The fluctuation of the beginning time of flood season in North China during AD1766–1911

Abstract

This study is based on the Qing government water level gauge records from Wanjingtan of the Sanmenxia Gorge in the middle reaches of the Yellow River, Muluandian in the lower reaches of Qinhe River, and Shijingshan and Lugou Bridge in the lower reaches of Yongding River. The beginning of the flood season has been reconstructed. The anomaly series of the flood season is assessed using pentad scales. The sequence represents the middle reaches of the Yellow River, the lower reaches of the Qinhe River, and the lower reaches of Yongding River. Within the research period, the middle reaches of the Yellow River and the lower reaches of the Qinhe River began the flood season on 6–10 July on average, and Yongding River began 16–20 July. The Yongding River is relatively stable and the start of rainy season of its upstream was basically maintained in early July. The middle reaches of the Yellow River and the lower reaches of the Qinhe River show great fluctuation, with discrepancies up to 3 months. The same anomalous fluctuation in the flood season of the middle reaches of the Yellow River and the Qinhe River is more obvious after the mid-19th century. The delay of flood season by about one pentad in the middle reaches of the Yellow River, Qinhe River, and Yongding River is in accordance with the relative low temperature of the Loess plateau in summer, while the early flood season of the middle reaches of the Yellow River and Qinhe River corresponds with the relative high summer temperature in the Loess Plateau. The beginning time of the flood season in the research area is the time the rainy season begins. The length of summer-monsoon rainy season during 1860s–1940s has been reconstructed, as the Little Ice Age transformed to the Modern Warm Period in China. Rapid warming could change the summer rainy season by 2–3 pentads.