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¿What is the SAVI Index?
The Soil-adjusted vegetation index is a vegetation index that attempts to minimize the influences of soil brightness by using a correction factor.
SAVI = ((NIR – Red) / (NIR + Red + L)) x (1 + L)
- NIR = near infrared pixel values.
- Red = pixel values of the near red band.
- L = amount of green vegetation coverage.
NIR and Red refers to the bands associated with those wavelengths. The L value varies depending on the amount of green cover. Generally, in areas without any green vegetation cover L=1; in areas with moderate green vegetation cover, L=0.5; and in areas with very dense vegetation cover, L=0 (which is equivalent to the NDVI method).
¿Where is it used?
It is often used in arid regions where vegetation cover is low and has a lower saturation than the NDVI at high index values.
¿When is it used?
In early stages of the cycle, with little vegetation or weeds. The SAVI index is a good indicator in the early stages of the crop, where plant cover is partial, and where the data that is acquired by the satellite is a combination of crop and soil. If the vegetation cover is very poor, the soil reflectance indexes affect the final value and this “soil effect” decomposes the NDVI values. By using the soil line as a correction factor (called the L-factor), this index allows to ignore the soil reflectivity, based on plant activity only.
This index oscillates between values -1 and 1, from brown to violet, where brown represents the optimum state of health of a crop, green represents the presence of small weeds and towards blue and violet the presence of weeds in a great state of growth. Once the crop has increased its vigor, reaching a good soil cover, the SAVI will no longer be so useful so it is advisable to migrate to another indicator.
The examples correspond to the same field, on different dates and we compare NDVI with SAVI. The first example corresponds to the month of April. The NDVI indicates that there is coverage of the entire field. However, with SAVI we can see that the green areas may be due to the presence of larger weeds and the brown/light brown areas, or may be due to low vegetation activity or bare soils. The actual situation can be verified by a field visit.
This example shows the same fields, one month later. With NDVI we observe a low presence of vegetation. When switching to the SAVI index, the dark brown areas will indicate a clean or weed-free soil.