Estimation of LAI, fAPAR and fCover of Sahel rangelands (Gourma, Mali)

Leaf area index, LAI, fraction of absorbed photosynthetic active radiation, fAPAR, and vegetation cover fraction, fCover, drive the main vegetation canopy processes and are key variables in ecosystem, climate, hydrology and biogeochemistry models. The present study aims to evaluate the joint use of hemispherical photographs and the CAN-EYE image processing software for deriving reliable estimates of these 3 variables in a semi-arid environment. First, LAI and fAPAR estimates are evaluated at a fine scale (about 1 m2) for which independent observations (destructive measurements for LAI, direct transmitted PAR radiation for fAPAR) are also accurately acquired. Second, a sampling methodology is defined to provide reliable estimations of these variables at the 1 km scale taking into account the spatial heterogeneity of the vegetation cover. Estimated accuracy for the 3 variables is about ±17.3% when 100 photographs are acquired along a 1 km transect. In addition, spatial and temporal variations of the clumping index of the herbaceous canopy are characterized. For pastoral sites of northern Sahel, measurements performed along one or two perpendicular 1 km transects provide reliable LAI/fAPAR/fCover estimates at that scale that should allow comparison with satellite products derived from medium resolution satellites such as the moderate resolution imaging spectroradiometer, MODIS.