降雨径流事件对三峡水库香溪河库湾甲烷释放的影响

Impact of Rainfall-Runoff Events on Methane Emission from Xiangxi Bay of the Three Gorges Reservoir

甲烷是一种重要的温室气体,水库的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非良好天气状况的考虑制约了CH4排放的准确估计.本研究以三峡水库香溪河库湾为研究对象,针对2019年夏季一场完整降雨径流事件在库湾中游断面开展跟踪连续监测,初步探讨了降雨以及来流过程对库湾甲烷浓度与释放的影响.结果表明,降雨事件前后,库湾中游监测断面处水-气界面甲烷通量变化范围为0.011~0.326 mg·(m^(2)·h)^(-1),表现为大气甲烷的"源".风速和降雨均能够通过调节气体传输速率影响水-气界面的甲烷排放,其中风速的驱动作用更为显著.流域降雨导致的底部异重流抵达监测断面时,水体底层甲烷浓度明显升高,很可能受到上游及沿程输入的影响.此次来流量偏小不足以破坏水温分层结构,底部高浓度甲烷向上扩散过程中,氧化消耗作用明显,对表层甲烷浓度以及水-气界面甲烷释放的影响十分有限.

Methane is an important greenhouse gas and whether reservoirs act as a source or sink of methane has attracted great attention worldwide.However,unrepresentative sampling periods and a lack of consideration of unfavorable weather conditions have limited the accurate estimation of CH4 emission from reservoirs.This study focused on the middle reach of Xiangxi Bay in the Three Gorges Reservoir to track an entire rainfall-runoff event via on-site measurements in the summer of 2019,and initiatively investigated the impact of rainfall and inflow processes on methane concentration and emission.Results showed that from before to after the rainfall event,methane flux at the air-water interface ranged between 0.011 and 0.326 mg·(m^(2)·h)^(-1),indicating a net source of methane to the atmosphere.Both wind velocity and rainfall affected methane evasion from the surface by altering the gas transfer velocity,with the effect of wind being more prominent.Methane concentrations at the bottom layer significantly increased when rainfall-induced density flow from the watershed arrived at the sampling section.This was likely due to methane export from upstream and along the flow path.During this event,discharge was too small to destratify the water column,and methane was strongly oxidized as it diffused upwards,having little impact on surface methane concentrations and air-water methane flux.