Assessment of land degradation using NASA GIMMS

Biomass is an integrated measure of productivity; its deviance from the norm may indicate land degradation or improvement. Biomass can be assessed by the normalized difference vegetation index (NDVI) derived from satellite data. Non11S may be established according to climate, soils and tenain and land use; deviance may be calculated regionally and combined globally to allow universal comparisons. As pati of a Global Assessment of Land Degradation and Improvement, spatial patterns and temporal trends of green biomass across Kenya were analysed using 23 years of fortnightly NOAA-AVHRR NDVl data and decadal precipitation. ArcGIS algorithms were used to calculate various biomass indicators; temporal trends were determined by regression at annual intervals and mapped to depict spatial changes. In Kenya over the period of 1981-2003, biomass increased over about 78% of the land area and decreased over 21 % - but the decrease has been across the most productive areas, the high-rainfall zones. A declining trend of biomass is strongly correlated with the cropped area and, in particular, the extension of cropping into marginal lands. To assess whether this trend represents land degradation or declining rainfall, we calculated rain-use efficiency, the ratio between green biomass (NDVI) and rainfall, which may be amore robust indicator of land degradation. The trends are similar but rain-use efficiency shows sharpest decline in two areas: the drylands around Lake Turkana and the whole of Kitui District in Eastern Province. 1. lntroduction