SAVI index and agricultural landscapes

SAVI is the acronym for Soil Adjusted Vegetation Index that, as the name suggests, it is one of the many indices for the calculation of the vegetation index.

The general formula for the SAVI index is (indexdatabase.de):

     800 nm - 670 nm
  ________________________  ( 1 + L)

   800 nm + 670 nm + L

   
where L is between -0.9 and 1.6.

In the case of Sentinel 2 data, SAVI uses two bands, B8 and B4, that have the maximum available resolution for Sentinel 2 images, i.e., 10 meters, so the results have a quite high spatial resolution.

B8 is in the NIR (842 nm) while B4 registers the red-NIR transition (665 nm).

For Sentinel 2, the index is therefore calculated as:

     B8 - B4
  _______________   ( 1 + L)

   B8 + B4 + L
   
The default value for L in indexdatabase is 0.5, and in fact testing for a few values (-0.9, -0.5, 0.0, 0.5, 1.0, 1.5) in the prescribed range, the most contrasted results are obtained for L = 0.5.

One information that is obtained from applying such an index to intensive farming landscapes is that it allows to delineate well the agricultural field limits, to compare the texture and the "spatial styles" of farming across different areas, for instances separated by rivers that may have acted as administrative or political borders.

In the following images, I present some examples of textures and styles of limits between fields for different areas in the Piedmont region, derived from a Sentinel 2B image, acquired on 2023/04/04 at 10:25. The SAVI index was calculated using the SNAP software by ESA.

The used L ("soil brightness correction factor") value is 0.5, chosen after some experimentation.

The figures below are generated from within SNAP, using a 8-4-3 composite for infrared view (upper window), while the SAVI index band is represented in the lower window. In the 843 composite, vegetation is red while rivers and roads are black.

Strong agricultural landscapes contrast between the western side (left) and eastern  side (right) of the Sesia river (central, with a North-South orientation). The western landscape is characterized by smaller sizes than in the eastern part, and is also more regular as geometries. Possibly it is the result of different political systems between the two sides in the previous centuries.

 Strong East-West differences in field sizes, orientations and textures without a clear separating physical boundary (such as the Sesia river in the previous example). It could be related to different grown agricultural products between the two zones (?rice to the East), possibly also related to distinct administrative/political situations.  

Irregular field borders probably due to the limiting rivers and their location changes with time.