Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River

International Journal of Sediment Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Original Research

Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River Minglong Dai a,b,n, Jun Wang b, Mingbo Zhang b, Xi Chen b a b

School of Hydropower & Information Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Bureau of Hydrology, Changjiang Water Resources Commission, Wuhan 430010, China

art ic l e i nf o

a b s t r a c t

Article history: Received 1 July 2016 Received in revised form 10 January 2017 Accepted 22 February 2017

The Three Gorges Project (TGP) is a world known project to utilize and manage the water resources of the Yangtze River. The reservoir stores water at the end of the flood season, and replenishes downstream reaches with water in dry seasons. In addition to such benefits, the TGP has irreversibly changed the hydrological process and the river-lake relation of the middle and lower reaches. In this paper, a hydrodynamic model was established to quantify the impact of the TGP's operation on the water exchange between Dongting Lake and the Yangtze River during 2009–2013. The results indicated that: the operation of the TGP has considerably reduced the peak discharge and the flood volume of the main stream and the Dongting Lake area. The inflow volume from the Yangtze River to Dongting Lake via three outlets decreased by 1.9–3.5 billion m3/yr, while the outflow volume from Dongting Lake to the Yangtze River at Chenglingji increased by 0.3–1.6 billion m3 in September and 0.4–0.6 billion m3 in October, respectively. This research provides valuable information for flood control, irrigation, and water allocation in the middle and lower reaches of the Yangtze River, and serves as a typical case for investigating the impact of other hydropower projects around the world. & 2017 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Keywords: Three Gorges Project Yangtze River Dongting Lake Three outlets diversion Water exchange

1. Introduction Economic development, industrialization, and urbanization have driven up the demands on water resources and hydropower, which can be met by building more dams and reservoirs. Approximately 50,000 large dams (4 15 m high) and millions of small reservoirs have been constructed to meet the needs of social and economic development throughout the world (World Commission on Dams, 2000). The global gross reservoir capacity is 10,000 km3, almost 5 times the aggregated river flow. More than half of the 300 biggest river systems around the world are controlled by or under the impact of dams. The relation between the main stream and tributaries as well as the aquatic ecological environment downstream are in the process of irreversible change (Nilsson & Berggren, 2000). There has been growing concern about the impact of dams on the aquatic ecological environment of drainage basins. Various studies show that large dams not only altered river flows but also resulted in negative ecological impacts, including the loss of biodiversity in aquatic and wetland n Corresponding author at: School of Hydropower and Information Engineering, Huazhong University of Science & Technology, Wuhan 430074, China. E-mail address: [email protected] (M. Dai).

ecosystems (Benjankar et al., 2012; Cappellen & Maavara, 2016; Grill et al., 2015; Pelicice et al., 2015). Therefore, dam construction is usually accompanied by disputes about the benefits of the dam and the costs of negative ecological impacts. The Three Gorges Project (TGP) is a key water resources project intended for the management and development of the Yangtze River. It has comprehensive benefits including flood control, power generation, navigation, etc. The TGP has many benefits, however, it has inevitably changed the natural hydrological situation of the downstream reaches. Sediment trapping and release of clear water cause scouring of the river channel in the lower reaches, which also makes the supplementary tributary flow for the main stream unsteady, and results in complicated changes to the runoff and water level of the Yangtze River main stream (Fang & Rodi, 2003; Fang & Wang, 2000). After the TGP was put into operation, the scouring of the river channel decreased the medium and low water levels, and reduced the water volume diverting from the main stream into the Dongting Lake area via the three outlets (namely Songzi, Hudu, and Ouchi). Since the operation of the TGP in 2003, water levels close to the historical lowest levels in the middle and lower Yangtze River were frequently observed in 2006, 2007, 2009, and 2011 (Min & Zhan, 2012). The three outlets river system continued to shrink while the water area of Dongting Lake decreased (Chang et al., 2010; Ou et al., 2014). The drop of water level in the main stream downstream

http://dx.doi.org/10.1016/j.ijsrc.2017.02.006 1001-6279/& 2017 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Dai, M., et al. Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.02.006i

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from the TGP during the storage period of the TGP accelerated the outflow from Dongting Lake, and made the dry season in the lake area come earlier (Lai et al., 2014; Wu et al., 2015). The hydrological situation of the Yangtze River influenced by the TGP needs to be clarified for the sake of providing better guidance to water administrative departments for water allocation and risk management. Fang et al. (2012) used a one-dimensional mathematical model to simulate flow and fluvial processes based on the field data of hydrology and sediment transport for forecasting the erosion and deposition in the middle and lower reaches of the Yangtze River over the next two decades. Lai et al. (2013) presented a coupled hydrodynamic analysis model designed for simulating the large-scale water system in the middle Yangtze River Basin considering seasonal wetting and drying controlled by complex river-lake interactions. They concluded that the hydrological situation in the middle and lower reaches of the Yangtze River would experience new changes in post TGP era. This study aims to determine the impact of the TGP's operation on the water exchange between Dongting Lake and the Yangtze River by comparing two scenarios, with and without the TGP operation. A description of the study area and river network is presented, followed by an introduction of the 1-D hydrodynamic model capable of simulating the flow processes in the study area. By comparing the results of the hydrodynamic outputs of two scenarios, the influence of the TGP on the water exchange between Dongting Lake and the Yangtze River is quantified, and finally conclusions are drawn after discussion.

2. Study area and river network The Yangtze River is the third longest river of the world with a catchment area of 1.8
106 km2. It is a golden waterway that links eastern, central, and western parts of China providing a lifeline of sustainable economic and social development. The TGP is located at Sandouping town, Yichang City, Hubei Province. The normal water level of the reservoir is 175 m, with a total capacity of 39.3 billion m3. Total length of the reservoir is more than 600 km with a water area about 1084 km2. The primary operation of the TGP began in June 2003 with the operation levels of 135 m in the flood season and 139 m in the dry season, respectively. In 2006, the operational water level became 144–145 m in the flood season and was raised to 156 m after the flood season. On October 26, 2010, the water level reached 175 m, making it possible to deliver the comprehensive benefits of flood control, power generation, navigation, etc, for which the TGP was designed. Dongting Lake is the second largest freshwater lake in China and an important part of flood control system of the middle and lower reaches of the Yangtze River. The natural water area of Dongting Lake is 2625 km 2 with a capacity of 16.7 billion m 3 and the annual average outflow is nearly 300 billion m 3 . Normally, Dongting Lake is divided into three sublakes, i.e. East Dongting Lake, West Dongting Lake, and South Dongting Lake. The entire lake has a very complex terrain and morphology. The bathymetry of most of the bottomlands ranges from 22 to 27 m in East Dongting Lake, 24 to 29 m in South Dongting Lake, and 27 to 30 m in West Dongting Lake (Lai et al., 2013). There are many traverse rivers with uncertain flow directions in the river network in the Dongting Lake area. All these tributaries form a complicated river network system with very perplexing hydraulic relation. After being regulated by the TGP, the water from the upper reaches of the Yangtze River flows through Zhicheng Station, and a portion enters Dongting Lake via the three outlets joining with the water from Xiangjiang, Zishui, Yuanjiang, and Lishui rivers, and

finally drains into the Yangtze River again at Chenglingji running through Luoshan Station on the main stream. The research region of this study is shown in the red boundary zone in Fig. 1. Information on the hydrological stations considered in this paper are listed in Table 1.

3. Model building and calibration 3.1. Model introduction The MIKE-11 modelling system (DHI, 2003) was applied in the study by using the 1-D hydrodynamic module (HD) and the Nedbør Affstrømnings Model (NAM) module (Singh et al., 2014). The NAM module is a lumped conceptual modelling tool for simulating rainfall-runoff processes. It accounts for the moisture content in four different and mutually interrelated storages that represent physical elements of the catchment. The four storages are: snow storage (optional), surface storage, lower zone storage, and ground water storage. These storages are connected by equations that simulate the land phase hydrological cycle (Solomatine & Torres, 1996). The HD module is the core of MIKE-11. An implicit finite-difference, 6-point Abbott-Ionescu scheme (Abbott & Ionescu, 1967) is implemented to solve the De Saint-Venant equations. The HD module receives the runoff hydrographs from the NAM module and treats them as boundary conditions for computation of water levels and discharges along the river system. Bed resistance can be described by Manning's roughness coefficients (DHI, 2003; Solomatine & Torres, 1996). In this study, The HD module was used for the flow calculations in the river network of the study area, while receiving runoff hydrographs from the uncontrolled sub-basins computed using the NAM module. 3.2. Model setting 3.2.1. River system generalization The river network for simulating the water exchange between Dongting Lake and the Yangtze River is generalized as shown in Fig. 2. Apart from the main stream between the TGP and Luoshan Station, the Qingjiang, Xiangjiang, Zishui, Yuanjiang, Lishui, Songzi, Hudu, Ouchi rivers, Dongting Lake area, and other diversion channels and secondary tributaries were included in the simulated network as well. 3.2.2. Sub-basins division and boundary conditions In this model, the Yichang hydrological station is the upper boundary on the main stream of the Yangtze River, while Luoshan Station is the outlet boundary. The study area can be divided into seven sub basins, i.e. Qingjiang Basin, Xiangjiang Basin, Yuanjiang Basin, Zishui Basin, Lishui Basin, Dongting Lake area, and YichangLuoshan area. The locations of these sub-basins are shown in Fig. 1 while detailed information on the sub-basins is listed in Table 2. Five of the sub-basins were measured by an individual hydrological station. For two of the sub-basins no measured data are available, and thus the NAM module was applied to simulate the hydrograph. The daily areal average precipitation data and the monthly potential evapotranspiration data for the same period were used as the input to the NAM module, and the calculated runoff was input to the HD module as surface source lateral inflow. 3.2.3. Profile data The river section measurements of the study area in 2012 were used in the simulations. The interval between measurements on the main stream of the Yangtze River between Yichang Station and Luoshan Station was about 2 km, while at least three

Please cite this article as: Dai, M., et al. Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.02.006i

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Fig. 1. Locations of the Three Gorges Project (TGP) and the Dongting Lake area.

representative sections of each tributary in the Dongting Lake region were used. 3.2.4. Roughness parameters According to the morphological characteristics of the river channels and observed hydrological data, roughness at different water levels was divided into three sections. Manning's roughness coefficients of the main stream at different sections is listed in Table 3. 3.3. Model calibration and validation The Nash-Sutcliffe coefficient of the model-fit efficiency, ENS, (Nash & Sutcliffe, 1970) and relative error, RE, (Bennett & Briggs, 2008) were used to evaluate the simulation accuracy of the model. The coefficients are computed as follows.

1. The Nash-Sutcliffe coefficient, ENS, was used to determine the degree of fit between simulated and observed flow. As the value of ENS gets closer to 1, this means the simulated flows are getting closer to observed flows. The calculation equation was as follows:

n P

ENS ¼ 1 

ðQ pi  Q oi Þ2

i¼1 n  P i¼1

Q oi  Q o

2

ð1Þ

where Q 0i is observed flow at time i, Q pi is simulated flow at time i, Q 0 is the average observed flow, and n is the number of observations.

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Table 1 Information on the main hydrological stations in the study area. Number

Station name

Longitude (deg.)

Latitude (deg.)

Catchment area (km2)

River

Data series

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Yichang Zhicheng Shashi Luoshan Xinjiangkou Shadaoguan Mituosi Ouchi (Kang) Ouchi (Guan) Chenglingji Xiangtan Taojiang Taoyuan Shimen Gaobazhou

111.28 111.50 112.23 113.37 111.78 111.92 112.12 112.30 112.32 113.13 112.92 112.10 111.48 111.38 111.37

30.70 30.30 30.32 29.67 30.18 30.18 30.22 29.73 29.73 29.42 27.87 28.55 28.90 29.62 30.42

1,005,501 1,024,131 1,032,033 1,294,911

Yangtze River Yangtze River Yangtze River Yangtze River Songzi River Songzi River Hudu River Ouchi River Ouchi River Outlet of Dongting Lake Xiangjiang River Zishui River Yuanjiang River Lishui River Qingjiang River

2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013 2009–2013

81,638 26,704 85,223 15,307 15,650

Table 3 Manning's roughness coefficient along the main stream of the Yangtze River.

Fig. 2. River system generalization map of the study area. Table 2 The detailed information on the sub-basins. Sub-basins

Catchment area (104 km2)

Measured at

Drain into

Qingjiang Basin Xiangjiang Basin Zishui Basin Yuanjiang Basin Lishui Basin Dongting Lake area Yichang-Luoshan area

1.63 8.54 2.34 8.26 1.67 3.13 2.03

Gaobazhou Station Xiangtan Station Taojiang Station Taoyuan Station Shimen Station Unmeasured Unmeasured

Yangtze River Dongting Lake Dongting Lake Dongting Lake Dongting Lake Yangtze River Yangtze River

2. Relative error, RE, was used to calculate the ratio of “the difference between simulated flow and observed flow” to “the observed flow”. It can reveal the bias of a simulation. The calculation equation is as follows.

RE ¼

Qp Qo  100 Qo

ð2Þ

where Q 0 is observed flow, and Q p is simulated flow. The evaluation criteria of accuracy (Moriasi et al., 2007) were used to quantify the simulation accuracy of the model. The detailed results of the Nash-Sutcliffe coefficient are listed in Table 4. The results indicated that the simulation accuracy of flow

Distance from TGP (km)

Low water level

Middle water level

Water level (m)

Roughness Water level (m)

Roughness Water level (m)

0 56.655 58.434 88.421 91.099 144.111 145.560

o 45 o 45 o 40 o 40 o 40 o 40 o 35.5

0.028 0.028 0.024 0.024 0.023 0.023 0.028

0.025 0.025 0.025 0.025 0.022 0.022 0.028

449 449 443 443 443 443 438.5

0.02 0.02 0.02 0.02 0.017 0.017 0.02

195.992

o 35.5

0.028

0.028

438.5

0.02

197.572 210.690 212.240

o 32 o 32 o 33

0.028 0.028 0.028

0.026 0.026 0.027

436 436 434.5

0.021 0.021 0.018

229.393

o 33

0.028

0.027

434.5

0.018

232.003 262.165 264.458 297.557 298.791

o 31 o 31 o 30 o 30 o 28

0.027 0.027 0.037 0.037 0.03

0.026 0.026 0.037 0.037 0.03

434 434 433 433 430.5

0.021 0.021 0.024 0.024 0.023

372.843

o 28

0.03

0.03

430.5

0.023

378.909 408.307 411.131

o 26 o 24 o 22

0.033 0.033 0.032

0.03 0.03 0.022

428 428 428

0.022 0.022 0.021

45–49 45–49 40–43 40–43 40–43 40–43 35.5– 38.5 35.5– 38.5 32–36 32–36 33– 34.5 33– 34.5 31–34 31–34 30–33 30–33 28– 30.5 28– 30.5 26–28 24–28 22–28

High water level Roughness

at Zhicheng and Luoshan stations is quite high. The value of NashSutcliffe coefficient was more than 0.99. The simulation accuracy of flow at the three outlets and Chenglingji was a little lower. However, the Nash-Sutcliffe coefficients were higher than 0.98 for all years and locations except for 2012 at Chenglingji. The relative errors between simulated and observed peak flow at several hydrological stations are listed in Table 5. The relative errors of the main stream and Chenglingji were excellent, and that of the three outlets were a little high because of their relatively low discharges, but the occurrence time of simulated peak flow was consistent with that of the observed peak flow. Overall, the hydrodynamic model has a quite good ability for simulating the flow process in the study area. To analyze the impact of the TGP's operation on the water exchange between Dongting Lake and the Yangtze River, two scenarios were built to run the model, i.e. the scenario with the TGP regulation and the

Please cite this article as: Dai, M., et al. Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.02.006i

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Table 4 Results of the Nash-Sutcliffe coefficient. Year

Zhicheng Shashi Luoshan Songzi outlet

Taiping outlet

Ouchi outlet

Chenglingji

2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

1.00 1.00 1.00 1.00 1.00 1.00 0.99 1.00 1.00 0.78 1.00

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.99 0.99 0.99 1.00

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Fig. 3. The capacity curve of the TGP.

TGP was propagated to Yichang Station using the Muskingum method (Cunge, 1969). Table 5 Relative errors of simulated and observed peak flow in percent. Year

Zhicheng Luoshan Songzi outlet

2009 2010 2011 2012 2013 2009–2013

1.77 0.24  1.07 0.86 2.33 0.83

4  1.28  2.17  5.25 3.14  0.31

 2.73  16.15 1.91  20.78  6.86  8.92

Taiping outlet

Ouchi outlet

Chenglingji

 8.7  26.21  1.83  9.69 10.48  7.19

9  12.83 23.97 4.02 19.87 8.81

1.81  2.14  3.62  1.7  4.57  2.04

4.1. Peak flow reduction

scenario without the TGP regulation, thus only changing the upper boundary (discharge of Yichang Station) condition while keeping other parameters and situations the same. 3.4. Impact evaluation The observed flow in the study area is influenced by the TGP regulation of flow, so comparing the simulated flow of the scenario without the TGP regulation with observed value, the contribution of the TGP regulation on the water exchange between the Yangtze River and Dongting Lake can be quantified. In order to reduce the system error generated from the hydrodynamic model, the simulated flow for the scenario with the TGP regulation was done applying the same model and same parameters was used instead of the observed value. The calculation equation of impact is as follows. Impact ¼ ðQ obs:sim  Q nat:sim Þ=Q nat:sim  100

ð3Þ

where Q obs:sim is the simulated flow at stations downstream when the observed flow of Yichang Station is set as upper boundary; Q nat:sim is the simulation flow at stations downstream when the restored natural flow of Yichang Station is set as upper boundary. 3.5. Restoration of flow of Yichang Station According to the actual operation records of the TGP from January 1, 2009 to December 31, 2013, the water level at the dam, the reservoir capacity curve (shown in Fig. 3), and the outflow discharges were used in a water balance equation to derive the inflow of the reservoir. The water balance equation is as follows: I ¼Oþ

ΔV loss þ ΔV Δt

4. Results

ð4Þ

where I is the daily inflow of the TGP, O is the daily outflow from the TGP, ΔV loss is the water loss of the TGP, ΔV is the change of storage volume of the TGP, and Δt is the time step (daily in this case). There is a long distance from the inflow area of the TGP to Yichang Station, therefore the natural daily inflow process of the

This study analyzed the impact of the TGP's operation on the peak flow in the lower reaches for 2009–2013. As shown in Fig. 4, flood flow at Zhicheng, Luoshan, the three outlets, and Chenglingji without the TGP were higher than those with the TGP, indicating that because of water regulation and flood control of the reservoir, the TGP had reduced the peak flow of the main stream in the lower reaches, three outlets, and Chenglingji during the flood season. The peak flow at Zhicheng Station was decreased by more than 20%, and the biggest change was 30%, which occurred in 2010. The flow reduction of the Songzi Outlet, Taiping Outlet, and Ouchi Outlet was less than 15%. The impact on the peak flow at Chenglingji was insignificant. After the confluence of Dongting Lake and the Yangtze River, the reduction rate of peak flow at Luoshan Station was only about 6%. The comparison of the occurrence time of peak flow at Zhicheng, the three outlets, Chenglingji, and Luoshan stations with and without the impact of the TGP from 2009 to 2013 is listed in Table 6. The TGP has changed the occurrence time of peak discharge at the measurement stations on the main stream, the three outlets, and Chenglingji as well as reducing the flood peak. In 2009–2013 (except 2011), the peak discharge date was changed 2– 6 days due to the regulation of the TGP. In the main flood season of 2011, the inflow from the upper reaches was relatively small, the peak discharge at Yichang Station occurred in the storage period (September 22). After regulation, the outflow was only 20,400 m3/ s. The peak flood of the main stream at Zhicheng Station and three outlets was recorded in June, July, and August, instead of September. The time distribution of peak flow and flood volume in 2011 at downstream stations were significantly changed after the regulation of the TGP. 4.2. Flood volume reduction The impact of TGP's operation on the 7-day, 15-day, and 30-day maximum flood volume is shown in Fig. 5. The operation of the TGP reduced 7-day maximum flood volume by 8.8–18.1% at Zhicheng, 4.6–10% (1.06% in 2011) at Luoshan, 9.1–18.2% at the Songzi Outlet, by 8.3–23.7% at the Taiping Outlet, and 6.6–32.4% at the Ouchi Outlet from 2009 to 2013. The impact on the flood volume at Chenglingji was trivial. For the 15-day and 30-day maximum flood volumes, the operation of the TGP had a smaller impact. The inflow from the upper reaches has a significant impact on the flood volume at Zhicheng Station and the three outlets, while

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Fig. 4. The hydrographs with and without the TGP's operation at Zhicheng Station, Luoshan Station, the three outlets, and Chenglingji Station. Table 6 Comparison of the occurrence time of peak flow. Year

2009 2010 2011 2012 2013

Zhicheng

Songzi outlet

Taiping outlet

Ouchi outlet

Chenglingji

Luoshan

TGP

Natu.

TGP

Natu.

TGP

Natu.

TGP

Natu.

TGP

Natu.

TGP

Natu.

8/05 7/27 8/06 7/28 7/20

8/08 7/22 9/22 7/26 7/23

8/06 7/27 6/27 7/30 7/21

8/08 7/22 9/22 7/26 7/23

8/05 7/27 7/09 7/29 7/21

8/08 7/22 9/22 7/26 7/23

8/07 7/28 6/28 7/29 7/21

8/08 7/23 9/23 7/27 7/24

7/09 6/26 6/17 6/14 5/19

7/09 6/26 6/18 7/20 5/19

8/10 7/30 6/29 7/29 7/25

8/10 7/24 6/28 7/28 7/27

Note: “TGP” means with TGP, “Natu.” means without TGP.

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Fig. 5. The reduction for the 7-day, 15-day, and 30-day maximum flood volume resulting from the TGP operation. Table 7 Comparison of three outlets diversion with and without the TGP's operation. Period

2009 2010 2011 2012 2013 2009–2013

With the TGP's operation

Without the TGP's operation

Zhicheng (109 m3)

Three outlets (109 m3)

Diversion ratio (%)

Zhicheng (109 m3)

Three outlets (109 m3)

Diversion ratio (%)

395.9 418.9 351.8 477.6 386.0 406.0

44.9 53.7 28.5 64.3 41.5 46.6

11.3 12.8 8.1 13.5 10.8 11.3

396.2 424.0 352.0 476.5 385.6 406.8

46.9 56.5 31.9 66.8 44.2 49.2

11.8 13.3 9.1 14.0 11.5 11.9

Diversion change (109 m3)

Relative change (%)

 1.9  2.8  3.5  2.5  2.6  2.7

 4.1  4.9  10.8  3.8  6.0  5.9

Fig. 6. Monthly water level at Chenglingji Station with and without the TGP's operation.

the impact on Chenglingji Station and Luoshan Station is smaller, which indicates that the impact of the TGP's operation on the middle and lower reaches becomes smaller as the distance from the TGP increases. For an individual station, the longer the statistical time, the smaller the impact of the TGP's operation on the flood volume. The impact on the 7-day flood volume is bigger than the impact on the 15-day flood volume, and the impact on the 15day flood volume is bigger than the impact on the 30-day flood volume.

4.3. Inflow to Dongting Lake The impact of the TGP's operation on the three outlets diversion is listed in Table 7. The total diversion water volume decreased by 1.9–3.5 billion m3/yr (the reduction was 3.8–10.8%), the diversion ratio dropped from 11.9% to 11.3% as well (a difference of 0.6%). The biggest reduction occurred in 2011, when the diversion volume dropped by 3.5 billion m3 and the reduction was

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10.8%, while the diversion ratio dropped from 9.1% to 8.1% (a difference of 1%).

4.4. Outflow from Dongting Lake The monthly average water level with and without TGP at Chenglingji Station, which is located at the point where Dongting Lake joins the Yangtze River, is shown in the Fig. 6. The water storage of Dongting Lake in the two scenarios was analyzed by considering the capacity curve of Dongting Lake (as shown in Fig. 7). The decrease of Dongting Lake's storage was consistent with the increase of outflow from Dongting Lake. Fig. 6 indicates that from 2009 to 2013, during the storage period of the TGP operation (from September to November), outflow from the TGP reduced, so the average water level at Chenglingji Station decreased by 0.76 m in September and 0.97 m in October. From January to June, outflow from the TGP increased, and the water level at Chenglingji Station increased by 0.15– 0.54 m. In July of 2010, 2012, and 2013, because of flood control at the TGP, the water level at Chenglingji Station decreased by 0.17, 0.32, and 0.20 m, respectively. Fig. 8 indicates that during the storage period of the TGP operation, the average water level at Chenglingji Station was under the natural conditions, and the gradient near the outlet of Dongting Lake increased. Thus the outflow volume from Dongting Lake via Chenglingji increased by 0.3–1.6 billion m3 in September and 0.4–0.6 billion m3 in October. On the contrary, from January to June, the water level of Dongting Lake slightly increased, and the outflow increased. In the same way, the outflow increased and decreased alternately in July and August of 2010, 2012, and 2013 because of the flood control of the TGP operation.

4.5. Impact on water-exchange The operation of the TGP has varied impacts on the inflow diversion into Dongting Lake via the three outlets and the outflow draining into the Yangtze River. Fig. 9 indicates that during 2009– 2013, from September to October, because of the storage in the TGP, the downstream water level dropped due to the reduction in flow of the main stream, thus, the inflow of Dongting Lake at the three outlets dropped dramatically. The inflow was reduced by 2.8 billion m3 in September and 2 billion m3 in October. The drop in the main stream water level further increased the water gradient of the river channel near the outlet of Dongting Lake, therefore, the outflow increased by 0.8 billion m3 in September and 0.5 billion m3 in October. In September and October, if the runoff from the Dongting Lake area is insufficient, or the inflow from the Xiangjiang, Zishui, Yuanjiang, and Lishui rivers is inadequate, severe drought in the Dongting Lake area may happen. From May to June, the TGP operations prepare for the flood season, so the outflow from the TGP was larger than the inflow to the TGP. The inflow of Dongting Lake from the three outlets increased by 1 billion to 1.5 billion m3, and the outflow from Chenglingji decreased by 0.3 billion to 0.5 billion m3. The water replenishment effect of the TGP to the Dongting Lake area is significant. Outflow from the TGP replenished the lower reaches with water from November to April during 2009–2013, i.e. the discharge of the lower reaches was larger than that in the scenario without the TGP, more water drained into Dongting Lake from the three outlets, and less water drained into the Yangtze River from Dongting Lake at Chenglingji. Both the diversion into Dongting Lake via the three outlets and the outflow draining into the Yangtze River from Dongting Lake were less than 0.1 billion m3. In the dry season, the TGP's replenishing effect to the lower reaches increased the available water resources in Dongting Lake, which could further alleviate the drought in the Dongting Lake area. From July to August, the inflow and outflow were alternately increasing or decreasing at the three outlets and Chenglingji due to the flood control of the TGP operation.

5. Conclusions

Fig. 7. The capacity curve of Dongting Lake.

The operation of the TGP has reduced the inflow that drained into Dongting Lake via the three outlets from September to October, while increasing the outflow from Dongting Lake at Chenglingji Station. During the storage period from September to October, the TGP decreases water resources of Dongting Lake significantly. From May to June, the reservoir replenishes the lower

Fig. 8. Monthly storage volume of Dongting Lake with and without the TGP operation.

Please cite this article as: Dai, M., et al. Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.02.006i

M. Dai et al. / International Journal of Sediment Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Fig. 9. Monthly variation of inflow to and outflow from the Dongting Lake in 2009– 2013.

reaches with water. The water replenishment effect increases the water resources of Dongting Lake considerably. From November to April, the operation of the TGP had little impact on the water resources of Dongting Lake. After the TGP was put into operation, the downstream hydrological regime has gone through dramatic changes. The water exchange between Dongting Lake and the Yangtze River has been irreversibly changed and will be further changed in the future. This research provides important information for mitigation of the impacts of the TGP on the middle and lower reaches, while offering a valuable reference for the flood control, irrigation, and water resources management for the cities in the middle and lower reaches. As the demand on energy increases (Stoeglehner et al., 2016), as well as the floods and droughts caused by climate change become more frequent (Barros & Field, 2014; Intergovernmental Panel on Climate Change, 2015; Ji et al., 2015), hydropower projects are still increasingly developing in the world. Before construction of big hydropower projects, experts must evaluate the risks of various aspects (Donnelly, 2006; Hartford & Baecher, 2004), identify the uncertainties, and reduce the possibilities of risks, including dam failure, and the irreversible impacts on the environment (Perdicoúlis et al., 2007). This research serves as a typical case and valuable experience for design and operation of other hydropower projects.

Acknowledgement This work was financially supported by the National Basic Research Program of China (973 Program) (Grant no. 2012CB417001) and the National Key Research Program of China (Grant no. 2016YFC0400901). The authors thank the editor and the anonymous reviews for their comments, which helped improve the quality of the paper.

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Please cite this article as: Dai, M., et al. Impact of the Three Gorges Project operation on the water exchange between Dongting Lake and the Yangtze River. International Journal of Sediment Research (2017), http://dx.doi.org/10.1016/j.ijsrc.2017.02.006i