDVI or any other indices, to determine the state of any crop at mid-to-late stages of development. On EOSDA Crop Monitoring, these and other indices are easy to use and will provide you with crucial data needed to grow healthy crops and improve yields.