The Satellite Saw What Imran Missed From the Bund

From the bund, the groundnut looked fine.

Imran walked his four acres near Kalyandurg in Anantapur district every evening, the way his father had taught him. Start at the gate, follow the main line along the bund, look across the rows, check the filter, turn back. The leaves were green, the rows were even, the drip was running. It was the rabi season, and for once, the bore well was giving enough water.

Then his nephew Faiz came home from his agriculture college for the weekend and opened his laptop on the cot.

"Mama, look at your field from the sky."

On the screen was a satellite image of the village, the fields coloured in patches of green, yellow and red. Faiz zoomed in on Imran's four acres. Most of it was green. But in the middle of the second plot, there was a long, narrow streak of yellow, running in a straight line.

Imran laughed. "That's the satellite's mistake. I walked there yesterday."

But the next morning, he went to the second plot and did something he did not usually do. He walked inside the rows, not along the bund.

Halfway down, the plants in one row were smaller, the leaves dull, the soil dry under the dripper. The lateral was blocked.

What is NDVI?

NDVI stands for Normalised Difference Vegetation Index. It is a number, worked out from satellite images, that shows how green and actively growing the plants in each patch of your field are. On a map, it is usually shown as colours.

You do not need to know the formula to use it. You only need to understand what the satellite is seeing.

How does a satellite know if a crop is healthy?

Healthy green leaves absorb most of the red light from the sun to make food, and strongly reflect near-infrared light, which our eyes cannot see. A satellite camera can see both. Where a crop is dense and healthy, the difference between the two is large, and NDVI is high.

When a plant is stressed by thirst, pests, disease or nutrient shortage, its leaves change inside before they change colour enough for our eyes to notice. They reflect less near-infrared light. NDVI drops, sometimes days before you would see it from the bund.

Bare soil, water and dry stubble reflect red and near-infrared light more evenly, so they show a low value.

What do the colours on an NDVI map mean?

Most maps use a simple traffic-light scale, though the exact colours can differ from one service to another.

• Dark green: dense, healthy, actively growing crop.
• Light green or yellow: thinner or stressed crop. Worth a look.
• Orange or red: very weak crop, bare soil, or something badly wrong.

The most useful thing is not a single colour but a difference. A yellow streak in an otherwise green field, or a patch that was green last week and is yellow this week, is a signal to go and look.

The shape of the patch is a clue too. A straight line often points to irrigation or machinery. A circle spreading outward can point to pests or disease. A low corner can point to waterlogging.

What are the limits of satellite crop monitoring?

Satellites cannot see through clouds, cannot see small details on small plots, and cannot tell you the cause of a problem. Knowing these limits keeps you from trusting the map too much.

1. Clouds. Most crop monitoring satellites use ordinary light, which cannot pass through clouds. In the monsoon, you may go weeks without a clear image just when you most want one.
2. Resolution. Free satellite images commonly used for crops see the ground in squares of around 10 metres by 10 metres. A one-acre field is only a few dozen of these squares. Small problems, single plants and narrow strips can be blurred or missed.
3. Timing. A new image comes every few days, not every hour, and only when the sky is clear.
4. Mixed pixels. At the edge of a field, one square may include your crop, the bund, a tree and the neighbour's field. Edges are less reliable.
5. Early crop. When plants are small and the soil shows between rows, NDVI is naturally low. That is normal, not a problem.
6. No diagnosis. A yellow patch could be thirst, pests, disease, nutrient shortage or weeds. The map only says "look here".

Why should you still walk the field?

Because the satellite can tell you where, but only you can find out why. NDVI is a pointer, not an answer.

Imran's yellow streak could have been many things. It took walking into the rows, kneeling at the dripper and running his finger through dry soil to know it was a blocked lateral, and not a pest or disease.

That is how to use satellite maps well:

• Check the map once a week, or whenever a new clear image is available.
• Look for differences, not perfect colours.
• Mark the weak spot and walk straight to it, inside the rows.
• Look closely: under the leaves, at the roots, at the dripper, at the soil moisture.
• Photograph what you find and record it with the date and location.
• Check the same spot again on the next image to see if your fix worked.

If you find spots or insects on the leaves, a clear phone photo is your next step. Read how AI photo diagnosis can identify crop problems.

What Imran did next

He cleaned the blocked lateral that morning, and then flushed the rest of the plot's laterals while he was at it. He found two more partly blocked, not yet bad enough to show on the map.

He also started keeping field scouting records in Mapis Farming. Every time he walked inside the rows, he logged what he saw with a photo, and when he cleaned the filter or flushed laterals, he recorded that too. Within a few weeks, he could see a pattern: the far end of each plot always choked first, and needed flushing more often.

On the next clear image, two weeks later, the yellow streak had faded to light green. By the end of the season, it was hard to tell from the rest of the plot. The yield from that row was likely lower than the others, but the damage stopped where it was.

"My eyes watch the field from the bund. The satellite watches it from above. Now I walk where it points."

If you run drip and want to make the most of every drop, read about drip irrigation, subsidies and saving water.

Frequently asked questions

What is NDVI in agriculture?

NDVI, or Normalised Difference Vegetation Index, is a measure of crop greenness and vigour calculated from satellite or drone images. It compares how much red and near-infrared light the plants reflect. High values mean dense, healthy growth, and low values mean weak crop, bare soil or stress.

What does a yellow or red patch on an NDVI map mean?

It means the crop there is thinner or less active than the rest of the field, or the ground is bare. The cause could be water stress, a blocked drip line, pests, disease, nutrient shortage or waterlogging. Walk to that spot and look closely to find out which.

Does satellite crop monitoring work during the monsoon?

Often not well. Most crop monitoring satellites use ordinary light and cannot see through clouds, so in cloudy weeks there may be no clear image. Some radar satellites can see through clouds, but their images are harder to interpret.

Is NDVI useful for small farms?

It can be, with limits. Free satellite images usually see the ground in squares of around 10 metres, so a small plot has only a few dozen squares and small problems may be missed. It works best for spotting larger patterns and changes over time, alongside regular field walks.

Can NDVI tell me what is wrong with my crop?

No. NDVI shows where a crop is weaker, not why. To find the cause, walk to the weak area, inspect the plants, soil and irrigation, and use a photo diagnosis or your agriculture officer to confirm any pest or disease.