Detecting Flood‐Rich and Flood‐Poor Periods in Annual Peak Discharges Across Europe.

This paper proposes a method from Scan statistics for identifying flood‐rich and flood‐poor periods (i.e., anomalies) in flood discharge records. Exceedances of quantiles with 2‐, 5‐, and 10‐year return periods are used to identify periods with unusually many (or few) threshold exceedances with respect to the reference condition of independent and identically distributed random variables. For the case of flood‐rich periods, multiple window lengths are used in the identification process. The method is applied to 2,201 annual flood peak series in Europe between 1960 and 2010. Results indicate evidence for the existence of flood‐rich and flood‐poor periods, as about 2 to 3 times more anomalies are detected than what would be expected by chance. The frequency of the anomalies tends to decrease with an increasing threshold return period which is consistent with previous studies, but this may be partly related to the method and the record length of about 50 years. In the northwest of Europe, the frequency of stations with flood‐rich periods tends to increase over time and the frequency of stations with flood‐poor periods tends to decrease. In the east and south of Europe, the opposite is the case. There appears to exist a turning point around 1970 when the frequencies of anomalies start to change most clearly. This turning point occurs at the same time as a turning point of the North Atlantic Oscillation index. The method is also suitable for peak‐over‐threshold series and can be generalized to higher dimensions, such as space and space‐time. Plain Language Summary: Flood studies usually assume that the statistical characteristics of flood discharges do not change over time. Here we propose a method for identifying changes in these characteristics. Specifically, we identify periods that exhibit unusually more floods above a threshold and periods with unusually more floods below a threshold. The method goes beyond trend analysis by providing more temporal detail on flood changes. We apply the method to 2,201 observed flood series in Europe between 1960 and 2010. We find that flood‐rich and flood‐poor periods occur in the data, as the number of periods is about 2 to 3 times larger than would be expected by chance. In the northwest of Europe, the number of flood‐rich periods tends to increase over time, while in the east and south of Europe, the opposite is the case. There appears to exist a turning point around 1970 when the frequency of unusual periods starts to change most clearly. This turning point occurs at the same time as a turning point of the North Atlantic Oscillation index suggesting a role in climate fluctuations in the frequency of flood‐rich periods. Key Points: A method from Scan statistics is proposed for identifying flood‐rich and flood‐poor periods in flood discharge recordsThere is evidence of flood‐rich and flood‐poor periods based on analyzing 2,201 flood series in Europe between 1960 and 2010In the northwest of Europe, the frequency of flood‐rich periods tends to increase over time, particularly for large return periods [ABSTRACT FROM AUTHOR]