Evaluation of climate modeling factors impacting the variance of streamflow

•For the first time, variance decomposition of GCM-forecasted streamflow is performed.•Forecasted streamflow variance depends on GCM type, GCM version, and downscaling.•Variance is reduced when streamflow forecasted with the delta change method is used.•A balanced GCM ensemble negates the use of excessive realizations of model runs.•Precipitation and streamflow increases of 10% and 11% were predicted.