Land degradation is defined as a long-term decline in ecosystem function and measured in terms of net primary productivity(NPP).
Long-term NPP measurement is not available; the remotely-sensed normalized difference vegetation index (NDVI) is used as
a proxy; its deviation from the norm may serve as an indicator of land degradation and improvement if other factors that may
be responsible (climate, soil, terrain and land use) are accounted for. NDVI is a ratio measuring of photosynthetically
active green biomass. The higher the NDVI, the more living green biomass can be found. There is a high correlation between
NDVI and NPP; the GIMMS NDVI time series has been translated to NPP using MODIS NPP data (Justice and others 2002, Running
and others 2004) for the overlapping period 2000-2003, i.e., NPP was estimated by correlation with MODIS 8-day NPP values
for the overlapping years of the GIMMS and MODIS datasets (2000-2003), re-sampling the annual mean MODIS NPP at 1km resolution
to 8km resolution using nearest-neighbour assignment.
NDVI from July 1981 to December 2003 are produced by the Global Inventory Modelling and Mapping Studies (GIMMS) group from
measurements made by the AVHRR radiometer on board US National Oceanic and Atmospheric Administration satellites. The fortnightly
images at 8km-spatial resolution are corrected for calibration, view geometry, volcanic aerosols, and other effects not related
to vegetation cover (Tucker and others 2004). These data are compatible with those from other sensors such as MODIS, SPOT
Vegetation, and Landsat ETM+ (Tucker and others 2005, Brown and others 2006).