Impacts of the Yangtze River water transfer on the restoration of Lake Taihu
Introduction
Lake Taihu is the third largest freshwater lake in China, with a water surface area of 2338 km2. Its basin area is 36,500 km2, belonging to Jiangsu, Zhejiang, and Anhei Provinces and Shanghai City (Sun et al., 1993). The basin area accounts for 0.4% of the Chinese territory land, but the GDP in the area accounts for 11% that of China (Yang et al., 2003). The population in the basin is 36.86 million, accounting for 3% in the whole country. GDP per capita in the area is US$ 3000, which is 3 times the country's average. The ratio of urbanization is 52%. All these data indicate that Lake Taihu has played an extremely important role in the social and economical development in China (Yang et al., 2003). Lake Taihu has multiple service functions (e.g., drinking water resource, irrigation, shipping, fishery, tourism, etc.).
With the economic and social development and the population increase, more pollutants have been discharged into Lake Taihu, resulting in water pollution and eutrophication in the lake. Algae bloom and its perils have become serious environmental problems in this area, which have led to the lack of qualified water resources for the local people and economy. In the summer of 1990, algae bloom distributed in the entire Meiliang Bay and lasted for more than a week, causing 118 plants to stop working and the townspeople to lack appropriate of qualified drinking water in Wuxi City (Pu et al., 1993). In recent years, the algae bloom has expanded from Meiliang Bay to the centre of Lake Taihu. The northern and western part of the lake in summer, autumn and even spring is often covered by algae bloom. Much more effort has been put into the restoration of the lake since 1991 (Pu et al., 1998).
To improve water quality and alleviate the algae bloom, two experimental water transfers from the Yangtze River to the Lake were conducted from 2002 to 2003. The water transfer route is shown in Fig. 1. Wangyu River is the channel of water from the Yangtze River to Lake Taihu, and is connected with many side rivers. These side rivers are seriously polluted. In order to ensure the water entering Lake Taihu is of relatively high quality, the floodgate of Wangyu River to Lake Taihu was regulated according to the condition of the river water quality. When the river water quality was not good, the floodgate was not raised to make the water level in Wangyu River rise, to extrude its polluted water to side rivers and to prevent the polluted water in these side rivers from entering it. When the water quality in Wangyu River was improved to the level of that in the Yangtze River, the floodgate was raised and the water entered Lake Taihu by Gonghu Bay, a bay of Lake Taihu with a surface area of 21,560 km2. In order to keep the water balanced and to avoid the peril of flood in rainy seasons, water in the lake was discharged through River Taipu on the southeast side of Lake Taihu. From 31 July to 23 August in 2000 about 222 million m3 of water was transferred from the Yangtze River for the preliminary experiment. In 2002, there were five experiments of water diversion, from 30 January to 4 April, from 7 to 12 August, from 26 September to 11 October, from 19 to 31 October and from 5 to 9 December, respectively. These experiments had transferred 681.8, 18.46, 24.31, 50.10 and 9.71 million m3 of water, respectively. The total amount of water transferred in 2002 was 784.38 million m3 (Hu, 2004). In 2003, 1109 million m3 of water in all was translated from 6 August to 17 December (Hu, 2004).
As the concentration of phosphorus is relatively high and the concentration of nitrogen is relatively low in the Yangtze River compared with those in the outflow of Lake Taihu, it is doubtful that the water transfer would lower the ratio of nitrogen to phosphorus to be close to 7:1 and benefit the growth of algae in the lake. Furthermore, it is arguable whether the water transfer could lower concentration of phosphorus and nitrogen in the lake, since the concentration of phosphorus and nitrogen in the transferred water is higher than that in the effluent through River Taipu. It is also argued that the more water is transferred to the lake, the more nitrogen and phosphorus Lake Taihu has, and the more eutrophic it becomes. This paper aims to assess the environmental effect of the water transfer and to discover the possibility of improving the lake water quality by transferring water from the Yangtze River. The following section describes the methods and the materials. Section 3 describes the results. Section 4 is the discussion. Finally, our conclusion is given in Section 5.
Section snippets
Material and methods
As the flux of water transferred to Lake Taihu is small compared with the amount of water in Lake Taihu, and the transfer has to last very long time, the natural factors such as wind, air temperature, solar radiation, precipitation, runoff and pollutant discharged would change during the transfer period. These changes would definitely result in the variations of nitrogen, phosphorus, algae biomass, dissolved oxygen, etc. (Hu et al., 1998a, Hu et al., 1998b, Hu et al., 2002, Cioffi and
Model parameters
Water transfer began on 4 January and ended on 4 April, and the transferred water entered Lake Taihu on 30 January. The cumulative amount of water transferred to Lake Taihu and the cumulative net input of water are shown in Fig. 5. The total amount of water transferred to Lake Taihu is 6.804 × 108 m3 and the total effluent amount of water via Taipu is 6.706 × 108 m3. The net input of water due to the water transfer in the entire period is only 0.1 × 108 m3, which could raise the water level by 0.43 cm.
The mechanism of change of water quality
The percentage of the area being improved in early days resulted from the change of location of the water body driven by the water transfer in Gonghu Bay. In general, as the influent rivers are located in the western and northern parts of the lake and the effluent rivers located in the eastern and southern parts of the lake and there is west to east and north to south current, the water quality becomes better and better from west to east and from north to south in Lake Taihu due to the self
Conclusion
The calibrated and verified EcoTaihu model was used to assess the environmental impact of two experimental water transfers from the Yangtze River to Lake Taihu. The results show the experimental water transfers in winter–spring 2002 and in summer–autumn 2003 both having notable positive effects on debasing the concentration of phytoplankton, total nitrogen and dissolved oxygen in some sub-areas. The positive effects depended on water parameters and sub-areas and fluctuated with the duration of
Acknowledgements
This work has been carried out within the frame work of the research project “A demonstrative research on lake ecosystem structurally dynamic model in Lake Taihu” (NSFC 30670351) and knowledge innovation major projects of the Ministry of Water Resource, P.R.C. The authors would like to thank the Water Resources Conservation Bureau, Taihu Basin Authority, MWR for providing monitoring data on Lake Taihu, the water fluxes and matter discharge of influent and effluent rivers, and Professor Sven E.
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