water

Article Hydrological Drought in Area () after the Running of and a Possible Solution

Yizhuang Liu 1,2,3, Shu-Qing Yang 4 , Changbo Jiang 1,2,3,*, Yuannan Long 1,2,3 , Bin Deng 1,2,3 and Shixiong Yan 1

1 School of Hydraulic Engineering, University of Science & Technology, Changsha 410114, China; [email protected] (Y.L.); [email protected] (Y.L.); [email protected] (B.D.); [email protected] (S.Y.) 2 Key Laboratory of Dongting Lake Aquatic Eco-Environmental Control and Restoration of Province, Changsha 410114, China 3 Key Laboratory of Water-Sediment Sciences and Water Disaster Prevention of Hunan Province, Changsha 410114, China 4 School of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong 2522, Australia; [email protected] * Correspondence: [email protected]; Tel.: +86-0731-8525-8437

 Received: 24 August 2020; Accepted: 25 September 2020; Published: 28 September 2020 

Abstract: Dongting Lake is located at the downstream of Three Gorges Dam (TGD) and the hydrological drought is intensified after the impoundment of TGD as the dry period has been extended from 123 days/year before the operation of TGD (1981–2002) to 141 days/year (2003–2016) on average. Particularly, the Dongting Lake’s water shortage becomes very severe. To solve the problem caused by upstream dams, an innovative flood control scheme (IFCS) was introduced, and its feasibility of application in Dongting Lake is studied using the hydrodynamic module of Mike 21. The results show the IFCS can effectively convert the peak discharge of floodwater in wet seasons into water resources in dry seasons as the IFCS could significantly increase the usable water storage of the lake. For example, the usable water storage could increase to 2.85 billion m3 and 1.81 billion m3 in the extreme drought year of 2006 and 2011, respectively. The average increment of the water level would be about 0.4 m, 0.6 m, and 0.5 m in the West Dongting Lake (WDL), (SDL), and the East Dongting Lake (EDL), respectively, if the water stored in the inner lake was discharged uniformly in 30 days (27 November to 27 December 2006) with the application of IFCS. This study may provide an innovative method to alleviate the water shortage problem in Dongting Lake and other similar lakes.

Keywords: hydrological drought; water shortage crisis; Dongting Lake; water supply; dry season

1. Introduction Lakes play an important role in the water supply for agricultural, industrial, commercial, and residential use. However, the extreme hydrological drought event happens more frequently in the lake area due to climate change and human activities in recent years [1]. The hydrological drought has attracted increasing worldwide attention for the ever-increasing demand on freshwater sources due to population growth, urbanization and industrialization. Climate change often results in effects such as higher evaporation, lower precipitation, and higher temperature, which would exacerbate the drought severity [2,3]. Meanwhile, numerous studies have shown that human activities (e.g., dam construction, river dredging, and sand mining, etc.) would decrease the inflow to the lake directly or accelerate the water flow out of the lake [4–8]. For example, the impoundment of the upstream dam leads to an overall

Water 2020, 12, 2713; doi:10.3390/w12102713 www.mdpi.com/journal/water Water 2020, 12, x FOR PEER REVIEW 2 of 14 accelerate the water flow out of the lake [4–8]. For example, the impoundment of the upstream dam leadsWater to an2020 overall, 12, 2713 reduction of the downstream water level, especially during the dry season;2 of 14 thus, the hydrological drought in the downstream-linked lakes is intensified [5,9,10]. reductionDongting ofLake, the downstream the second-largest water level, freshwater especially lake during in China, the dry is season; located thus, in the the middle hydrological reach of Yangtzedrought River. in the Its downstream-linked hydrological cycle lakes has is intensifiedbeen changed [5,9,10 significantly]. by the Three Gorges Dam [1,5,11–13]Dongting as its inflow Lake, theto second-largestthe lake has been freshwater reduced lake in in dry China, periods, is located especially in the middle from reachSeptember– of DecemberYangtze in River. the recent Its hydrological decades. cycle Consequently, has been changed its water significantly level byand the water Three Gorgessurface Dam areas [1,5 ,have11–13 ]been reducedas its significantly, inflow to the lake which has beenis caused reduced by in the dry TGD periods, after especially 2003 [12]. from More September–December specifically, thein statistics the recent decades. Consequently, its water level and water surface areas have been reduced significantly, show that, when compared with the pre-TGD period, the starting date of the dry season occurs earlier which is caused by the TGD after 2003 [12]. More specifically, the statistics show that, when compared and the duration of the dry season lasts longer after TGD’s operation in 2003 [8,14–16]. For example, with the pre-TGD period, the starting date of the dry season occurs earlier and the duration of the Dongtingdry season Lake has lasts experienced longer after TGD’sextreme operation drought in events 2003 [ 8in,14 2006–16]. and For 2011 example, with Dongtingthe water Lake surface has area 2 of aboutexperienced 350 km extreme (i.e., 1/8 drought of the eventssurface in area 2006 of and Dongting 2011 with Lake the water in the surface wet season) area of aboutin the 350 dry km season2 [17]. (i.e.,Moreover, 1/8 of the in surface 2011, areaextremely of Dongting low Lakewater in level the wet pe season)riod lasted in the dry80 days season in [ 17the]. Moreover,Dongting in Lake 2011, area and hundredsextremely lowof thousands water level periodof residents lasted 80suffered days in the the water Dongting shortage Lake area problem and hundreds [17]. of thousands ofPrevious residents studies suffered mainly the water focus shortage on problemthe identification [17]. and characteristics of the hydrological drought Previousas well as studies the mainlyfactors focusthat oninduce the identification the extreme and drought characteristics event of in the the hydrological Dongting drought Lake area [1,8,9,14–16].as well as Few the factors studies that investigate induce the extremehow to droughtenhance event the water in the Dongtingsupply of Lake Dongting area [1, 8Lake,9,14– 16[18,19].]. Actually,Few studies apart from investigate the environmental how to enhance change the water (e.g., supply climate of Dongting change Lake and [ 18human,19]. Actually, activities), apart one from of the the environmental change (e.g., climate change and human activities), one of the main reasons for the main reasons for the water shortage problem in the Dongting Lake area is that the lake cannot contain water shortage problem in the Dongting Lake area is that the lake cannot contain its floodwater till the its floodwater till the dry period since it is connected with River directly, as shown in Figure dry period since it is connected with Yangtze River directly, as shown in Figure1. Therefore, to solve 1. Therefore,its water shortageto solve problem, its water one shortage has to convert proble itsm, floodwater one has into to waterconvert resources its floodwater in the dry into period. water resourcesYang etin al. the [20 dry] and period. Liu et Yang al. [21 ]et proposed al. [20] and an innovative Liu et al. flood[21] proposed control scheme an innovative (IFCS) as shownflood control in schemeFigure (IFCS)1. Together as shown with in the Figure existing 1. dike Together surrounding with thethe lake,existing they suggestdike surrounding building an innerthe lake, dike they suggestwith building several gates an inner to form dike an innerwith lakeseveral for mitigationgates to form of flood an inner disaster lake in wet for seasons.mitigation At theof flood beginning disaster in wetof wetseasons. seasons, At the lakebeginning water is of lowered wet seasons, to its dead the water lake level.water Duringis lowered the flood to its period, dead thewater peak level. Duringfloodwater the flood could period, be diverted the peak into floodwater the inner lake could for storage be diverted by opening into the the gates, inner thus lake the for inner storage lake by openingfunctions the gates, as a giant thus flood-detentionthe inner lake pond.functions More as details a giant could flood-detention be found in thepond. previous More study details [21 could]. be foundThe stored in the water previous will be study released [21]. when The the stored gates wa areter open will to regulatebe released the outflowwhen the in the gates dry are seasons. open to Therefore, a huge amount of floodwater stored in the inner lake can be converted to water resources in regulate the outflow in the dry seasons. Therefore, a huge amount of floodwater stored in the inner dry seasons. lake can be converted to water resources in dry seasons.

FigureFigure 1. 1.TheThe schematic schematic of of the the IFCS inin Dongting Dongting Lake. Lake. In this paper, the effect of TGD on the hydrological drought was analyzed using 35 years (1981–2016) ofIn hydrologicalthis paper, the data effect observed of TGD at Chenglingji on the hydrological station, which drought is the typical was analyzed station to representusing 35 theyears water (1981– 2016)level of hydrological in Dongting Lake data [8 observed]. The feasibility at Chenglingji of applying station, the IFCS which to increase is the the typical usable station water storage to represent of the waterDongting level Lake in Dongting was examined Lake using [8]. The the hydrodynamic feasibility of applying module of the Mike IFCS 21. Theto increase main objectives the usable of this water storagepaper of wereDongting to: (1) analyzeLake was the eexaminedffect of the TGDusing on the the hydrologicalhydrodynamic drought module in the of Dongting Mike 21. Lake The area; main objectives(2) examine of this the paper usable were water to: storage (1) analyze of Dongting the ef Lakefect of after the applying TGD on the the IFCS; hydrological (3) study its drought feasibility in the Dongting Lake area; (2) examine the usable water storage of Dongting Lake after applying the IFCS; (3) study its feasibility for enhancing the water supply in the Dongting Lake area. This study may

Water 2020, 12, x FOR PEER REVIEW 3 of 14 provide an innovative method to alleviate the water shortage problem in Dongting Lake and other similar lakes.

2. Materials and Methods

Water 2020, 12, 2713 3 of 14 2.1. Study Area Dongting Lake (28°20′–30°20′ N, 111°30′–113°15′ E) is the second-largest freshwater lake in for enhancing the water supply in the Dongting Lake area. This study may provide an innovative China, which provides water for millions of people who live in the surrounding area and method to alleviate the water shortage problem in Dongting Lake and other similar lakes. downstream of the lake. The lake is located at approximately 400 km downstream away from Three Gorges2. MaterialsDam (TGD, and Methodssee Figure 2) and thereby is considerably affected by TGD. Dongting Lake exhibits seasonal variability of the water surface area from about 2670 km2 in the wet season to 2.1. Study Area approximately 710 km2 in the annual dry season. The variation of the annual water level is 12.7 ± 1.8 m based Dongtingon the Lakedaily (28 measured◦200–30◦20 0 waterN, 111◦ 30level0–113 from◦150 E) 1961–2014 is the second-largest at Chenglingji freshwater station, lake in China,which is consideredwhich provides as the typical water for hydro-station millions of people of Dongin who liveg inLake the surrounding[22]. The mean area andannual downstream precipitation of the and evaporationlake. The in lake the is Dongting located at Lake approximately area are about 400 km 1363 downstream mm/year away and from729 mm/year, Three Gorges respectively Dam (TGD, [23]. see Figure2) and thereby is considerably a ffected by TGD. Dongting Lake exhibits seasonal variability The whole lake is divided into three sub-lakes (i.e., West Dongting Lake, WDL; South Dongting of the water surface area from about 2670 km2 in the wet season to approximately 710 km2 in the Lake, SDL; and East Dongting Lake, EDL, see Figure 2b). It receives inflow mainly from the “four annual dry season. The variation of the annual water level is 12.7 1.8 m based on the daily measured ± rivers”water (Xiangjiang, level from 1961–2014Zishui, Yuanshui, at Chenglingji and station,Lishui) which and Yangtze is considered River as through the typical the hydro-station “three channels” of (Songzi,Donging Hudu Lake and [22 Ouchi).]. The mean Other annual inflows precipitation sources include and evaporation local rainfall in the and Dongting small Lakestreams area around are lake shoreline.about 1363 The mm/ yearlake anddischarges 729 mm /toyear, the respectively Yangtze River [23]. through Chenglingji station.

(a)

(b)

FigureFigure 2. The 2. locationThe location of Dongting of Dongting Lake. Lake. The Thelake lakecolle collectscts water water from from the theYangtze Yangtze River River through through ‘three channels’‘three (Songzi, channels’ Hudu, (Songzi, and Hudu, Ouchi) and Ouchi)and ‘four and rive ‘fourrs’ rivers’ (Xiangjiang, (Xiangjiang, Zishui, Zishui, Yuanshui, Yuanshui, and and Lishui) Lishui) and dischargesand discharges to the Yangtze to the Yangtze River Riverat Chenglingji. at Chenglingji. (a) The (a) Thelocation location of ofDongti Dongtingng Lake Lake catchment; catchment; (b) Dongting(b) Dongting lake and lake its and main its tributaries. main tributaries.

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The whole lake is divided into three sub-lakes (i.e., West Dongting Lake, WDL; South Dongting Lake, SDL; and East Dongting Lake, EDL, see Figure2b). It receives inflow mainly from the “four rivers” (Xiangjiang, Zishui, Yuanshui, and Lishui) and Yangtze River through the “three channels” (Songzi, Hudu and Ouchi). Other inflows sources include local rainfall and small streams around lake shoreline. The lake discharges to the Yangtze River through Chenglingji station.

2.2. Method

2.2.1. M-K Method The Mann-Kendall (M-K) test [24,25] is a non-parametric test to identify the long-term trend of the duration of the dry period in this study. It is widely used in the analysis of hydrological time series [26]. Given a time series of x , i = 1, 2, 3, ...... , n for each variable, the M-K statistic (d ) is defined as: { i } k Xk dk = mi k = 2, 3, 4, ...... , n (1) t=1 ( +1 x > x where m = i j j = 1, 2, ...... , i i 0 x x i ≤ j The mean and variance of dk were:

k(k 1) E(d ) = − (2) k 4

k(k 1)(2k + 5) D(d ) = − 1 < k n (3) k 72 ≤

The statistic value UF(dk) is calculated by:

dk E(dk) UF(dk) = p− (4) D(dk)

The backward sequence (UB) is calculated using the same method that applied to the inverted series of xi. A positive UF(dk) indicates an increasing trend, while a negative value represents a declining trend. A change-point occurs at the intersection of UF and UB within the confidence interval.

2.2.2. Hydrodynamic Model A previously calibrated 2D depth-averaged hydrodynamic model (Mike 21 FM) was used in this study since it could simulate the hydrodynamic process well in Dongting Lake [21]. The model covers the whole area of Dongting Lake, “three channels”, “four rivers” and a part of the middle reach of Yangtze River. It was constructed by 210,341 triangular elements to capture the complex lake bathymetry (see Figure3). The element size varies from 30 m in narrow channels to 700 m in the lake area by trial and error. The upstream boundary conditions were specified as the daily inflow records of Yangtze River (i.e., Zhicheng station) and the four rivers ( station, Taojiang station, Taoyuan station and Jinshi station). The downstream boundary condition used the rating curve observed at Luoshan station on Yangtze River [21]. The wetting and drying process function [27] was used as the water surface varies significantly in the lake area [28]. Previously calibrated parameters such as Manning roughness number (from 32 m1/3/s to 60 m1/3/s) of the lake and the minimum time step (i.e., 2s) were used in the current module. More details of the model could be found in the previous study [21], thus a brief description of the model is given here. Water 2020, 12, 2713 5 of 14 Water 2020, 12, x FOR PEER REVIEW 5 of 14

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Figure 3. The unstructured grid for the Mike 21 model. FigureFigure 3.3. The unstructured grid for the MikeMike 2121 model.model.

The TheThe2D 2Dhydrodynamic2D hydrodynamichydrodynamic model model model was was validatedvalidated validated using using the thethe measured measured measured water water water level level atlevel theat the stationsat thestations stations in the in in the lakelakethe lake (i.e., (i.e., Nanzui,Nanzui, Nanzui, Yintan, Yintan, Yintan, Lujiao Lujiao Lujiao and and Chengljiand Chenglji Chenglji stations, stations, see Figure see see Figure Figure2) during 2) 2)during theduring dry the seasonthe dry dry season of 2006.season of of 2006.Figure2006. Figure Figure4 shows 4 shows 4 theshows comparisonthe the comparison comparison of the measuredof of thethe measuredmeasured and simulated andand simulated simulated water level water water process level level atprocess the process selected at the at the selectedstationsselected stations instations Dongting in Dongtingin Dongting Lake. The Lake. Lake. results The The show results results that theshowshow model thatthat can thethe simulate model model can the can watersimulate simulate level the fluctuation thewater water level in level the lake successfully since the simulated results agree well with the measured data. The uncertainty of fluctuationfluctuation in the in thelake lake successfully successfully since since the the simusimulated resultsresults agree agree well well with with the the measured measured data. data. theThe model uncertainty was estimated of the model by the Nash-Sutcliwas estimatedffe e ffibyciency the Nash-Sutcliffe coefficients (NSE), efficiency determination coefficients coe ffi(NSE),cient The uncertainty2 of the model was estimated by the Nash-Sutcliffe efficiency coefficients (NSE), (Rdetermination), the difference coefficient of the maximum(R2), the difference water level, of the and maximum the root-mean-square water level, errorand the (RMSE) root-mean-square (see Table1). determination coefficient (R2), the difference of the maximum water level,2 and the root-mean-square Theerror result (RMSE) shows (see that Table the 1). NSE The value result in shows ranges that of 0.887 the NSE to 0.981, value the in Rrangesvalues of are0.887 greater to 0.981, than the 0.96, R2 error (RMSE) (see Table 1). The result shows that the NSE value in ranges of 0.887 to 0.981, the R2 thevalues RMSE are valuesgreater are than less 0.96, than the 0.2 RMSE m and values the average are less water than level0.2 m di andfferences the average are within water 0.15 level m, values are greater than 0.96, the RMSE values are less than 0.2 m and the average water level whichdifferences indicates are within that the 0.15 hydrodynamic m, which indicates model that could the represent hydrodynamic the flow model process could of the represent lake during the flow the differencesdryprocess season. are of the within lake during0.15 m, the which dry season.indicates that the hydrodynamic model could represent the flow process of the lake during the dry season.

(a) (b)

Figure 4. Cont. (a) (b)

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(c) (d)

FigureFigure 4.4. ComparisonComparison ofof waterwater levellevel processprocess duringduring thethe drydry seasonseason (November(November toto December)December) inin 2006.2006. (a(a)) Xiaohezui; Xiaohezui; ( b(b)) Yingtian; Yingtian; ( c()c) Lujiao; Lujiao; ( d(d)) Chenglingji. Chenglingji.

2 TableTable 1. 1.The The Nash–Sutcli Nash–Sutcliffeffe e efficiencyfficiency coe coefficientsfficients (NSE), (NSE), RR2 andand thethe rootroot meanmean squaresquare errorerror (RMSE)(RMSE) calculatedcalculated atat the the main main hydrological hydrological stations stations in in Dongting Dongting Lake. Lake.

Average Stations NSE R2 RMSE Average Stations NSE R2 RMSE Difference (m) Difference (m) Xiaohezui 0.887 0.962 0.086 0.07 YingtianXiaohezui 0.979 0.887 0.9900.962 0.086 0.124 0.07 0.10 Lujiao 0.932 0.992 0.195 0.12 ChenglingjiYingtian 0.981 0.979 0.9940.990 0.124 0.10 0.10 0.08 Lujiao 0.932 0.992 0.195 0.12 2.3. The Preliminary Design of IFCS in Dongting Lake Chenglingji 0.981 0.994 0.10 0.08 The inner dikes with sluice gates of the IFCS were proposed in Figure5, where the gates are represented by the void red circle with a cross inside and the inner dikes are marked by the solid yellow 2.3. The Preliminary Design of IFCS in Dongting Lake line. The inner dikes are built along the mainstream in the Dongting Lake, thus the safe floodwater couldThe be dischargedinner dikes to with the downstreamsluice gates smoothlyof the IFCS through were proposed the mainstream in Figure [21]. 5, The where area the of thegates inner are lakerepresented of WDL, by SDL, the and void EDL red is circle 190 km with2, 560 a cross km2, andinside 969 and km 2the, respectively. inner dikes The are water marked level by should the solid be loweredyellow line. to its The dead inner water dikes level are (i.e., built 29.0 malong in WDL, the mainstream 27.5 m in SDL, in andthe 26Dongting m in EDL) Lake, in thethus inner the lakesafe atfloodwater the end of could the dry be discharged season [21] to and the thereby downstream there issmoothly enough through flood control the mainstream capacity to [21]. mitigate The area the floodof the disaster. inner lake During of WDL, the flood SDL, season, and EDL the is excess 190 km floodwater2, 560 km could2, and be969 discharged km2, respectively. into the innerThe water lake bylevel opening should the be sluice lowered gate to when its dead the water peakflood level wave(i.e., 29.0 arrives m in [20 WDL,] and 27.5 the m safe in floodwaterSDL, and 26 would m in EDL) flow toin thethe downstreaminner lake at throughthe end theof the mainstream dry season by [21] closing and thereby the sluice there gates. is enough Therefore, flood a hugecontrol amount capacity of floodwaterto mitigate isthe stored flood in disaster. the inner During lake after the theflood flooding season, period, the excess which floodwa couldter be usedcould to be alleviate discharged the waterinto the shortage inner crisislake by during opening the drythe season.sluice gate More wh detailsen the could peak beflood found wave in thearrives previous [20] and study the [21 safe]. floodwater would flow to the downstream through the mainstream by closing the sluice gates. Therefore, a huge amount of floodwater is stored in the inner lake after the flooding period, which could be used to alleviate the water shortage crisis during the dry season. More details could be found in the previous study [21].

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FigureFigure 5. 5.The The layoutlayout of the IFCS in Dongting Dongting Lake. Lake. Figure 5. The layout of the IFCS in Dongting Lake. 3.3. Results Results 3. Results 3.1.3.1. Water Water Level Level in in the the Dry Dry Season Season 3.1.To WaterTo analyze analyze Level the in the the long long Dry term term Season change change of of the the waterwater levellevel in the dry dry season season of of Dongting Dongting Lake, Lake, the the period period ofof 1981–2016 1981–2016To analyze was was dividedthe divided long intoterm into two change two typical typical of the periods, water periods, i.e.,level i.e., 1981–2002 in the1981–2002 dry (pre-TGD)season (pre-TGD) of Dongting and and 2003–2016 Lake,2003–2016 the (post-TGD), period (post- accordingTGD),of 1981–2016 according to the was operation to divided the operation year into (i.e., two year 2003) typical (i.e., of TGD periods,2003) [ 8of,9 ].TGDi.e., Normally, 1981–2002 [8,9]. Normally, the (pre-TGD) dry season the dryand starts season 2003–2016 from starts November (post- from toNovemberTGD), March according (next to March year) to the in(next theoperation year) Dongting in yearthe Dongting Lake(i.e., 2003) area. La ofke The TGD area. water [8,9]. The impoundmentwaterNormally, impoun the dmentdry period season period is usuallystarts is usually from from thefromNovember late the September late to September March to (next December to year) December in while the whileDongting the the discharge dischargeLake area. period period The water is is from from impoun January Januarydment to May, May,period but but is itusually it would would bebefrom adjusted adjusted the late depending depending September on onto theDecember the inflow inflow while fromfrom the thethe discharge upstreamupstream period of TGD is from[29]. [29]. JanuaryFigure Figure 6 6to showsshows May, butthe the itmonthly monthlywould averageaveragebe adjusted water water depending level level during during on thethe inflow two periods. from the The The upst water waterream level levelof TGDincreases increases [29]. from Figure from January 6 January shows to Marchthe to Marchmonthly due dueto totheaverage the discharge discharge water oflevel of TGD, TGD, during while while the ittwo it decreased decreased periods. considerThe considerably waterably level from from increases September September from Januaryto to December December to March due due due to to theto the impoundmentimpoundmentthe discharge of ofof TGD. TGD,TGD. Therefore, whileTherefore, it decreased thethe droughtdrought consider severityseveriablyty could couldfrom beSeptember be alleviated alleviated to fromDecember from January January due to to toMarch Marchthe (i.e.,(i.e.,impoundment the the average average of monthly monthly TGD. Therefore, water water level level the increasedincreased drought byseveriby aboutaboutty could 0.6 m), m), be while whilealleviated it it would would from be beJanuary exacerbated exacerbated to March from from NovemberNovember(i.e., the average (i.e., (i.e., the the monthly mean mean monthly watermonthly level waterwater increased levellevel decreaseddecreas by abouted 0.6 by m),0.88 0.88 while m) m) and and it would December December be exacerbated (i.e., (i.e., the the average averagefrom monthlymonthlyNovember water water (i.e., level level the decreasedmean decreased monthly by by 0.35 0.35water m) m) level underunder decreas thethe eeffectffedect by of 0.88 TGD. m) and December (i.e., the average monthly water level decreased by 0.35 m) under the effect of TGD.

Figure 6. The average monthly water level in the two typical periods. Figure 6. The average monthly water level in the two typical periods. Figure 6. The average monthly water level in the two typical periods.

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The dry dry spell spell of of Dongting Dongting Lake Lake can can be defined be defined as a asperiod a period when when the water the waterlevel declines level declines to 23 m to at Chenglingji station. Figure 7a shows the duration of the dry period from 1981 to 2016. The result 23 m at Chenglingji station. Figure7a shows the duration of the dry period from 1981 to 2016. shows that the average duration is about 123 days/year during 1981–2002, while it increases to 141 The result shows that the average duration is about 123 days/year during 1981–2002, while it increases days/year after the operation of TGD (2003–2016). The duration of the dry period increased by 18 to 141 days/year after the operation of TGD (2003–2016). The duration of the dry period increased by days compared to the period of 1981–2002. Moreover, there is only one year that the duration is longer 18 days compared to the period of 1981–2002. Moreover, there is only one year that the duration is than 150 days during 1981–2002. However, 4 years that the duration is longer than 150 days occurred longer than 150 days during 1981–2002. However, 4 years that the duration is longer than 150 days during 2003–2016. It indicates that a long duration of the dry period becomes more frequent after the occurred during 2003–2016. It indicates that a long duration of the dry period becomes more frequent operation of TGD. Meanwhile, the longest duration (i.e., 207 days) of the dry season in the past 4 after the operation of TGD. Meanwhile, the longest duration (i.e., 207 days) of the dry season in the past decades occurred in 2011. Figure 7b shows the MK test of the duration of the dry season from 1981 4 decades occurred in 2011. Figure7b shows the MK test of the duration of the dry season from 1981 to to 2016. There is a sudden change in 2003, which is consistent with the operation of TGD. It could 2016. There is a sudden change in 2003, which is consistent with the operation of TGD. It could also be also be found that there is an increasing trend of the duration after the year 2003 as the UF value is found that there is an increasing trend of the duration after the year 2003 as the UF value is positive. positive.

(a) (b)

Figure 7. TheThe long-term long-term change change of of the the duration duration of of the the dry dry period. period. (a ()a The) The duration duration of of the the dry dry period period from 1981 to to 2016; 2016; ( (bb)) the the M-K M-K test test of of the the duration duration..

3.2. Application Application of of I IFSCFSC to to Dongting Dongting Lake Lake As discussed in in Section Section 3.1, 3.1 ,the the monthly monthly average average water water level level decreases decreases significantly significantly in inNovember November and December,December, and and the the duration duration of theof the dry perioddry period become become longer longer after theafter operation the operation of TGD. of Therefore, TGD. theTherefore, water shortage the water crisis shortage becomes crisis more becomes and moremore seriousand more in theserious Dongting in the LakeDongting area. Lake There area. is an There urgent needis an urgent to enhance need theto enhance water level the water in November level in November and December and December in the Dongting in the Dongting Lake area. Lake The area. IFSC, asThe described IFSC, as described in Section in 2.3 Section and the2.3 and previous the previo studyus [study21], could [21], could mitigate mitigate the flood the flood by storing by storing peak floodwaterpeak floodwater into theinto inner the inner lake and lake this and amount this amount of floodwater of floodwater could becould used be to used alleviate to alleviate the drought the severitydrought inseverity the dry in season. the dry It season. might beIt amight solution be a for solution the water for shortagethe water crisis shortage in Dongting crisis in LakeDongting area as itLake has area a huge as it potential has a huge water potential storage water capacity. storage capacity.

3.2.1. The The Usable Usable Water Water Storage Storage of of Dongting Dongting Lake Lake The usable water storage storage in in each each year year ca cann be be estimated estimated by by the the following following equation: equation:

𝑆×((ℎ −ℎ) ℎ < ℎ V=S×∆h= S (hmax he) hmax < hd (5) V = S ∆𝑆×h = (ℎ ×−ℎ ) − ℎ ≥ ℎ (5) × S (h he) hmax h × d − ≥ d where V is the usable water storage, S is the surface area of inner lake; hmax is the maximum water where V is the usable water storage, S is the surface area of inner lake; hmax is the maximum water level of each element in Dongting Lake; he is the dead water level (i.e., 29.0 m in WDL, 27.5 m in SDL, level of each element in Dongting Lake; he is the dead water level (i.e., 29.0 m in WDL, 27.5 m in SDL, and 26 m in EDL); hd is the designed water level of the embankment of each sub-lakes (i.e., 34.0 m in andWDL, 26 33.1 m in m EDL); in SDLh dandis the 32.6 designed m in EDL). water If the level maximum of the embankment water level is ofover each the sub-lakes designed (i.e.,water 34.0 level, m in WDL,we use 33.1 the mdesigned in SDL level and 32.6 to estimate m in EDL). the usable If the maximum water storage. water level is over the designed water level, we useFigure the designed8 shows levelthe usable to estimate water the storage usable of water Dongting storage. Lake from 1996 to 2016. According to EquationFigure (5),8 showsthe usable the water usable storage water storagereaches ofup Dongtingto 2.85 billion Lake m from3 and 1996 1.81 tobillion 2016. m3 According even in the to Equation (5), the usable water storage reaches up to 2.85 billion m3 and 1.81 billion m3 even in the

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Water 2020, 12, x FOR PEER REVIEW 9 of 14 extreme drought year of 2006 and 2011, respectively, while the total water storage content was less extreme drought year of 2006 and 2011, respectively, while the total water storage content was less than 1 billion m3 during these two extreme drought years in the current condition [30]. than 1 billion m3 during these two extreme drought years in the current condition [30].

FigureFigure 8. 8. TheThe usable usable water water storage storage capacity capacity in in Dongting Dongting Lake Lake in in each each year year (1996–2016).

3.2.2.3.2.2. The The Effect Effect of of the the IFSC IFSC on the Water Supply FromFrom the abovementioned analysis, analysis, there there is is sti stillll a a great potential water storage capacity in in DongtingDongting Lake Lake even even in the extreme drought years such as 2006 and 2011. In In this this section, section, the the dry dry period (from 27 NovemberNovember toto 2727 December)December) of of extreme extreme drought drought year year 2006 2006 was was taken taken as as an an example example to toestimate estimate the the eff ectivenesseffectiveness of theof th watere water supplement supplement of theof the IFCS. IFCS. Four Four gates gates (i.e., (i.e., #1, #1, #3, #5#3, and#5 and #8, see#8, seeFigure Figure5) were 5) were used used to regulate to regula thete water the water from thefrom inner the lakeinner to lake the canalto the to canal increase to increase the water the supply. water 3 supply.The usable The water usable storage water of storage WDL, SDLof WDL, and EDLSDL wasand calculated EDL was bycalculated Equation by (5) Equation (i.e., 0.17 (5) billion (i.e., m 0.17in 3 3 billionWDL, 0.84m3 in billion WDL, m 0.84in billion SDL and m3 1.84in SDL billion and m1.84in billion EDL). m We3 in assumed EDL). We the assumed water stored the water in the stored inner inlake the would inner belake discharged would be indischarged 30 days (from in 30 27days November (from 27 to November 27 December to 27 2006) December uniformly, 2006) and uniformly, the flow andrate the at each flow gate rate wasat each shown gate in was Table shown2. in Table 2.

TableTable 2. The flow flow rate at each gate. Location Gate V (Billion m3) Discharge (m3/s) Location Gate V (Billion m3) Discharge (m3/s) #1 32 WDL #1 0.17 32 #3 32 WDL 0.17 SDL#3 #5 0.84 32432

SDL EDL#5 #80.84 1.84 710324

EDL #8 1.84 710 The water level change response to the operation of the IFCS in Dongting Lake under one of the extremeThe water drought level change years, response i.e., 2006, to was the simulatedoperation basedof the IFCS on the in hydrodynamicDongting Lake modelunder one (Mike of the 21). extremeFigure9a drought shows the years, variation i.e., 2006, of water was simulated level in the based BPC ofon West the hydrodynamic Dongting Lake model (WDL, (Mike Nanzui 21). station), Figure 9aSouth shows Dongting the variation Lake (SDL, of water Yingtian level in station), the BPC and of EastWest Dongting Dongting Lake Lake (EDL, (WDL Chenglingji, Nanzui station), station) South after Dongtingthe application Lake of(SDL, IFCS. Yingtian The results station), show and that East the Do waterngting level Lake increases (EDL, significantly Chenglingji instation) Dongting after Lake the applicationwith the IFCS. of IFCS. Figure The9b presents results show the water that levelthe water di fference level atincreases all selected significantly hydro-stations in Dongting in Dongting Lake withLake the with IFCS. the applicationFigure 9b presents of IFCS. the The water result level shows difference that the IFCSat all imposesselected morehydro-stations influence in on Dongting the water Lakelevel inwith SDL the than application the other of two IFCS. sub-lakes The result (i.e., WDLshows and that EDL). the ThisIFCS is imposes because more the SDL influence receives on water the waterfrom thelevel inner in SDL lake than of WDL the other and two SDL sub-lakes and thereby (i.e., the WDL increment and EDL). of theThis water is because level inthe SDL SDL is receives greater waterthan thatfrom in the SDL. inner For lake the of EDL, WDL the and water SDL level and increasesthereby the less increment than that of in the SDL water because level the in EDLSDL is greateraffected than by Yangtze that in SDL. River For since the the EDL, EDL the connected water level the increases Yangtze less River than directly, that in although SDL because the discharge the EDL isis theaffected largest by in Yangtze EDL. The River average since increments the EDL ofconne the watercted the level Yangtze were about River 0.4 directly, m, 0.6 m, although and 0.5 mthe in discharge is the largest in EDL. The average increments of the water level were about 0.4 m, 0.6 m, and 0.5 m in WDL, SDL and EDL, respectively, while the largest increments of the water level were

Water 2020, 12, 2713 10 of 14 Water 2020, 12, x FOR PEER REVIEW 10 of 14

WDL,0.5 m, SDL 0.81 and m, EDL,and respectively,0.7 m, respectively. while the This largest indicates increments that ofthe the IFCS water increases level were the 0.5 water m, 0.81 level m, andsignificantly 0.7 m, respectively. in Donging This Lake indicates, and thus that it the could IFCS alleviate increases the the wa waterter shortage level significantly in the Dongting in Donging Lake Lake,area. and thus it could alleviate the water shortage in the Dongting Lake area.

(a) (b)

FigureFigure 9.9. The variation of the water levellevel withwith IFCSIFCS (27(27 NovemberNovember toto 2727 DecemberDecember 2016):2016): (aa)) thethe fluctuationfluctuationof of the the water water level level at the at selectedthe selected station; station; (b) the average(b) the andaverage maximum and maximum water level water differences level indifferences the Dongting in the Lake. Dongting Lake.

4.4. Discussions HumanHuman activitiesactivities suchsuch asas damdam construction,construction, riverriver channelization,channelization, landland reclamation,reclamation, etc.etc. havehave significantsignificant impactsimpacts on on the the hydrological hydrological cycle cycle [4 –[4–8,31].8,31]. Dam Dam construction construction is consideredis considered to beto onebe one of the of mainthe main factors factors that contributethat contribute to the to low the water low water level during level during the dry the season dry inseason the Dongting in the Dongting Lake area Lake [8]. Forarea example, [8]. For example, the discharge the discharge was reduced was by reduce 64%d after by the64% impoundment after the impoundment of TGD in of 2006 TGD compared in 2006 withcompared the annual with averagethe annual discharge average in discharge the pre-TGD in the period, pre-TGD which period, results which in an results obvious in reduction an obvious of thereduction water levelof the at water the downstream level at the of downstream TGD [19]. It of also TGD could [19]. be It found also could that the be waterfound levelthat decreasesthe water significantlylevel decreases after significantly September (seeafter Figure September6) due (see to the Fi impoundmentgure 6) due to periodthe impoundment of TGD, and period the dry of period TGD, startsand the earlier dry period and lasts starts a long earlier duration and lasts (i.e., a the long duration duration is 18(i.e., days the longerduration than is that18 days in the longer pre-TGD than periodthat in ofthe 1981–2002). pre-TGD period of 1981–2002). FigureFigure 1010 showsshows thethe long-term long-term change change of of the the inflow inflow from from Yangtze Yangtze River River to Dongtingto Dongting Lake Lake via thevia “threethe “three channels” channels” (see Figure(see Figure2). It can 2). beIt foundcan be that found the averagethat the dischargeaverage discharge from the “threefrom the channels” “three decreasedchannels” fromdecreased 135 m from3/s to 135 113 m m3/s3/ tos, although113 m3/s, although the average the discharge average discharge from the from upstream the upstream of Yangtze of RiverYangtze has River increased has increased by 18% (from by 18% 5710 (from m3/ s5710 to 6747 m3/s m to3/ 6747s) in them3/s) dry in seasonthe dry after season the after operation the operation of TGD. Thisof TGD. can This be ascribed can be ascribed to the lower to the water lower level water at level the middle at the middle reach of reach Yangtze of Yangtze River, onRiver, average on average about 1.6about m lower1.6 m lower in the in same the drysame period dry period after theafter running the running of TGD of TGD [32]. [32]. Now, Now, it is moreit is more diffi cultdifficult for thefor waterthe water flowing flowing to the to the “three “three channels” channels” from from the the Yangtze Yangtze River River by by gravity. gravity. Similarly, Similarly, the the lowerlower bedbed scouredscoured byby clean clean water water from from the the TGD TGD can can increase increase the outgoing the outgoing discharge discharge from the from lake the to thelake Yangtze to the River.Yangtze Consequently, River. Consequently, less water less can water be stored can be in stored the lake in inthe dry lake seasons. in dry seasons. On the otherOn the hand, other several hand, largeseveral dams large have dams been have built been on the built “four on rivers” the “fou duringr rivers” 1995–2016, during which 1995–20 leads16, to which the reduction leads to of the inflowreduction from of “fourthe inflow rivers” from with “four a decreasing rivers” with rate a of decreasing14.3 m3 rates 1/year of −14.3 (see m Figure3 s−1/year 11). (see Other Figure reasons 11). − − likeOther the reasons climate like change the climate would alsochange affect would the low also water affect level the low during water the level dry seasonduring asthe the dry net season rainfall as decreasedthe net rainfall slightly decreased in the post-TGD slightly in period the post-TGD [7,8,16] andperiod thereby [7,8,16] might and intensify thereby might the water intensify shortage the problem.water shortage But other problem. studies But show other that studies the e showffect of that future the effect climate of onfuture discharge climate in on the discharge Yangtze in River the BasinYangtze is still River uncertain Basin is [33still,34 uncertain]. [33,34].

Water 2020, 12, 2713 11 of 14 Water 2020, 12, x FOR PEER REVIEW 11 of 14 Water 2020, 12, x FOR PEER REVIEW 11 of 14

,

(a) (b) (a) (b) Figure 10. The long term change of the inflow from (a) “three channels” (b) Yangtze River (Zhicheng Figure 10. 10. TheThe long long term term change change of the of inflow the inflowfrom (a from) “three (a )channels” “three channels” (b) Yangtze (b )River Yangtze (Zhicheng River station). (Zhichengstation). station).

Figure 11. The long term change of the inflow from “four rivers”. Figure 11. The long term change of the inflow inflow from “four rivers”. Apart from the decreasing amount of inflow to Dongting Lake, the water storage capability of Apart from the decreasing amount of inflow to Dongting Lake, the water storage capability of the lakeApart also from plays the an decreasing important amount role in ofwater inflow supply to Dongting during Lake,the dry the season. water storageFigure 12 capability shows the of the lake also plays an important role in water supply during the dry season. Figure 12 shows the thewater lake surface also playsarea of an Dongting important La roleke during in water the supply dry season during in the2006, dry which season. looks Figure like several 12 shows rivers, the water surface area of Dongting Lake during the dry season in 2006, which looks like several rivers, waterand the surface amount area of ofthe Dongting water storage Lake duringcontent the was dry only season 0.6 billion in 2006, m3 which [30], about looks 1/10 like severalof the annual rivers, and the amount of the water storage content was only 0.6 billion m3 [30], about 1/10 of the annual andwater the storage amount in ofDongting the water Lake. storage Liu et content al. [18] was stud onlyied a 0.6 scheme billion called m [30 “central], about lake 1/10 reservoir” of the annual into water storage in Dongting Lake. Liu et al. [18] studied a scheme called “central lake reservoir” into waterthe WDL, storage which in Dongting could enhance Lake. Liu the et water al. [18 supply] studied du aring scheme the calleddry season. “central However, lake reservoir” it has intolimited the the WDL, which could enhance the water supply during the dry season. However, it has limited WDL,effect on which the water could enhancelevel except the waterin WDL supply (e.g., duringthe water the level dry season.only increase However, about it has0.15 limited m in EDL effect with on effect on the water level except in WDL (e.g., the water level only increase about 0.15 m in EDL with the“central water lake level reservoir”) except in WDLdue to (e.g., the thelimited water water level storage only increase of the aboutreservoir. 0.15 mBut in the EDL IFSC with has “central huge “central lake reservoir”) due to the limited water storage of the reservoir. But the IFSC has huge lakepotential reservoir”) water storage due to the(see limitedFigure water8) in Dongting storage of Lake, the reservoir.with which But the the water IFSC level has would huge potential increase potential water storage (see Figure 8) in Dongting Lake, with which the water level would increase waterat least storage 0.4 m (see even Figure in the8) inextreme Dongting drought Lake, withyear which (i.e., 2006). the water Although level would the IFSC increase could at leastefficiently 0.4 m at least 0.4 m even in the extreme drought year (i.e., 2006). Although the IFSC could efficiently evenmitigate in the the extreme flood disaster drought [21] year and (i.e., alleviate 2006). the Although water shortage the IFSC crisis, could the effi operationciently mitigate of the IFSC the flood and mitigate the flood disaster [21] and alleviate the water shortage crisis, the operation of the IFSC and disasterits effect [on21 ]the and environment alleviate the needs water further shortage study. crisis, the operation of the IFSC and its effect on the environmentits effect on the needs environment further study. needs further study.

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FigureFigure 12.12. TheThe waterwater surfacesurface areaarea ofof DongtingDongting LakeLake inin thethe drydry seasonseason (2006).(2006).

5.5. ConclusionsConclusions DamDam construction construction could could alter alter streamflow streamflow significantly significantly and thereby and thereby affects theaffects hydrological the hydrological drought indrought its downstream in its downstream area. Dongting area. Dongting Lake, which Lake, is wh locatedich is atlocated 400 km at downstream400 km downstream of Three of Gorges Three DamGorges (TGD), Dam has(TGD), been has su ffbeenering suffering a longer a drylonger season dry season due to due the impoundmentto the impoundment of TGD. of ApartTGD. Apart from thefrom decreased the decreased inflow, inflow, another another reason reason for the for water the water shortage shortage problem problem in the in the Dongting Dongting Lake Lake area area is thatis that it lacks it lacks the the water water storage storage capability capability since since it is it connected is connected with with the Yangtzethe Yangtze River River directly. directly. In this In study,this study, a flood a flood control control scheme scheme called called IFCS, IFCS, which which could could store store a huge a huge amount amount of water, of water, was appliedwas applied into Dongtinginto Dongting Lake toLake enhance to enhance the water the water supply supply during during the dry the season. dry season. The following The following conclusions conclusions can be drawncan be fromdrawn the from current the current study: study: (1)(1) TheThe TGDTGD causescauses aa waterwater shortageshortage crisiscrisis inin thethe water-richwater-rich DongtingDongting areaarea andand thethe droughtdrought severityseverity wouldwould bebe exacerbated,exacerbated, e.g.,e.g., thethe averageaverage monthlymonthly waterwater levellevel decreaseddecreased by by 0.880.88 mm andand 0.350.35 mm inin November November and and December, December, respectively. respectively. The The duration duration of the of dry the period dry period (2003–2016) (2003–2016) is 18 days is 18 longer days thanlonger that than in thethat period in the beforeperiod the before construction the construction of TGD of (1981–2002). TGD (1981–2002). (2)(2) TheThe waterwater crisiscrisis isis solvablesolvable asas DongtingDongting LakeLake hashas aa hugehuge potentialpotential forfor waterwater storagestorage capabilitycapability 3 3 withwith the the application application of of IFCS. IFCS. The The usable usable water water storage storage reaches reaches up up to 2.85to 2.85 billion billion m andm3 and 1.81 1.81 billion billion m evenm3 even in the in extremethe extreme drought drought year year of 2006 of 2006 and and 2011, 2011, respectively. respectively. (3)(3) TheThe averageaverage incrementincrement ofof thethe waterwater levellevel waswas aboutabout 0.40.4 m,m, 0.60.6 m,m, andand 0.50.5 mm inin WDL,WDL, SDL,SDL, andand EDL,EDL, respectively,respectively, ifif the water stored in in the the in innerner lake lake was was discharged discharged uniformly uniformly in in 30 30 days. days. It Itindicates indicates that that the the water water level level could could be be increased increased significantly significantly by the application ofof IFCS,IFCS, andand thusthus itit couldcould alleviatealleviate thethe waterwater shortageshortage problem problem in in the the Dongting Dongting Lake Lake area. area.

Author Contributions: Conceptualization, S.-Q.Y.; Formal analysis, Y.L. (Yizhuang Liu) and S.Y.; Supervision, C.J.; Validation,Author Contributions: Y.L. (Yuannan Conceptualization, Long), B.D.; Writing—original S.-Q.Y.; Formal draft, analysis, Y.L. (Yizhuang Y.L. (Yizhuang Liu); Writing—review Liu) and S.Y.; Supervision, & editing, S.-Q.Y.,C.J.; Validation, C.J. All authors Y.L. (Yuannan have read Long), and agreed B.D.; toWriting—or the publishediginal version draft, ofY.L. the (Yizhuang manuscript. Liu); Writing—review & editing, S.-Q.Y., C.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Research Foundation of Education Bureau of Hunan Province, China (GrantFunding: No. This 19C0059 research and No.was 19B036)funded by and the the Research National Founda Naturaltion Science of Education Foundation Bureau of China of Hunan (Grant Province, No. 52079010). China Conflicts(Grant No. of Interest:19C0059 Theand authors No. 19B036) declare and no conflictthe National of interest. Natural Science Foundation of China (Grant No. 52079010).

Conflicts of Interest: The authors declare no conflict of interest.

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