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The profitability of hydropower is affected by soil erosion and sedimentation in dam reservoirs, which are influenced by land use, infiltration and aquifer interactions with surface water. In order to promote the payment of Hydrological Environmental Services (HES) in Costa Rica, a quantitative assessment of the impact of land uses (such as coffee farming) on the functioning of drainage basins is required. This thesis seeks: 1) to study the water balance partitioning in a newly installed coffee agroforestry basin 2) to estimate the water and sediment yield at various spatio-temporal scales: from plot to basin and from event to annual scale; and 3) to simulate the water and sediment yields, at both annual and peakflow scale, by including the surface runoff from hillslopes and roads. The main hydrological, ecophysiological and sediment processes were monitored during one year at the basin (rainfall, streamflow, evapotranspiration, soil humidity, aquifer level, turbidity) and at the plots (surface runoff and erosion). A new eco-hydrological model was developed to close the water balance, and the annual sediment yield was also quantified. Improvements are in progress to take into account the effect of roads in surface runoff generation. The low surface runoff, low plot erosion and low basin sediment yield observed under the current biophysical conditions (andisol) and management practices (no tillage, planted trees, bare soil kept by weeding), offer potential HES by reducing the superficial displacement capacity for fertilizers and pesticides, yielding low sediment loads and regulating streamflow variability through highly efficient mechanisms of aquifer recharge-discharge., La production hydroélectrique est fortement impactée par la sédimentation dans les retenues de barrages, qui sont influencés par l'occupation du sol, l'infiltration et les interactions entre les eaux de surface et les aquifères. Pour le paiement des Services Hydrologiques Environnementaux au Costa Rica, une quantification de l'impact des utilisations des terres cultivées en café sur le fonctionnement des bassins est nécessaire. Cette thèse vise à : 1) étudier les composantes du bilan hydrologique dans un bassin agroforestier récemment installé 2) à quantifier les flux d'eau et de sédiments à différentes échelles : de la parcelle au bassin et de l'événement de crue à l'échelle annuelle, et 3) à simuler le bilan hydrique et la production de sédiments en tenant compte du ruissellement sur les versants et les routes. Les processus hydrologiques, écophysiologiques et de transfert de sédiments ont été suivis sur le bassin (pluie, débit, évapotranspiration, humidité du sol, niveau de l'aquifère, turbidité) et les parcelles (ruissellement de surface et érosion). Un nouveau modèle éco- hydrologique a été développé pour simuler les termes du bilan hydrologique et de sédiments. Des améliorations sont en cours pour tenir compte de l'effet des routes sur la genèse du ruissellement de surface et de l'érosion. Le faible ruissellement de surface, la faible érosion des parcelles et la faible production de sédiments, compte-tenu des conditions biophysiques et des pratiques de gestion (arbres d'ombrage, désherbage), offrent clairement des Services Ecosystémiques en réduisant les transferts superficiels de pesticides et de sédiments et en permettant une bonne régulation du débit de rivière.

International audience; Environmental services (ES) provided by agroforestry systems require accurate evaluation in order to encourage their payment and the adoption of appropriate practices by farmers. We aimed at quantifying the hydraulic erosion and its subsequent sediment yield in a 900 ha homogeneous watershed of Costa Rica, displaying adult coffee plants shaded by high Erythrina poepiggiana. This watershed is connected to a large hydropower project, where reservoir sedimentation could become a critical economical issue. At the outlet of the watershed we monitored the river runoff (using calibrated hydraulic flume and pressure transducer) and water turbidity (using optical turbidimeter) over 8 months and correlated them with rainfall. We calibrated in situ the turbidimeter rating curve, which relates its measurements (NIR backscatter detected between 140° and 180°) with reference to suspended sediment concentrations (laboratory analyses), and calculated the instantaneous and accumulated sediment transport. In order to assess the impact of shade trees, and to compare with the full-sun situation, we monitored surface runoff and turbidity at the outlet of 2 neighbouring coffee plots (1600 m2 each, with and without trees). This, together with ancillary measurements, allowed us to test the hypothesis about the reduction of: a) splash erosion by canopies; and b) sheet and rill soil transport capacity, by either larger infiltration (less surface flow), larger surface roughness (due to leaf litter) or the increase in soil retention (by rooting systems). Various flow separation methods and models were tested, in order to scale-up from shaded/full-sun plots to the whole watershed. The partitioning of base and storm sediment contributions was inferred. These results are a first step towards the development of objective criteria to assess the impacts of agroforestry practices in coffee plantations, their cumulative effect at watershed scales and their future for hydrological and sediment ES in Costa Rica