Stable isotope compositions (18 O and 2 H), determined for underground and surface waters from the watershed of a hill reservoir and downstream from the reservoir, suggest that (i) the reservoir water, which is more or less evaporated, represents a mixture between surface waters (rainfall, runoff) and the upstream alluvial groundwater meteoric in origin; (ii) the downstream alluvial groundwater have a stable isotope composition of a previously infiltrated reservoir water. The 18 O isotope enrichment modelling of the mixed reservoir water shows that an input flux of 50 m3 day−1 is balanced by an output flux of 300 m3 day−1 when the reservoir water level is above 4.5 m, and by an output flux of 170 m3 day−1 when water levels are lower. The contri- bution of hill reservoirs to local groundwater movements must be considered in any regional scale hydrological investigations of areas that contain hill reservoirs.