| Research on the hydrological cycle under changing environment is one of the key scientific questions addressed by the Global Water System Project (GWSP) and one of the current hot issues in the hydrology science as well. Flood is one of the most frequent disaster in the word and it's also quite often in China. With the strong impact of climate change and human activities on the process and factors of the hydrological cycle, frequent hydrological cycle anomalies (HCA) resulted in flood disasters caused by the extreme precipitation event. Therefore, studies on the impact of HCA on flood disaster under the changing environment is of great scientific importance and a fundamental science issue.Located in the climatically south-north transitional zone, Lixiahe plain river region situates in the Lower Reaches of the Yangtze-Huai river basin which is greatly controlled by the monsoon circulation system, the direct influencing factor of the flood is the extreme precipitation anomalies in this area. Lixiahe region had undergone serious floods recently wit a total flood economic losses of 8.1 billion Chinese Yuan in 2003. Thus, the understanding of how the flood responds to the HCA plays a theoretical and practical critic role in this region.It takes the inner Lixiahe region, climatically south-north transitional zone, as the study area to explore the impact of the HCA on the flood systematically utilizing GIS technique, time series analysis, ranking classification, wavelet detection and the atmospheric circulation reanalysis. Focused on the key scientific questions of the process and influencing factor of the HCA and the flood chain constructing, from four inter-related aspects, which are trend detection, response process, impact factors and driving mechanism, we try to discus how the precipitation changes on a multiple temporal scale, how the flood water level responses to the precipitation anomalies, what the atmospheric circulation configuration is for HCA and what flood chain is under the impact of the HCA. Thereby, we cover our research on the main four parts which are as follows:(1) Precipitation variation characteristics on a multiple temporal scale. The annual rainfall, rainfall in flooding season, rainfall of different days maximum and the Plum rain show an insignificant decreasing trend in the inner Lixiahe region in the recent 50 years, and serious flood occurred in the year of plum rain anomalies, this is the main background of the HCA in the study area. The annual rainfall, rainfall in flooding season had undergone a higher amplitude fluctuation and the 1970s witnessed a significant lower precipitation on decadal scale, and a 3 year and a 6-8 year period are detected in both annual rainfall, rainfall in flooding season rainfall series. Abrupt changes are detected in 1966,2003,2005 in annual rainfall and the late 1960s, early 1970s and 1980 in the rainfall in flooding season respectively.Both annual precipitation concentration period and the concentration degree show a slight decreasing trend in recent 50 years. However, due to precipitation concentration degree anomalies, serious flood occurred in 1991 though decadal concentration degree is relatively low in 1990s which implied a HCA event. Precipitation days of higher grade in flooding season decreases slightly, this implies that the precipitation in flooding season could be more concentrated and the middle rank days of precipitation contributes more to the flood precipitation. Flood ranking classification based on the standardized precipitation index in flooding season can be closer to the actual flood in the study area. Typical flooding years in the inner Lixiahe region could be representative of two flood types, the plum rain caused flood (1980,1991,2003) and flood caused by plum rain plus typhoon precipitation (1962,1965).(2) Response of flood to the precipitation change. Annual daily mean water level maximum shows a nice response to the precipitation change, the year of 1962,1965 and 1980 demonstrated to be an effect of continuous long time of relatively high water level, while the flooding year 1991, and 2003 are the effect of the significant higher water level. High grade water level days have a relatively decreasing trend while middle and lower grade water level days increase, this suggests a fact that the water level tends to be higher in the typical years. However days of warning water level increase gradually from 1970s to present, it could be a composite precipitation anomalies and human activities induced HCA effect.Both annual daily mean water level, days of warning water level show a positive correlation with annual and flood season rainfall, but the high grade water level days has a negative correlation with rainfall in flood season, and the precipitation is more concentrated, the less days of high grade water level are, and the water level either. Plum rain and the water level has a positive relationship, but it varies in the south and the north, it is more significant in the south and less correlated in the north, and the Xinghua, Qintong, Anfeng and Yancheng, Sheyangzhen, Jianhu hydrological stations are the corresponding stations respectively.Rainfall of relative small return period at 2-5 years can result in water level over 2.0 m warning level which shows a flood effect of short-period high rainfall intensity, it is the direct flood response to precipitation variation. Similarly, rainfall of large return period and long-period precipitation can lead to much higher water level, reflecting an accumulative effect of continuous precipitation in the study area. Thirty-day precipitation totals maximum constitutes the main water source for the annual daily mean water level maximum, however, three-day, seven-day and fifteen-day precipitation totals maximum are more useful in flood practice.(3) Identification of the specific atmospheric circulation configuration driven by monsoon system. Monsoon system is the dominating factor for the precipitation anomalies in the study area, more positive precipitation anomalies can appear in weak monsoon year while relatively less positive precipitation in strong monsoon year, but the typical flooding years are mostly constituted by strong precipitation in weak monsoon year. At period of about 3 yr and 6 yr, nice in-phase and anti-phase behavior can be found between East Asia summer monsoon and precipitation in summer. As for the main atmospheric circulation factors, the intensity and area of west Pacific subtropical high are positive to the rainfall in summer while the west Pacific subtropical high ridge line position and western point had a negative relationship with rainfall in summer. Asia zonal circulation and Asia meridian circulation had negative and positive correlation with summer precipitation respectively, but the Asia zonal circulation is more significant in correlation to the summer precipitation. The Asia polar vortex intensity index is relatively lower in the corresponding weak monsoon year which is helpful to more precipitation in the inner Lixiahe region but that is higher in strong monsoon years. And results of the teleconnections between ENSO and summer precipitation show that positive precipitation anomalies years following the ENSO event are consist of strong precipitation in weak and strong monsoon year, which also confirmed that the monsoon is the most dominant driving forces for the summer precipitation in the study area.Based on the analysis of different year groups positive precipitation anomalies, typical HCA related atmospheric circulation configuration is in tight connected with monsoon system, west Pacific subtropical high, ENSO and etc, the feature atmospheric circulation configuration can be concluded into two types, one is that when the monsoon system is weaker and the west Pacific subtropical high position lies more to the west and the north but the main part is in the south, the east Asia meridian circulation appears to be a "+,-,+" geopotential height anomaly, southwest wind prevails in the middle and lower troposphere and there is an obvious plum rain frontal surface in the inner Lixiahe region which is beneficial for the staying of the precipitation belt, this specific atmospheric circulation configuration tends to be more precipitation; and the other is that when the monsoon system is stronger and the west Pacific subtropical high position tends to shrink back to the east and the north, the east Asia meridian circulation appears to be a "-,-" geopotential height anomaly, southwest wind prevail in the lower troposphere and southeast wind and east wind in the middle and high troposphere respectively, there is no obvious plum rain frontal surface, this specific atmospheric circulation configuration tends to be relatively less positive precipitation anomalies in the study area. These two types of atmospheric circulation configuration nicely reveal the background of the general atmospheric circulation that is directly connected to the HCA induced flood chain, which can be identified as a "monsoon-hydrological cycle anomalies-precipitation anomalies"(4) Synthetical analysis for the flood mechanism. For the natural causes, rainstorm anomalies can be regard as the direct flood factors, and the pot shaped geomorphology constructs the main flood breeding environment while the complicated lower plain river network intensified the flood disaster. And more, irrational anthropogenic activity is another part for the flooding mechanism, accumulation of mud caused by dam construction resulted in the lower capacity of river channel, the building of dyke and embankment greatly decreases the lake area which evidently weakens the capacity of the lake system made the continuance of high flooding water level, and the increase of runoff coefficient caused by hydrological effect of urbanization such as the increase of imperviousness with the sprawl of urban also leads to great flood in the inner Lixiahe region. Therefore, HCA induced flood mechanism in the inner Lixiahe region can be integrated from two aspects, natural cause and anthropogenic factor. The driving mechanism for the flood disaster can be generalized as "monsoon-hydrological cycle anomalies-precipitation anomalies-flood disaster", a coupled process of double impacts of the atmospheric and land surface hydrological cycle anomalies in the inner Lixiahe region.
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