Two-band EVI
Two reasons drive the search for a two-band EVI:
1. Extending the EVI back in time, using the AVHRR record. The AVHRR sensors lacks a blue band, hence using a three-band EVI version is not possible. This could potentially lead to a 30 year EVI record that complements the NDVI record.
2. The blue band has always been problematic, and its Signal to Noise ratio (S/N) quite poor. This is mainly due to the nature of the reflected energy in this part of the spectrum over land, which is extremely low.
As such, there are currently proposals to design a 2-band EVI. In designing this two-band EVI a non-physically based mathematical approach is employed. We'll call the two-band EVI EVI_2, and the three-band EVI simply EVI:
Approximate EVI by EVI2, where EVI_2=f(red,NIR)
f(red,NIR) = G*(NIR-RED)/(L+NIR+C*Red) Find G,L, and C with G as (organic) that minimize the difference between EVI_2 and EVI This leads to multiple (infinite) solutions but few conditions could be imposed on the solution to generate the best coefficients.EVI2=2.5*(NIR-Red)/(NIR+2.4*Red+1)
A linearity-adjustment factor β is proposed and coupled with the soil-adjustment factor L used in the soil-adjusted vegetation index (SAVI) to develop EVI2. EVI2 has the best similarity with the 3-band EVI, particularly when atmospheric effects are insignificant and data quality is good. EVI2 can be used for sensors without a blue band, such as the Advanced Very High Resolution Radiometer (AVHRR), and may reveal different vegetation dynamics in comparison with the current AVHRR NDVI dataset.
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