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Savanna is one of the main African ecosystems. The tree community in the savanna is a key element and provides many ecosystem services. Unmanned aerial vehicles (UAVs) combined with photogrammetric analysis are useful tools to produce accurate 3D maps, which, in turn, help describe the structure of the tree populations. The purpose of this study was to evaluate the use of commercial economical UAVs to assess tree characteristics in the savanna. A Dji Spark UAV was used to map 24 1-ha plots in northern Senegal. The images were processed using Pix4D software to produce a high-resolution canopy height model (CHM). A total of 239 trees were selected and their heights and crown areas were manually measured in the field. A strong correlation was found between UAV and field measurements with R2 = 0.84 for height and R2 = 0.93 for crown area. Based on tree canopy colours and tree morphologies measured by UAV, it was possible to predict tree species with an error rate of 20%, using a random forest classification. This study thus confirms the possibility of using low-cost UAVs to assess tree structures in the savanna not only for research on tree communities in savanna, but also by forestry agencies to inform stakeholders.

La contribution des ligneux sahéliens à de nombreux services écosystémiques est communément admise. Cependant, elle est difficilement quantifiable et rarement évaluée. Nous présentons ici une approche par expertise qui se base sur un outil d’évaluation multicritère, TATALE. Sur base de caractéristiques de la végétation, qui sont reliées à des services par des expert·e·s, l’outil génère des scores synthétiques. Ceux-ci sont ensuite utilisés comme proxy de 4 services : la production de fourrage, la production de nourriture, la production de bois et la création d’un microclimat favorable. Les avantages majeurs de l’outil résident dans sa simplicité de mise en oeuvre et d’interprétation. Cette première approche bénéficiera dans un second temps de l’expertise d’autres scientifiques avant d’être utilisée pour déterminer l’influence de facteurs environnementaux sur la fourniture des services.

Les parcours sahéliens sont peuplés d’une communauté d’herbacées principalement annuelles en interaction avec une communauté de ligneux. Le lien statistique entre la composition botanique des communautés ligneuses et celle des herbacés est étudié. Les observations analysées sur la région du Ferlo de 1970 à 1981 sont extraites de la base de données FLOTROP (base d’observations multi-sources et multi-dates sur la végétation d’Afrique subsaharienne). Les compositions botaniques des deux communautés sont faiblement liées entre elles, et cela de façon variable d’une année sur autre. Ce résultat indique une sensibilité différente des deux communautés aux variables de l’environnement. La composition floristique des communautés ligneuses est liée à la position géographique, à la pluviosité moyenne ou le type de sol alors que les communautés d’herbacées varient d’une année sur l’autre et sont principalement sensibles à l’année d’observation

The profitability of hydropower in Costa Rica is affected by soil erosion and sedimentation in dam reservoirs, which are in turn influenced by land use, infiltration and aquifer interactions with surface water. In order to foster the provision and payment for Hydrological Environmental Services (HES), a quantitative assessment of the impact of specific land uses on the functioning of drainage-basins is required. The present paper aims to study the water balance partitioning in a volcanic coffee agroforestry micro-basin (1 km2, steep slopes) in Costa Rica, as a first step towards evaluating sediment or contaminant loads. The main hydrological processes were monitored during one year, using flume, eddy-covariance flux tower, soil water profiles and piezometers. A new Hydro-SVAT lumped model is proposed, that balances SVAT (Soil Vegetation Atmosphere Transfer) and basin-reservoir routines. The purpose of such a coupling was to achieve a trade-off between the expected performance of ecophysiological and hydrological models, which are often employed separately and at different spatial scales, either the plot or the basin. The calibration of the model to perform streamflow yielded a Nash-Sutcliffe (NS) coefficient equal to 0.89 for the year 2009, while the validation of the water balance partitioning was consistent with the independent measurements of actual evapotranspiration (R2 = 0.79, energy balance closed independently), soil water content (R2 = 0.35) and water table level (R2 = 0.84). Eight months of data from 2010 were used to validate modelled streamflow, resulting in a NS = 0.75. An uncertainty analysis showed that the streamflow modelling was precise for nearly every time step, while a sensitivity analysis revealed which parameters mostly affected model precision, depending on the season. It was observed that 64% of the incident rainfall R flowed out of the basin as streamflow and 25% as evapotranspiration, while the remaining 11% is probably explained by deep p

The profitability of hydropower in Costa Rica is affected by soil erosion and sedimentation in dam reservoirs, which are in turn influenced by land use, infiltration and aquifer interactions with surface water. In order to foster the provision and payment for Hydrological Environmental Services (HES), a quantitative assessment of the impact of specific land uses on the functioning of drainage-basins is required. The present paper aims to study the water balance partitioning in a volcanic coffee agroforestry micro-basin (1 km2, steep slopes) in Costa Rica, as a first step towards evaluating sediment or contaminant loads. The main hydrological processes were monitored during one year, using flume, eddy-covariance flux tower, soil water profiles and piezometers. A new Hydro-SVAT lumped model is proposed, that balances SVAT (Soil Vegetation Atmosphere Transfer) and basin-reservoir routines. The purpose of such a coupling was to achieve a trade-off between the expected performance of ecophysiological and hydrological models, which are often employed separately and at different spatial scales, either the plot or the basin. The calibration of the model to perform streamflow yielded a Nash-Sutcliffe (NS) coefficient equal to 0.89 for the year 2009, while the validation of the water balance partitioning was consistent with the independent measurements of actual evapotranspiration (R2 = 0.79, energy balance closed independently), soil water content (R2 = 0.35) and water table level (R2 = 0.84). Eight months of data from 2010 were used to validate modelled streamflow, resulting in a NS = 0.75. An uncertainty analysis showed that the streamflow modelling was precise for nearly every time step, while a sensitivity analysis revealed which parameters mostly affected model precision, depending on the season. It was observed that 64% of the incident rainfall R flowed out of the basin as streamflow and 25% as evapotranspiration, while the remaining 11% is probably explained by deep p