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Mapping methane emissions from the three gorges reservoir area: A feasibility study
Step one: Delineate the study area
The study area includes the area that will be flooded in 2009. To delineate the study area, digital elevation model (DEM) and land cover map could be used. They both exist over the study area. And, radar data and TM/ASTER images could be used to improve the classification accuracy for the purpose of identifying the sources and sinks of methane, especially the classification of wetlands in this area.
Step two: Develop methane emissions model based on field measurements
Field measurements will be based on different land cover types. For non-inundated area, soil moisture, soil temperature, slope, aspect and vegetation biomass can be measured; for inundated area, water depth and aboveground vegetation biomass can be measured. The methane emissions can be measured by gas chromatography with the use of static chamber.
Based on the field measurement of potential affecting environmental variables, different multivariate analysis can be applied to derive methane emissions model for different land cover types for this area.
Step three: Map Vegetation biomass, soil moisture, soil temperature, slope, aspect and water depth
To spatially extrapolate methane emissions values for the study area, maps representing environmental variables relevant to methane emissions according to the above-mentioned model could be derived from satellite data and DEM:
Vegetation biomass map can be derived from NDVI image using an empirical relationship between field measurements of NDVI and image NDVI. The results from Li and Dong (1996) indicated that microwave remote sensing would be great potential for monitoring soil moisture [6]. Soil moisture map can be derived from Radar SAR data using an empirical relationship between SAR signal (after geometric and radiometric corrections) and soil moisture measured in the field. Soil temperature map can be calculated by many approaches [5], among which is using the relationship between soil temperature field measurements and net solar radiation derived from DEM. Slope and aspect can be derived directly from DEM. Water depth of water body and wetlands could be derived using both existing hydrological data and DEM.
Step four: Map methane emissions
The maps of affecting environmental variables will then be combined by using the empirical methane emissions model to map and predict the methane emissions over the three gorges reservoir area.
Conclusions
For such a huge anthropic activity, three gorges dam project deserves long-term concern on its impact on environment. Mapping methane emissions over the area is possible. By means of a remote sensing approach, maps representing environmental variables relevant to methane emissions can be derived and then are combined to map and predict the methane emissions over the three gorges reservoir area. Soil moisture, soil temperature, slope, aspect, vegetation biomass and water depth are considered as affecting parameters in mapping methane emissions. The result of this study shows the viability of remote sensing as an indirect means of mapping and predicting methane emissions from inundated lands. This made possible that mapping the distribution (sources/sinks) of methane emissions of current year and 2009; mapping the changes of methane emissions caused by the ongoing dam project and calculate the sum of methane emissions to present the impact of three gorges dam on methane emissions and furthermore, long-term impact on climate. A small final comment is that as the reliability of the result heavily depends on empirical models based on field sampling, the dynamics of emissions rate deserves further study to find the representative sampling time.
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