The Normalised Difference Vegetation Index (NDVI) and the Microwave Polarisation Difference Index (MPDI) for vegetation

The effects of desertification on human societies, and also its potential to initiate a host of other environmental feedbacks, makes it an important process to monitor, learn about past episodes, and to predict the scale of spread or decline in the future under a number of inputs. A classic paper by Becker and Choudhury (1988) discusses how the Normalised Difference Vegetation Index (NDVI) and the Microwave Polarisation Difference Index (MPDI) for vegetation can help to analyse desertification processes.

As previously highlighted desertification is associated with the reduction of crop yields, reduction of biomass, river flow and groundwater depletion, encroachment of sand sheets over settlements or productive lands, and significant social disruption. Furthermore, existing species may be preferentially replaced by less desirable species leading to the depletion of livestock materials etc. Monitoring global vegetation can hence provide a sensitive indicator of environmental changes.

Becker and Choudhury state that ‘several research efforts have been undertaken in order to find relevant indices characterizing these processes and which are observable from satellites. Among them, albedo (a), surface temperature (T), and Normalized Difference Vegetation Index (NDVI) have received much attention’ and that more recently ‘another index could be of great value… namely the normalized difference of brightness temperatures in horizontal and vertical polarization measured at 37 GHz by SMMR on board Nimbus 7’ (MPDI).

So how do they work as a tool in monitoring desertification? NDVI is correlated to leaf area index (LAI), defined by The Global Climate Observing System as ‘one half the total green leaf area per unit ground surface area’, and the vegetation cover fraction. The key mechanism is the absorptivity and reflectivity of biomass due to chlorophyll absorption. Hence chlorophyll concentrations received from satellite technology plays an important part in the NDVI calculation and hence the quantity of biomass at a given location. The MPDI method incorporates brightness temperatures in horizontal and vertical polarisation and is sensitive to the water content of plants rather than chlorophyll absorption. NDVI is generally considered more appropriate for monitoring vegetation. 

                                                         Global NDVI

Further advancement of these techniques will equip us with the tools to build high resolution, high accuracy models that help us to predict regions most susceptible to the threat of desertification and hence implement policies and practices designed to mitigate the ill effects it can bring. Remote sensing will no doubt be a key feature of desertification management and monitoring.