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

The major droughts and floods in the Hanjiang River basin have a significant impact on the flood prevention and 15 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 Applying the yearly drought and flood grades reconstructed based on historical documents, and the criteria developed for identifying extreme droughts and floods in historical periods, this paper constructs a time series of extreme droughts and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% 20 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 Niño and La Niña) 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 25 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 Niño events and large volcanic eruptions. These 30 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.

precipitation. The annual precipitation increases from northwest to southeast and mainly concentrates in summer and autumn, 140 with July to September accounting for about 70% of the annual total precipitation. Droughts and floods are both severe in the HRB, mainly because the river's upper reaches are mountainous, with narrow, deep, meandering channels and fast currents.
Flash floods and prolonged droughts can cause severe impacts. In the middle and lower reaches of the Hanjiang River, the slow currents flow due to the low slope of the Jianghan Plain leads to unstable channels and inadequate discharge capacity, resulting in frequent floods. Since the 1990s, the HRB has been experiencing continuous drought, severely impact the 145 ecological environment, the rational allocation of water resources, and water supply in the basin. (1) Historical drought and flood sequences and documentation The information used in this study to reconstruct the drought and flood sequences was divided into two parts: historical data and instrumental data. Sources of historical data included local chronicles, Qing Dynasty archives (memorials, Shangyu) 160 etc. Primary historical data source was "A Compendium of Chinese Meteorological Records of the Last 3000 Years" (Zhang, 2004). This collection of materials systematically compiled various kinds of written records on weather and climate in China for more than 3000 years from the 13th century AD to 1911 AD. In addition, other datasets were also collected and used, including "The historical Documents on Flood and Waterlogging of Southwest International Rivers in the Yangtze River https://doi.org/10.5194/cp-2021-43 Preprint. Discussion started: 5 May 2021 c Author(s) 2021. CC BY 4.0 License. Basin in Qing Dynasty " (Yang and Guo, 1991), "The Disaster Annals in Modern China" and "The Continuation of Disaster 165 Annals in Modern China (1919( -1949( ) "(Li et al., 19901993), "Zaixu Xingshuijinjain-Yangtze River Volume" (Wu and Zhao, 2004), and "Compilation of the Memorials to the Throne in Qing Dynasty: Agriculture·Environment " (Ge, 2005). The data were mainly derived from official documents, notes, letters, local chronicles, inscriptions, newspapers, magazines and river worker file transcripts of the Qing dynasty. Moreover, drought and flood records from 1911 to 1949 AD in "The China Meteorological Disaster Dictionary" (Shaanxi Volume, 2005;Henan Volume, 2005;Hubei Volume, 2007) were also 170 collected and used as supplementary data.
In terms of the overall distribution of information, the records of drought/flood in the local chronicles are more continuous and complete. They can also reflect information on the extent of disasters and disaster relief in each prefecture and county, effectively showing disaster's spatial distribution and temporal change. The archival information is of the highest credibility (Ge and Zhang, 1990). It provides a primary basin-wide picture of droughts/floods with a clearer spatial and temporal 175 resolution accurate to the county level.
The instrumental data comes from the monthly precipitation dataset "China National Ground Meteorological Station Homogenized Precipitation Data Set (V1.0)". This set of precipitation observations were quality controlled and tested, and adjusted for inhomogeitieies caused by non-climatic factors such as the relocation of stations and instrumentation. This study uses precipitation data from 8 sites (Hanzhong, Ankang, Yunxi, Nanyang, Xiangyang, Zhongxiang, Qianjiang and Wuhan) in 180 and around the HRB (Figure 1).
(2) Other historical data series Asian monsoon index. The East Asian summer monsoon index dataset  reconstructed by the δ 18 O content of Vientiane Cave by Zhang et al. (2008) and the South Asian summer monsoon index dataset (1426-2000) reconstructed using 185 Indian tree ring data by Shi et al. (2017) were used in this study. ENSO Sequence Chronology (1525-2002. The El Niño and La Niña events in the historical ENSO chronology reconstructed by Gergis and Fowler (2009) from tree-ring, ice-core, coral records and historical documents were used. These

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(1) Reconstruction method of historical drought and flood Based on the criteria of "The Atlas of Drought and Flood Distribution in China in the Last 500 Years" (Central Meteorological Bureau, 1981), single-station and regional drought/flood series were established using the grading method.

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The degree of drought/flood or precipitation was divided into five grades: Grade 1-Flood, Grade 2-Mild Flood, Grade 3-Normal, Grade 4-Mild Drought, Grade 5-Drought. The drought and flood grades at each site indicated the degree of regional precipitation anomalies within a specific range represented by that site. Historical drought/flood grades were mainly based on historical records. In assessing the drought/flood grades for a region based on several drought/flood records for a given year, the primary considerations were the precipitation conditions in spring, summer and autumn, as well as the timing, be based on most counties' situation. If a site has a gap in records of less than 3 years, it is considered to have no drought or flood and was graded 3; a site with a gap in records of more than 3 years was not graded.
In order to take into account the frequency of occurrence of each grade, the ideal frequency criteria of 10% (Grade 1-Flood; Grade 5-Drought), 20%-30% (Grade 2 Mild-Flood; Grade 4-Mild-Drought), 30%-40% (Grade 3-Normal) (Central 215 Meteorological Bureau, 1981) were used to adjust the classification of drought and flood grades in the HRB throughout the study period. When precipitation records were available, the May-September precipitation for the area where the site is located was used to be consistent with the frequency of drought and flood grades obtained from historical data. Table 1 shows the division criteria into various grades and their typical descriptions in historical sources and the criteria for grading precipitation.  at least 75% of the sites have droughts or floods at the same time, meanwhile, at least 2 of these adjacent sites were experiencing either severe drought (i.e. grade 5 drought) or severe flood (i.e. grade 1 flood) at the same time. If the above two conditions were met, the year could be identified as a year of extreme drought or flood in the basin.

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(2)Chi-square test ( 2 ) The chi-square test is a way of inferring whether the overall distribution is significantly different from the expected distribution or a particular theoretical distribution, based on the current state of the sample distribution. In this study, the correlation between extreme droughts/floods and the occurrence of strong ENSO and large volcanic eruptions was calculated and detected using a chi-square test ( 2 ). Northwestern China in 1928, with annual precipitation comparable to that during the Ming Chongzhen drought (Tan, 2003).

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(4) The 20th century was a common high period of extreme drought/flood events, with 14 extreme droughts and 14 extreme floods occurred, and approximately 1 extreme drought or flood event average 3-4 years. Tree-ring reconstruction studies (Zhang et al., 2005;Liang et al., 2003) indicated that the monsoon precipitation variability in northern China over the last 100 years has been high, with significant wet and dry changes and widespread extreme drought events. Ding et al. (2006)

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The HRB is a transitional zone between northern and southern China and between the subtropical and warm temperate zones, so it is susceptible to monsoon strength changes. In addition, the Ba shan Mountains to the south and the Qinling Mountains to the north of the Hanjiang River have a common blocking effect on airflow, allowing the front of subtropical high pressure to remain in the region for a more extended period, amplifying the effect of the monsoon on precipitation.
Therefore, when ENSO events occur, the HRB is vulnerable to both drought and flood conditions. However, due to the lower 435 Hanjiang River's southerly latitude, the response of the EI Niño to precipitation in the lower Hanjiang River may be different from that in the upper. This may be why the extreme drought/flood in the whole HRB have no obvious correlation with the EI Niño. Volcanic eruptions can reduce the global average temperature (Li et al., 1994). Meanwhile, volcanic ash and sulfur dioxide gas produced by volcanic eruptions increase the hygroscopic condensation nuclei in the atmosphere, which has a 455 catalytic effect on the occurrence and intensification of precipitation (Mann et al., 1998;Liu et al., 2016). Moreover, the aerosols formed during the eruption affect solar radiation, indirectly changing atmospheric circulation, thereby causing precipitation and distribution changes. Zhang and Zhang (1994) ( Zhou, 1981;Xu, 1986).