Nationwide changes in spatially well-resolved patterns of British runo were investigated under two climate change scenarios derived from general circulation model (GCM) output. A physical process-based hydrological model (HYSIM) was used to simulate eective runo across a 10 km ร 10 km British grid under baseline and future climate conditions. A gridded baseline climatology for precipitation and the Penman variables was used to validate HYSIM across Britain using grid cell-speciยฎc parameters derived from land use and soil type. The climate change scenarios were constructed from the Hadley Centre's high resolution equilibrium GCM (UKHI) for 2050 and transient GCM (UKTR) for 2065. Future eective runo was simulated under both scenarios by applying changes in precipitation and the Penman variables to the baseline climatology. Annual eective runo is shown to increase throughout most of Britain under the UKHI scenario for 2050, whilst it decreases over much of England and Wales under the UKTR scenario for 2065. Both scenarios show an increasing gradient in runo between a wetter northern Britain and a drier south-eastern Britain. This gradient is more pronounced under the UKTR scenario. Changes in eective runo for winter and summer show an increase in seasonality under both scenarios. Winter runo is shown to increase most in northern Britain under both scenarios, whilst summer runo is shown to experience major reductions over much of England and Wales under the UKTR scenario. If these simulations are realized, Britain may expect an accentuated north to south-east imbalance in available water resources. If this is combined with a temporal imbalance suggested by the increased seasonality, there could be problems for the future management of British water resources.