Increasing reliance on the rivers of southern Alberta for irrigation and other uses has heightened vulnerability to hydrological drought. While water supplies have been monitored since the early 20th century, these hydrometric records do not capture the full range in the severity and duration of hydrological drought as revealed by previous research on the paleohydrology of the South Saskatchewan River Basin (SSRB). Using a two-stage approach to modeling runoff from tree-ring data, we reconstructed 1000 years of annual and seasonal streamflow at 75 gauges in the SSRB. The two stages – single site reconstructions and principal component analysis – take full advantage of our network of 599 tree-ring chronologies to develop multiple linear regression models that account for up to 80% of the variance in naturalized streamflow. These long runoff records underscore the risks in relying solely on instrumental data for informing water resource policy, management practices and planning. They also provide critical context for assessing uncertainty in the climate model projections that are used to force hydrological models for the prediction of hydrological drought. Research suggests that natural hydroclimatic variability, as recorded by radial tree growth, is the dominant source of uncertainty for the model projection of water balance variables.
