Use of MODIS NDVI to evaluate changing latitudinal gradients of rangeland phenology in Sudano-Sahelian West Africa

This study is concerned with the implications of changing latitudinal gradients in vegetative phenology (green-up, senescence, and length of growing season) for the management of long-distance seasonal movements of livestock herds in Sudano-Sahelian West Africa. For a study area covering much of the southern half of Mali, phenological parameters were estimated using a double-logistic function fitted to seasonal NDVI trajectories for 1 km² MODIS data over the period 2000-2010. Green-up dates, senescence dates and length of growing season were all found to more strongly vary by latitude (+ 9 days/degree, − 5 days/degree and − 14 days/degree, respectively) than across years (+ 0.42 days/year, + 0.86 days/year and + 0.44 days/year respectively). Interannual and spatial variability of these parameters are highest at lower latitudes within the study area. The slopes of the relationship of phenological parameters with latitude change across the latitudinal range studied. Breakpoint analysis of annual green-up versus latitude curves identifies a mean inflection point of 13° north latitude above which the positive slope declines significantly. This previously-undescribed pattern is consistent with recent work on monsoonal dynamics showing rainfall onset being associated with an abrupt shift in the location of the ITCZ (monsoon onset) at latitudes north of 13° north latitude. The effects of the observed variation in latitudinal gradients of phenological variables on the direction and timing of regional livestock movements are discussed.