Abstract The study used a monthly water balance model to examine the factors controlling the effects of climate change on catchments in the UK. Fifteen case study catchments representing a range of climatic and geological conditions were used together with a number of realistic climate change scenarios. The effects of climate change on average annual runoff depend on the ratio of average annual runoff to average annual rainfall, with the greatest sensitivity in the driest catchments with lowest runoff coefficients. The greater the concentration of a given annual rainfall change in winter, the greater the effect of that change on runoff. Several different empirical formulae gave results inconsistent with those from the monthly water balance model. Changes in monthly runoff are controlled by catchment geology and the current summer balance between rainfall and potential evapotranspiration. A catchment where summer rainfall is currently close to potential evapotranspiration shows the greatest proportional change in runoff in summer, whilst flows in catchments with large groundwater storages may be maintained even during warmer, drier summers if winter rainfall increases.