How it works
Healthy leaves absorb red light for photosynthesis and strongly reflect near-infrared light. NDVI compares the two: (NIR - Red) / (NIR + Red). Bare soil sits near 0.1 to 0.2, a sparse crop around 0.3, and a dense, healthy canopy above 0.6.
Sentinel-2 measures these bands at 10 m resolution every few days, so NDVI can be tracked for each field across a season.
Why it matters for Indian farmers
A fall in NDVI on part of a field, compared with the field's own recent history, is an early sign that something has changed: water stress, a nutrient shortage, pest damage, waterlogging or simply harvest. It tells you where to look, not what the cause is.
How VarshaKrishi uses it
VarshaKrishi computes NDVI (with NDRE and EVI) over the cloud-free pixels inside each registered field and compares it with that field's own baseline. A meaningful change becomes a Watch message asking the farmer to check a specific part of the field.
Methods and evidence A year of real Sentinel-2 NDVI for one field (open data) What NDVI means for an orange orchard (guide)
Questions about NDVI
What is a good NDVI value?
It depends on the crop and stage. A dense, healthy canopy is usually above 0.6; young crops and orchards with bare soil between trees read lower. Comparing a field with its own history is more useful than any fixed number.
Can NDVI tell which disease a crop has?
No. NDVI shows that greenness changed, not why. A field visit, or an agronomist, identifies the cause.
Does NDVI work on small plots?
Approximately. Each Sentinel-2 pixel is 10 m by 10 m, so a plot under half a hectare contains few clean pixels. On small plots we report the direction of change only.
NDVI kya hai? (NDVI म्हणजे काय?)
NDVI (Normalized Difference Vegetation Index) फसल की हरियाली मापने का अंक है, जो सैटेलाइट की लाल और इन्फ्रारेड रोशनी से निकलता है. मराठीत: NDVI म्हणजे उपग्रहातून मोजलेली पिकाची हिरवळ; जास्त अंक म्हणजे दाट, निरोगी पीक.