The index that we use is the NDVI (Normalized Difference Vegetation Index). It is a mathematical calculation that allows us to quantify the intensity of chlorophyll activity or simply the photosynthetic activity in the plants.
In the remote sensing this index is used primarily in environmental studies to allow analysis about the vegetal coverage in specific regions. With the NDVI is possible to identify the existence or the absence of vegetation, compare the photosynthetic activity of a farming through the time, as in different areas.
The remote sensing is the science to get information about a specific target without make any kind of contact. The remotely collected data can be: variation in the forces distribution, of mechanical and electromagnetic waves (light). The capturing information can be done in many platforms, also in airplanes, satellites or even in laboratories.
In the Agrosmart platform we make use of the satellite Sentinel-2, that belongs to the European Spatial Agency (ESA), in this satellite there’s a sensor able to capture information in many bands of the electromagnetic spectrum that covers from visible to infra-red, including the red band (RED) and the Near Infra-Red (NIR) both used to the NDVI calculation.
NDVI = (NIR - RED) / (NIR + RED)
In a practical way the healthy leaf absorb a lot of light from the RED band (or have a tiny reflection, around 8%) to do the photosynthesis process and reflects a lot of infra-RED (Ie. 50%), because the chlorophylle blocks your passage. The dry plant does not absorb the light from RED (reflects a lot, Ie. 30%), because it does not do photosynthesis and reflects less (ie. 40%) the infra-red because there’s less chlorophylle to block the passing light.
Figure 1: Example of the NDVI calculation and the healthy leaves in the left and the dry ones in the right.
In the NDVI images from the Agrosmart platform these values are converted in colors to make them easy to identify, values near of zero of NDVI with the red color means absence of vegetation, yellow means dry vegetation and the green color means high NDVI index with high taxes of photosynthesis.
Agrosmart’s patterns of color scale
The pattern of color adopted by the Agrosmart follow the following NDVI values:
Figure 2: Agrosmart’s NDVI scale.
This pattern refers to the values achieved by calculations over the satellite imagery. In The next chapter we will explain the NDVI’s applications.
The NDVI image below (figure 3) shows the soybean crop under a central pivot. With irregular relief it is possible to see the water tanks around the pivot, because the DVI filter reveals with the red color the absence of vegetation in these areas. Is possible to see one yellowish part of the NDVI because of a lower intensity of hotosynthesis, meaning possible failures or even careers, paths and roads.
Figure 3: Soybean crop’s NDVI under a central pívot irrigation system.
An interesting case study is the use of leaf Nitrogen (N) in the corn crop, and the result can be seen in the image below (Figure 3). The fertilization response can be seen by the high index of vegetation shown by the darker green belt in the pivot, meaning a higher activity of photosynthesis.
Figure 4: Corn crop’s NDVI under a central pívot irrigation system.
Another example is a coconut crop under a drip irrigation system in the image below (Figure 5). Perceives the parallels divisions of the careers by the red and orange colors from the NDVI, also can be seen some fails in the crop and some stains with different intensities of NDVI.
Can be seen that the left part has a NDVI more yellowish than the right portion of the image, that has the highest photosynthetic rate of plants. Another good point to notice is the lighter spots spread in the crop, showing planting failures or clearings.
Figure 5: Coconut crop’s NDVI in a drip irrigation system.
With a broader view of the farm we can compare the NDVI of different areas. Areas with it’s totality in red means absence of vegetation, full harvest or even exposed soil. In more orangeish parts with some yellow means the presence of straw, one time that the yellow in NDVI means low rate of photosynthesis of the plants. The intense green from NDVI represents vegetation in full photosynthetic production.
In the image below we can see a farm with it’s irrigation pivots. Detail that the pivot positioned at the bottom of the image has differences in NDVI, allowing the interpretation of a harvest or some different type of management along the pivot. Most pivots are without vegetation, orange or yellow, and only one with vegetation.