A comparison of delta change and downscaled GCM scenarios for three mountainous basins in the United States

Simulated daily precipitation, temperature, and runoff time series were com pared in three mountainous basins in the United States: (1) the Animas Rive r basin in Colorado, (2) the East Fork of the Carson River basin in Nevada and California, and (3) the Cle Elum River basin in Washington State. Two m ethods of climate scenario generation were compared: delta change and stati stical downscaling. The delta change method uses differences between simula ted current and future climate conditions from the Hadley Centre for Climat e Prediction and Research (HadCM2) General Circulation Model (GCM) added to observed time series of climate variables. A statistical downscaling (SDS) model was developed for each basin using station data and output from the National Center for Environmental Prediction/National Center for Atmospheri c Research (NCEP/NCAR) reanalysis regridded to the scale of HadCM2. The SDS model was then used to simulate local climate variables using HadCM2 outpu t for current and future conditions. Surface climate variables from each sc enario were used in a precipitation-runoff model. Results from this study s how that, in the basins tested, a precipitation-runoff model can simulate r ealistic runoff series for current conditions using statistically downscale d NCEP output. But, use of downscaled HadCM2 output for current or future c limate assessments are questionable because the GCM does not produce accura te estimates of the surface variables needed for runoff in these regions. G iven the uncertainties in the GCMs ability to simulate current conditions b ased on either the delta change or downscaling approaches, future climate a ssessments based on either of these approaches must be treated with caution .