GCM and downscaling uncertainty in modelling of current river flow: why is it important for future impacts?

This paper compares the uncertainty in flow simulation due to the modelling of precipitation (using three global circulation Models, GCMs, and two downscaling techniques) with the current natural flow variability in three catchments in Scotland. The reference daily flow series, obtained by running a calibrated continuous rainfall-runoff model with observed catchment rainfall and potential evaporation, is compared with simulations from a range of different precipitation scenarios, representative of the climate modelling uncertainty and cut-rent natural variability. Results show that the natural variability can be large and differs from one catchnient to another. The reproduction of climate variability from different GCMs is not always within this natural variability and there is some regional variation in the GCMs' and downscaling techniques' ability to reproduce the current climate. The uncertainty due to GCMs is consistently larger than that of downscaling techniques.