Estimates of changes in design rainfall values for Canada Donald H. Burn * and Amir Taleghani Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, CANADA Abstract: Annual maximum rainfall data from 51 stations in Canada were analyzed for trends and changes by using the Mann–Kendall trend test and a bootstrap resampling approach, respectively. Rainfall data were analyzed for nine durations ranging from 5 min to 24 h. The data analyzed are typically used in the development of intensity-duration-frequency (IDF) curves, which are used for estimating design rainfall values that form an input for the design of critical water infrastructure. The results reveal more increasing than decreasing trends and changes in the data with more increasing changes and larger changes, noted for the longer rainfall durations. The results also indicate that a traditional trend test may not be sufficient when the interest is in identifying changes in design rainfall quantiles. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS intensity-duration-frequency curves; climate change; rainfall quantiles; Mann–Kendall test; bootstrap resampling approach; Canada Received 26 July 2011; Accepted 2 February 2012 INTRODUCTION Design rainfall events are used in the sizing of critical water infrastructure and are generally derived from rainfall intensity-duration-frequency (IDF) curves. An IDF curve for a location can be estimated from the observed (historical) rainfall record. Climate change impacts are expected to include changes in precipitation patterns, intensity and extremes, and can thus be expected to result in changes in IDF curves, hence, changes in design rainfall events. Examination of observed rainfall records and the results from global climate modelling suggest that increases in extreme rainfall will be larger than increases in mean rainfall (Meehl et al., 2007; Bates et al., 2008; Mailhot et al., 2011). Min et al. (2011) show, through modelling, that the observed intensification of extreme precipitation can be attributed to human-induced increases in greenhouse gases. More frequent heavy rainfall events are expected to increase the risk of flash floods and urban flooding (Bates et al., 2008; Willems et al., 2009; Mailhot et al., 2010). Changes in the meteorological variables that drive the hydrological cycle, such as precipitation, can be expected to affect the spatial and temporal distribution of water, which can affect the capability of the impacted population to cope with natural hazards related to water excess or shortage. Markus et al. (2007) examined changes in the design precipitation in northeastern Illinois, which resulted from the analysis of different periods recorded and the selection of different methods for estimating the design values. Their work emphasized the importance of estimating design events using the most recent data available. Madsen et al. (2009) report on the updating of regional IDF curves for Denmark by using additional data available for 1997 to 2005. For durations of 30 min to 3 h and return periods of around 10 years, with characteristics typical of many urban drainage designs, the increase in design rainfall intensity was of the order of 10%. Villarini et al. (2011) examined the frequency of heavy rainfall events for the midwest portion of the United States. They found evidence for an increase in annual maximum daily rainfall but less evidence for increases in extreme events on the basis of results from quantile regression. Douglas and Fairbank (2011) examined extreme precipitation in New England and identified a need to update design storms to reflect the most recent conditions. In Canada, Groisman et al. (1999) found an increase in the probability of heavy precipitation but no increase in the frequency of the occurrence of precipitation. Stone et al. (2000) examined trends in precipitation intensity in Canada and found increases in heavy precipitation events, especially in the Canadian north. Adamowski and Bougadis (2003) applied trend analysis to extreme rainfall events of different durations for 44 stations from the province of Ontario, for a 20-year period. They found both decreasing and increasing significant trends, assessed on a regional basis, for short duration rainfall events. Jakob et al. (2003) used linear regression analysis to detect trends in rainfall intensity data from 11 rain gauge stations in the Greater Vancouver, British Columbia area. There was no systematic pattern of change in rainfall intensity for the study area; strongest trends were found for the shorter durations. Watt et al. (2003) examined the potential impacts of climate change in the context of urban stormwater infrastructure and recom- mend increasing design storm magnitudes by 15% until better estimates of the potential impacts are available. Coulibaly and Shi (2005) investigated the trend character- istics for annual maximum daily precipitation data for eight *Correspondence to: Donald Burn, Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, CANADA. Email: dhburn@civmail.uwaterloo.ca HYDROLOGICAL PROCESSES Hydrol. Process. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9238 Copyright © 2012 John Wiley & Sons, Ltd.