Changes in the hydrological character of rainfall on the Canadian prairies

摘要

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