长江源多年冻土退化对地下水和地表径流的影响机制研究

The majority of the Yangtze River source regions is underlain by permafrost. Recent studies indicated that permafrost degradation has been occurring under the scenarios of accelerating climatic warming since the end of 1990s. While the impacting mechanism of permafrost degradation on the ground water and surface runoff was unclear until now, which would result in the difficulty in understanding the dynamic processes of runoff and in the prediction of runoff in the source regions of major rivers. This proposal suggests to select the Dongkemadi Basin in the Yangtze River source regions which is underlain by continuous permafrost as the study region. The geophysical investigation, borehole drilling, testpit excavation, long-term continuous observation and sampling soil and water will be conducted to reveal the structure of water storage in permafrost regions. In the meanwhile, the isotope tracing data and water chemistry data were analyzed to identify the runoff processes of groundwater and its interaction with surface runoff in permafrost regions. Finally, the coupled groundwater flow and heat transportation numerical model would be established according to the constraining of the model with the baseflow separation results obtained from isotopic end-member mixing model.The influence of permafrost dynamics and active layer freezing/thawing cycles on the ground water and surface runoff would be analyzed. The results are expected to contribute to learning about the key hydrological processes, providing crucial parameters for hydrological models, and presenting example reference for the establishment of hydrological processes model in cold regions.

长江源大部分主要产流区均位于多年冻土区，近期研究表明这一地区多年冻土在气候加速变暖背景下发生了广泛的退化，然而目前我们对该地区多年冻土退化对地下水和地表径流影响机制的认识基本空白，严重制约了对江河源区径流形成过程和演化规律的认知和对径流变化的预测。本项目拟选取长江源冬克玛底河小流域产流区为研究区，通过地球物理勘查、钻探、坑探、定点定期观测和土壤水体样品采集等手段，揭示多年冻土区地下水含水系统的结构；利用同位素与水化学示踪数据，识别多年冻土区地下水的流动过程及其与地表水的转换关系；在此基础上，以同位素径流分割结果为约束，构建多年冻土区地下水热运移耦合数值模型，综合分析多年冻土分布格局和活动层冻融过程对地下水和地表径流的影响机制。本项目的实施可为寒区水文过程模型提供过程认识、关键参数和参考范例。

多年冻土退化对地表径流和地下水的影响是冰冻圈水文学研究的重要内容，定量识别多年冻土退化对不同形式水体转换规律的影响，可以对深入理解多年冻土变化的水文过程及影响机理提供基础依据。.本课题通过大量野外调查，借助同位素技术和相关模型，明晰了研究区降水、河水、活动层水、多年冻土上限附近地下冰、多年冻土水等多个水体间的转换过程和机制，并对影响冻土水文过程模型的关键参数做了有效评估和改进。经过4年研究，取得了如下成果和新认识：1）利用探地雷达和瞬变电磁探测手段，获取了研究区多年冻土的垂直结构和水平区域差异，为进一步分析多年冻土区深层水文地质结构和地下水循环过程提供了资料基础；2）定量评估了多年冻土区地表热通量的遥感估算模型的适用性，为进一步评估气候变化背景下多年冻土水热状况变化提供了输入变量的关键参数化方案；3）比较和评估了普通和极端降水事件对活动层水热过程的影响，提升了对冻土水文过程的认识；4）使用同位素和水化学手段，揭示了降水、活动层水、上限附近地下冰和多年冻土水的水文过程，发现降水是活动层水、上限附近地下冰和多年冻土水的主要补给源。在青藏公路沿线地区，活动层水分对上限附近地下冰的贡献比率介于58.9%与87.0%之间，而多年冻土水分对上限附近地下冰的贡献比率介于13.0%与41.1%之间；5）使用同位素技术明晰了局地再循环水汽对多年冻土区降水的贡献，发现在当前气温和地温升高、降水和土壤水分增加，植被变绿和热融湖塘等水体扩张的背景下，下垫面和气候条件的变化将显著改变蒸散发的组成，未来多年冻土区的局地再循环水汽比例极有可能呈现增加的趋势。.项目共计发表标注论文42篇，包括SCI论文38篇，中文核心论文4篇，其中第一标注SCI论文2篇，第二标注SCI论文12篇。两名项目参与人员由中级职称晋升为副高职称。已经培养9名博士研究生和1名硕士研究生完成了学位论文，初步形成了一支研究基础扎实和创新能力卓越的多年冻土环境科研团队。