A Vegetation Indices (VI) is a spectral transformation of two or more bands that is intended to improve the contribution of vegetation features and allow for valid geographical and temporal inter-comparisons of terrestrial photosynthetic activity and canopy structure alterations.
There are numerous Vegetation Indices (VIs), many of which are functionally equivalent. Many of the indices use the inverse relationship between red and near-infrared reflectance, which is associated with healthy green vegetation. Scientists have used satellite remote sensing to monitor changes in vegetation at the Earth’s surface since the 1960s. The leaf area index (LAI), percent green cover, chlorophyll content, green biomass, and absorbed photo synthetically active radiation are all measures of vegetation attributes (APAR).
Vegetation Indices (VIs) have historically been classified based on a variety of characteristics, such as the number of spectral bands (2 or greater than 2); the method of calculation (ratio or orthogonal) depending on the required objective; or their historical development (classified as first generation VIs or second generation VIs).
Lyon, Yuan, et al. (1998) classified 7 VIs based on their computation methods in order to compare the effectiveness of different VIs (Subtraction, Division or Rational Transform). High-resolution reflectance spectrums that can be used with traditional multispectral VIs are now available as a result of advances in hyperspectral remote sensing technology. Furthermore, VIs have been developed specifically for use with hyperspectral data, such as the use of Narrow Band Vegetation Indices.
Vegetation condition index (VCI)
It is a satellite-based drought indices which is depend up on the satellite image collected using different satellite sensors, Proposed by Kogan, 1990.
Compares the current NDVI to the range of values observed in the same period in previous years. The VCI is expressed in % and gives an idea where the observed value is situated between the extreme values (minimum and maximum) in the previous years. Lower and higher values indicate bad and good vegetation state conditions, respectively.
And it is calculated by the following formula
where VCIijk is the VCI value for the pixel i during week/month/DOY j for year k, VIijk is the weekly/monthly/DOYs VI value for pixel i in week/month/DOY j for year k whereby both the NDVI or EVI can utilized as VI, VIi, min and VIi, max is the multiyear minimum and maximum VI, respectively, for pixel i.
ranges for Vegetation condition index (VCI)
| Value | Classes |
| >50-100 | No drought |
| 40%-50% | No drought |
| 30%-40% | Light drought |
| 20%-30% | Moderate drought |
| 10%-20% | Severe drought |
| 0%-10% | Extreme drought |
Enhanced Vegetation index (EVI)
The enhanced vegetation index (EVI) was created as a replacement for the NDVI to solve some of the NDVI’s shortcomings. The EVI was created with the following goals in mind:
- be more sensitive to changes in high-biomass areas (a key flaw in NDVI),
- limit the impact of atmospheric circumstances on vegetation index values, and
- adjust for canopy background signals
The Enhanced Vegetation Index (EVI) from Landsat is comparable to the Normalized Difference Vegetation Index (NDVI) in that it can be used to measure the greenness of vegetation. EVI, on the other hand, compensates for various atmospheric variables and canopy background noise, and is more sensitive in densely vegetated areas.
EVI is more sensitive to changes in the plant canopy, such as leaf area index (LAI), canopy structure, plant phenology, and stress, than NDVI, which primarily responds to the amount of chlorophyll present.
It includes a “L” value for canopy background, “C” values for air resistance coefficients, and values from the blue band (B). In most cases, these improvements allow for index calculation as a ratio of the R and NIR values, while minimizing background noise, ambient noise, and saturation.
enhanced vegetation index can be calculated by the following formula :-
EVI = G * ((NIR – R) / (NIR + C1 * R – C2 * B + L))
the calculation procedure for EVI is different depending up on the series of Landsat satellite
for instance
in Landsat 4-7, EVI = 2.5 * ((Band 4 – Band 3) / (Band 4 + 6 * Band 3 – 7.5 * Band 1 + 1)).
In Landsat 8, EVI = 2.5 * ((Band 5 – Band 4) / (Band 5 + 6 * Band 4 – 7.5 * Band 2 + 1)).
The value range for EVI is –1 to +1, and for healthy vegetation,
NASA adopted EVI as a standard MODIS product that is disseminated by the USGS with the launch of the MODIS sensors. calculated through
where NIR, RED, and BLUE are atmospherically adjusted (or partially atmospherically corrected) surface reflectances, respectively, and C1, C2, and L are atmospheric correction coefficients (i.e., aerosol resistance). L=1, C1=6, and C2=7.5 for the typical MODIS EVI product.
