【作者】 刘东；

【导师】 潘德炉； 白雁；

【作者基本信息】 浙江大学 ， 地图学与地理信息系统， 2017， 博士

【摘要】 河流连通着地球最大的两个碳库—陆地和海洋生态系统碳库。全球河流每年可以输运约 219.50±54.36 Tg C/yr 的溶解有机碳(DOC),204.00±21.73 Tg C/yr 的颗粒有机碳(POC)进入边缘海。河流有机碳输运不仅影响着全球碳循环,而且会影响河口和边缘海的生态环境,输运量也反映了流域的自然过程和人为活动的变化。在自然变化和人为活动等多种因素的影响下,河流有机碳通量具有显著的时空变异特征,这些时空变异增加了通量监测和估算的难度。目前,国际上关于河流有机碳输运的研究大多关注欧美大陆河流,这些河流积累了长时间序列的有机碳(DOC、POC)观测资料。而东亚季风影响下的亚洲河流有机碳输运报道较少,且缺乏不同河流有机碳输运的空间变异和同一河流的时间变异研究。基于此,本文基于不同入海河流有机碳实测数据与流域特征数据,分析了中国大陆入海河流有机碳通量的空间变异;并进一步以长江为重点研究区,在长江河流(大通水文站断面)和河口分别开展了月尺度和季节尺度的光学-化学同步现场观测,利用多源遥感和实测资料估算了长江长时间序列的有机碳通量,并讨论了流域特征对长江有机碳输运时空变异的影响,为进一步估算全球河流有机碳输运通量打下基础。本文的主要研究内容和取得的结论如下:(1)中国大陆入海河流有机碳输运通量及空间变异。通过分析中国大陆不同入海河流有机碳(DOC、POC)数据与流域特征数据,构建了中国大陆入海河流年平均DOC浓度和POC浓度的估算模型,并以2010年为代表,估算了中国大陆不同入海河流的DOC年通量和POC年通量。2010年,中国大陆入海河流有机碳输运通量约为5.05 Tg C的DOC和7.00TgC的POC;其中,34.65%的DOC和42.43%的POC由长江输运。空间分布上,虽然降雨会稀释河流DOC浓度和总悬浮物(TSM)中的POC重量百分比(POC(%TSM)),但单位面积的流域DOC与POC输出量都与降雨量的空间分布一致。中国大陆东南部降雨多的流域,单位面积的流域DOC和POC输出量都较高。(2)长江河流(大通水文站断面)DOC通量估算。以流量、流域耕地面积比和总初级生产力为自变量,构建了长江大通DOC浓度的多层回馈神经网络估算模型,并结合大通实测流量估算了 2000-2013年长江大通水文站的DOC月通量。2000-2013年,长江大通DOC月通量变化范围为3.88-42.78×104tC,平均值为13.49±6.73×104tC/month。大通DOC月通量主要是由流量决定的,两者存在显著的正相关线性关系(R=0.8914,P<0.0001)。(3)长江河流(大通水文站断面)POC通量估算。根据大通的光学-化学同步观测数据和生物光学模型模拟数据,构建了适用于Landsat数据(空间分辨率30 m)的POC浓度遥感算法。大通断面上,POC浓度与TSM浓度之间存在正相关线性关系,且空间变异不显著。结合大通流量,利用2000-2016年的TM/Landsat-5和ETM+/Landsat-7数据,估算了大通长时间序列的POC月通量。2000-2016年,长江大通的POC月通量变化范围为1.4-52.04×104tC,平均值为13.04±7.17×104tC/month。大通POC月通量与流量之间存在显著的正相关线性关系(R=0.7143,P<0.05)。过去十几年,大通POC与DOC的平均月通量基本相等,并未呈"高POC输运"的特征。(4)长江河口(徐六泾水文站断面)POC通量估算。根据长江河口的光学-化学同步测量数据,构建了适用于GOCI数据(空间分辨率500 m)的POC浓度遥感算法。长江河口,POC浓度与TSM浓度存在显著的正相关线性关系;徐六泾断面的流量剖面分布呈线性函数变化,POC浓度剖面分布呈指数函数变化。结合大通水文站的流量,利用GOCI数据估算了徐六泾断面的POC月通量。2015年5月至2016年4月,长江徐六泾POC月通量平均为7.14×104tC,比大通POC月通量实测值平均高44.58%。长江河口复杂的水动力环境(潮汐、风)及颗粒再悬浮等会增加该区域的POC通量,且在冬季的增加效应更明显。(5)长江河流(大通水文站断面)有机碳输运时间变异和流域特征的影响。综合分析长时间序列的长江大通有机碳(DOC、POC)月通量和流域特征遥感数据,发现:因降雨的稀释作用,大通DOC浓度表现为"湿季低、干季高"的季节特征(湿季为5月至10月),干季DOC浓度平均是湿季的1.14倍;因降雨的冲刷作用,大通POC浓度表现为"湿季高、干季低"的季节特征。长江三峡大坝正常运行之后,大通POC浓度的季节差异正逐渐减弱;由于流量的调控作用,DOC与POC通量都表现为"湿季高、干季低"的季节特征。2000-2013年湿季DOC通量平均占全年通量的65.89%,2000-2016年湿季POC通量平均占全年通量的75.01%。2000-2013年,长江大通DOC浓度和DOC通量都没有发生显著变化;但2000-2016年,长江大通POC浓度和POC通量都呈指数显著降低。总的来说,本文利用遥感和实测数据估算了中国大陆入海河流有机碳输运通量及长江长时间序列的有机碳输运通量,并分析了中国大陆入海河流有机碳输运的时空变异。下一步将考虑河流输运的有机碳在经过河口过程(如转化、分解、沉降等)后的通量,进一步研究河流输运有机碳的有效入海通量。更多还原

【Abstract】 River connects the two largest carbon reservoirs on the earth,the terrestrial and marine ecosystems.On the global,rivers transport about 219.50±54.36 Tg C/yr(1 Tg =1×1012 g)of dissolved organic carbon(DOC)and 204.00±21.73 Tg C/yr of particulate organic carbon(POC)into marginal seas annually.The lateral transport of riverine organic carbon not only impacts global carbon balance,but also reflects natural processes and human activities in river watershed.Moreover,riverine organic carbon can have impacts on ecological environment characteristics of estuary and marginal sea.Under the influences of precipitation,air temperature,land use change,human activities,etc.in watershed,transports of riverine organic carbon are tempo-spatially varied.Under the influences of natural processes and human activities,transport of riverine organic carbon is featured by highly tempo-spatial variations,which make it difficult to estimate riverine organic carbon flux.So far,most of the reported studies about riverine organic carbon transport focused on non-monsoon rivers in the Europe and America,for which there are long-time sequenced in-situ DOC and POC data.For Asian rivers under the influence of East Asian Monsoon,however,there are few reports about organic carbon transport,especially for spatial variations of different rivers and temporal variations of a specific river.Based on these,this study focused on spatial variations of organic carbon flux from mainland Chinese rivers.Moreover,taking the Changjiang River as an important study area,we synchronously measured optical and chemical in-situ data at Datong hydro metric station section(monthly)of the Changjiang River and its estuary(seasonally),estimated long-time sequenced DOC and POC flux,and discussed the impacts of basin characteristics on tempo-spatial variations of riverine organic carbon transport.This study laid a foundation for estimation of organic carbon flux from global oceanic rivers.Main contents and conclusions of this study are shown as below.(1)Spatial variations of organic carbon transport in mainland Chinese rivers.Based on data of riverine organic carbon(DOC and POC)and basin characteristics,we built estimation models of annual mean DOC and POC concentrations in mainland Chinese rivers respectively.By combining annual water discharge,we further calculated annual DOC and POC fluxes from mainland Chinese rivers in 2010.In 2010,mainland Chinese rivers transported about 5.05 Tg C of DOC and 7.00 Tg C of POC.Among them,about 34.65%of DOC and 42.43%of POC were transported by the Changjiang River.In the spatial distribution,precipitation decreased riverine DOC concentration and POC(%TSM)(POC content in total suspended matter(TSM)),but DOC and POC exports from mainland Chinese river watersheds are spatially consistent with precipitation,both with higher values in the southeast.Moreover,air temperature,watershed cropland area,watershed slope,etc.can also impact the spatial distributions of riverine organic carbon transport in mainland Chinese rivers.(2)Estimation of DOC flux in the Changjiang River(Datong section).Taking river discharge,cropland area ratio and mean gross primary productivity in watershed as independent variables,we built a multilayer back-propagation neural network model(MBPNN)to estimate monthly mean DOC concentration at Datong section.By combining monthly water discharge at Datong,we calculated monthly DOC fluxes from 2000 to 2013.Monthly DOC flux ranged from 3.88×104 t C to 42.78×104 t C,with mean of 13.49±6.73×104 t C/month.We found that DOC flux at Datong was determined by water discharge.There was significantly linear relationship between monthly DOC flux and water discharge(R=0.8914,P<0.0001).(3)Estimation of POC flux in the Changjiang River(Datong hydrometric station section).Based on simulated data from a bio-optical model and photo-chemically synchronous in-situ data,we built a POC remote sensing algorithm which can be applied to Landsat satellite data(with spatial resolution of 30 m).At Datong section,POC concentration was linearly related to TSM concentration,and with obviously spatial variations.By combining monthly water discharge at Datong,using data of thematic mapper onboard Landsat-5 satellite(TM/Landsat-5)and enhanced thematic mapper onboard Landsat-7 satellite(ETM+/Landsat-7)we calculated monthly POC fluxes at Datong from 2000 to 2016.Monthly POC flux ranged from 1.4×104 t C to 52.04×104 t C,with mean of 13.04±7.17×104 t C/month.We found that there was significantly linear relationship between monthly DOC flux and water discharge(R=0.7143,P<0.05).At Datong hydrometric station of the Changjiang River,monthly POC flux was almost equal to monthly DOC flux.The Changjiang River was not featured by high POC transport.(4)Estimation of POC flux in the Changjiang River Estuary(Xuliujing hydrometric station section).Based on photo-chemically synchronous in-situ data,we built POC remote sensing algorithms which can be applied to satellite data of Geostationary Ocean Color Imager(GOCI)(with spatial resolution of 500 m).In the Changjiang River Estuary,POC concentration was linearly related to TSM concentration.At Xuliujing section,current velocity linearly decreased with the increasing of water depth;however,POC concentration exponential increased with the increasing of water depth.By combining monthly water discharge at Datong,using GOCI data we calculated monthly POC fluxes at Xuliujing.From May 2015 to April 2016,mean monthly POC flux was 7.14×104t C/month at Xuliujing section,which was 44.58%higher in average than in-situ values at Datong section.We found that tide,wind,etc.in the Changjiang River Estuary could increase POC flux from the Changjiang River to marginal sea.(5)Temporal variations of riverine organic carbon transport in the Changjiang River(Datong section)and impacts of basin characteristics.Based on long-time sequenced monthly DOC and POC fluxes,we discussed temporal variations of organic carbon transport in the Changjiang River(Datong section).Due to dilution effects of precipitation water,DOC concentration was usually lower in the wet season(from May to October)than that in the dry season.Mean DOC concentration in the dry season is 1.14 times of that in the wet season.Due to flushing effects of precipitation water,however,POC concentration was usually higher in the wet season than that in the dry season.Under the influence of the three gorges dam(TGD),temporal variations of POC concentration was gradually weakened.From 2000 to 2013,both DOC concentration and DOC flux at Datong did not changed significantly.However,both POC concentration and POC flux at Datong decreased exponentially significantly from 2000 to 2016.To summary,this study estimated total organic carbon fluxes from mainland Chinese rivers and long-time sequenced DOC and POC fluxes from the Changjiang River,and discussed tempo-spatial variations of organic carbon transport in mainland Chinese rivers.However,transformation,decomposition,and deposition,etc.may happen to riverine organic carbon in the estuary,so it is necessary to further study effective transport of riverine organic carbon flux into marginal oceans in the future.更多还原