Spectral Reflectance of Crops and Soils
Spectral Reflectance of Crops
Remote and ground-based proximal sensing of vegetation is mainly performed by obtaining the electromagnetic wave reflectance information from canopies using passive sensors. It is well known that the reflectance of light spectra from plants changes with plant type, water content within tissues, and other intrinsic factors. The different components of the canopy structure can reflect energy at different wavelengths depending on the molecules in the tissues. The spectral bands used in precision farming include visible (VIS; 400–700nm), near-infrared (NIR; 700–1,000nm), short-wave infrared (SWIR; 1,000–2,500nm), and thermal infrared (TIR; 8,000–14,000 nm). These spectral bands allow the calculation of vegetation indices (VI) that are useful in assessing plant physiological parameters, e.g., the Normalized Difference Vegetation Index (NDVI), Crop Water Stress Index (CWSI), Normalized Difference Red Edge Index (NDRE), Normalized Difference Water Index (NDWI), etc. RGB, multispectral, and hyperspectral sensors can calculate vegetation indices due to spectral coverage in visible and near-infrared bands. In this regard, remote and ground-based proximal sensors include optical cameras distinguished by various factors such as the type of operation to carry out, the type of acquisition, and the number of spectral bands.
Vegetation Indices
The vegetation index (VI) is one of the most commonly used characteristic parameters in the field of crop growth monitoring. Vegetative indices are dimensionless numerical measures that provide information related to the vegetation's activity and its interactions along the electromagnetic spectrum, such as leaf area index (LAI), percent vegetation cover, chlorophyll content, and biomass, among others.
Greenness and Biomass Indices
Vegetation indices (VIs) related to greenness and biomass are designed to estimate plant vigor, canopy development, and overall crop productivity by analyzing how plants reflect light in specific spectral bands—especially red, green, and near-infrared (NIR). These indices are widely used in agriculture for monitoring crop growth, estimating yield, and guiding input decisions (like irrigation or fertilization).
Normalized Difference Vegetation Index (NDVI)
Green Normalized Difference Vegetation (GNDVI)
Soil Adjusted Vegetation Index (SAVI)
Enhanced Vegetation Index (EVI)
Triangular Vegetation Index (TVI)
Ratio Vegetation Index (RVI)
Chlorophyll and Nutrient Indices
Vegetation indices (VIs) that target chlorophyll content and nutrient status—especially nitrogen—are crucial in precision agriculture. These indices rely on spectral bands that are sensitive to leaf pigment concentrations, particularly chlorophyll, which is strongly correlated with nitrogen availability in plants.
Normalized Difference Red Edge Index (NDRE)
Modified Chlorophyll Absorption Ratio Index (MCARI)
Chlorophyll Index Red-Edge Index (CIre)
Water Content and Stress Indices
Vegetation indices (VIs) for water content and stress detection are designed to assess the hydration status, drought response, and physiological stress in plants. These indices rely on spectral bands that are sensitive to leaf water absorption, canopy temperature, or photosynthetic activity.
Moisture Stress Index (MSI)
Normalized Difference Water Index (NDWI)
Normalized Difference Moisture Index (NDMI)
Canopy Structure and Leaf Area Indices
Vegetation indices (VIs) for canopy structure and leaf area are designed to estimate the Leaf Area Index (LAI), canopy architecture, and fractional vegetation cover—critical for understanding crop development, photosynthesis potential, and yield forecasting.
Leaf Area Index (LAI)
Enhanced Vegetation Index (EVI)
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