Regional precipitation-frequency analysis and spatial mapping for 24-hour and 2-hour durations for Washington State

This preamble explains why a paper on precipitation probabilities at the 2-hour and 24-hour time periods for the State of Washington, USA, was solicited for a special symposium honouring Dr. McCulloch's contributions to hydrology. Indeed, the specific subject of the paper has no particular connection with Dr. McCulloch's technical specialties or the thrust of IH work, but it does have much to do with Dr Me Culloch's prowess and vision for running a research institute. In 1984, I was invited to spend a Sabbatical year at IH to work on the then new technique of Regional Probability Weighted Moments (PWM) in connection with Generalised Extreme Value (GEV) distribution. The IH Flood Studies Report (FSR) had used the GEV distribution with a graphical method involving rather arbitrary and subjective steps in its fitting procedure for determining the regional distributions to use in different parts of the UK. Because physically impossibly-large flood probabilities had been produced by the FSR in connection with some Scottish dams, there was a controversy with large economic and social implications. I, working together with Jon Hosking, a young, very bright mathematical statistician employed by IH, addressed the problem and devised a PWM solution for the GEV so that the FSR flood probability estimation difficulty was resolved which was invaluable to IH at the time. After much more research in the UK and the USA, a book Regional Frequency Analysis; An Approach Based on L-Moments by J.R.M. Husking and J.R. Wallis was published by Cambridge University Press. It is worth noting that the Regional L-moment algorithm is numerically equivalent to the Regional PWM algorithm, but the Regional L-moment approach is much more complete and powerful; and has appeared in a myriad of other investigations published in hydrological and meteorological journals worldwide, as well as in other studies reported at this General Assembly. Presumably Regional L-moments would have been discovered eventually, but their prompt appearance can be largely attributed to Dr. McCulloch's stewardship of IH, and in particular on his insistence on the value of inviting outside scientists to IH.