Relative sensitivity of normalized difference vegetation Index (NDVI) and microwave polarization difference Index (MPDI) for vegetation and desertification monitoring

Abstract

Given that a Microwave Polarization Difference Index (MPDI) is rather sensitive to the water content of plant, while the Normalized Difference Vegetation Index (NDVI) is sensitive to chlorophyll absorption, there should be a correlation between these two indices, depending upon the relationship between water content and chlorophyll absorption occuring in the plant itself. Assuming a simple form for this relationship, a simple equation relating MPDI and NDVI was derived. Comparing with actual data from Nimbus 7/SMMR at 37 GHz and NOAA/AVHRR, Channels 1 and 2, the proposed formula represents correctly the general tendency of data. It is then shown that there exists a limit characteristic of a particular type of cover which has to be analyzed, given by NDVI = 0.13 ± 0.02, for which both indices are equally sensitive to the variation of vegetation, and below which MPDI is more efficient than NDVI. The scatter of the data around the theoretical curve obtained is briefly analyzed. It is suggested that, despite the dispersion due to the processing itself, this spreading could give some insight into the relationship between water content and chlorophyll absorption at pixel size scales. In this respect, particular biologic cycles, which have to be confirmed, are observed and discussed.