Impacts of shade trees on hydrological services and erosion in a coffee AFS of Costa Rica; Scaling from plot to watershed

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