暴雨洪水事件对三峡水库香溪河库湾甲烷排放的影响机制

Dam construction leads to greenhouse gas (GHG) emissions via changing the land use, which has become a scientific issue of great interest to IHA. However, there are large uncertainties in reservoir GHG fluxes estimated by the scientific community. This is due to the high heterogeneity between reservoirs and within a reservoir, while another important reason is the inadequate representativeness of sampling period. Since the observations in current studies were almost implemented under normal weather conditions, resulting in that the hydrological processes within the reservoir have not been fully considered. This would also restrict the development of high-quality GHG models. Taking Xiangxi Bay of the Three Gorges Reservoir as the object, this study will be carried out through in-situ measurements and indoor experiments, with storm events being entirely tracked. CH4 flux across the air-water interface and dissolved CH4 concentration in water at all depths will be monitored continuously, combined with real-time meteorological and aquatic environmental observations, to reveal the mechanism of rainfall and wind speed impacting on the air-water interface flux, and to elucidate the influence of destratification caused by storm floods on vertical transportation and transformation of methane. By means of sediment incubation experiments under on-site conditions of major aquatic elements, the effect of storm-induced density flow on CH4 release from sediments will be investigated. In the context of more frequent occurrence of extreme rainfall events resulting from global warming, this study would provide scientific basis for more accurate estimation of methane emission in aquatic ecosystems.

水库的修建改变土地利用方式而产生温室气体释放，成为当前IHA关注的热点科学问题。然而，学术界对于水库温室气体的通量估计存在较大不确定性，除了水库间和水库内高度的空间异质性之外，另一重要原因是观测时间的代表性不足。由于已有观测基本在良好天气状况下进行，导致典型的水库水文过程未得到充分考虑，这也限制了高质量的水库温室气体模型开发。本课题拟以香溪河库湾为对象，采用野外原位观测和室内培养模拟相结合的方法，跟踪数场暴雨洪水事件，连续观测水-气界面CH4通量和垂向剖面CH4浓度，同步监测气象和水环境因子，揭示降雨和风速等对水-气界面通量的影响机制，阐明暴雨洪水造成的分层水体扰动对CH4垂向上迁移和转化的影响；通过模拟现场主要水环境因子下的沉积物培养，探讨暴雨带来的顺坡底部异重流过程对沉积物CH4释放的影响。在全球变暖造成极端降雨事件频发的背景下，本研究为更准确的估算水域生态系统CH4排放提供科学依据。

甲烷是一种重要的温室气体，水库甲烷源汇效应备受关注，然而观测时间的代表性不足以及缺乏对不利天气因素的考虑使水库温室气体排放估计存在较大不确定性。本项目以香溪河库湾等河道型水库为例，追踪数场强降雨径流事件，在特征断面连续监测水-气界面CH4通量、表层水体和垂向剖面CH4浓度，同步监测气象和水环境因子，探讨了强降雨径流过程对CH4产生、传输、消耗及排放的影响。研究结果表明，降雨事件前后水库水体均表现为大气甲烷的源。强降雨过程中水-气界面CH4通量及气体传输速率受降雨和风速的共同影响，风速的主导更为明显。强降雨后水体底层CH4浓度升高倾向于受上游输入的影响，水体垂向掺混深度和持续时间十分有限，垂向上CH4氧化消耗作用明显。暴雨过程中，水-气界面CH4交换速率对风速和降雨的响应表现出阶段性差异，说明传统静态浮箱法在强降雨条件下是否适用可能存在雨强阈值，有必要结合广泛采用的涡度相关法等手段探讨适宜不利天气条件的温室气体通量观测方法。本研究成果源自针对强降雨事件开展甲烷原位高频连续监测的首次尝试，虽受限于追雨实验的不确定性等困难，但在全球变暖造成极端降雨事件频发的背景下，能够为湖库水域生态系统CH4排放的准确监测和估算提供科学参考。