| Abstract/Description or Keywords |
Changes in the magnitude and frequency of high intensity rainfall are investigated for the Greater Vancouver Regional District (GVRD). Data on precipitation intensity from five minutes to 24 hours duration for nine GVRD rain gauges and three Meteorological Service of Canada (MSC) rain gauges were gathered. Results show only two stations with a statistically significant upward trend for the five minutes to two hours durations for the annual time series. At one station (DN2S) this apparent trend was attributed to local site/exposure changes rather than a real change in local precipitation intensities. A month-by-month analysis for all analysed stations demonstrates statistically significant upward trends particularly for the months of April, May and June, which are also months of high convective activity but low overall rainfall. Strongest trends were found for the shorter durations (up to two hours). High-intensity rainfall threshold exceedances were computed for all stations and durations to investigate possible long-term changes in the frequency of high intensity rainfall. This analysis demonstrates an increasing trend in threshold exceedances particularly during the 1990s. A significant increase in threshold exceedances is noticeable between the last Pacific Decadal Oscillation (PDO) cool phase that ended in 1977, and the last warm phase that likely ended in 1998. Variability of rainfall that was correlated to PDO and ENSO (El Niño Southern Oscillation) indices appears greater than the signal attributed to the enhanced greenhouse effect. The results of this study are consistent with the findings from recent scientific literature suggesting that there is, as yet, little evidence for an increase in high intensity precipitation over southwestern British Columbia. These results suggest that there is no urgent need to upgrade the sewerage and drainage system. However, a continuation of the observed increase in threshold exceedances during the 1990s would imply an increase in flood flows resulting in more channel erosion and nuisance flooding which would have implications for managing stream health and flood management planning. Without considering changes in precipitation due to climate change, given the correlation between PDO and ENSO cycles and high rainfall intensity occurrence, then lower-than-normal rainfall intensities can be expected during the next PDO cool phase. |
