【作者】 叶正伟；

【导师】 许有鹏；

【作者基本信息】 南京大学 ， 自然地理学， 2011， 博士

【摘要】 “变化环境下的水循环研究”是全球水系统计划(GWSP)的核心科学问题,也是当前水文科学的热点问题之一。洪涝是全球最为频繁的自然灾害之一,我国洪涝亦呈高频态势。随着气候变化和人类活动对水循环过程与要素影响的加剧,偏离常态的水循环变异越来越频繁,往往导致极端降水洪涝事件。因而,在变化环境背景下,关于水循环变异对洪涝的影响研究是一个重要的基础科学问题。里下河平原水网地区位于江淮下游,地处我国南北气候过渡带区域,受季风环流影响显著,降水异常是该地区洪涝的直接致灾因子。近年来,里下河地区洪涝严重,2003年洪涝直接经济损失高达81亿元。因此,探讨该地区水循环变异对洪涝的影响,是一个在理论和实践上都具有重要意义的课题。本文以低海拔的里下河腹部平原水网地区为研究对象,综合应用GIS技术、时间序列检验、等级分析、小波分析和大气环流场分析等方法,围绕水循环的过程、要素变异及洪涝成灾机制链分析等关键问题,从检测分析、响应过程、影响因子、驱动机制四个层面,分析了降水的多时间尺度变化规律,探讨了洪涝的响应特征,阐明了水循环变异的大气环流配置形势,揭示了洪涝成灾机制。主要研究涉及以下四个方面：(1)降水的多时间尺度变化分析。研究区近50年的年、汛期、时段及梅雨等不同时间尺度的降水量皆呈减少趋势,但变化趋势不显著,这是水循环降水要素变异的宏观背景,且梅雨异常偏多年都出现了严重洪涝。年、汛期降水量的年际波动较大,1970年代显著偏少,且都存在3年、6-8年左右的变化周期。其中,年降水在1966、2003、2005年,汛期降水在1960年代中后期、1970年代初和1980年都出现了降水异常的突变过程。降水强度上,年降水集中期与集中度皆为减小趋势,1990年代降水集中度偏低,但在1991年变异偏高导致大洪涝。在汛期降水趋于减少的背景下,汛期高等级降水日数呈微弱减少趋势,说明降水更为集中,且中等级降水的贡献率相对较大。基于汛期标准化降水指数的洪涝等级划分可更好的反映洪涝实际,典型丰水年分别代表了梅雨(1980、1991、2003年)、梅雨加台风(1962、1965年)两大洪涝类型。(2)洪涝水位的响应特征。年最高日均水位较好的响应于降水变异的直接影响,典型丰水年1962、1965、1980年的洪涝由长时段的持续较高水位引发,而1991、2003年则是显著集中的高水位洪涝效应。低、中等级水位日数呈增加趋势,而高等级水位日数为较大减少趋势,说明了近年来洪涝水位趋于升高的事实。尽管汛期降水量趋于减少,但超警戒水位日数均值却自1970年代以后逐渐增加,显示了降水强度异常和人类活动影响的综合水循环变异结果。最高日均水位、超警戒水位日数与年、汛期降水量呈正相关,汛期降水越集中,高等级水位日数越少,水位越高,且降水的贡献率大小决定水位日数。梅雨降水与水位呈较好的正相关,但具南北空间差异,南部兴化、溱潼、安丰站相关程度较大,而北部射阳镇、盐城和建湖站较小。汛期2-5年左右重现期的降水即可导致超过2.0m的警戒水位出现,表明短时强降水的直接洪涝效应；而较大重现期的汛期、长时段降水往往导致更高水位,是持续降水的累积效应。30dMax降水是年最高水位的主要水量来源,但水位对3dMax、7dMax、15dMax降水量的显著响应关系更具实践参考价值。(3)季风驱动下水循环过程变异的大气环流分析。季风驱动是研究区水循环中降水要素异常的主导因素,研究区以弱季风强降水类型最多,其次为强季风强降水类型,且丰水年以弱季风强降水居多。东亚夏季风与汛期降水变化在3、6年左右的共振周期上存在较好的位相对应关系。西太平洋副高强度、面积与汛期降水呈正相关,而其脊线位置、西伸脊点则反之。汛期降水同亚洲区纬向环流呈负相关,而与经向环流略呈正相关,但纬向环流的影响更为显著。弱季风强降水年纬向环流指数偏低、经向环流指数相对较高,而强季风强降水年则反之。弱季风强降水年的亚洲区极涡强度和面积指数皆偏小,这一关系有利于降水偏多,而强季风强降水年二者指数则皆偏大。且ENSO遥相关分析表明,ENSO事件翌年降水偏多者皆为弱季风强降水和强季风弱降水类型,也印证了季风是水循环变异的最重要因子。大气环流场的综合分析发现,降水变异与季风、西太平洋副高、ENSO等及其相互影响密切关联。降水偏多的大气环流异常配置可概括为两类,其一为：季风偏弱时,西太平洋副高位置偏西偏北,但主体偏南,东亚经向环流呈“+、-、+”位势高度距平,研究区中低层为偏西南风,存在明显的梅雨锋面,利于雨带停留,这一形势降水量总体更为偏多；其二为：季风偏强时,副高位置偏东偏北收缩,东亚经向环流呈“-、-”位势高度距平,研究区低层为西南风距平,中层为偏东南风、高空亦为偏东风,缺少梅雨锋面,这一形势降水偏多程度相对较小。这两类大气环流形势较好的揭示了“季风驱动-水循环变异-降水异常”灾害链的大气环流异常背景。(4)洪涝成灾机制的综合分析。自然因素上,暴雨降水异常是洪涝的直接致灾因子,“锅底洼”地形是洪涝的孕灾环境大背景,低海拔平原的河网水系格局加剧了洪涝灾情。而人类活动影响上,闸坝建设导致河道淤积致使河道容蓄能力降低,大范围的圩垸围垦导致湖荡萎缩,减弱了河湖水体的调蓄能力,使得洪涝水位趋高。同时,城镇化的不利水文效应也加强了洪涝效应,反映了地表水循环过程变异的影响。结合降水变异的大气环流配置异常分析、自然因素和人类活动影响的综合分析表明,研究区的洪涝成灾机制是大气环流过程异常导致降水变异和地表水循环过程变异双重影响下的“季风驱动-水循环变异-降水异常-洪涝事件”作用链过程。更多还原

【Abstract】 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.更多还原