Is a Dammed Lake Poyang a Damned Lake Poyang?
China's largest freshwater lake is shrinking.
Lake Poyang is China’s largest freshwater lake and is the habitat for half a million migratory birds and an important resource that affects the livelihood of many.
The Gan, Xin, and Xiu Rivers connect to the Yangtze River which directly feeds into the lake. Water is being pulled upstream from the Yangtze River which is causing Lake Poyang to shrink and create negative environmental and economic impacts.
Spanning the Yangtze River to the north is the Three Gorges Dam which was designed to generate electricity, increase the shipping capacity of the river, and reduce the potential for flooding downstream. However, the construction of the dam has resulted in ecological and economic issues.
The Three Gorges Dam spanning the Yangtze River (Chang Jiang) near Yichang, Hubei province, China. https://www.britannica.com/topic/Three-Gorges-Dam
The area of Lake Poyang naturally fluctuates between wet and dry seasons, but satellite images over the past years have revealed that it appears the overall area of the lake is shrinking at an alarming rate. Landsat imagery and spatial analysis techniques in ArcGIS Pro were used to compare the size of the lake in June of 1984, June of 2001, and May of 2014.
Landsat images of Lake Poyang from 1984, 2001, and 2014 (left to right).
Analysis of Landsat Imagery Using ArcGIS Pro
Step 1: Iso Cluster Unsupervised Classification Tool.
This tool uses statistical analysis to decide which pixel values are similar enough to group together into classes. Each image was processed into four classes. The default output for Value 1 is gray and represents water pixels.
Comparison of the three Landsat images after applying the Iso Cluster Unsupervised Classification tool. 1984, 2001, and 2014 (left to right)
Step 2: Majority Filter Tool
This tool uses generalization analysis tools to smooth water boundaries and remove smaller water features that aren’t part of the lake. Cells in the image are replaced based on the value of the majority of neighboring cells. The resulting layers from Iso Cluster Unsupervised Classification were filtered to smooth boundaries.
Visible differences in the number of small water pixels between the 1984 layer after Step 1 (left) and Step 2 (right).
Step 3: Boundary Clean Tool
Smooths boundaries between classes by expanding boundaries then shrinking them back to the original size. It removes individual pixels and replaces them with the value of pixels around them. The smoothed boundaries were cleaned up and resulted in a final layer to use for area calculations.
Visible differences in the lake boundaries between the 2014 layer after Step 2 (left) and Step 3 (right).
Step 4: Calculate Lake Area Over Time
A new field, “Hectares,” was added to the attribute table of each final layer. Each cell size is 30m x 30m. Using this information, the “Hectares” field was calculated using the formula Hectares = (!count!*900)/10000.
Results
The analysis results were published as a web map ( Lake Poyang Levels_ESchneider - Overview (arcgis.com) ) and area values are summarized in Table 1.
Web map of Lake Poyang Levels in June of 1984 (orange), June of 2001 (dark blue), and May of 2014 (light blue).
In a 30-year period, Lake Poyang has lost 67,345.8 hectares of area covered by water. As of May 2014, nearly 25% of the total area from June 1984 is now dried, exposed lakebed.
June 1984 | June 2001 | May 2014 | |
|---|---|---|---|
Total Lake Area (Hectares) | 269,654.9 | 248,038.5 | 202,309.1 |
% Change from Previous Image Year | ______ | 8% | 18.4% |
% Change from June 1984 (30 years) | ______ | ______ | 25% |
Table 1: Lake area and percent changes calculated from each Landsat image.
Conclusion
With longer dry seasons and no change in water management, Lake Poyang will undoubtedly continue to face a shrinking shoreline. With this trend, it is important to evaluate water manipulation and usage then develop sustainable ways to continue to use this resource while mitigating the rate of lake area being lost. A proposal for the construction of a dam to maintain water levels in the lake is currently undergoing an environmental impact assessment. This dam would consist of a sluice wall across the connection between the lake and the Yangtze River and some environmentalists argue that artificially controlling water levels will negatively impact wildlife. What cannot be argued, however, is the fact that as water supply decreases, the severity of hardships for humans and organisms that depend on this resource will increase.
