Regional Frequency Analysis Estimates of Extreme Rainfall Events under Climate Change

Abstract

Extreme rainfall events have a long history of causing large economic damages in urban areas and even loss of human life. Reliable estimates of extreme rainfall intensities for different rainfall durations are essential for the effective planning of drainage systems under climate change to balance the construction costs and potential damages caused by future extreme rainfall events. The information required for design rainfall events can be obtained through frequency analysis of extreme rainfall. However, extreme rainfall quantiles obtained from the traditional approach of frequency analysis have become increasingly unreliable under climate change. With increasing global temperatures and the uneven distribution of atmosphere moisture, the frequency and magnitude of extreme rainfall events can experience accelerated changes. Thus, urban drainage systems designed based on extreme rainfall quantiles obtained from historical records are becoming increasingly ineffective. Under the impacts of climate change, extreme rainfall events are becoming one of the most destructive natural hazards in the world. Frequency analysis of the extreme rainfall events used to estimate the probability of exceedance of extreme rainfall events of a given magnitude in the future context can generate unreliable estimates under climate change because of two issues. Firstly, there are often insufficient data records available for the quantification of extreme rainfall events of interest from a design perspective. Since extreme rainfall events are rare, there is large uncertainty in quantile estimates obtained from using only the information from the site of interest. Thus, regional frequency analysis, which expands the data records through gathering information from sites sharing similar rainfall patterns, is widely used and is applied in this research. Secondly, the traditional assumption that there is a repetitive pattern in the occurrences of extreme rainfall events has become invalid in a nonstationary environment. Since extreme rainfall patterns can be altered in the future, estimates for rainfall quantiles obtained from using frequency analysis in a historical stationary environment can be unreliable when applied for future conditions. Further research is required into applying the regional frequency analysis approach for the estimation of extreme rainfall quantiles under climate change. To provide reliable regional estimates of rainfall quantiles for different rainfall durations under climate change, this research improves regional frequency analysis through exploring the following issues: 1) An improved procedure for homogeneous group formation for historical stationary periods. Extreme rainfall events have been affected by climate change. A three-layer searching algorithm is proposed for homogeneous group formation in a stationary environment for the consideration of climate change impacts on the spatial distribution of extreme rainfall events. 2) An adjustment procedure for homogeneous group formation in the future stationary environment. Under the assumption that extreme rainfall patterns remain stationary within a 30-year period, a procedure is proposed to adjust the optimal homogeneous group formation from the previous temporal periods to reflect conditions in future 30-year periods. 3) A procedure used for rainfall quantile estimation in a future nonstationary environment. Under the assumption that the extreme rainfall series exhibit nonstationary behavior during the whole future period, a one-step forward procedure is constructed based on the unscented Kalman filter to consider the potential non-monotonic change behavior of extreme rainfall events at different return periods. In this approach, the homogeneous groups are formed using a trend centered pooling approach. The proposed methodology fills the gaps of considering climate change impacts on homogeneous group formation in both historical and future stationary environments and challenges the assumption of monotonic change behavior of extreme rainfall quantiles used in the traditional regional frequency analysis for stations exhibiting nonstationary behavior. The proposed procedures have been extensively tested using large sets of climate data in both historical and future contexts and have been shown to improve the extreme rainfall quantile estimates in both historical and future contexts.