Onset of flood season in the middle reach of Yellow River,Qinhe River and Yongding River and summer monsoon strength in the Qing dynasty

The time series of flood season onset date anomalies are established on a pentad scale (or a 5-day scale) in the Qing dynasty. A water level rise of 0.64 m marks the beginning of a flood season. The historical water level records of the Qing Government are obtained for Wanjingtan near the Sanmenxia Gorge in the middle reach of the Yellow River, and Muluandian in the lower reach of Qinhe River, as well as Shijingshan and Lugouqiao Bridge in the lower reaches of Yongding River. The records start respectively in 1766, 1736 and 1761 for the Yellow River, the Qinhe River and the Yongding River. During the study periods, the flood season usually started between July 6 and July 10 in the middle reach of the Yellow River and the lower reach of the Qinhe River; whereas in the Yongding River, the season normally started between July 16 and July 20. The starting date is found to be more stable in the Yongding River comparing to the other two rivers. This is due to the fact that the rainy season is very much regular in early July in the upstream region of the Yongding River. However, the rainy season showed a great deal of fluctuation for up to three months in the middle reach of the Yellow River and the lower reach Qinhe River. The synchronous phase fluctuation in flood season onset date anomalies was more significant in the middle reach of the Yellow River and the lower reach Qinhe River after the mid-19th century. The flood season was generally postponed by 5 days in all three rivers during the period 1870-1990, which corresponded well with relatively cool summer temperatures observed on the Loess Plateau. The opposite was true in the middle reach of the Yellow River and in the lower reach of Qinhe River, which also corresponded well with relatively high summer temperatures observed on the Plateau.