Water Stewardship Information Sources
| Citation | Murdock, TQ, Bennett, KE and Werner, AT. 2007. GVRD historical and future rainfall analysis update. Pacific Climate Impacts Consortium. Report prepared for Metro Vancouver. August 2007. |
|---|---|
| Organization | Metro Vancouver |
| URL | http://www.metrovancouver.org/services/liquid-waste/LiquidWastePublications/HistoricalRainfallAnalysisUpdate.pdf |
| Abstract/Description or Keywords | The final report of a 2002 study (KWL-02) on rainfall extremes prepared by Kerr Wood Liedal Associates Ltd. and published by the Greater Vancouver Regional District (GVRD) recommended conducting updates to the report every five years. Since the time of the original report, the following have become available: • additional GVRD historical rainfall data (mainly 2002-2005 observations) • additional data and analysis of the state of the Pacific Decadal Oscillation (PDO) • regionally-averaged future climate model projections Furthermore, recent extreme weather events of the last five years provide added motivation to improve understanding of extreme rainfall in the GVRD. Making use of the new information, Pacific Climate Impacts Consortium staff repeated select analysis from the original report. In some cases, slight modifications to the methodology were made although the scope of the work remained an update to the original report. In addition to presenting the results of the updated analysis, this report also includes a large amount of technical information regarding the analysis performed to facilitate comparison of methods for future updates. However, this document neither replaces the original report nor can it serve as a complete analysis of GVRD rainfall. The update considers extreme rainfall recorded at twelve precipitation gauges located throughout the GVRD for two measures: rate of rainfall (intensity) and how often large events occur (threshold exceedance). Firm conclusions about trends are not possible because records are too short and stations are located in disparate locations. In addition, there are large data gaps that may skew some results. A cautious consideration of uncertainty also guides interpretation of the results in sections that address effect on GVRD rainfall of climate variability and climate change. Despite the limitations, results provide an update for planning that depends on GVRD rainfall variability and change as well as a discussion of potential opportunities for future work. An overview of findings is as follows: Recent trends and patterns In general, patterns of increased rainfall, as noted in KWL-02, are still occurring and many have become more accentuated since 2001. Statistically significant trends were found at some stations, particularly in April to June, and to a lesser extent October to January. At the station where the most significant trends (DN25 North Vancouver) were found, the largest trends were those with shortest durations. Climate variability The potential relationship between known aspects of climate variability and GVRD rainfall was investigated. Larger rainfall events appeared to occur during the positive phase of the PDO. However, separating this from the underlying long-term trend was beyond the scope of this analysis. Climate change Future precipitation projections derived from 29 experiments with 7 global climate models show a considerable range in projections for the GVRD region. A large range indicates considerable uncertainty. However, models consistently project a 14% to 33% decrease in summer precipitation by the middle of the century. In addition, most models project small increases in precipitation during all other seasons, particularly in winter (4% to 14% increase by the middle of the century). The projected net annual increase in precipitation of 1% to 5% by the middle of the century is too small (and uncertain) to be used for planning purposes. The small range indicates that neither very large decreases nor increases in the annual average are to be expected. However, important seasonal changes are projected despite the small annual average change: large winter precipitation increases (4% to 14%) and summer decreases (-33% to -24%) by the middle of the century. Furthermore, consistent projections for warming in the region (2.1°C to 2.6°C) will cause increased winter and spring rainfall due to a larger fraction of total precipitation falling as rain than snow. An overview of recommendations for future updates and further analysis is as follows: Data quality and local station limitations • continue using recently digitized data • fill missing values • use rehabilitated data • analyse regionally-averaged results beyond the GVRD to allow for identification of more climatologically robust regional trends (and to include a larger number of stations) Methods: beyond rainfall intensity and exceedance • assess vulnerability to rainfall changes, climate change and extreme events • conduct literature review prior to routine updating to ensure the study of most useful parameters, use of most appropriate research methods, and to understand the limitations • investigate changes to synoptic weather types • include snow and temperature trends and projections in addition to rainfall • event-based analysis as a foundation for examining rainfall and streamflow record • hydrologic modelling • apply trend analysis to seasonal results rather than grouping monthly trends by season • use consecutive water years rather than calendar years and a quantitative comparison of the wet and dry seasons • select exceedance thresholds using percentiles |
| Information Type | report |
| Regional Watershed | Lower Fraser |
| Sub-watershed if known | Metro Vancouver |
| Aquifer # | |
| Comments | |
| Project status | |
| Contact Name | Trevor Murdock |
| Contact Email | [email protected] |