Influence Of The Change Of Water Level Onthe Soil Gas Release In Three Gorges Reservoir

The change of water level is an important restrictive factor for the release of gas from the riverbank soil,which regulates the carbon cycle and climate change of the Earth system.How the change of water level in the Three Gorges reservoir area affects the release of soil gas in the reservoir area is urgently to be studied.In this paper,the spatial-temporal distribution of soil gas concentration and release rate in the downstream of the pengxi River and its correlation with the indicators of water index and soil physical and chemical properties were analyzed.Preliminarily clarified the key factors affecting the gas release process of the beach soil in the Three Gorges reservoir area.The conclusions are as follows:(1)Water regime characteristics in the Three Gorges reservoir area: It is mainly affected by the water flow from the upper reaches of the Yangtze River and the tributaries of the reservoir area.The change of water regime in the centre area of the Three Gorges reservoir is similar to that of the tributary flow,but the trend of the water level changes is contrary.In the flood season,the water level of the main stream is relatively low and the tributary water level is relatively high,while in the dry season,the water level is opposite.At the same time,the water level of the Wujiang River which in the Three Gorges reservoir and the Jialing River in the terminal reservoir area is affected by the return water while reservoirs storing water in autumn and winter.(2)Under the condition of the water level lower in the Three Gorges Reservoir area,there is a high positive correlation between CO2 concentration and release rate in the riverbank soil,and the temporal and spatial variation trends of the two are basically consistent,indicating that the soil CO2 concentration has a strong regulation effect on its release rate.On the monthly scale,with the gradual subsidence of the water level,the CO2 concentration and gas release rate in the near-surface soil are gradually increased with the decrease of altitude.At the same time,the soil CO2 concentration and its release rate both showed a minimum at 170.5 meters at the end of January,and they also showed a maximum at the end of March.When the water level decreased to the lowest reservoir of water level(145 m),the soil CO2 concentration in the natural section at 165.7 m was significantly higher than the temporary ploughing area above this elevation.(3)In the case of changes in water level in the Three Gorges Reservoir Area,cluster analysis was made on the important factors affecting the release rate of soil gas.The results showed that the influencing factors can be divided into two categories: soil physic-chemical properties(soil moisture content,nutrients,ammonium nitrogen,organic carbon and heavy metals)and external conditions(temperature).Further subdivision can be divided into 5 categories,namely soil moisture content,nutrient salt,soil organic carbon,total heavy metals and temperature.(4)The correlation analysis showed that the nonlinear correlation between soil gas release rate and the impact factors was higher than the linear correlation,with the highest correlation with organic carbon content and soil moisture content,followed by temperature,heavy metals and other impact factors.When the temperature is between 20~25°C and the soil moisture content is about 30%,the soil gas release rate can maintain a relatively high value;with the shorter the water level receding,although the organic matter content in the beach soil is high,The rate of soil gas release was significantly reduced.(5)Factor analysis shows that many influencing factors can be combined into two principal components,that is,principal component 1 that characterizes soil heavy metal conditions and principal component 2 that characterizes soil nutrient conditions.Factor analysis functions F1 and F2 were calculated and a non-linear curve between soil gas release rate and F1 as well as F2 was fitted.The results showed that the nutrient component has a stronger influence level with the release rate of soil gas.Considering comprehensively,the restrictive factors are mainly soil moisture content and temperature,and the remaining indicators can better explain the changes in soil gas release.