Understanding how today’s agricultural data were generated
Data from satellite earth observations provide regular updates on the current biophysical/biochemical state of vegetated surfaces including crop production areas. The Moderate Resolution Imaging Spectroradiometer Mission (MODIS) collects data on 36 spectral bands at a spectral resolution of 1 km every 1 or 2 days. Landsat data collects data on 11 spectral bands at a 30m spatial resolution every 16 days. Analysis of this data can indicate areas of healthy and stressed plant growth which is invaluable in understanding the state of cultivated areas in a near real-time manner.
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Various indices of vegetation state are constructed using MODIS image analysis including the following:
- Normalized Difference Vegetation Index (NDVI) essentially assesses chlorophyll density in a given image pixel by comparing the proportion of reflectance in the near-infra-red and visible light wavelengths. Calculations of NDVI for a given pixel always result in a number that ranges from zero to one ., Pixels with no green leaves have a value close to zero meaning an absence of vegetation, and values close to 1 indicate the highest possible density of green leaves.
- Enhanced Vegetation Index (EVI) is more responsive to canopy structure variation than NDVI. EVI is calculated similarly to NDVI but it corrects for distortions in the reflected light caused by particles in the air as well as ground cover below the vegetation.
- Leaf Area Index (LAI) characterizes plant canopies. It is defined as the one-sided green leaf area per unit ground surface area (LAI = leaf area (m2)/ ground area (m2)) in broadleaf canopies. In conifer canopies, three definitions for LAI have been used including total needle surface area per unit ground area. LAI ranges from 0 (bare ground) to over 10 (dense conifer forests)
- Fraction of Photosynthetically Active Radiation. Photosynthetically active radiation (PAR) is the spectral range from 400-700nm that is used by plants in photosynthesis. The fraction of PAR (fPAR) is a parameter used in remote sensing and in ecosystem modeling that signifies the portion of PAR used by plants. It is an important parameter in measuring biomass production because vegetation development is related to the rate at which radiant energy is absorbed by vegetation.