The enhanced vegetation index (EVI) was developed as an alternative vegetation index to address some of the limitations of the NDVI. The EVI was specifically developed to:
EVI tends to be more sensitive to plant canopy differences like leaf area index (LAI), canopy structure, and plant phenology and stress than does NDVI which generally responds just to the amount of chlorophyll present. With the launch of the MODIS sensors, NASA adopted EVI as a standard MODIS product that is distributed by the USGS (see below).
EVI is calcualted as
where NIR, RED, and BLUE are atmospherically-corrected (or partially atmospherically-corrected) surface reflectances, and C1, C2, and L are coefficients to correct for atmospheric condition (i.e., aerosol resistance). For the standard MODIS EVI product, L=1, C1=6, and C2=7.5.
The output of EVI is a single image layer with values typically from 0.0 to 1.0.
Most of the rangeland applications of EVI have, to date, been regional-scale to global-scale assessments of rangeland parameters. EVI has mostly been used for assessments of biomass, biophysical properties like leaf area index, quantification of evaoptranspiration or water-use efficience, or assessments of change over large areas. In addition to the citations below, RangeView includes a standard EVI component in it's web-based rangeland assessment tools.
One of the biggest current limitations to implementing EVI is that it needs a blue band in order to be calculated. Not only does this limit the sensors that EVI can be applied to (e.g., ASTER has no blue band), but the blue band typically has a low signal-to-noise ratio. Research is ongoing to develop a two-band EVI that can be calculated from just red and near infrared bands (see Jiang et al. 2008).
EVI requires surface reflectance measurements from blue, red, and near-infrared bands.
Given the appropriate inputs, EVI is fairly easy to calculate with image processing software (e.g., ERDAS Imagine, ENVI, IDRISI) or GIS software that can do raster processing (e.g., ArcGIS with Spatial Analyst Extension, GRASS). Most applications of EVI, however, have made use of the standard MODIS EVI products that can be downloaded from USGS LPDAAC (see below).
Image source: RangeView MODIS Dynamic Animation Tool, http://rangeview.arizona.edu
MODIS EVI (right) compared to NDVI (left) for New Mexico over the same time period in 2006. The NDVI image shows a greater area in dark green because NDVI loses sensitivity to changes in vegetation in areas of higher biomass (forests in this case). The EVI image maintains a more consistent sensitivity to changes in vegetation and, in this example, has a more even distribution of vegetation greenness values.
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