A 6,300-m2 catchment in SW Sweden was covered by a roof and irrigated by deacidified lake water. The Δ18O of lake water differed from that of soil water and groundwater in the catchment, making it possible to follow the replacement of old (preirrigation) water by new (irrigation) water. After 7.5 months of irrigation the old water in the catchment had been replaced by new water. The breakthrough curve for new water gave a nearly exponential distribution of flow corrected transit times, with a mean transit time corresponding to a runoff of 54 mm. The estimated outflow of old water by runoff and evapotranspiration, 186 mm, compared well with independent estimates of the water storage at the onset of irrigation, 205 mm. The spatial pattern of the Δ18O of soil water and groundwater within the catchment demonstrated the strong influence of the topography on the water flow. ¿ American Geophysical Union 1996