Analysing the impact of future climate change on hydrological regimes is hampered by the disparity of scales between general circulation model (GCM) output and the spatial resolution required by catchment-scale hydrological simulation models. In order to overcome this, statistical relationships were established between three indices of atmospheric circulation (vorticity and the strength and direction of geostrophic windflow) and daily catchment precipitation and potential evapotranspiration (PET) to downscale from the HadCM2 GCM to the Upper Wye experimental catchment in mid-Wales. The atmospheric circulation indices were calculated from daily grid point sea-level pressure data for: (a) the Climatic Research Unit observed data set (1975–90); (b) the HadCM2SUL simulation representing the present climate (1980–99); and (c) the HadCM2SUL simulation representing future climate conditions (2080–99). The performance of the downscaling approach was evaluated by comparing diagnostic statistics from the three downscaled precipitation and PET scenarios with those recorded from the Upper Wye catchment. The most significant changes between the downscaled HadCM2SUL 1980–99 and 2080–99 scenarios are decreases in precipitation occurrence and amount in summer and autumn combined with a shortening of mean wet spell length, which is most pronounced in autumn. A hydrological simulation model (HYSIM) was calibrated on recorded flow data for the Upper Wye catchment and ‘forced’ with the three downscaled precipitation and PET scenarios to model changes in river flow and hillslope hydrological processes. Results indicate increased seasonality of flows, with markedly drier summers. Analysis of extreme events suggests significant increases in the frequency of both high- and low-flow events. Copyright © 2002 John Wiley & Sons, Ltd.