夏季降雨事件对水库温室气体通量变化的影响:来自湖北官庄水库的高频观测

Impacts of summer rainfall events on the dynamics of greenhouse gas fluxes revealed by high-frequency observation from Guanzhuang Reservoir,Hubei Province

湖库淡水水域对温室气体排放的贡献不容小觑,然而观测时间的代表性不足以及缺乏对降雨因素的考虑制约了碳排放的准确估计.本研究以湖北宜昌境内官庄水库为例,选取强降雨多发的夏季时段,针对水气界面温室气体通量、水体表层和垂向剖面气体溶存浓度及环境因子开展了为期1周的原位高频观测,以探讨夏季降雨事件对水库温室气体通量变化的影响.结果表明,观测期内官庄水库水气界面CH_(4)通量变化范围为0.007~0.077 mg/(m^(2)·h),CO_(2)通量范围为5.48~57.57 mg/(m^(2)·h),白天和夜晚均表现为大气的碳源.小雨、中雨乃至暴雨天气条件下,CH_(4)和CO_(2)日均通量均较低,日通量倾向于受风速和温度调控.CH_(4)和CO_(2)通量变化趋势较为一致,观测期内日间排放量高于夜间排放量出现的次数更多,昼夜差异对降雨天气状况无明显响应,风速是CH_(4)和CO_(2)通量昼夜变化的主导因素.暴雨过程中,CH_(4)-k 600和CO_(2)-k 600与水气温差存在显著正相关,但水体垂向混合过程十分短暂.在平均雨强为3.8 mm/h的暴雨第I阶段,CH_(4)-k 600对风速和降雨的响应明显,而在雨强更大(8.5 mm/h)的第II阶段,CH_(4)-k 600与风速、降雨均未表现出相关性,通量箱在强降雨条件下的适用性可能存在雨强阈值.

Lakes and reservoirs are recognized to significantly contribute to global emissions of greenhouse gases,but unrepresentative sampling periods and a lack of consideration of rainfall conditions have limited the accurate estimation of carbon emission.This study took the Guanzhuang Reservoir in Yichang,Hubei Province,as an example,selecting summertime which has more frequently intense precipitation.In-situ high-frequency observations of greenhouse gas fluxes at the air-water interface,dissolved gas concentrations and environmental variables at the surface and over the vertical profile were implemented for a week,to investigate the impacts of summer rainfall events on the dynamics of greenhouse gas fluxes.Results showed that CH_(4) flux ranged between 0.007 and 0.077 mg/(m^(2)·h),and CO_(2) flux was from 5.48 to 57.57 mg/(m^(2)·h)at the air-water interface of the Guanzhuang Reservoir during the monitoring period,indicating a net source of carbon to the atmosphere during both day and night.Under light rain,moderate rain,and even storm conditions,daily average fluxes of both CH_(4) and CO_(2) appeared to be relatively low.Daily fluxes tended to be regulated by wind speed and temperature.CH_(4) and CO_(2) fluxes exhibited similar trends of variation,and daytime flux>nighttime flux was more often detected during the observation period.Difference between daytime and nighttime fluxes was not influenced by rainy weather conditions,but wind speed was the dominant factor affecting diurnal variations of CH_(4) and CO_(2) fluxes.During the storm event,CH_(4)-k 600 and CO_(2)-k 600 were significantly positively correlated with the temperature difference between water and air,but the convective mixing was in a very short time.At phaseⅠof the storm with an average rainfall intensity of 3.8 mm/h,CH_(4)-k 600 responded to wind speed and rainfall significantly.While during phaseⅡof the storm with a higher rainfall intensity(8.5 mm/h),CH_(4)-k 600 was not correlated with wind speed and rainfall.It suggested that there might be a threshold value of rainfall intensity for the applicability of floating chambers under intense rainfall conditions.