Comparison of regional and at-site frequency analysis methods for the estimation of southern Alberta extreme rainfall

Abstract

At least one national meteorological organization has updated their rainfall intensity–duration–frequency (IDF) estimates using a regional frequency analysis approach. In this study, the regional frequency analysis L-moments approach was applied to data from 12 rain gauge sites in southern Alberta, for rainfall durations from 1 hour to 24 hours. Five candidate frequency distributions were tested, and the best-fit regional frequency distribution was used to estimate rainfall quantiles for return periods up to 1:1000 years. The quantiles were compared to the quantiles from the conventional at-site Gumbel approach. The results showed that the conventional approach tends to underestimate the quantiles relative to the regional approach, mainly for the sub-daily rainfall durations. The accuracy of the two approaches was tested using Monte Carlo simulations, and the results showed that the root mean square error (RMSE) using the conventional approach is approximately 2–2.5 times greater than the error using the regional approach, for the 1:100-year return period. The full benefits of regional frequency analysis have not yet been realized in Canada, which could benefit from a national program to update rainfall IDF estimates using these more statistically robust approaches.