To Harvest or not to Harvest

NDVI correlation with wheat harvest

In this article, we will be going micro. We will zoom in to a single farm patch, and see what we can gather from it, by analyzing the satellite image data of the patch over a period of time.

This patch of interest lies in Dildarnagar, Uttar Pradesh.

Now, we know the following about the patch:

1. Location (coordinates)
2. Area (2.25 acres)
3. Crop (Wheat)
4. Harvest date (16th April 2021)

We know this because one of the wheat harvesters in our fleet worked on this patch on 16th April 2021, when wheat harvesting was going on in full swing across Uttar Pradesh.

While building the satellite image analytics capabilities within Carnot, this appeared to be a suitable patch to get started with. So our team evaluated the NDVI (Normalized Difference Vegetation Index) values for this patch from January till May 2021. We took the SentinelSat images (10m resolution) for this region (for all the dates in this period for which they were available).

In step 1, we simply visualized the output.

What you see above is a visual representation of how the NDVI changes from 16th January 2021 to 5th May 2021. If we plot the mean NDVI plot for these dates, this is what we get:

The plot above correlates well visually with the image (the greener the image, the higher the NDVI). We know that harvesting happened on 16th April 2021. NDVI was well below its peak and was decaying further (although slowly) around that time. Was this the right time for harvest? Or should harvest have happened earlier, when NDVI was the highest? We looked up literature.

We found this variation of indices (including NDVI), in this publication. It is reproduced below with proper citations:

Figure 4. Variation of indices values for NDVI, SAVI and EVI for different phenological stages of wheat. Temporal MODIS Data for Identification of Wheat Crop using Noise clustering Soft Classification Approach — Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Variation-of-indices-values-for-NDVI-SAVI-and-EVI-for-different-phenological-stages-of_fig2_275956960

As can be seen, the decline in NDVI from the peak corresponds to a period of maturity, and harvesting happens toward the end of the decline when the NDVI value is quite close to the NDVI at the time of sowing.

So looking at the above, it seems like the harvesting did happen alright. It could have been a bit sooner, but it definitely wasn’t premature harvesting.

Enjoyed this article? Then be on the lookout for another two weeks later. Till then, for any data-related discussions, feel free to drop us a line at datalabs@carnot.co.in. And don’t forget to follow Data Kisaan on Medium.