NDVI full form and meaning: what NDVI is and what it tells you about an orange orchard

What is the full form of NDVI?

NDVI is the Normalized Difference Vegetation Index. In plain words, a greenness index. The Marathi and Hindi plain-word versions on this site are our own, not official terms.

NASA researchers started using the index in 1974 to monitor vegetation from satellite images (Rouse and others). Today it is worked out daily on the images of free satellites such as Sentinel-2.

What is the NDVI formula and how does it work?

The formula is NDVI = (NIR - Red) / (NIR + Red). NIR is near-infrared light, which the eye cannot see. Healthy leaves absorb most visible red light because of chlorophyll and reflect a large share of NIR because of their leaf structure. Sparse or unhealthy leaves reflect more red and less NIR (NASA). So more green leaf means more NIR, less red, and a higher NDVI.

In Sentinel-2, NIR is band B8 and red is band B4, so NDVI = (B8 - B4) / (B8 + B4) (Sentinel Hub). A worked example, with numbers chosen only to explain, not measured: if NIR is 0.5 and red is 0.1, then (0.5 - 0.1) / (0.5 + 0.1) = 0.67. If NIR is 0.3 and red is 0.2, then (0.3 - 0.2) / (0.3 + 0.2) = 0.2.

What do NDVI values mean?

The value runs from -1 to +1. Near zero means little or no vegetation (NASA). In Sentinel Hub's guide, negative values are water and values near zero are bare ground such as rock, sand or snow. Roughly 0.2 to 0.3 is shrubland and grassland, and 0.6 to 0.8 is dense forest (NASA).

For an orange orchard there is no single good number. The value shifts with the bare ground between trees, weeds and the age of the trees. So compare your orchard with its own earlier weeks and with the same time last year, not with your neighbour's orchard.

Why does NDVI dip in an orchard?

Some dips are part of the plan. Trees are put under deliberate water stress in November and December for ambia bahar and in April and May for mrig bahar (ICAR-CCRI), and dead wood is pruned after harvest. That can lower greenness a little; this is our reading, not a measured result.

Other causes: unwanted water stress, leaf drop, disease or pests, the ground between trees being ploughed or weeds drying, and cloud, haze or shadow in the image. Do not assume one cause when the number falls; check the image for cloud first, then walk the orchard.

What can NDVI not tell you?

NDVI does not name a disease, identify a pest, measure which nutrient is short, measure soil moisture directly, or tell you the number or quality of fruit. It shows only a change in the greenness of leaves.

In one 10 m pixel, tree and the ground between the trees blend. So in small fields and at the edges the numbers are less reliable. To find the cause of a change, walk the orchard and ask the KVK. Do not take a spray or fertiliser decision from NDVI.

How should I use NDVI for my orchard?

NDVI is a warning, not a diagnosis. Check your orchard's number week by week. If it falls below its usual level, or one strip of the orchard starts to look different from the rest, go and look at the leaves, the soil moisture and the trunk there. Our weekly message does exactly this: what changed, from which date's image, and whether there was cloud. It never gives a spray, fertiliser or water amount.