黄河源区河流泥沙和碳输移过程及梯级水库的影响研究

River systems are important linkages between the land and ocean, transporting huge amounts of terrestrially-derived materials (e.g., sediment and carbon) into the oceans. Recent studies show that the total carbon entering into inland waters, including rivers, lakes, wetland and reservoirs, is estimated at 5.7 Pg per year. Of the carbon originated from terrestrial ecosystems, however, only 0.9 Pg is finally discharged into the oceans through fluvial export. This implies that more than 80% of the carbon is buried in sediment on landscape or is biogeochemically processed and degassed into the atmosphere as greenhouse gases. It is therefore evident that construction of reservoirs on river channels would significantly alter riverine carbon export, and understanding the impact of reservoir construction and operation on riverine carbon transport is of paramount importance for riverine carbon cycle and beyond. Focusing on 4 cascading reservoirs on the Yellow River mainstem channel in its headwater region, this proposal is aimed to elucidate the impact and mechanisms of reservoir operation on riverine sediment and carbon export. Based on field sampling and lab analyses, this proposal will systematically investigate the changes in lateral delivery of riverine sediment and carbon and quantify sediment and carbon burials inside cascading reservoirs as well as carbon degassing across water-air interface. Particularly, a more detailed assessment of riverine sediment and carbon dynamics will be carried out in the Longyangxia and Liujiaxia reservoirs by establishing sediment and carbon budgets. In combination with future changes in sediment and carbon in the headwater region of the Yellow River as affected by climate change and human activities, this project will also predict the potential impacts of cascading reservoirs on future riverine and carbon export. These findings will shed light on the ongoing water resources management and hydropower exploitation projects in southwestern rivers.

作为陆地生态系统与海洋之间相互联系的重要纽带，河流是陆源物质向海洋输移的主要通道。每年自陆地生态系统进入内陆水体的碳约为5.7 Pg ，然而只有0.9 Pg最终经由河流系统输移入海洋，即>80%的碳被埋藏在内陆水体或经历复杂的生物地球化学过程后被氧化分解并进入了大气系统中。在河流上修建水库将影响河流系统原有的泥沙和碳输移规律，探讨其影响机理对于理解河流碳输移过程及对碳循环研究具有重要意义。首先选取黄河源区的4个梯级水库作为研究对象，深入分析黄河源区梯级水库对河流泥沙和碳输移过程的沿程影响，建立龙羊峡和刘家峡典型水库的泥沙、碳收支平衡方程，进一步系统地分析梯级水库对河流泥沙、碳横向输移、埋藏和碳逸出的影响机理；结合黄河源区在气候变化影响下的泥沙、碳输移变化趋势，还将预测评估梯级水库运行对其的潜在影响。研究成果将为西南河流开发和管理提供理论依据和指导原则。

河流是陆地生态系统与海洋之间相互联系的重要纽带，是陆源物质向海洋输移的主要通道。每年自陆地生态系统进入内陆水体的碳约>80%的碳被埋藏在内陆水体或经历复杂的生物地球化学过程后而被氧化分解并进入了大气系统中。在河流上修建水库将影响河流系统原有的泥沙和碳输移规律，探讨其影响机理对于理解河流碳输移过程及对碳循环研究具有重要意义。首先选取黄河源区的12个梯级水库作为研究对象，深入分析黄河源区梯级水库对河流泥沙和碳输移过程的沿程影响，建立龙羊峡和刘家峡典型水库的泥沙、碳收支平衡方程，进一步系统地分析梯级水库对河流泥沙、碳横向输移、埋藏和碳逸出的影响机理；结合黄河源区在气候变化影响下的泥沙、碳输移变化趋势，评估梯级水库运行对其的潜在影响。本研究对若尔盖湿地、黄河干流以及海拔4200米以上冰川冻土区实施重点布点监测，共设计采样点36个。春季自4月中旬到5月中旬，夏季六月下旬至七月下旬，秋季自8月中旬到9月中旬，以及冬季10月中旬到11月上旬监测源区河流径流泥沙、碳输移及CO2逸出通量的变化。利用LI-7000 CO2/H2O分析仪和声学多普勒流速剖面仪(ADCP)采用静态漂浮箱法对二氧化碳(CO2)温室气体逸出展开监测，并观测海拔、气温、风速、水温、盐度、流速、pH值、总溶解固体量(TDS)、水中溶解氧等环境物理指标，同时收集水样分析。同时，结合黄河上游自龙羊峡至刘家峡12个梯级水库的水库特征和运行方式，采用Li-7000静态箱法监测水库入库前、库中和出库河道内的二氧化碳分压、CO2逸出量，并测定了相关水化学因素，包括pH，溶解氧（Dissolved Oxygen, DO）和水温T，综合分析了水库碳逸出的时空变化规律及其影响因素，监测在梯级水库调节造成流速和水温等环境指标。结果表明梯级水库库区逸出夏秋两季明显高于春冬两季，但是总是明显低于入库前和坝下；大多数黄河源区河流是碳源，二氧化碳分压介于181 to 2441 μatm，逸出介于-221 to 6892 g C m–2 yr–1，冻土区较低，若尔盖湿地最高。源区河流CO2每年逸出量约为0.37±0.49 Tg C。