城市化对平原河网水系结构及功能的影响——以苏州市为例

针对城市化对平原河网水系结构的影响所引起的洪涝灾害频发等一系列水文问题,以我国典型平原河网地区苏州市为例,根据不同城市化程度分为主城区、市辖区、其他市县区,基于1991、2001和2015年三期遥感影像与1960s、1980s和2010s三个时期的水系数据,应用RS/GIS等技术,构建水系结构参数指标,重点探讨了城市化对河网水系结构及功能的影响.结果表明：城镇用地迅速增长,主要以牺牲水田、水域等土地利用方式为代价,到2015年全区城镇用地面积所占比重已达到41.35%,土地利用类型的变化规律与城市化进程的差异性保持一致;水系结构变化主要受城市化影响,且基本与城市化进程呈现同步性.近50年来,全区的水面率、河网密度、支流发育系数、主干河流面积长度比、河网复杂度和河网结构稳定度分别减少了19.63%、6.91%、7.34%、1.06%、5.49%和7.87%,城市化水平与各指数均呈负相关关系;人类活动不仅直接影响河流功能,也间接地通过改变平原河网的水系结构导致其功能发生改变,如河网调蓄能力下降、河流生态功能受损等.该研究为城市化地区河流水系保护及防洪减灾提供参考与理论支撑.     

基于连通性指数(IC)的近30年鄱阳湖流域水系结构与水文连通演变评估

由于气候变化和人类活动等多重影响,流域河湖水系格局与连通程度发生了显著变化,进而引发洪涝灾害等一系列水资源问题。本文以鄱阳湖流域为研究区,基于Google Earth Engine(GEE)提取1989—2020年5期水系数据,采用图论方法构建水系评价体系,定量分析该地区近30年来水系格局和结构连通性的时空演变特征,并结合该时期地形、土地利用和归一化植被指数(NDVI)等数据,利用连通性指数(index of connectivity,IC)评估功能连通性的动态变化,进而探讨水文连通与径流量和输沙量的联系。结果表明,近30年来鄱阳湖流域水系结构趋于复杂化,主要体现在流域北部。除干流外,其他等级河流的数量和长度均有所增加,其中Ⅲ级河流最为明显。河网密度、水面率、河网复杂度和发育系数均呈增加趋势,2000年后的变化率约为2000年前的两倍。水系连通环度、节点连接率和水系连通度总体增加,结构连通性呈好转趋势且变化幅度较小。功能连通分析表明,近30年来大部分流域IC减少,流域下游靠近主河道的平坦地区IC较高,上游远离河道的植被密集区域IC较低。此外,IC与年径流量和输沙量表现为显著的正相关性(...     

近30年来南四湖流域城市化进程中的水系变化分析

城市化对水系演化影响的研究国内目前多集中在快速、高速城市化地区,而对大流域、城市化发展较缓地区的研究比较薄弱.以南四湖流域为研究区,基于1987、2000和2014年3期遥感影像,分析了流域城市化进程中的下垫面变化特征;选取流域1980s、2003和2014年的地形图进行水系提取,从数量参数、结构参数和连通性参数3个角度分析近30年城市化进程中水系结构的时空变化特征.结果表明:(1)近30年来流域建设用地增加了1568.06 km~2,2000年以后城市建设用地扩张显著,2012年流域人口城市化率为32%;(2)1980s—2010s流域总河流长度、面积和河网密度均呈现出持续减少趋势,分别减少了135.46 km、2.75 km~2和0.49 km/km~2,各级河流表现出不同的变化特点,较低等级河流受到的影响较大;而流域水面率持续增加,近30年共增加了59.79%;(3)流域水系总体上还保持着自然状态下的空间格局,但结构特征发生了较大改变,河网结构稳定度减少了4.30%,连接率和实际结合度分别减少了21.82%和21.62%;子流域内部距湖区越远的空间城市扩展强度指数值越大,城市化对水系的影响越显著.该研究将补充对不同空间尺度、不同城市化水平地区河网水系演化影响的案例,并为研究区河网水系的保护提供支持与参考.     

城市化对水系结构及其连通性的影响——以秦淮河中、下游为例

针对当前城市化所引起水系衰减、河流连通受阻以及由此所引起洪涝与水环境的问题,以秦淮河中、下游为例,选取1979和2006年两期流域遥感影像,分析了城市化影响下的下垫面变化特征;选取1980s和2009年的地形图对河流水系进行提取,借鉴景观生态学中河流廊道空间结构分析方法,通过不同时期水系分级,探讨了城市化对水系结构及其连通性的影响.结果表明:(1)城市化使得2006年城镇用地面积相比1979年增加84.54 km²,增加了9倍多,大量林草地、耕地以及水域转变成城镇用地;(2)河流长度在过去的30年里减少了41%,河道主干化趋势明显;河流发育呈现由多元到单一、由复杂到简单的趋势;(3)连通性参数连接率、实际结合度分别由原来的1.28、0.43下降到0.79、0.26,河流的连通性呈下降趋势.该研究将为城市化地区河流水系保护提供支持与参考.     

太湖流域水文连通性:现状、研究进展与未来挑战

水文连通性作为连接河湖水体间物质、能量及信息传递与交换的关键纽带,对水环境、水生态和生境状况具有联动与触发反馈作用,已成为水文学、水利学和生态学等诸多领域的研究热点.太湖流域作为全国典型的流域性密集平原河网区,在快速城镇化背景下,河湖水文连通关系变化剧烈并引起了一系列生态环境效应.本文以水文连通性概念与内涵为背景,对太湖流域水文连通性研究进行了综述.太湖流域水文连通性评价方法以图论法、景观格局指数及水文连通性函数等方法联合使用为主,且聚焦于区域尺度研究;演变过程按人类活动影响强度大致划分为自然水系形成阶段、古代人类活动影响阶段和现代人类活动影响阶段;气候变化和人类活动共同影响着太湖流域水文连通性变化,近百年来水利工程建设和城市化进程等人类活动的影响尤为剧烈;良好的水文连通性有利于汛期减轻流域洪涝灾害及非汛期保障水资源供给,但水文连通性的提高对水环境和水生态的效应由于涉及因素众多尚存在争议.针对当前研究现状和存在问题,提出(1)平原河网区水文连通性的定量表征与评估是水文连通性研究的前提;(2)定量解析流域水文连通性的驱动机制是水文连通性研究的重点;(3)深入跟踪大型引水调水工程对流域水文连通影响及其效应是一项长期任务;(4)兼顾环境生态效应的水利工程生态化改造研究,开展工程控制背景下的流域水文连通多目标优化调控,是实现太湖流域洪水调蓄、水资源供给、水环境净化、生物多样性维持等生态系统服务协同提升的重要途径.     

太湖河湖水系连通需求评价初探

水系连通需求分析是河湖水系连通战略研究的重要内容.水系连通需求指人类社会经济发展对水资源调配、防洪排涝、改善生态环境等方面的需求及对河湖水系健康的维护,它源于水系能够提供各种水生态系统服务功能的生态学特性.通过分析水系连通需求、水生态系统服务功能和水系连通工程之间的内在关系,认为水系连通需求评价可转化为对水生态系统服务功能的评价.运用水当量评价方法,建立了河湖水系连通需求评价的方法体系,并以河网地区的典型代表——太湖为例,定量评价了太湖在水资源调配、调蓄洪水、水环境净化和维持生境等方面的连通需求.结果表明,太湖水系年均连通需求最大的是净化入湖废污水需水量,其次为水资源调配需水量,而太湖调蓄洪水的需求减少;湖水自身净化需水量较大,且为一次性需水.水环境净化的需求需要通过降低污染物入湖量,进行湖泊生态修复等主要措施以及引清水增加环境容量这一辅助连通措施共同完成.水系连通需求的定量评价可为水系连通战略及工程的规划设计提供理论基础.     

太湖流域南河水系无尺度结构

基于图论和无尺度结构视角及节点度指标初步探讨太湖流域南河水系结构的数量特征和空间秩序,为辨识和模拟域水文情势提供重要参数.主要结论如下:太湖流域南河水系结构的数量节点度不符合幂次分布,而长度和等级节点度符台无尺度结构;南河水系结构符合Horton第一定律和第二定律,同时此类特征亦符合无尺度结构,但指数函数回归模拟的R<'2>相比乘幂函数回归模拟的R<'2>较低;由幂次定律所支配的这种水系网络特征状况是由地区地形、农业灌溉、圩区建设和城市化过程共同推动;从节点度角度看,南河下游圩区使得河网数量节点度增加,等级节点度和长度节点度降低.     

基于动态贝叶斯网络的道路交通系统抗震韧性评价

以道路交通系统在地震作用下的连通性随时间变化的曲线作为其抗震韧性量化评价的基础,构建了基于动态贝叶斯网络的抗震韧性评价模型。该模型采用动态贝叶斯网络表征道路交通系统抗震功能的连通逻辑以及路段连通性随时间的变化趋势,并基于网络中各变量的概率与条件概率表输出不同时刻系统的连通性,绘制连通性曲线。最后以青岛市的局部城市路网为应用实例,实现了对道路交通系统全过程抗震韧性的评价。结果表明：所提出的韧性指标可全面描述震后系统连通性退化与恢复全过程;构建的动态贝叶斯网络能直观表征系统抗震功能的连通逻辑与变化;全过程韧性评价结果能够为系统震前预防与震后恢复策略的制定提供参考。     

洞庭湖流域常德柳叶湖及其邻近水体轮虫群落结构变化及其对环境因子的响应

城市河-湖连通使不同水文特征的水体连通构成多样化水环境,环境异质性为提高浮游动物多样性提供了多样性生境条件.为了解湖南常德柳叶湖及其邻近水体轮虫群落结构变化对不同水体连通性和异质性的响应,于2018年对柳叶湖、穿紫河和沅江常德市区河段共15个站点进行了每季度1次的轮虫群落结构和环境特征调查分析.调查期间,不同水体的营养水平呈现:穿紫河柳叶湖沅江,穿紫河和柳叶湖呈富营养化状态,沅江呈中营养状态.本研究共检出轮虫88种(包括亚种8种和变种1种),其中,长肢多肢轮虫(Polyarthra dolichoptera)为水系第一优势种.轮虫最高丰度为2535ind./L,全年平均丰度空间分布呈现:柳叶湖穿紫河沅江;季节分布呈现:6月 9月 3月 12月.轮虫群落对水体连通性的响应表现为:柳叶湖、穿紫河和沅江3个水体轮虫种类组成具有一定的相似性且第一优势种相同.对异质性的响应表现为:不同水体轮虫群落年平均丰度和多样性指数存在空间差异.典范对应分析表明水温是影响轮虫群落结构变化的主要环境因子.     

太湖流域典型滨湖河网水动力与水质时空异质性

基于中国太湖梅梁湾东部的无锡市滨湖区河网29个监测点在丰水期、平水期和枯水期的流速和水质监测数据,将河网分为梁溪河、曹王泾、骂蠡港、城市河网南区以及城市河网北区5个区域,对流速和典型水质指标的时空异质性进行分析,结合主成分分析和相关性分析,得到各区域水动力与水质现状及其成因.结果显示:梁溪河和曹王泾的水质条件和水动力条件较好,多数水质因子与流速表现出了强相关性;骂蠡港的水质和流速区域变化明显,表现弱相关性;城市河网北区和南区的流速较缓,河道污染负荷较大,流速与水质因子之间的相关性较低.通过在滨湖河网开展流速和水质的野外监测,分析流速对于河网水环境的实际效果,验证不同水质指标与流速之间的响应关系,为滨湖河网区水质保护和科学的水污染治理技术提供基础支撑.     

Changes in the plain river system and its hydrological characteristics under urbanization – case study of Suzhou City,China

ABSTRACT

Rapid urbanization in China has severely disturbed the underlying surface and river systems. The stream structure parameters of Suzhou City were analysed to study the evolution and spatial differentiation of a water system undergoing urbanization. The influencing pattern of different urbanization processes on river system structure and hydrological processes was detected by statistical methods and simulation analysis. The results show that urban sprawl is influenced by both the natural environment and the social economy. At different stages of urbanization, the spatial urban expansion and the natural river characteristics influence the evolution of the river network: during rapid urbanization, the decline in surface water ratio and river density is more intense. The decrease of surface water ratio and river density in rapidly urbanized areas was greatest, followed by that in area urbanized in the 1980s, with few changes in the old town. Under high urbanization, river system indicators tend to stabilize. The rivers’ hydrological features were affected by urbanization, with water yield in the study area increasing from 0.81 to 0.95 m³/m² (1991–2015). The same rainfall intensity results in higher flood levels and greater risk of flooding under rapid urbanization.     

River networks system changes and its impact on storage and flood control capacity under rapid urbanization

Urbanization significantly affects both the drainage network structure and the storage capacity of river channels in lowland plain river networks. The Yangtze River Delta region has experienced a rapid and profound urbanization of any region worldwide in the past 30 years. The southeast Yinfeng plain in the Yangtze River Delta region was used as the study area to assess changes in the river network structure from 1990 to 2010, using river feature parameter analyses and GIS spatial analysis. The elastic method was adopted to analyse river network pattern response to both urbanization and climate change, and the rates at which these factors contributed to changes in the river network pattern were investigated. Changes in the storage capacity and flood control capacity of the channel network over the past 21 years were then analysed. The results indicated that: (1) the number of river networks, and the complexity of the river network spatial structures were reduced, the drainage density decreased by 20%, the water surface ratio by 36%, the river area–length ratio by nearly 18% and the fractal dimension of the river networks by 4.5%; (2) the river network changes were due more to human activities than to climate change in this study period. Limited changes occurred in the structure of the river system, as indicated by an urbanization variability rate of less than 20% and precipitation variability range of ?5% to ?15%, although the stream structure gradually decreased with increasing urbanization variability; and (3) the storage capacity of the main river network decreased earlier (1990–2003) and was restored later (2003–2010) because of dredging and widening activities. In addition, the number of lower‐order rivers decreased, which resulted in significant decreases in the storage and flood control capacities of the river networks. Copyright © 2016 John Wiley & Sons, Ltd.     

Influence of changes in river system structure on hydrological processes in Taihu Basin,China

ABSTRACT

With the rapid economic development and urbanization in Taihu Basin (eastern China), the river system has decayed and the connectivity of rivers and lakes has weakened, resulting in frequent floods. The impact of changes in river system structure on hydrological processes in the plain river network area was analysed against a background of urbanization. An indicator system was built to describe the quantitative, morphological and spatial structure of the river system. Analysis of the change in annual average and extreme water levels revealed the influencing mechanism of the changes in river system structure on the hydrological processes. The results indicate a decreasing tendency in the density of the river system in the study area, with a reduction in water surface ratio by about 20% in the past 50 years. Since the 1960s, the maximum and annual average water levels have increased. The degree of change in the mean monthly, annual lowest and annual highest water levels was great, with that in the non-flood season being higher than in the flood season. The decrease in the number of rivers directly reduced the storage and adjustable capacity of the basin.     

Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

Restoring hydrologic connectivity between channels and floodplains is common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact stream hydraulics, ecology, biogeochemical processing, and pollutant removal, yet rigorous field evaluations of surface water–groundwater exchange within floodplains during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection by pumping stream water onto an existing floodplain swale. Floods were conducted throughout the year to capture seasonal variation and each involved two replicate floods on successive days to test the effect of varying antecedent moisture. Water levels and specific conductance were measured in surface water, soil, and groundwater within the floodplain, along with surface flow into and out of the floodplain. Vegetation density varied seasonally and controlled the volume of surface water storage on the floodplain. By contrast, antecedent moisture conditions controlled storage of water in floodplain soils, with drier antecedent moisture conditions leading to increased subsurface storage and slower flood wave propagation across the floodplain surface. The site experienced spatial heterogeneity in vertical connectivity between surface water and groundwater across the floodplain surface, where propagation of hydrostatic pressure, preferential flow, and bulk Darcy flow were all mechanisms that may have occurred during the five floods. Vertical connectivity also increased with time, suggesting higher frequency of floodplain inundation may increase surface water–groundwater exchange across the floodplain surface. Understanding the variability of floodplain impacts on water quality noted in the literature likely requires better accounting for seasonal variations in floodplain vegetation and antecedent moisture as well as heterogeneous exchange flow mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.     

Modelling hydrological connectivity of tropical floodplain wetlands via a combined natural and artificial stream network

The ecological condition and biodiversity values of floodplain wetlands are highly dependent on the hydrological connectivity of wetlands to adjacent rivers. This paper describes a method for quantifying connectivity between floodplain wetlands and the main rivers in a wet tropical catchment of northern Australia. We used a one‐dimensional hydrodynamic model to simulate time‐varying water depths across the stream network (i.e. rivers, streams and man‐made drains). The timing and duration of connectivity of seven wetlands (four natural and three artificial) with the two main rivers in the catchment were then calculated for different hydrological conditions. Location and areal extent of the wetlands and the stream network were identified using high‐resolution laser altimetry, and these data formed key inputs to the hydrodynamic model. The model was calibrated using measured water depths and discharges across the floodplain. An algorithm was developed to identify contiguous water bodies at daily time steps, and this gave the temporal history of connection and disconnection between wetlands and the rivers. Simulation results show that connectivity of individual wetlands to both rivers varies from 26 to 365 days during an average hydrological condition. Location, especially proximity to a main river, and wetland type (natural stream or artificial drain) were identified as key factors influencing these levels of connectivity. Some natural wetlands maintain connection with the river for most or all of the year, whereas the connectivity of some artificial wetlands varies from 26 to 36 days according to their patterns of network connection to adjacent rivers – a result that has important implications for the accessibility of these types of wetland to aquatic biota. Using readily available river gauge data, we also show how connectivity modelling can be used to identify periods when connectivity has fallen below critical thresholds for fish movement. These connectivity patterns within the floodplain network are central to the setting of river flows that will meet environmental requirements for biota that use floodplain wetlands during their life history. Copyright © 2013 John Wiley & Sons, Ltd.     

Headwater stream length dynamics across four physiographic provinces of the Appalachian Highlands

Understanding patterns of expansion, contraction, and disconnection of headwater stream length in diverse settings is invaluable for the effective management of water resources as well as for informing research in the hydrology, ecology, and biogeochemistry of temporary streams. More accurate mapping of the stream network and quantitative measures of flow duration in the vast headwater regions facilitate implementation of water quality regulation and other policies to protect waterways. We determined the length and connectivity of the wet stream and geomorphic channel network in 3 forested catchments (<75 ha) in each of 4 physiographic provinces of the Appalachian Highlands: the New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. We mapped wet stream length 7 times at each catchment to characterize flow conditions between exceedance probabilities of <5% and >90% of the mean daily discharge. Stream network dynamics reflected geologic controls at both regional and local scales. Wet stream length was most variable at two Valley and Ridge catchments on a shale scarp slope and changed the least in the Blue Ridge. The density and source area of flow origins differed between the crystalline and sedimentary physiographic provinces, as the Appalachian Plateau and Valley and Ridge had fewer origins with much larger contributing areas than New England and the Blue Ridge. However, the length and surface connectivity of the wet stream depended on local lithology, geologic structure, and the distribution of surficial deposits such as boulders, glacially derived material, and colluival debris or sediment valley fills. Several proxies indicate the magnitude of stream length dynamics, including bankfull channel width, network connectivity, the base flow index, and the ratio of geomorphic channel to wet stream length. Consideration of geologic characteristics at multiple spatial scales is imperative for future investigations of flow intermittency in headwaters.