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MCARI (Modified Chlorophyll Absorption in Reflectance Index)

Definition

The Modified Chlorophyll Absorption in Reflectance Index (MCARI) is designed to quantify the depth of chlorophyll absorption in the red region relative to the green reflectance peak. It emphasizes changes in the red absorption feature while attempting to minimize the effects of non-photosynthetic materials (soil, senescent vegetation) and canopy background.

Application

MCARI is widely used in agricultural remote sensing for estimating chlorophyll content, assessing crop nitrogen status, and monitoring plant stress. It is often combined with other indices (e.g., in the TCARI/OSAVI ratio) to improve chlorophyll estimation accuracy. Applications include precision agriculture, crop modeling, and vegetation stress detection in both managed and natural ecosystems.

FAQ

What is the formula for MCARI?

MCARI is calculated as: MCARI = [(RedEdge - Red) - 0.2 * (RedEdge - Green)] * (RedEdge / Red). This formula quantifies the depth of the chlorophyll absorption feature at the red band relative to the red edge and green bands, with the term (RedEdge/Red) providing normalization.

How does MCARI minimize soil background effects?

MCARI incorporates several design features to reduce soil effects:

1) The term (RedEdge - Green) helps account for baseline reflectance variations;

2) The specific coefficients (0.2) were optimized to minimize sensitivity to soil brightness;

3) The ratio (RedEdge/Red) provides normalization that reduces soil-induced variability.

However, for optimal soil resistance, MCARI is often used in combination with soil-adjusted indices like OSAVI.

Why is MCARI often used in combination with OSAVI?

MCARI is frequently combined with OSAVI as the ratio MCARI/OSAVI (or the similar TCARI/OSAVI) because:

1) MCARI is sensitive to chlorophyll content but also to canopy background (soil, senescent vegetation);

2) OSAVI minimizes soil background effects;

3) The ratio helps isolate the chlorophyll signal from canopy structural effects, providing more robust chlorophyll estimation across different canopy densities and soil types.

What are the limitations of MCARI?

Limitations include:

1) Sensitivity to canopy structure and leaf area index, not just chlorophyll content;

2) Requirement for specific bands (red edge, green, red) not available on all sensors;

3) Potential saturation at high chlorophyll concentrations;

4) When used alone, still affected by soil background, especially in sparse vegetation;

5) More complex interpretation than simpler ratio indices like NDVI or NDRE.