Estimating low flow frequencies in the mid to late 21st century for two basins in Central Wales

Abstract

The research reported here uses a methodology that has been developed to generate riverflow scenarios for future climate change from GCMs. The method involves simulation of daily flows to assess the degree of change in flow frequencies, lengths of low flow periods and seasonal distributions in catchments in mid-Wales under two climate change scenarios derived from the Hadley Centre HadCM2 GCM. The method consists of: (1) establishing statistical relationships between interval-scale airflow indices (vorticity, the strength of the geostrophic wind, and its zonal and meridional directional components) and recorded precipitation and potential evapotranspiration (PET) for the period of instrumental record within each catchment, (2) using the derived transitional probabilities and correlations as input to a daily Stochastic Weather Generator, and (3) inputting this synthesised daily weather sequence into a physicallybased hydrological simulation model (HYSIM). Split-sample tests indicate good agreement between predicted and recorded daily flow frequencies, with a Nash-Sutcliffe efficiency criterion of 0.7 for daily flows and 0.9 for monthly totals. The results suggest that there will be increased frequency of low flow days during the summer and a slight increase in the length of individual low flow spells.