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Despite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy-intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc-excess) and soil water content. Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the "drier" season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc-excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the "drier" season, leading to enhanced rainfall interception and a negative relationship between lc-excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc-excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared with under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season, there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics.

Abstract. 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 microbasin (1 km 2 , 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 Correspondence to: F. Gómez-Delgado (fgomezd@ice.go.cr) validation of the water balance partitioning was consistent with the independent measurements of actual evapotranspiration (R 2 = 0.79, energy balance closed independently), soil water content (R 2 = 0.35) and water table level (R 2 = 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 percolation, measurement errors and/or inter-annual changes in soil and aquifer water stocks. The model indicated an interception loss equal to 4% of R, a surface runoff of 4% and an infiltration component of 92%. The modelled streamflow was constituted by 87% of baseflow originating from the aquifer, 7% of subsurface nonsaturated runoff and 6% of surface runoff. Given the low surface runoff observed under the current physical conditions (andisol) and management practices (no tillage, planted trees, bare soil kept by weeding), this agroforestry system on a volcanic soil demonstrated potential to provide valuable HES, such as a reduced superficial displacement-capacity for fertilizers, pesticides and sediments, as well as a streamflow regulation function provided by the highly efficient mechanisms Published by Copernicus Publications on behalf of the European Geosciences Union. 370 F. Gómez-Delgado et al.: Modelling hydrological behaviour of coffee agroforestry basin in Costa Rica of ...