Rain-use efficiency (RUE; the ratio of vegetation productivity to annual precipitation) has been suggested as a measure for assessing land degradation in arid/semi-arid areas. In the absence of anthropogenic influence, RUE has
been reported to be constant over time, and any observed change may therefore be attributed to non-rainfall impacts. This study presents an analysis of the decadal time-scale changes in the relationship between a proxy for
vegetation productivity (SNDVI) and annual rainfall in the Sahel-Sudanian zone of Africa. The aim is to test the quality of data input and the usefulness of both the RUE approach and an alternative method for separating the effects on vegetation productivity of rainfall change and human impact. The analyses are based on earth observation of both rainfall (GPCP (Global Precipitation Climatology Project), 1982–2007 and RFE (Rainfall Estimate) (1996–2007)) and SNDVI (AVHRR GIMMS NDVI dataset, 1982–2007). It is shown that the increase in SNDVI has been substantial in the Sahel-Sudanian zone over the 1982–2007 period,whereas for the period 1996–2007 the pattern of SNDVI trends is more complex. Also, trend analysis of annual rainfall from GPCP data (2.5° resolution) and RFE data (0.1° resolution) suggests that rainfall has increased over both periods. Further it is shownthat RUE values are highly correlated to rainfall, undermining the use of earth observation (EO)-based RUE (using SNDVI) as a means of separating rainfall impacts from other factors. An alternativemethod identify temporal trends in residuals of SNDVI, after regressing it against annual rainfall, is tested, yet is shown to be useful only where a high correlation between
SNDVI and annual rainfall exists. For the areas in the Sahel-Sudanian zone forwhich this condition is fulfilled, trend analyses suggest very limited anthropogenic land degradation in the Sahel-Sudanian zone.