• Berno Greyling

NDVI – Over-promising and easily under-delivering

NDVI (Normalized Difference Vegetative Index) has emerged as one of the leading vegetative health-indexes in recent times, although, how valuable is this index really in determining a plant’s health? There have been many claims about what value NDVI can offer in terms of monitoring plant health and has often been thought of as the holy grail of plant monitoring. In light of this, the team at Revolute Systems would like to provide some insight into this topic and caution farmers against the limitations of applying this index to real-life production scenarios.

What is NDVI?

NDVI works on the principle that a plant will reflect and absorb different wavelengths of light. These wavelengths of light that are either absorbed or reflected depend the health of the plant, which is in turn, dependent on the environmental conditions. An example of this that can be seen with the naked eye is when a leaf starts to die. During this time, you will notice that it starts to lose it’s lush green colour and starts turning various shades of red and brown. The nifty thing about the NDVI index is that the cameras used can detect very small changes taking place during this transition which are not visible to the eye.

What do NDVI values mean?

NDVI values indicate the ratio of near-infrared light to visible red light and range from -1 to 1. In summary, water is indicated by a value less than 0, bare soil ranges between 0 - 0.1, while very healthy and actively growing leaves are close to 0.8.

We now understand that NDVI is impacted by the plant’s health and the environmental conditions that the plant finds itself in, although, here is where we would like to caution farmers: NDVI values are also highly impacted by factors such farming practices, the stage of season and even the time of day that these values are measured. This should raise a large red flag when wanting to compare NDVI value and begs the question, how comparable are values given through by agri-tech companies – are these factors being taken into account?

Considerations when choosing service provider

When monitoring a block with NDVI maps, it is highly important to keep into consideration human and environmental factors that could influence measurements. For example, if you would like to track the change in growth over the course of the season, pruning your block (i.e. introducing another changing variable), will have an impact on the resultant NDVI value and therefore the measurement taken before and after pruning the block will no longer be comparable. The part of the vineyard block that has just been pruned will show lower NDVI values, even though the plants’ health did not change. Satellites ability to take multiple images in a season gives us an affordable tool to monitor these changes over time in great detail.

So timing is everything, and how to make the decision can be extremely difficult sometimes. At Revolute we have been focusing a lot on how to guide our partners and clients through these decisions as each crop has its own challenges. A nice trick we can give you for summer tree crops, is over Christmas and New Years there is minimal amount of activities on farms, providing an ideal gap for stable images to be taken. Some service providers might be on Hermanus beach at that time, but we are open for high-resolution bookings and the low-resolution monitoring never sleeps.

With low resolution options we also have to take into account the cover crop of the production areas. Over time we have found that cover crop has a bigger impact with younger tree crops, and at the start of the season before the canopy of the crops have started to mature. This means, NDVI should not be used for any big decisions in the scenarios just mentioned, but rather just as a tool for helping you inspect variation while in the field.

Applying NDVI to Decision making at Revolute

Due to the number of factors influencing NDVI values and how easily they can be misread, at Revolute we look at the range of NDVI values across a particular block at a certain point in time, and from this we aim to identify management zones or problem areas to investigate. Although this is where the use of NDVI stops as NDVI can only tell you whether a plant is healthy or stressed but gives no indication to what the under-lying cause of the stress is.

As it would be unwise to make a judgment call solely based on NDVI, this is where Revolute Systems can make a difference and add value to you. We are here to help you understand the factors that actually affect the plant’s health rather than just giving you a report of healthy vs. stressed, therefore you are not left asking yourself, “So what now?”

As plants are affected by a multitude of factors, we try to take as many of these factors into consideration as possible, such as the soil (by use of our (EMI) Electro-Magnetic Induction instrument), the terrain, as well as the irrigation layout, and therefore we at Revolute are able to provide farmers with a multivariate solution to a multivariate system.

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