During 19-21 June 2013 a heavy precipitation event affected southern Alberta and adjoining regions, leading to severe flood damage in numerous communities and resulting in the costliest natural disaster in Canadian history. This flood was caused by a combination of meteorological and hydrological factors, which are investigated from weather and climate perspectives with the fifth generation Canadian Regional Climate Model. Results show that the contribution of orographic ascent to precipitation was important, exceeding 30 % over the foothills of the Rocky Mountains. Another contributing factor was evapotranspiration from the land surface, which is found to have acted as an important moisture source and was likely enhanced by antecedent rainfall that increased soil moisture over the northern Great Plains. Event attribution analysis suggests that human induced greenhouse gas increases may also have contributed by causing evapotranspiration rates to be higher than they would have been under pre-industrial conditions. Frozen and snow-covered soils at high elevations are likely to have played an important role in generating record streamflows. Results point to a doubling of surface runoff due to the frozen conditions, while 25 % of the modelled runoff originated from snowmelt. The estimated return time of the 3-day precipitation event exceeds 50 years over a large region, and an increase in the occurrence of similar extreme precipitation events is projected by the end of the 21st century. Event attribution analysis suggests that greenhouse gas increases may have increased 1-day and 3-day return levels of May-June precipitation with respect to pre-industrial climate conditions. However, no anthropogenic influence can be detected for 1-day and 3-day surface runoff, as increases in extreme precipitation in the present-day climate are offset by decreased snow cover and lower frozen water content in soils during the May-June transition months, compared to pre-industrial climate.

The ACRU agro-hydrological modeling system provided the framework, containing code to simulate all major hydrological processes, including actual evapotranspiration estimates, to simulate the impacts of climate change in the Cline River watershed, Alberta. Canada, under historical (1961-1990) and a range of future climate conditions (2010-2039, 2040-2069, and 2070-2099). Whilst uncertainties in the estimation of many hydrological variables were inevitable, verification analyses carried out for the historical baseline period resulted in good to very good simulations of a range of hydrological processes, including daily air temperature, snow water equivalent and streamflow. Five climate change scenarios were selected to cover the range of possible future climate conditions. In order to generate future climate time series, the 30-year baseline time series was perturbed according to predicted changes in air temperature and precipitation. Projected increases in air temperature and precipitation resulted in mean annual increases in potential and actual evapotranspiration, groundwater recharge, soil moisture, and streamflow in the Cline River watershed. Increases in both high and low flow magnitudes and frequencies, and large increases to winter and spring streamflow are predicted for all climate scenarios. Spring runoff and peak streamflow were simulated to occur up to 4 weeks earlier than in the 1961-1990 baseline period. Predicted changes were simulated to progressively increase into the future. A clear shift in the future hydrological regime is predicted, with significantly higher streamflow between October and June, and lower streamflow in July-September. (C) 2011 Elsevier B.V. All rights reserved.