Relating Changes in Extreme Streamflow Events to Changes in Extreme Rainfall Events in Canada

Abstract

The changes in extreme events caused by climate change are likely to have significant impacts in the future. It is expected that climate change will cause changes in rainfall intensities and magnitudes and consequently extreme streamflow events. Incorporation of nonstationarity and improved understanding of the changes expected to occur from climate change will lead to improved management of our water systems. In this study, the effects of climate change in Canada are examined; specifically changes in extreme rainfall are related with corresponding changes in flood events.

The Mann-Kendall non-parametric trend test was used to identify trends in Intensity-Duration-Frequency (IDF) data for various rainfall durations at stations across Canada. Although most of the IDF sites found with a significant trend were showing signs of increasing rainfall amounts, more streamflow sites were found to have a decreasing trend for both trends in the annual maximum and peaks over threshold streamflow event series. No increasing streamflow trends were found where a decreasing IDF trend was identified, however a decreasing streamflow trend was identified for increasing IDF trends of all duration lengths. Trends in the peaks over threshold streamflow series were similar to the annual maximum streamflow with a higher level of agreement obtained. Reviewing the seasonality of the sites supports that many of the streamflow sites that are decreasing in trend are from a nival snowmelt regime, and may be changing towards a mixed regime, making rainfall driven events more important. The number of peak streamflow events per year also was generally increasing across the country.

The majority of decreasing streamflow trends were a result of decrease in the spring freshet. After removing the snowmelt period, rainfall driven streamflow events were evaluated for trend and found to be mainly increasing. While the overall agreement in trend direction between the extreme rainfall and streamflow events increased, disagreement still occurred in the East Coast sites and much of southern Ontario, where unlike the rest of the country, a cluster of IDF stations experienced decreasing trends in extreme rainfall.