Many studies have examined trends in the amount and phase of precipitation on the Canadian prairies over the period of record but without considering the unusual hydrology and hydrography of the region. On the Canadian prairies, runoff is primarily due to spring snowmelt over frozen soils but can also be caused by intense rainfall from summer thunderstorms. The fraction of spring snowmelt forming runoff is strongly influenced by the rate of melt and the presence of ice layers near the surface in frozen soils or at the base of the snowpack, all of which can be influenced by rainfall in the spring and late fall. Precipitation intensities sufficient to cause runoff are generally due to small, intense convective storms, which are prevalent during the summer months.
Historical records of the fraction of monthly precipitation falling as rain, obtained from the Historical Adjusted Climate Database for Canada (HACDC), were found to display statistically significant increasing trends over the periods 1901–2000 and 1951–2000 at many locations on the Canadian prairies. The fraction of stations showing significant trends, and the importance of the trends, were strongly dependent on the month of the year.
Single-day summer rainfalls are believed to be primarily convective in the Canadian prairies. Historical records obtained from HACDC indicate that the hydrological importance of single-day summer rainfalls has not increased and has shown significant decreases at many locations over the periods 1901–2000 and 1951–2000. Conversely, the hydrological importance of summer multiple-day rain events has not decreased and has significantly increased at many locations over the periods analysed.
Multiscaling analyses of summer rainfall events demonstrated that the temporal uniformity of rainfall on the Canadian prairies has increased over the periods 1901–2000 and 1951–2000. Analyses of the ratios of rainfall over multiple days demonstrate significant trends over the same periods, confirming the general tendency to temporal uniformity over scales between 1 and 32 days. Longer rain events strongly suggest greater spatial extents for storms and therefore the potential for increasing tendencies to promote basin-scale rainfall–runoff events such as seen in 2011 in the region. Copyright © 2012 John Wiley & Sons, Ltd.