Résumé
It has been shown that vegetation growth in semi-arid regions is important to the global terrestrial CO 2 sink, which indicates the strong need for improved understanding and spatially explicit estimates of CO 2 uptake (gross primary production; GPP) in semi-arid ecosystems. This study has three aims: (1) to evaluate the MOD17A2H GPP (collection 6) product against GPP based on eddy covariance (EC) for six sites across the Sahel; (2) to characterize relationships between spatial and temporal variability in EC-based pho-tosynthetic capacity (F opt) and quantum efficiency (α) and vegetation indices based on earth observation (EO) (normal-ized difference vegetation index (NDVI), renormalized difference vegetation index (RDVI), enhanced vegetation index (EVI) and shortwave infrared water stress index (SIWSI)); and (3) to study the applicability of EO upscaled F opt and α for GPP modelling purposes. MOD17A2H GPP (collec-tion 6) drastically underestimated GPP, most likely because maximum light use efficiency is set too low for semi-arid ecosystems in the MODIS algorithm. Intra-annual dynamics in F opt were closely related to SIWSI being sensitive to equivalent water thickness, whereas α was closely related to RDVI being affected by chlorophyll abundance. Spatial and inter-annual dynamics in F opt and α were closely coupled to NDVI and RDVI, respectively. Modelled GPP based on F opt and α upscaled using EO-based indices reproduced in situ GPP well for all except a cropped site that was strongly im-pacted by anthropogenic land use. Upscaled GPP for the Sa-hel 2001-2014 was 736 ± 39 g C m −2 yr −1. This study indicates the strong applicability of EO as a tool for spatially explicit estimates of GPP, F opt and α; incorporating EO-based F opt and α in dynamic global vegetation models could improve estimates of vegetation production and simulations of ecosystem processes and hydro-biochemical cycles.