Reconstruction and analysis of extreme drought and flood events in the Hanjiang River basin since 1426

Abstract. The major droughts and floods in the Hanjiang River basin have a significant impact on the flood prevention and control in the middle reaches of the Yangtze River and water resources management in the areas of the South-North Water Diversion Middle Line Project of China. However, there is a lack of understanding of the multi-decadal to century-scale patterns of droughts and floods in the Hanjiang River Basin. Applying the yearly drought and flood grades reconstructed based on historical documents, and the criteria developed for identifying extreme droughts and floods in historical periods, this paper constructs a time series of extreme droughts and floods (i.e., the event with occurrence probability less than 10 % from 1951–2017) in the Hanjiang River basin from 1426–2017. The possible linkages of the extreme droughts and floods with Asian monsoon (i.e., East Asian monsoon and South Asian monsoon), strong ENSO (i.e., El Nino and La Nina) and large volcanic eruptions are also discussed. The results show that there were 45 extreme droughts and 51 extreme floods in the Hanjiang River basin over the past 592 years. The frequency of extreme droughts was high during the 15th century, early 16th century, the 17th, and the 20th centuries, with the 20th century being the highest. For extreme floods, the frequency was high in the 16th century, the 17th century, the 19th century, and the 20th century, with the 19th to 20th centuries being the highest. The 18th century was a common low period of extreme droughts and floods, while the 20th century saw a high frequency of both. When the Asian monsoon is weak, extreme droughts were more likely to occur; and when the Asian monsoon is strong, extreme floods were more likely to occur. Furthermore, on multi-decadal scale, extreme floods were found to become more frequent with the increase in numbers of strong El Nino events and large volcanic eruptions. These results are informative for the study of mechanisms and predictability of decadal to century scale variability of extreme hydro-climatic events in the Hanjiang River basin.