Assessing and modeling impacts of different inter-basin water transfer routes on Lake Taihu and the Yangtze River, China
Introduction
Eutrophication has become ubiquitous in many lakes, reservoirs and other freshwater bodies affected by anthropogenic nutrient inputs over the past few decades (Paerl and Huisman, 2008, Qin et al., 2010, Schindler and Vallentyne, 2008, Smith and Schindler, 2009). Water transfer engineering, an important method for lake restoration, has been successfully used in many water bodies for accelerating water exchange, diluting polluted water, improving water quality and mitigating eutrophication issues by transferring large volumes of water from a relatively clean source to a severely polluted water body. There are currently over 160 large-scale inter-basin water transfer projects in 24 countries (Ghassemi and White, 2007, Wang, 2004), including the famous Snowy Mountain Scheme in Canada (Pigram, 2000), the California State Water Project in the United States (Davies et al., 1992), the Northern Siberian Rivers Diversion in the former Soviet Union (Voropaev and Velikanov, 1985) and the Ganges Water Diversion in India (Mirza, 2004). In China, major inter-basin water transfer projects include the man-made Great Canal from Beijing to Hangzhou city (Yao, 1998), the South-to-North Water Diversion Project transferring water from southern China to northern China by three different transfer routes (Liu and Zheng, 2002) and the water transfer project from the Yangtze River to Chaohu Lake (Xie et al., 2009).
The Yangtze River Water Diversion, transporting water from the Yangtze River to Lake Taihu, is another example of inter-basin water transfer engineering in China. Lake Taihu, the third largest freshwater lake in China, is suffering from severe eutrophication problems, threatening the water supply for the surrounding cities (Hu et al., 2008, Yang and Wang, 2003). In order to relieve eutrophication by enhancing water exchange in Lake Taihu, the Yangtze River Water Diversions have been built (Qin et al., 2010). Up to now, four different routes of this project have been implemented or planned (Fig. 1). Route One (the original route), implemented in 2002, transfers freshwater from the Yangtze River into Lake Taihu via the Wangyu River and discharges water through the Taipu River. Previous literature showed that Route One, as an emergency measure, could temporarily improve water quality and mitigate algal bloom in some lake regions excluding the most polluted area in Lake Taihu (i.e., Meiliang Bay, Zhushan Bay, Northwest and Southwest Zones) (Hu et al., 2008, Hu et al., 2010, Li et al., 2011a, Zhai et al., 2010). Route Two (the improved route) was applied in 2004 to improve the water exchange in Meiliang Bay in the northern lake region by adding two additional pump stations named Meiliang and Xingou around Meiliang Bay (Fig. 1) (JWRA, 2006). Li et al. (2013) showed that Route Two played a supplementary role for Route One in enhancing water exchange directly in Meiliang Bay. The optimal flow rate from the Wangyu River (Route One) was predicted to be 120 m3/s, and the corresponding appropriate outflow rate from the Meiliang pump station (Route Two) was about 15–20 m3/s based on the multi-objective optimization method. However, both Route One and Route Two did not significantly enhance water exchange in the northwest and western regions, the heavily polluted areas of Lake Taihu. Hence, Routes Three and Four have been recently designed to enhance water exchange in those polluted lake regions. Route Three is planned to bring fresh water from the Yangtze River to the northwest region via the Xinmeng River. Route Four is planned to take fresh water via the Changxing River to the southwest lake region. The design concept is to transfer water from the clean source into the specified hyper-eutrophic areas directly and flush the pollutants out of Lake Taihu. However, whether these diversions will work or not is difficult to predict due to the complex hydrodynamics of Lake Taihu.
Although previous papers have evaluated the effect of Routes One and Two on accelerating water exchange in Lake Taihu and obtained the optimal transferred flow rates for them (Li et al., 2011a, Li et al., 2013), it still remains unclear about the effects of Routes Three and Four. For example, how will the new-planned Routes Three or Four work for enhancing water exchange in the lake? How to set the optimal transferred flow rate for the single route or combination of the four routes to improve the lake's water exchange with a minimal economical cost? Additionally, the impact of water transfer diversions should focus on both the “receiver” (Lake Taihu) and “donator” (Yangtze River). However, previous research only assessed the impacts on the receiving system (Hu et al., 2008, Hu et al., 2010, Li et al., 2011a, Li et al., 2013, Zhai et al., 2010), ignoring the donating systems.
Thus, the hydrodynamic and hydrological impacts on both Lake Taihu and the Yangtze River and appropriate flow rates of different inter-basin water transfer routes will be assessed using the concept of water age and particle tracking based on a three-dimensional Environmental Fluid Dynamic Code (EFDC) model in this paper. The detailed objectives of this study were to: (1) understand the effect of single route of inter-basin water transfer on the hydrodynamic process of Lake Taihu and its appropriate transferred flow rate based on a multi-objective optimization program; (2) identify the optimal combinations and the corresponding flow rates for Lake Taihu in the algal bloom seasons and the non-algal bloom seasons, respectively; (3) assess the impact of water diversions on the hydrodynamic processes of the Yangtze River. This study aimed to help the local government and other decision-makers to better understand the effect of water transfer projects on the physical and hydrodynamic processes in Lake Taihu and the Yangtze River.
Section snippets
Study area
Lake Taihu, a well-known large shallow lake, is located in the lower Yangtze River delta between 30°56′–31°33′ N and 119°53′–120°36′ E, with an area of 2338 km2 and a mean depth of 1.9 m (Fig. 1, Qin et al., 2010). The lake retention time was about 181 days during the period from 1951 to 1988 and 309 days after the 1990s (Qin, 2008). Wind is a key driving force in the hydrodynamic processes of Lake Taihu. Wind direction around the lake changes with the seasons, with southeasterly winds prevailing
Numerical model description
EFDC, a three dimensional numerical model, initially developed by the United States Environmental Protection Agency (USEPA), was used to simulate the impact of water transfer on Lake Taihu and the Yangtze River including water level, current, water age (WA) and Lagrangian particle tracking (LPT). EFDC has been successfully applied to a wide range of environmental studies simulating circulation, thermal stratification, sediment transport, tracers, water quality and eutrophication process in
Effect of Route Three on water age in different lake regions of Lake Taihu
The effectiveness of Route Three on WA in Lake Taihu was assessed by the simulation scenarios of Group One (Table 1). The results for these scenarios showed that WA exhibited great spatial variability. Route Three mainly helped decrease WA in the western lake regions, and winds had strong impacts on the spatial distribution of WA (Fig. 4). Taking a 100 m3/s of inflow rate from Route Three as an example, for the entire lake, WA with no wind (235 days) was younger than that with the dominate wind
Effect of water transfer routes on Lake Taihu
Water transfer could enhance hydrodynamic processes, dilute polluted water, improve water quality, and lower the lake trophic level when the water quality of inflow from the “supplier” is relatively better than that in the “receiver” (Oglesby, 1969, Welch, 1981, Welch et al., 1992). The water transfer from the Yangtze River to Lake Taihu with various routes could play an important role as a short-term (emergency) measure to temporarily dilute the heavily polluted water, enhance water exchange
Conclusion
The impact of different inter-basin water transfer routes on Lake Taihu and the Yangtze River was assessed based on the EFDC model by using water age and Lagrangian particle tracking. The results showed that the appropriate transferred inflow rates were quite different for a single route or for the combination of routes, depending on the priorities of interest targets, such as lowest economical cost, maximal environmental improvement for specific lake regions or the entire lake. In general, two
Acknowledgments
The research was supported by Grant # 2010CB429003, 2010CB951101, Chinese National Science Foundation (51379061, 51009049 and 51179053), and Jiangsu Province Science Foundation (BK20131370). The research was also supported by Program for Excellent Talents in Hohai University, Qing Lan Project, the Innovation Program of Graduate Students in Jiangsu Province (CXZZ13_0270), China Scholarship Council, Grant # 40911130507, 2012ZX07506-002, IRT0717, 1069-50986312. We would like to thank the Taihu
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