Water-Exchange Response of Downstream River–Lake System to the Flow Regulation of the Three Gorges Reservoir, China

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Article Water-Exchange Response of Downstream River–Lake System to the Flow Regulation of the Three Gorges Reservoir, China

Junhong Zhang 1,2, Tao Huang 1, Lu Chen 3,* , Xiaofang Liu 4, Lingling Zhu 5, Luojie Feng 6 and Yunping Yang 7

1 College of Resources and Environmental Science, South-central University for Nationalities, Wuhan 430074, China; [email protected] (J.Z.); [email protected] (T.H.) 2 China Institute of Water Resources and Hydropower Research, Beijing 100038, China 3 College of Hydropower & Information Engineering, Huazhong University of Science & Technology, Wuhan 430074, China 4 Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; [email protected] 5 Bureau of hydrology, Changjiang water resources Commission, Wuhan 430010, China; [email protected] 6 Guangzhou Institute of Geography, Guangzhou 510070, China; [email protected] 7 Tianjin Research Institute for Water Transport Engineering, Tianjin 300456, China; [email protected] * Correspondence: [email protected]; Tel.: +86-027-6784-3918

Received: 3 September 2019; Accepted: 11 November 2019; Published: 15 November 2019

Abstract: Hydrological regime changes in the river–lake system and their influences on the ecological environment downstream dams have attracted increasingly more attention all over the world. The Dongting lake downstream of the Three Gorges Dam (TGD) in the Yangtze River has been experiencing a series of hydraulic and hydrological changes over the last decade. The hydrological and ecological influences of the TGD flow regulation on the Dongting river–lake system and its functional mechanism during the impounding periods remain extremely unclear. This study examines the hydrological changes in the Dongting river–lake system based on a 1D/2D coupled hydrodynamic model. In particular, the inflow boundary of the model with and without the TGD was applied with the outflow and inflow of the TGD, respectively, during the same regulation periods. The results show that the diverted flow from the Yangtze River into the Dongting lake and outflow from the lake back to the river drastically decreased during the impounding periods, especially in October. The decreased water exchange between the Yangtze River and the Dongting lake impaired the water residence capacity to some extent in the lake. Stage decrease in the lake area resulted in a significant reduction in the water volume of the Dongting lake with the same time percentage. In addition, the obvious drainage effect in Dongting lake due to the increased stage difference and current speed after the TGD operation was the essential cause of hydrological changes in the lake area. These results provide an improvement in the understanding of impoundment influences on the large river–lake system and give some practical information for ecological environment management in similar river–lake systems.

Keywords: Three Gorges Reservoir; impoundment inﬂuences; river–lake system; water exchange

1. Introduction Lakes, as one kind of the most important water providers, widely exist on the earth. Most of these lakes connect with some rivers, so the river–lake system is the common form in natural basins [1,2]. A ﬁfth of the world’s surface freshwater was held in the ﬁve Great Lakes and their connecting rivers in North America [3]. Other lakes such as Saimaa Lake in Finland, Peace-Athabasca Delta system in

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Canada, and Tonle Sap Lake in Cambodia, as part of the river–lake system, provide adequate water to their adjoining residents [4–6]. Lakes in the river–lake system receive and store plenty of water from storms and floods during flood seasons and release it steadily; this plays a significant role in flood control for the downstream rivers, as well as the nutrient substance and part of sediment deposited in the lake area while the water flows through a lake. Therefore, lakes in a river–lake system provide the necessary elements for plant growth and the important habitats for wildlife [7]. However, with the intensification of anthropogenic activities, the health of the river–lake system also faces many challenges. There are increasingly more pollutant loads in lakes all over the world, owing to the extensive human activities [8–12]. More important, a series of dams in upstream channels drastically changed the hydrological regime of a river–lake system located in the middle and lower part of catchments [13–15]. Therefore, significant water-exchange variations occurred in the river–lake system during different reservoir-impounding periods. Consequently, water-resource utilization, waterway maintenance, ecological conservation, and water environmental management in lakes are affected to some extent [16–18]. The Yangtze River, as the third longest river, contributed a lot to the integrated utilization of water resources in China [19,20]. Water-resource development based on cascade reservoirs in the Yangtze River basin has achieved great progress over the past 50 years, especially in regard to the construction of the Three Gorges Dam (TGD) [21–23]. However, a series of hydrological and hydraulic changes inevitably occurred in the downstream channels [24,25]. Therefore, the Dongting river–lake system, 260 km downstream of the TGD, suffered from severe hydrological variations during the different flow regulation periods. Increasing attention has been attracted to the hydrological changes in the Dongting river–lake system in recent years. Hayashi et al. simulated the daily runoff processes based on an integrated catchment model in the Yangtze River basin and examined the hydraulic effect of the mainstream on the outflow of the Dongting lake; the results showed a good agreement of hydrologic budget between the observed and simulated ones [26,27]. Lai et al. [28] quantified the impoundment effects of the TGD during the dry seasons, based on a coupled model, and came to the conclusion that the extremely low water level that occurred downstream of the TGD principally resulted from the reduced inflow to the Yangtze River basin and the precipitation decrease. Wang et al. recently confirmed Lai et al.’s results again and quantified the influence of human water consumption on the last decadal lake stage decline [29]. These studies significantly contributed to revealing the hydrological changes in the Dongting river–lake system. However, more work should be done to quantify the changes in the outflow volume from the Dongting lake to the Yangtze River channels and its functional mechanism. The evaluation of hydrological changes in the Dongting river–lake system has been carried out so far from different perspectives based on various kinds of methods. Jiang et al. [30] investigated the flood response to polder restoration in Dongting lake and explored the formation mechanism of frequent flood disasters after the TGD operation, which provided vital insights into the lake ecosystem restoration downstream of dams. Using the measured hydrological data before and after the operation of the TGD, Chang et al. [31] conducted an evaluation of the influences of the TGD during its initial operation periods and quantified the annual average changes in flow and sediment discharge in the river–lake system, which provided a scientific basis for optimizing the operation program of the Three Gorges Reservoir. Sun et al. [32] examined the topographical changes downstream of TGD and analyzed its influences on the hydrological cycle around the Dongting lake basin based on the BP neural networks. The results indicated that the inundation patterns of the lake wetlands have been changing since the TGD operation. In addition, combining the observed field and remote sensing data and hydrodynamic models, Liu et al. [33] found that the TGD resulted in great downstream landscape changes, which significantly contributed to the Dongting shrinkage and its severe drought. Therefore, physically mathematical models were highly effective means to examine the hydrological changes in the complex river–lake system. Water 2019, 11, 2394 3 of 16

Despite several attempts that have been made to reveal the hydrological changes that occurred after the TGD operation, some problems remain unclear regarding the TGD impoundment impact on the Dongting river–lake system and their hydrodynamic mechanism of this system. In particular, whether the lake outflow during the impounding periods is increasing or not has not drawn a definitive conclusion. Current research was mainly focused on the TGD impoundment impact by comparing with hydrological data series and the fluvial processes before and after the TGR operation; little information has been provided about the different influences of the TGD inflow and outflow processes on the downstream river–lake system, let alone the studies on the root cause analysis for the hydrological variations. The research objectives of this paper are to (1) examine the changes in the water exchange with and without the TGD and to evaluate the variation in water retention capacity of the Dongting lake based on a 1D/2D coupled hydrodynamic model; (2) analyze the variations in the lake level and water volume during the impounding periods after the operation of the TGD; and (3) further explore the formation mechanism and its dominant factors of the hydraulic and hydrological changes in the river–lake system, which will be significantly conductive to relieve the adverse influences on the river–lake system.

2. Materials and Methods

2.1. Study Area and Data The Yangtze River, located in South Central China, is 6300 km long and ranks as the third longest river on the earth. The TGD in the middle Yangtze River was completed in 2003, and it is the largest dam structure in the world. One of the most important objectives of the TGD was the flood control in the Yangtze river basin. Therefore, significant hydrological changes occurred downstream of the TGD [34]. Dongting lake, situated downstream of the TGD (111◦190–113◦340 E, 27◦390–29◦510 N) with a drainage area of 2579 km2, is the second-largest freshwater lake in China (Figure1). The Dongting lake was mainly fed by the Yangtze River through three inlets (Ouchi, Taiping, and Songzi) and four tributaries (Zi River, Yuan River, Xiang River, and Li River,). In addition, the lake empties its water through the Chenglingji outlet (Figure1). The climate of the lake area is characterized by hot summers and relatively mild winters, which belongs to the subtropical monsoon climate. Its annual precipitation ranges from 1100 to 1400 mm, and the wet seasons are from May to October. According to the operation rules of the TGD, the complete run cycle was divided into four continuous flow regulating periods: (1) flood pre-discharged regulation, emptying the reservoir volume to the downstream during this period; (2) flood regulation, the floods from the upstream were regulated during this period; (3) reservoir impoundment, partial water inflow was stored in the reservoir; and (4) water supplement, the reservoir increased its outflow to the downstream in order to meet irrigation water needs and keep the ecological balance and water depth of the waterways [35]. The above operational procedure is repeated year by year. The reservoir impoundment generally began from September to October, after the main flood seasons. The water-holding capacity test of the TGD was completed in 2008, and then normal flow regulation was carried out. Figure1 presents the inflow and outflow processes of the TGD from 2008 to 2015. For a comparative analysis of hydrological and hydrodynamic changes in the Dongting river–lake system, the meteorological and hydrological data (e.g., water level, discharge, precipitation, and evaporation) were collected from the Bureau of Hydrology, Changjiang Water Resources Commission, and the Weather Bureau of China. In addition, the outflow and inflow data of the TGD were collected from the Three Gorges Corporation, China. The meteorological and hydrological data cover the concerned sites in this study area. The detailed locations of the gauging stations are presented in Figure1. Moreover, the digital elevation model including rivers and lake terrain used in this study was surveyed from 2008 to 2012, which was also provided by the Changjiang Water Resources Commission (see Supplementary Materials). Water 2019, 11, 2394 4 of 16 Water 2018, 10, x FOR PEER REVIEW 4 of 16

Figure 1. The physical location of the Dongting river–lake system: (a) sketch map with the information Figure 1. The physical location of the Dongting river–lake system: (a) sketch map with the of gauging stations; (b) regular scheduling chart of the Three Gorges Dam (TGD); (c) the hydrograph of information of gauging stations; (b) regular scheduling chart of the Three Gorges Dam (TGD); (c) the hydrographthe inflow and of the outflow inflow of and the TGDoutflow from of 2008 the TGD to 2015. from 2008 to 2015. 2.2. 1D/2D Coupled Hydrodynamic Model For a comparative analysis of hydrological and hydrodynamic changes in the Dongting river– lake Asystem, 1D/2D the coupled meteorological model constructed and hydrological by Zhang data et al. (e.g., [36] waswater used level, to examinedischarge, the precipitation, hydrological andimpact evaporation) of the TGD were on the collected Dongting from river–lake the Bureau system of during Hydrology, the water Changjiang impoundment Water periods. Resources The Commission,coupled model and was the set Weather up based Bureau on the of moduleChina. In of addition, MIKE FLOOD, the outflow which and is aninflow integrated data of modeling the TGD wereand analysis collected platform from the developed Three Gorges by theCorporation, Danish Hydraulic China. The Institute meteorological (DHI) and and extensively hydrological applied data coveraround the the concerned world [37 –sites40]. Thein this 1D /study2D coupled area. modelThe detailed used in locations this study of covered the gauging the Dongting stations lakeare presentedand the connected in Figure branches 1. Moreover, in the the middle digital Yangtze elevation River model [36]. including The river rivers network and in lake the studyterrain area used was in thismodeled study by was using surveyed the MIKE from 11 hydrodynamic 2008 to 2012, module,which was while also the provided MIKE 21 hydrodynamicby the Changjiang module Water was Resourcesapplied to simulateCommission the lake(see area.Supplementary There were Materials 862 cross-sections,). and 23,436 grid cells in the 1D and 2D hydrodynamic modules respectively. The MIKE 11 and MIKE 21 hydrodynamic modules are described 2.2.by the 1D/2D Saint-Venant Coupled Hydrodynamic equations and Model the 2D shallow-water equations, respectively [41,42]. The standard linkage in the MIKE FLOOD was used in this study to connect the hydrodynamic modules; thus, water A 1D/2D coupled model constructed by Zhang et al. [36] was used to examine the hydrological exchanges between the modules occurred through the overlapped MIKE 21 grid cells and the MIKE impact of the TGD on the Dongting river–lake system during the water impoundment periods. The 11 network. Therefore, the hydrodynamic processes of the Dongting river–lake system during the coupled model was set up based on the module of MIKE FLOOD, which is an integrated modeling impounding periods were simulated by using the coupled model in this research. and analysis platform developed by the Danish Hydraulic Institute (DHI) and extensively applied The daily streamflow gauged at the tributary gauging stations (Gaobazhou, Taojiang, Taoyuan, around the world [37–40]. The 1D/2D coupled model used in this study covered the Dongting lake Shimen, and Xiangtan) was specified as the upstream boundary conditions for the coupled model, and the connected branches in the middle Yangtze River [36]. The river network in the study area while the regular inflow and outflow of the TGR were applied as the mainstream boundary conditions. was modeled by using the MIKE 11 hydrodynamic module, while the MIKE 21 hydrodynamic The downstream boundary condition was applied as the rating curve at the Luoshan station. Zhang et module was applied to simulate the lake area. There were 862 cross-sections, and 23,436 grid cells in al. [36] calibrated and validated the coupled model against the observed data in the study area for the the 1D and 2D hydrodynamic modules respectively. The MIKE 11 and MIKE 21 hydrodynamic periods of January–December 2008 and January–December 2011, respectively. The coupled model was modules are described by the Saint-Venant equations and the 2D shallow-water equations, respectively [41,42]. The standard linkage in the MIKE FLOOD was used in this study to connect the

Water 2019, 11, 2394 5 of 16 quantitatively evaluated by using the Nash–Sutcliffe efficiency coefficients (NSE) and the normalized root mean squared error (RMSE); the results show that there is a fairly close agreement between the simulated and observed values at the main stream, tributaries, and the lake area. The NSE value is larger than 0.89 and RMSE is smaller than 0.025 at the river–lake system stations, which indicates that the coupled model has a high simulation accuracy to predict and evaluate the hydrological changes in the study area [36].

2.3. Hydrologic Residence To eliminate the eﬀects of data random ﬂuctuation on the research results, the gauged stage and water volume data in the lake area were processed by using the moving average method, as follows:

5 1 X h(y) = H(y, t) (1) 5 t=1

5 1 X v (y) = V(y, t) (2) 5 t=1 where h(t) and v(t) are the data series of moving averaged stage and water volume, respectively, which are the results of a daily averaged sequence of H(y,t) and V(y,t); y and t denote the year and day in the time series. The fluctuation of stage and water volume in the lake was on account of the flow difference between the lake inflow and outflow. Therefore, the ratio between the volume difference and the inflow volume was used to show the water residence rate as follows: R (qin(t) qout(t))dt Rresi = R − (3) qin(t)dt where Rresi is the water residence rate (WRR) in the lake; qin(t) and qout(t) are the daily water inflow and outflow rate of the Dongting lake. In addition, the residence time is an important variable to present the hydrological changes in the study area, which can be addressed as follows: R tqout(t)dt Tresi = R , qin>qout (4) qout(t)dt where Tresi is the mean residence time (MRT), which is the cumulative time that inflow is larger than the outflow.

2.4. Evaluation of Hydrologic Regime Changes during the Impounding Periods To accurately quantify the influence of the TGD impoundment on the river–lake system, the following two research scenarios were performed: Scenario 1 (S1), simulating the actual hydrological processes under the TGD regulation during 2008–2011; Scenario 2 (S2), simulating the hydrological processes that without the operation of the TGD during the same periods. For the second scenario (S2), the flow boundary at the Yichang station was replaced by the upstream inflow of the TGD. The other boundary conditions were kept the same as scenario 1. The gauged estuary station data of the tributaries (Xiang River, Zi River, Yuan River, and Li River) were applied as the inflow boundary conditions to the lake area. Several complete scheduling cycles were covered in the selected simulation periods; therefore, the hydraulic and hydrological changes during the different run phases of the TGD can be detected by comparing the two simulation scenarios. Water 2018, 10, x FOR PEER REVIEW 6 of 16 second scenario (S2), the flow boundary at the Yichang station was replaced by the upstream inflow of the TGD. The other boundary conditions were kept the same as scenario 1. The gauged estuary station data of the tributaries (Xiang River, Zi River, Yuan River, and Li River) were applied as the inflow boundary conditions to the lake area. Several complete scheduling cycles were covered in the selected simulation periods; therefore, the hydraulic and hydrological changes during the different run phases of the TGD can be detected by comparing the two simulation scenarios. Water 2019, 11, 2394 6 of 16

3. Results 3. Results 3.1. Water Exchange between the Dongting Lake and the Yangtze River 3.1. Water Exchange between the Dongting Lake and the Yangtze River There were obvious hydrological variations after the TGD was put into operation; as shown in FigureThere 2, the were diverted obvious flow hydrological through the variations Taiping, Songzi, after the and TGD Ouchi was inlets put into to the operation; tributaries as showndropped in Figureobviously.2, the Compared diverted flow with through the flow the changes Taiping, in Songzi, September, and Ouchi the flow inlets drop to the occurred tributaries much dropped more obviously.seriously during Compared October with due the flowto the changes increasing in September, water impoundment the flow drop of occurredthe TGD. much The mean more seriouslydiverted duringflow through October the due three to the inlets increasing was less water than impoundment 1000 m3/s during of the October, TGD. The after mean the diverted TGD operation. flow through The theflow three drop inlets was most was less serious than at 1000 the mOuchi3/s during inlet, October,which was after nearly the TGDdrying operation. up during The the flow October drop 2009 was mostand 2011. serious In addition, at the Ouchi there inlet, was which a high was agreemen nearlyt drying between up the during mainstream the October flow 2009 and andthe diverted 2011. In addition,flow during there the was impoundment a high agreement periods between at the the three mainstream inlets of flowthe tributaries and the diverted with and flow without during the impoundmentTGD. Consequently, periods there at the was three an ev inletsident of flow the tributaries drop in the with mainstream and without and the diverted TGD. Consequently,flow after the thereTGD wasoperation. an evident When flow the drop flow in discharge the mainstream in the mainstream and diverted was flow lower after than the TGD 4800, operation. 7200, and When 6700 them3/s, flow there discharge would be in no the flow mainstream diversion was at the lower Songzi than, Taiping, 4800, 7200, and and Ouchi 6700 inlets, m3/s, respectively there would (Figure be no flow2). diversion at the Songzi, Taiping, and Ouchi inlets, respectively (Figure2).

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0. 0 2. 0 4. 0 6. 0 8. 0 0. 0 0. 5 1. 0 1. 5 2. 0 0. 0 1. 0 2. 0 3. 0 Flow diversion (×103m3/s) Flow diversion (×103m3/s) Flow diversion (×103m3/s) Figure 2. Monthly diverted flow through the three inlets in September and October, from 2008 to 2011, andFigure the 2. relationship Monthly diverted between theflow mainstream through the flow three and inle thets diverted in September flow. Natural and October, processes from indicate 2008 the to hydrological2011, and the processes relationship are thebetween ones thatthe withoutmainstream the TGDflow influences,and the diverted while theflow. regulation Natural representsprocesses indicate the hydrological processes are the ones that without the TGD influences, while the the hydrological processes that are under the TGD flow regulation. (a) Monthly average diverted flow regulation represents the hydrological processes that are under the TGD flow regulation. (a) through the Songzi outlet with and without the TGD, (b) Monthly average diverted flow through the Monthly average diverted flow through the Songzi outlet with and without the TGD, (b) Monthly Taiping outlet with and without the TGD, (c) Monthly average diverted flow through the Ouchi outlet average diverted flow through the Taiping outlet with and without the TGD, (c) Monthly average with and without the TGD, (d) Relationship between the diverted flow and the mainstream flow at diverted flow through the Ouchi outlet with and without the TGD, (d) Relationship between the the Songzi outlet, (e) Relationship between the diverted flow and the mainstream flow at the Taiping

outlet, (f) Relationship between the diverted ﬂow and the mainstream ﬂow at the Ouchi outlet.

Figure3 presents the outflow processes at the Chenglingji outlet during September and October, from 2008 to 2011, with and without the TGD, which indicates that water impoundment of the TGD indeed leads to a series of hydrological changes in the lake outflow. Although there were both a flow increase and a flow decrease after the TGD operation, the flow decrease was more apparent on the Water 2018, 10, x FOR PEER REVIEW 7 of 16

diverted flow and the mainstream flow at the Songzi outlet, (e) Relationship between the diverted flow and the mainstream flow at the Taiping outlet, (f) Relationship between the diverted flow and the mainstream flow at the Ouchi outlet.

Figure 3 presents the outflow processes at the Chenglingji outlet during September and October, from 2008 to 2011, with and without the TGD, which indicates that water impoundment of the TGD indeed leads to a series of hydrological changes in the lake outflow. Although there were bothWater 2019a flow, 11, 2394increase and a flow decrease after the TGD operation, the flow decrease was more7 of 16 apparent on the whole compared with the processes without the TGD. During the periods of TGD impoundmentwhole compared from with September the processes to withoutOctober thein TGD.each Duringyear from the periods2008 to of2011, TGD the impoundment maximum flow from 3 decreaseSeptember was to up October to 4860, in 2012, each 2610, year fromand 9551 2008 m to/s, 2011, and thethe maximummean flow flowdecrease decrease was about was up 1610, to 4860,790, 3 1029,2012, 2610,and 1035 and 9551m /s mthan3/s, andthe processes the mean flowwithout decrease TGD. was Furthermore, about 1610, the 790, dashed 1029, and lines 1035 in mFigure3/s than 3 presentthe processes the beginning without TGD. day Furthermore,that the TGD theimpoundment. dashed lines inThere Figure was3 present a sharp the increase beginning in daythe thatlake outflowthe TGD when impoundment. the TGD began There to was store a sharp water. increase But then in the the lake outflow outflow reduced when sharply the TGD because began to of store the waterwater. volume But then decrease the outflow in Dongting reduced lake. sharply because of the water volume decrease in Dongting lake.

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( ( ( ( e e e g e c c r g n n a r e e h a r r c h e e s c f f i f f s i i D i D D D -505 05 024681 0481 -2 -10 01/ 09 10/ 09 19/ 09 28/ 09 07/ 10 16/ 10 25/ 10 01/ 09 10/ 09 19/ 09 28/ 09 07/ 10 16/ 10 25/ 10 Ti me DD/ MM Ti me DD/ MM ( ) ( ) Figure 3. Hydrograph at the Chenglingji outlet during the impoundment periods from 2008 to 2011, Figure 3. Hydrograph at the Chenglingji outlet during the impoundment periods from 2008 to 2011, and the flow difference with and without the TGD. The positive flow difference denotes the outflow and the flow difference with and without the TGD. The positive flow difference denotes the outflow without TGD was larger than the one under the flow regulation of TGD and vice versa. without TGD was larger than the one under the flow regulation of TGD and vice versa. In addition, the flow difference between lake inflow and outflow directly caused the water Table 1. Computing results of the water residence rate and mean residence time in the Dongting lake exchangewith and variation without in the the TGD. river–lake system. Therefore, the water volume and its residence time in the lake area unavoidably changed with the water exchange. The water residence rate and mean residence Without TGD TGD time wasTime calculated via Equations (3) and (4), and the results were presented in Table1. The water residence rate indicatesWRR the water retention MRT (Day) capacity of the Dongting WRR lake; as shown in MRT Equation (Day) (3), its negative2008 value implies−0.234 the outflow is larger18 than the inflow. Although−0.242 there was an evident17 decrease in 2009 −0.285 12 −0.351 10 the inflow and outflow from September to October, the water residence rate under the TGD regulation 2010 −0.191 16 −0.198 15 was smaller than the one without TGD, which indicated the water volume gap between the lake inflow 2011 −0.038 25 −0.05 23 from the Yangtze River and the lake outflow to the Yangtze River increased after the TGD operation. Note: WRR denotes the water residence rate; MRT denotes the mean residence time, day. The inflow decrease was larger than the outflow decrease after the TGD operation. Consequently, the mean residence time was also reduced due to the decreased water retention capacity. As shown in Table1, there were about 1–2 days reduced in the water residence time after the TGD operation.

Water 2019, 11, 2394 8 of 16

Table 1. Computing results of the water residence rate and mean residence time in the Dongting lake with and without the TGD.

Without TGD TGD Time WRR MRT (Day) WRR MRT (Day) 2008 0.234 18 0.242 17 − − 2009 0.285 12 0.351 10 − − 2010 0.191 16 0.198 15 − − 2011 0.038 25 0.05 23 − − Note: WRR denotes the water residence rate; MRT denotes the mean residence time, day.

3.2. Changes in the Hydrological Regime of the Dongting Lake The changes of water exchange between the Dongting lake and the Yangtze River inevitably lead to the variation in the hydrological regime of the Dongting lake. Figure4 shows the lake-stage changes from September to October, with and without the TGD. The water level decreased apparently at all the gauging stations during the same periods. The stage diﬀerence at the Nanzui station was more insigniﬁcant than the other lake stations. In addition, there was a similar stage-changing trend at the Heyehu, Lujiao, and Chenglingji stations. Compared with the processes without TGD, the lake level decreased greatly from late September, after the TGD impoundment. The maximum and mean lake level decrease at the Nanzui station was 2.18 and 0.46 m during the impounding periods from 2008 to 2011. Similarly, there was a larger-lake level decrease at the other stations, especially at the Chenglingji station. The maximum lake-level decrease occurred at the Chenglingji station and was up to 3.39 m, and the mean lake-level decrease at the Heyehu, Lujiao, and Chenglingji stations was about 1.17, 1.19, Water 2018, 10, x FOR PEER REVIEW 9 of 16 and 1.20 m.

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l 24 - e k a L 22

18 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 9/ 9/ 9/ 9/ 9/ 0/ 0/ 0/ 0/ 0/ 9/ 9/ 9/ 9/ 9/ 0/ 0/ 0/ 0/ 0/ 6/ 2/ 8/ 4/ 0/ 1 1 1 1 1 6/ 2/ 8/ 4/ 0/ 1 1 1 1 1 1 1 2 3 6/ 2/ 8/ 4/ 0/ 1 1 2 3 6/ 2/ 8/ 4/ 0/ 1 1 2 3 1 1 2 3 Ti me D/ M/ Y Ti me D/ M/ Y ( ) ( ) FigureFigure 4. Lake-level 4. Lake-level changes changes from from SeptemberSeptember to Octobe October,r, with with and and without without the theTGD. TGD. Continuous Continuous lineslines represent represent hydrological hydrological processes processes without without the the TGD; TGD; dashed dashed lines denote lines denote the TGD the regulated TGD regulatedflow ﬂowprocesses processes..

Figure 5 presents the stage-change rate in the lake area. The stage change rate was the daily mean water level variation from September to October. As shown in Figure 5, the stage drop under the TGD flow regulation from September to October was more severe than the one in the processes without TGD. After the TGD operation, the lake level decreased about 0~0.6 m per day, while the stage drop was only 0~0.45 m per day in the processes without TGD. It should be noted that the flood regulation of the TGD before the impoundment periods had also some influences on the stage change rate. The peak-flow reduction during the flood periods (from July to August) drastically decreased the flow diversion to the Dongting lake, which had an influence on the lake-level changes to some extent. For this reason, the stage-change rate in 2011 under the TGD operation was slightly smaller than the one without TGD.

Water 2019, 11, 2394 9 of 16

Figure5 presents the stage-change rate in the lake area. The stage change rate was the daily mean water level variation from September to October. As shown in Figure5, the stage drop under the TGD flow regulation from September to October was more severe than the one in the processes without TGD. After the TGD operation, the lake level decreased about 0~0.6 m per day, while the stage drop was only 0~0.45 m per day in the processes without TGD. It should be noted that the flood regulation of the TGD before the impoundment periods had also some influences on the stage change rate. The peak-flow reduction during the flood periods (from July to August) drastically decreased the flow diversion to the Dongting lake, which had an influence on the lake-level changes to some extent. For this reason, the stage-change rate in 2011 under the TGD operation was slightly smaller than the one Water 2018, 10, x FOR PEER REVIEW 10 of 16 without TGD.

Figure 5. Rate of stage change in the Dongting lake area from September to October with and without Figure 5. Rate of stage change in the Dongting lake area from September to October with and the TGD. without the TGD.

Water exchange in the river–lake system also had a direct impact on the water volume in the lake area. The relationship between the water volume of the Dongting lake and the stage at the Chenglingji outlet from September to October was shown in Figure 6. The Chenglingji outlet is situated at the lowest lake site, and its stage changes greatly depended on the lake volume. Therefore, the relationship shown in Figure 6 showed the volume changes due to the water exchange in the Dongting river–lake system. The water volume decreased more or less after the TGD operation, especially during the impoundment periods in 2011. The decrease in the smallest lake volume during the impounding periods was about 3.61 × 109 m3, 1.47 × 109 m3, 0.93 × 109 m3, and 0.63 × 109 m3 in each year, from 2008 to 2011, which contributed a lot to the severe drop of lowest lake level in the Dongting lake.

Water 2019, 11, 2394 10 of 16

Water exchange in the river–lake system also had a direct impact on the water volume in the lake area. The relationship between the water volume of the Dongting lake and the stage at the Chenglingji outlet from September to October was shown in Figure6. The Chenglingji outlet is situated at the lowest lake site, and its stage changes greatly depended on the lake volume. Therefore, the relationship shown in Figure6 showed the volume changes due to the water exchange in the Dongting river–lake system. The water volume decreased more or less after the TGD operation, especially during the impoundment periods in 2011. The decrease in the smallest lake volume during the impounding periods was about 3.61 109 m3, 1.47 109 m3, 0.93 109 m3, and 0.63 109 m3 in each year, from Water 2018, 10, x FOR PEER REVIEW× × × × 11 of 16 2008 to 2011, which contributed a lot to the severe drop of lowest lake level in the Dongting lake.

30 30 (a) 2008 (b) 2009 28 28

26 26

24 24

h(t)(m) 22 22

26 26

24 24

h(t)(m) 22 22

20 20

18 18 3 6 9 12 15 18 21 24 27 30 33 36 3 6 9 12 15 18 21 24 27 30 33 36

v(t)(109m3) v(t)(109m3)

Figure 6. Relationship between water volume of the Dongting lake and stage at the Chenglingji outlet. Figure 6. Relationship between water volume of the Dongting lake and stage at the Chenglingji outlet.The impoundment impact on the water volume of the Dongting lake was not instantaneous but lasted all the impounding periods. As shown in Figure7, compared with the processes without TGD, thereThe was impoundment a significant decrease impact on in the the water water volume volume of during the Dongting most time lake periods was not under instantaneous the TGD flow but lastedregulation all the from impounding September periods. to October. As Theshown mean in water-volumeFigure 7, compared decrease with with the the processes time percentage without TGD,from 50%–80%there was wasa significant about 3.32 decrease109 m in3, the 3.27 wate10r9 volumem3, 3.72 during109 mmost3, and time 2.05 periods109 munder3 in each the TGD year, × × × × flowfrom 2008regulation to 2011. from In addition, September the waterto October. volume Th withe mean the largest water-volume time percentage decrease of the with Dongting the time lake percentagewas still smaller from than50%–80% the one was during about the 3.32 same × 10 periods9 m3, 3.27 of processes× 109 m3, without3.72 × 10 TGD,9 m3, and which 2.05 resulted × 109 m from3 in eachthe drastic year, from water 2008 exchange to 2011. between In addition, the Dongting the wate laker volume and the with Yangtze the largest River. The time TGD percentage impoundment of the Dongtingnot only increased lake was thestill outflow smaller from than the the Dongting one during lake the but same also seriouslyperiods of decreased processes the without inflow TGD, to the whichlake area. resulted from the drastic water exchange between the Dongting lake and the Yangtze River. The TGDFlow-field impoundment variation not inevitably only increased occurred the due outflo to thew variations from the inDongting lake level lake and but water also volumeseriously in decreasedthe lake area. the Figureinflow8 to presents the lake the area. flow velocity variations at the gauging stations of lake area, with and without the TGD.40 The changes in the flow velocity40 did not maintain consistency, because of the

complicated hydraulic35 relation in(a)2008 the great lake area. The35 mean ﬂow(b)2009 velocity at the Chenglingji, Lujiao, ) Fl ow regul ati on 30 30 3

Heyehu stationsm increased to some extent Natr afterual proc theess TGD operation, while mean ﬂow velocity reduced 9

0 25 25

at the Shawan and1 Nanzui stations, to a diﬀerent degree, with an especially drastic decrease at the × ( 20 20 e

Nanzui station.m The15 maximum velocity increase, about15 0.067 m/s in 2008, occurred at the Chenglingji u l o v outlet of the eastern 10 part of the Dongting lake. There1 was0 a high agreement of current speed variation r e t a

between the LujiaoW and Heyehu stations. On the contrary, the maximum velocity decrease at the 05 05 Nanzui station was about0 2 0.1280 4 m0 /s in60 2009.80 100 0 20 40 60 80 100

40 40

m 30 30 9 0

1 25 25 × (

e 20 20 m u l

o 15 15 v

e 10 10 t a W

05 05 0 20 40 60 80 100 0 20 40 60 80 100 Percent age of ti me vol u me exceeded % Percent age of ti me vol u me exceeded % ( ) ( )

Figure 7. Volume duration curves from September to October, with and without the TGD.

Water 2018, 10, x FOR PEER REVIEW 11 of 16

30 30 (a) 2008 (b) 2009 28 28

26 26

24 24

h(t)(m) 22 22

26 26

24 24

h(t)(m) 22 22

20 20

18 18 3 6 9 12 15 18 21 24 27 30 33 36 3 6 9 12 15 18 21 24 27 30 33 36

v(t)(109m3) v(t)(109m3)

Figure 6. Relationship between water volume of the Dongting lake and stage at the Chenglingji outlet.

The impoundment impact on the water volume of the Dongting lake was not instantaneous but lasted all the impounding periods. As shown in Figure 7, compared with the processes without TGD, there was a significant decrease in the water volume during most time periods under the TGD flow regulation from September to October. The mean water-volume decrease with the time percentage from 50%–80% was about 3.32 × 109 m3, 3.27 × 109 m3, 3.72 × 109 m3, and 2.05 × 109 m3 in each year, from 2008 to 2011. In addition, the water volume with the largest time percentage of the Dongting lake was still smaller than the one during the same periods of processes without TGD, which resulted from the drastic water exchange between the Dongting lake and the Yangtze River. The TGD impoundment not only increased the outflow from the Dongting lake but also seriously Water 2019, 11, 2394 11 of 16 decreased the inflow to the lake area.

40 40

35 (a)2008 35 (b)2009 ) Fl ow regul ati on 30 30 3

m Natrual process 9

0 25 25 1

× ( 20 20 e

m 15 15 u l o v

10 10 r e t a W Water 2018, 10, x FOR PEER05 REVIEW 05 12 of 16 0 20 40 60 80 100 0 20 40 60 80 100

40 40

Flow-field variation35 inevitably(c)2010 occurred due to 3th5 e variations in(d)2011 lake level and water volume in ) 3 the lake area. Figurem 30 8 presents the flow velocity variations30 at the gauging stations of lake area, with 9 0

1 25 25 × and without the ( TGD. The changes in the flow velocity did not maintain consistency, because of the

e 20 20 m

complicated hydraulicu relation in the great lake area. The mean flow velocity at the Chenglingji, l

o 15 15 v

Lujiao, Heyehu e stations10 increased to some extent after10 the TGD operation, while mean flow velocity t a reduced at the ShawanW and Nanzui stations, to a different degree, with an especially drastic decrease

05 05 at the Nanzui station.0 The20 maximum40 60 velocity80 increase,100 0 about20 0.06740 m/s60 in8 02008,10 0occurred at the Percent age of ti me vol u me exceeded % Percent age of ti me vol u me exceeded % ( ) Chenglingji outlet of the eastern part of the Dong( ) ting lake. There was a high agreement of current speed variation between the Lujiao and Heyehu stations. On the contrary, the maximum velocity Figure 7. Volume duration curves from September to October, with and without the TGD. decrease at the Nanzui station was about 0.128 m/s in 2009. Figure 7. Volume duration curves from September to October, with and without the TGD.

(a)2008 (b)2009 1. 0 0. 8 Natrual process Fl ow regul ati on 0. 8 0. 6

0. 6

0. 4 0. 4

Current speed (m/s) Current 0. 2 0. 2

0. 0 0. 0

Chenglingji Lujiao Heyehu Shawan Nanzui Chenglingji Lujiao Heyehu Shawan Nanzui

1. 0

0. 6 0. 8

0. 6 0. 4

0. 4 Current speed(m/s) Current 0. 2 0. 2

0. 0 0. 0

Chenglingji Lujiao Heyehu Shawan Nanzui Chenglingji Lujiao Heyehu Shawan Nanzui Gauging stations Gauging stations

Figure 8. Flow-velocity variations at the gauging stations of the lake area, with and without the TGD. Figure 8. Flow-velocity variations at the gauging stations of the lake area, with and without the TGD. 4. Discussion 4. Discussion Impoundment of the TGD reduced the outflow to the downstream during its impounding periods, whichImpoundment had a complex of influencethe TGD reduced on the hydraulic the outflow interaction to the downstream between the during Yangtze its River impounding and the periods,Dongting which lake. had There a complex was a positive influence correlation on the hydraulic between theinteraction mainstream between flow the and Yangtze the diverted River flowand theat the Dongting three inlets lake. of There the Dongting was a positive lake (Figure correlation2). The between reduced the outflow mainstream from theflow TGD and significantlythe diverted flowdecreased at the the three diverted inlets flow of the during Dongting impounding lake (Figure periods, 2). whichThe reduced contributed outflow a lot from to the the volume TGD significantlyreduction of thedecreased lake area. the Moreover, diverted flow there during was an im evidentpounding stage periods, decrease which at the contributed Chenglingji a lake lot to outlet the volumethat resulted reduction from of the the reduced lake area. flow Moreover, discharge there in the was Yangtze an evident River. stage As presented decrease in at thethe FigureChenglingji9, the water-headlake outlet that difference resulted increased from the to reduced a different flow degree discharge after thein the TGD Yangtze was put Rive intor. operation;As presented there in wasthe Figureespecially 9, the a larger water-head water-head difference difference increased between to the a different eastern and degree western after part the of TGD the Dongtingwas put lake,into operation; there was especially a larger water-head difference between the eastern and western part of the Dongting lake, e.g., the maximum water-head difference was much higher than 1.5 m between the Chenglingji and Nanzui (Shawan). It was the very reason that the outflow from the Chenglingji outlet had a transitory increase at the beginning of TGD impoundment. However, limited to the water volume and the rapid stage reduction in the lake area, the lake outflow significantly decreased subsequently (Figure 3, Figure 5, and Figure 7).

Water 2019, 11, 2394 12 of 16 e.g., the maximum water-head difference was much higher than 1.5 m between the Chenglingji and Nanzui (Shawan). It was the very reason that the outflow from the Chenglingji outlet had a transitory increase at the beginning of TGD impoundment. However, limited to the water volume and the rapid stage reduction in the lake area, the lake outflow significantly decreased subsequently (Figure3, Water 2018, 10, x FOR PEER REVIEW 13 of 16 Figure5 and Figure7).

3. 0 3. 0

(a)2008 (b)2009 ) 2. 5 Z - Z 2. 5

m H- C , R H- C , N

( ) ( ) ( Z - Z

2. 0 S- C , R S- C , N 2. 0 ( ) ( ) e

c Z - Z n N- C , R N- C , N ( ) ( ) e 1. 5 1. 5 r

e Z - Z

f L- C , R L- C , N f ( ) ( ) i

d 1. 0 1. 0

d a

e 0. 5 0. 5 h

r e t

a 0. 0 0. 0 W

-0. 5 -0. 5

-1. 0 -1. 0 008 008 008 008 008 009 009 009 009 009 01/ 09/ 2 16/ 09/ 2 01/ 10/ 2 16/ 10/ 2 31/ 10/ 2 01/ 09/ 2 16/ 09/ 2 01/ 10/ 2 16/ 10/ 2 31/ 10/ 2 3. 0 3. 0

2. 5 2. 5 )

m 2. 0

2. 0 (

e 1. 5 c

n 1. 5 e r

e 1. 0 f f i

d 1. 0

d 0. 5 a e

h 0. 5

0. 0 r e t a 0. 0

W -0. 5

-1. 0 -0. 5 010 010 010 010 010 011 011 011 011 011 01/ 09/ 2 16/ 09/ 2 01/ 10/ 2 16/ 10/ 2 31/ 10/ 2 01/ 09/ 2 16/ 09/ 2 01/ 10/ 2 16/ 10/ 2 31/ 10/ 2 Date D/ M/ Y Dat e D/ M/ Y ( ) ( )

Figure 9. Water-headWater-head difference difference between the stations, wi withth and without the TGD. The water-head differencedifference waswas thethe valuevalue that that the the water water head head between between the the stations stations without without the th TGDe TGD subtracted subtracted from from the thewater water head head between between the stationsthe stations with with the TGD.the TGD. ZH-C, Z RH-C,,Z RS-C,, ZS-C, R,Z R, N-C,ZN-C, R R,, and and Z ZL-C,L-C, RR denote the water head between one of the stations (Heyehu, Shawan Shawan,, Nanzui, Nanzui, and and Lujiao) Lujiao) and and the the Chenglingji Chenglingji station, station, respectively, underunder thethe flow flow regulation, regulation, and and the the ZH-C, ZH-C, N ,ZN, S-C,ZS-C, N N,Z, ZN-C,N-C, N ,, and and Z L-C, N N presentpresent the the water head between one of the stations (Heyehu, Shawan Shawan,, Nanzui, Nanzui, and and Lujiao) Lujiao) and and the the Chenglingji Chenglingji station, station, respectively, withoutwithout thethe TGD.TGD.

On the other hand, the decrease in the water volume and lake level of Dongting lake also had an impact onon thethe hydrological hydrological regime regime of theof the Dongting Dongting river–lake river–lake system. system. Lacking Lacking adequate adequate water supply,water supply,the Dongting the Dongting lake provided lake provided much less much water less to the wate downstreamr to the downstream channels to channels maintain to the maintain water depth the waterof the depth waterway of the in waterway the following in the dry following seasons dry [43 ].seasons The increased [43]. The volume increased gap volume between gap the between inflow theand inflow outflow and of theoutflow Dongting of the lake Dongting contributed lake contri greatlybuted to the greatly low stage to the of thelow lakestage area. of the Furthermore, lake area. Furthermore,the decrease inthe the decrease lake level in the and lake water level volume and water greatly volume changed greatly the hydrodynamicchanged the hydrodynamic conditions in conditionsthe lake area, in whichthe lake were area, precisely which necessarywere precisely conditions necessary for the conditions migration for and the transformation migration and of transformationnutrient content of in nutrient the lake content flow [ 44in ].the The lake abrupt flow [44]. hydrological The abrupt changes hydrological during changes the impoundment during the impoundmentperiods were extremely periods were adverse extremely to the stability adverse of to the the lake stability ecological of the environment, lake ecological e.g., environment, the 1–2 days e.g.,difference the 1–2 in days residence difference time indicatedin residence the time time indica decreaseted the that time the lakedecrease inflow that from the the lake Yangtze inflow Riverfrom thewas Yangtze larger than River the was lake larger outflow than to the the lake Yangtze outflo River,w to whichthe Yangtze means River, the aquatic which environment means the aquatic of the environmentDongting lake of changed the Dongting acutely lake within changed two months acutely impoundment within two months periods. impoundment The increased periods. water head The increasedbetween thewater stations head contributed between the to stations the increase contributed in the flow to the velocity increase and in shorting the flow the velocity watercycle and shortingduration, the which water reduced cycle duration, the risk of which water reduced eutrophication the risk in of the water lake eutrophication area. The water-head in the dilakefference area. Thenear water-head the lake outlet difference resulted near in an the increase lake outlet in the hydraulicresulted in gradient, an increase which in significantlythe hydraulic contributed gradient, whichto the velocitysignificantly increase contributed in the eastern to the lakevelocity area. incr It isease well-known in the eastern that enoughlake area. current It is well-known speed is the that enough current speed is the necessary hydrodynamic conditions for nutrient transport. Therefore, the ecological environment will be influenced by the velocity variation in the eastern Dongting lake area. In addition, the change of the hydrological regime did not show well consistent among subareas of the lake due to the complicated hydraulic relationship in the river–lake system. Therefore, the influencing mechanism of the reservoir impoundment to the river–lake system should be further researched. It should be noted that this study examined the impoundment impact, but the

Water 2019, 11, 2394 13 of 16 necessary hydrodynamic conditions for nutrient transport. Therefore, the ecological environment will be influenced by the velocity variation in the eastern Dongting lake area. In addition, the change of the hydrological regime did not show well consistent among subareas of the lake due to the complicated hydraulic relationship in the river–lake system. Therefore, the influencing mechanism of the reservoir impoundment to the river–lake system should be further researched. It should be noted that this study examined the impoundment impact, but the other influence factors remain unchanged. The inflow and outflow of the TGD, as the flow boundary conditions in the model research, more directly revealed the impoundment impact on the river–lake system. In spite of some results that have been achieved based on the effective analysis in this paper, more research on the influence of other factors is still worth carrying out.

5. Conclusions Impoundment impact, generally, is one of the most important influence factors to the hydrological regime of the river basin. In this study, a physically coupled hydrodynamic model and extensive hydrological data were combined to quantify the impoundment influences on the Dongting river–lake system. The TGD inflow and outflow, as the upstream flow boundary conditions, were the first attempt to explore the impoundment impact in the large Dongting river–lake system, which provided insights into the changes in the hydrological regime. The results revealed that water volume and lake level in the lake area decreased to a different degree under the flow regulation of the TGD, which resulted from the water-exchange changes in the river–lake system. The water inflow to the Dongting lake drastically decreased in total during the impounding periods of the TGD, which not only resulted in a great reduction in water volume and lake stage in the Dongting lake but also decreased the outflow to the downstream channels. In addition, compared with the different upstream flow conditions for the coupled hydrodynamic model, the impoundment impact on the hydraulic interaction between the Yangtze River and the Dongting lake during the impounding periods was examined. The results implied that the flow velocity in the eastern part of the Dongting lake increased due to the enlarged water head to the lake outlet. However, the flow velocity decreased in the western lake area due to the drastic inflow reduction from the mainstream and the weak hydraulic relationship between the western and eastern parts of the Dongting lake. It indicated that the hydraulic and hydrological regime in the Dongting river–lake system was complicated, which also had some other influence factors worthy of further investigation. Moreover, the results indicated that the water residence rate decreased to some extent during the different impounding periods, which implied the water retention capacity of the Dongting lake was impaired. Consequently, the water residence time also decreased during impounding periods. This study focused on the impoundment impact on the hydrological and hydraulic regime changes in the Dongting river–lake system and allowed us to better understand the potential influences of the changes in the water exchange after the TGD operation. Finally, the scientific knowledge derived from this study can provide a reference for the administrative departments to make a scientific and rational decision for water-resource, water–ecological, and environmental management in the similar river–lake systems.

Supplementary Materials: The model input data of the two scenarios investigated in this paper can be accessed through the following websites: http://www.cjh.com.cn and http://sw.hubeiwater.gov.cn. Author Contributions: J.Z. conducted the modeling, performed the analysis, and drafted the manuscript; T.H. acquired, processed, and analyzed the data. L.C. conceived of and designed the study and interpreted the results; X.L. and L.F. prepared the input data for the simulations. L.Z. provided observational data and helped with the analysis; Y.Y. gave constructive comments on the results in this study. All the authors edited the manuscript. Acknowledgments: This paper was financially supported by the National Key Research and Development Plan (2016YFC0400901) and the National Natural Science Foundation of China (51509273, 41601006, 51679094, and 51409085). Conflicts of Interest: The authors declare that they do not have individual or collective conflicts of interest. Water 2019, 11, 2394 14 of 16

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