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Mapping methane emissions from the three gorges reservoir area: A feasibility study
Hu Yong
School of Resource and Environmental Science, Wuhan University,
Luoyu Road 129#, Wuhan City, 430079,P.R.China
Email: yonghu@itc.nl
Introduction
The three gorges dam (China) is under construction and will be finished in 2009. This ongoing dam project has been attracting worldwide attention. It will be the largest hydroelectric dam in the world. Both positive and negative effects exist. Generation of hydro-electricity will reduce the pressure from lack of power for the rapidly developing country; seasonally flooded downstream of Yangtze River will be under better control. However, issues like displacement of almost two million people, rapid silting up of reservoir, destroy of history cultural sites and rare biotic species received disputation.
As a large area to be flooded with the river closure work going on, methane will be generated by microorganisms living in the oxygen-poor environments. The three gorges dam is thought to be significant source of atmospheric methane and consequently emitted methane, as an important greenhouse gas, will affect the climate.
Surprisingly, no scientific study on the total amount and spatial distribution of methane emissions over the three gorges reservoir area is carried out yet. Only very recently, the concern about the emissions of methane from flooded lands as a result of constructing huge dam has been expressed [4].
Some research indicates that dams and their associated reservoirs are globally significant sources of emissions of the greenhouse gases, carbon dioxide and, in particular, methane. Vincent St. Louis estimated that reservoirs of all types and sizes worldwide release 70 million tons of methane annually, which equals about one-fifth of the estimated methane emissions from all other human activities [4].
Thus, carrying out a research aiming to map the methane emissions from the three gorges reservoir area would be of significance. The major benefits from the research results to the society and the major scientific significance include:
- Develop innovative methods to predict the spatial distribution of methane emissions over large reservoir area which is under construction;
- Give a scientific answer on the amount of methane emissions caused by this controversial hydroelectric project;
- Provide the Chinese government with necessary information for management of the project to reduce the emissions of green house gas;
- Provide with scientific data for the study on China's climate change or even global climate change.
This paper will explore the feasibility of utilizing remote sensing (RS) data for mapping CH4 emissions from the three gorges reservoir area. It will start with an overview of a few methane emissions mapping projects, which were already carried out, followed by a background introduction on three gorges dam project and its flooded area. Next will be a discussion on the feasibility of a proposed research approach, comprising four steps. Finally it will end up with a conclusion.
An overview of methane emissions mapping projects
The concern on the global warming made the study on the measurement of methane emissions a hot research field. Researches have been carried out over many areas that are major sources of CH4. Among those, paddy fields have been widely and frequently studied globally [2,13,14,15]. Some researchers showed interests in the emissions from wetlands [8,10]. There is only limited study on the swine houses [11] and landfills [7]. One study was on the Arctic tundra by using a remote sensing approach to map the methane emissions [12]. There is no study over reservoir areas and flooded areas except that Prsenqvist, et al (1998) estimated methane emissions from the Jau river floodplain in Brazil [9].
Generally two approaches were taken in these studies. Field measurement based approach was most frequently applied. Static chambers were often used to collect methane in sample areas and then followed by measuring the sample gas using gas chromatography. The total emissions over the study area will be a product of direct measurement and areas of same type of fields. It can provide accurate results in a homogeneous area. Wang et al (1994) designed continuous measurement system for methane emissions from paddy field for prediction and this system could reduce uncertainty in the estimation of CH4 [14]. While studying on a large scale where the area is heterogeneous, indirect approach was introduced. Empirical models were often developed to relate CH4 emissions to affecting factors [2,13,15]. Soil properties, such as soil temperature, soil moisture, and other environmental variables were found that there are strong relationship between them and CH4 emissions. When studying over a large area, remote sensing becomes an interesting approach. Balloon-mounted/ airplane-mounted laser spectrometry was used to directly measure methane emissions [3]; satellite images were also used in some studies in a regional scale [15], but in an indirect way. For instance, Yan M.,et al (2000) tried satellite data to derive the areal extent of study area to calculate the total emissions over Changchun area.
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