###### Abstract  
Water loss is a crucial factor for vegetation in the semi-arid Sahel region of Africa. Global satellite-driven estimates of plant CO\({}_{2}\) uptake (gross primary productivity, GPP) have been found to not accurately account for Sahelian conditions, particularly the impact of canopy water stress. Here, we identify the main biophysical limitations that induce canopy water stress in Sahelian vegetation and evaluate the relationships between field data and Earth observation-derived spectral products for up-scaling GPP. We find that plant-available water and vapor pressure deficit together control the GPP of Sahelian vegetation through their impact on the greening and browning phases. Our results show that a multiple linear regression (MLR) GPP model that combines the enhanced vegetation index, land surface temperature, and the short-wave infrared reflectance (Band 7, 2105-2155 nm) of the moderate-resolution imaging spectroradiometer satellite sensor was able to explain between 88% and 96% of the variability of eddy covariance flux tower GPP at three Sahelian sites (overall = 89%). The MLR GPP model presented here is potentially scalable at a relatively high spatial and temporal resolution. Given the scarcity of field data on CO\({}_{2}\) fluxes in the Sahel, this scalability is important due to the low number of flux towers in the region. Sahel; drought; gross primary productivity; Earth observation; plant-available water; soil moisture; vapor pressure deficit; plant stress; greening; browning figure  
over the 18-year period. The key suggested mechanism behind the greening is increased rainfall [7], and to a lesser extent, improved land use and migration-induced land abandonment [3]. Gross primary productivity (GPP) is the total amount of C plant extract from the atmosphere through photosynthesis. Some of this C goes to maintaining plant tissue, and is thus lost through autotrophic respiration (Ra). The remaining C is stored as phytomass (net primary productivity, NPP = GPP \(-\) Ra). The capacity of Sahelian ecosystems to provide essential food, fuel, feed and fiber is a function of available NPP [8]. Since these ecosystem services are essential, evaluating the environmental controls that modulate GPP dynamics is necessary for accurate accounting of primary productivity. The Sahel has a single growing season that starts in July and ends in October [9] with considerable inter-annual variability in the spatial distribution and quantity of rainfall [10; 11]. The driest parts of the northern Sahel receive an annual average of 150 mm of rain, while the southern parts receive an annual average of ~750 mm [12] (Figure 1). The Sahel (Figure 1) is dominated by plants that have the C\({}_{4}\) photosynthetic pathway [13], which are adapted to warm, arid environments and are composed mainly of grasses, herbs and crops. The mean tree canopy cover is approximately 7.3% and comprises trees that use the C\({}_{3}\) photosynthetic pathway [14; 15]. It is well known that moisture availability controls C fluxes in drylands such as the Sahel, and several studies have attempted to explain the underlying mechanisms of its modulation of primary productivity in these regions [7; 16; 17; 18; 19; 20; 21]. [PERSON] et al. [22] found that mean annual rainfall is strongly correlated with maximum photosynthetic capacity and is the predominant factor driving C fluxes across Africa. With rainfall, the greening phase commences (Figure 2), and soil moisture is replenished, which impacts the amount of energy partitioned into evapotranspiration [21; 23; 24]. Rainfall also increases humidity, which lowers the difference between the vapor pressure deficit inside the leaf and that of the air. This difference in vapor pressure deficit was found to be a key factor affecting stomatal conductance of Sahelian plants [25; 26].