Modelling spatial and temporal dynamics of GPP in the Sahel from earth observation based photosynthetic capacity and quantum efficiency

International audienceIt has been shown that vegetation growth in semi-arid regions is an important sink for human induced fossil fuel emissions of CO2, which indicates the strong need for improved understanding, and spatially explicit estimates of CO2 uptake (gross primary productivity (GPP)) in semi-arid ecosystems. This study has three aims: 1) to evaluate the MOD17A2H GPP (collection 6) product against eddy covariance (EC) based GPP for six sites across the Sahel. 2) To find evidence on the relationships between spatial and temporal variability in EC based photosynthetic capacity (Fopt) and quantum efficiency (α) and earth observation (EO) based vegetation indices 3) To study the applicability of EO up-scaled Fopt and α for GPP modelling purposes. MOD17A2H GPP (collection 6) underestimated GPP strongly, most likely because the maximum light use efficiency is set too low for semi-arid ecosystems in the MODIS algorithm. The intra-annual dynamics in Fopt was closely related to the shortwave infrared water stress index (SIWSI) closely coupled to equivalent water thickness, whereas α was closely related to the renormalized difference vegetation index (RDVI) affected by chlorophyll abundance. Spatial and inter-annual dynamics in Fopt and α were closely coupled to the normalized difference vegetation index (NDVI) and RDVI, respectively. Modelled GPP based on Fopt and α up-scaled using EO based indices reproduced in situ GPP well for all but a cropped site. The cropped site was strongly impacted by intensive anthropogenic land use. This study indicates the strong applicability of EO as a tool for parameterising spatially explicit estimates of photosynthetic capacity and efficiency; incorporating this into dynamic global vegetation models could improve global estimations of vegetation productivity, ecosystem processes and biochemical and hydrological cycles