## 5 Conclusions  
Hydrological processes generally control vegetation dynamics at multiple spatial and temporal scales. This is particularly true for the Sahel, where the short unimodal rainy season induces a net CO\({}_{2}\) uptake that is driven mainly by the herbaceous canopy [13]. Except in highly humid environments, all plants undergo photosynthesis at the expense of losing moisture to the atmosphere. This moisture loss is particularly significant for dryland vegetation, where an imbalance can occur between the amount of water plants require and the limited amount that is available in the ecosystem. In this study, we found that the two main moisture-related biophysical limitations, PAW and VPD together control plant CO\({}_{2}\) uptake in the semi-arid conditions. With the start of the dry season in November, low PAW is present in the drying soils of the Sahel, which leads to decreased availability of water for uptake by plant roots (Figures 3 and 4). This, in turn, reduces evapotranspiration (latent heat) due to drier soils and plants close their stomata to prevent moisture loss.  
Considering both the importance of drylands in the global C cycle [85] and the dependence of human livelihoods on the products of photosynthesis [8], it is essential to account for water stress controls in order to better model primary productivity in this region. This is particularly important considering the fact that there is a chronic lack of field data on C fluxes in the Sahel. We found that the combination of MODIS NBAR Band 7 as a proxy for soil moisture variability, LST as a proxy for VPD, and EVI as a proxy of photosynthetic activity in an MLR model was able to explain 89% of the variability of EC GPP across the three Sahelian sites (Figures 8 and 9). Since EVI, LST, and MODIS NBAR Band 7 are based on Earth observation data, the MLR model presented here is potentially scalable with repeatable estimates of GPP at relatively high spatial and temporal resolutions. Moreover, the availability of standardized EC flux measurements, such as the recently released FLUXNET2015 database, can potentially eliminate variability in systematic and processing-induced uncertainties between sites. This can further improve model estimates when relating EC GPP with Earth observation data. Studies of the C cycle often focus on the growing season as it is the principal period where C assimilation takes place, however, the inclusion of dry season dynamics can potentially improve the explanatory capacity of GPP models by accounting for land memory effects. Further evaluation of this approach using more dryland sites from the FLUXNET2015 database as well as other types of in-situ validation data is required to test its efficiency in modeling GPP in water-limited areas.  
**Acknowledgments:** [PERSON] received support from the Lund University Center for Studies of Carbon Cycle and Climate Interactions (LUCCI) and the Royal Physiographic Society in Lund. Data for the Wankama (Niger) site were produced through the AMMA-CATCH regional observatory, funded by the French IRD and INSU national research institutes. Funding for this study was provided by the Swedish National Space Board (contract No. 100/11 to [PERSON]). The authors would like to thank all three reviewers of this manuscript for providing thoughtful and helpful comments.  
**Author Contributions:** [PERSON] conceived and designed the study, conducted the analysis, interpreted the results, and drafted the paper. [PERSON], [PERSON] and [PERSON] assisted with the study design and the interpretation of the results. [PERSON] contributed to the analysis and the interpretation of the results. [PERSON] provided site data for Wankama and approved the final draft. All authors discussed the results and revised the manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.