Chlorophyll and Red Edge Bands: Sentinel-2's Secret Weapon for Crop Health
Quick Answer: Sentinel-2's red edge bands (B5: 705nm, B6: 740nm, B7: 783nm) sample the sharp reflectance transition between chlorophyll absorption and leaf structure reflection. This transition shifts with chlorophyll concentration — stressed plants with less chlorophyll show a blue-shifted red edge. Red edge indices like NDRE = (B8A - B5)/(B8A + B5) detect nitrogen deficiency and early stress 1-2 weeks before standard NDVI changes. This makes red edge data essential for precision agriculture variable-rate applications.
A precision agriculture consultant in the Netherlands once told me that red edge data saved his client about €15,000 in a single growing season. The fields looked uniformly green to the naked eye and even in standard NDVI. But a red edge index revealed a nitrogen deficiency zone in the southwest corner — invisible to conventional analysis — that, left untreated, would have reduced yield by 20% in that section.
That's the practical value of spectral information most satellites simply don't have.
What Is the Red Edge?
The "red edge" isn't a single wavelength — it's a spectral region between approximately 680 and 780 nanometers where vegetation reflectance increases dramatically. On one side (red, ~680 nm), chlorophyll absorbs light for photosynthesis, producing low reflectance. On the other side (NIR, ~780 nm), the internal leaf structure (spongy mesophyll cells) scatters light, producing high reflectance.
This transition happens over a narrow ~100 nm window, and its exact position and steepness carry information about the plant's biochemistry:
- More chlorophyll → stronger red absorption → the transition stays anchored at longer wavelengths (red-shifted edge)
- Less chlorophyll (stress, nitrogen deficiency, senescence) → weaker red absorption → the transition moves to shorter wavelengths (blue-shifted edge)
- Steeper transition → healthier, denser canopy
- Shallower transition → reduced vigor, mixed pixels, or stressed vegetation
The shift is subtle — typically 5 to 20 nm — but Sentinel-2's three dedicated red edge bands are positioned precisely to capture it.
Sentinel-2's Red Edge Bands
| Band | Central Wavelength | Bandwidth | What It Captures |
|---|---|---|---|
| B5 | 705 nm | 15 nm | Lower red edge — still influenced by chlorophyll absorption |
| B6 | 740 nm | 15 nm | Mid red edge — the inflection point region |
| B7 | 783 nm | 20 nm | Upper red edge — transitioning to NIR plateau |
| B8A | 865 nm | 20 nm | Narrow NIR — full NIR reflectance reference |
These bands are at 20-meter resolution, coarser than the 10-meter visible and broadband NIR (B8). For field-level precision agriculture, 20 meters is sufficient — most management zones are larger than individual pixels.
Red Edge Indices
NDRE (Normalized Difference Red Edge)
NDRE = (B8A − B5) / (B8A + B5)
This is the most widely used red edge index. It's structurally identical to NDVI but replaces the red band with the red edge band B5. Because B5 sits at the start of the red edge transition, it's sensitive to chlorophyll concentration changes that have already saturated the deeper red band used by NDVI.
Why NDRE outperforms NDVI for dense vegetation: In a mature crop canopy, the red band (665 nm) is almost completely absorbed regardless of whether chlorophyll is at 40 or 60 μg/cm². Red is "saturated" — it can't tell you anything more. But at 705 nm (B5), absorption is partial, meaning there's still dynamic range to detect chlorophyll differences.
NDRE values typically range from 0.1 to 0.6, with higher values indicating more chlorophyll/healthier vegetation.
CIre (Chlorophyll Index Red Edge)
CIre = (B7 / B5) − 1
A simpler ratio index that's linearly related to canopy chlorophyll content. Studies have shown CIre explains 75–85% of the variance in measured chlorophyll across a range of crop types.
MCARI (Modified Chlorophyll Absorption in Reflectance Index)
MCARI = ((B5 − B4) − 0.2 × (B5 − B3)) × (B5 / B4)
More complex but designed to minimize soil background effects while being sensitive to chlorophyll. Works well in sparse canopies where simple indices are contaminated by soil reflectance.
Practical Application: Nitrogen Mapping
Chlorophyll production requires nitrogen. When nitrogen is deficient, chlorophyll concentration drops, and the red edge shifts. This makes red edge indices effective proxies for crop nitrogen status.
A typical workflow for variable-rate nitrogen application:
- Acquire a mid-season Sentinel-2 image (when canopy is established but before flowering)
- Compute NDRE or CIre for each field
- Zone the field into management units based on index values (typically 3–5 zones)
- Calibrate zones against ground samples (leaf nitrogen measurements from a few representative points)
- Generate a variable-rate prescription map: less nitrogen where the crop is already healthy, more where it's deficient
The economic value comes from precision: instead of applying a uniform rate, the farmer applies nitrogen only where needed, reducing waste and environmental impact while maintaining yield.
Red Edge vs Standard NDVI: When Does It Matter?
The advantage of red edge indices is most pronounced when:
Vegetation is dense (LAI > 3): NDVI saturates; NDRE continues to show variation. This applies to mid-to-late season crops, dense grasslands, and forest canopies.
Stress is subtle: Mild nitrogen deficiency, early drought stress, beginning of disease impact — these reduce chlorophyll before they cause visible wilting or browning. Red edge detects the biochemical change; NDVI doesn't register it yet.
You need to differentiate nitrogen from water stress: Both reduce NDVI, but they affect the red edge differently. Nitrogen stress reduces chlorophyll (red edge shifts); water stress initially changes leaf water content (SWIR response) without immediately reducing chlorophyll. Combining red edge and SWIR indices helps separate these two stress types.
When is NDVI adequate? For low-to-moderate vegetation density, coarse monitoring (healthy vs. not healthy), and when 10-meter resolution matters more than spectral sensitivity. NDVI at 10m may outperform NDRE at 20m for small-plot agriculture.
Seasonal Monitoring Pattern
Through a typical temperate crop growing season, I observe this pattern:
Early season (emergence): Both NDVI and NDRE are low. Soil still visible. SAVI may be more appropriate than either.
Vegetative growth: NDVI and NDRE rise together. Differences between the two are minimal — neither is saturated yet.
Peak canopy: NDVI saturates at 0.85–0.90. NDRE continues to differentiate chlorophyll concentration. This is the critical window for red edge analysis.
Senescence: Both decline, but NDRE often drops first as chlorophyll degrades while leaf structure (NIR reflectance) initially persists.
The practical takeaway: if you're monitoring dense crops or forests and need to detect within-canopy variation in health, red edge indices are not optional — they're essential. Sentinel-2 is one of the few free satellite platforms that provides this capability, and it's one of the strongest reasons to prefer it over alternatives.
