*Article* **Quantitative Assessment of the Influences of Three Gorges Dam on the Water Level of Poyang Lake, China**

#### **Dan Wang 1,2, Shuanghu Zhang 2,\*, Guoli Wang 1, Qiaoqian Han 3, Guoxian Huang 4, Hao Wang 2, Yin Liu <sup>2</sup> and Yanping Zhang <sup>2</sup>**


Received: 4 June 2019; Accepted: 18 July 2019; Published: 22 July 2019

**Abstract:** Lakes are important for global ecological balance and provide rich biological and social resources. However, lake systems are sensitive to climate change and anthropogenic activities. Poyang Lake is an important wetland in the middle reach of the Yangtze River, China and has a complicated interaction with the Yangtze River. In recent years, the water level of Poyang Lake was altered dramatically, in particular showing a significant downward trend after the operation of the Three Gorges Dam (TGD) in 2003, thus seriously affecting the lake wetland ecosystem. The operation of the TGD changed both the hydrological regime and the deeper channel in the middle reach of the Yangtze River, and affected the river–lake system between the Yangtze River and Poyang Lake. This study analyzed the change in the water level of Poyang Lake and quantified the contributions of the TGD operation, from the perspectives of water storage and erosion of the deeper channel in the middle reach of the Yangtze River, through hydrodynamic model simulation. The erosion of the deeper channel indicated a significant decrease in annual water level. However, due to the water storage of the TGD in September and October, the discharge in the Yangtze River sharply decreased and the water level of Poyang Lake was largely affected. Especially in late September, early October, and mid-October, the contributions of water storage of the TGD to the decline in the water level of Poyang Lake respectively reached 68.85%, 59.04%, and 54.88%, indicating that the water storage of the TGD was the main factor in the decrease in water level. The erosion of the deeper channel accelerated the decline of the water level of Poyang Lake and led to about 10% to 20% of the decline of water level in September and October. Due to the combined operation of the TGD and more reservoirs under construction in the upper TGD, the long-term and irreversible influence of the TGD on Poyang Lake should be further explored in the future.

**Keywords:** water level; Three Gorges Dam; hydrodynamic model; river–lake system; Poyang Lake

#### **1. Introduction**

Lakes provide valuable ecosystem services for riparian communities and play an important role in sustaining ecological security and sustainable development [1,2]. Water levels and surface areas of lakes are sensitive to climatic factors and anthropogenic activity and greatly influence the species distribution and functions in lake ecosystems [3,4]. Many lakes are connected with rivers to form complicated river–lake systems [5]. Anthropogenic activities in rivers and lakes, such as dam construction, reservoir operation, farming, and landscape modification, would influence the water levels of lakes and river–lake systems. The variation in water level is the direct response to the hydrological regime change and anthropogenic activities in lakes, and influences lake productivity, stability, species diversity, and succession of wetland vegetation communities [6,7].

Poyang Lake in the middle reach of the Yangtze River is the largest freshwater lake in China and one of the world's most important wintering grounds of migratory birds in the center of East Asian–Australasia flyway. It was added to the List of Wetlands of International Importance (the Ramsar List) in 1992 and also designated as one of the world's important ecological zones by the World Wide Fund for Nature. There is a complicated river–lake system between Poyang Lake and the Yangtze River, and the hydrological regimes of Poyang Lake catchment and the Yangtze River contribute to the water level of Poyang Lake differently in over the year [8].

Since the twenty-first century, especially after the operation of Three Gorges Dam in 2003, the water level of Poyang Lake was extremely low during the dry season and lake and wetland areas varied significantly [3,9–11]. This phenomenon has led to the seasonal bottomlands appearing in advance, reducing the suitable habitats of mainly migratory birds. In addition, the living spaces of fishes and migration of Yangtze finless porpoises were seriously influenced. The operation of the Three Gorges Dam (TGD) has greatly changed the runoff regime in the middle and lower reaches of the Yangtze River Basin. In particular, water storage of the TGD in September every year leads to the lower water level and earlier dry season of the Yangtze River [12,13]. Therefore, the supporting force of the Yangtze River on the water level of Poyang Lake is decreased, leading to the declining water level of Poyang Lake, the earlier dry season, and the altered seasonal inundation pattern [14–16]. Seasonal bottomlands appeared one month in advance, thus decreasing community species diversity and seriously damaging the wetland ecosystem and habitats of migratory birds [17,18].

The variation in water level of Poyang Lake is related to climatic factors and anthropogenic activities, such as hydrological regimes of lake catchment and the Yangtze River, dam construction and reservoir operation in the lake and Yangtze River, and geographical changes caused by river channel erosion and sand extraction [7,10]. In recent years, the influences of the TGD and climatic change on the variation of water level of Poyang Lake have been extensively explored. The dominant role of the TGD or climate change in the variation of water level of Poyang Lake is controversial [19–21]. Mei et al. [10] considered Poyang Lake as a frustum, quantified the influences of precipitation variation and anthropogenic activities in Poyang Lake catchment on the water level of Poyang Lake, and then got the contribution of the TGD. Mei et al. [10] indicated that the operation of the TGD was the main cause for the lowered water level of Poyang Lake in October. However, the variation in water level of Poyang Lake is a consequence of water storage of the TGD and the erosion of the deeper channel in the middle reach of the Yangtze River [22], Mei et al. [10] did not differentiate the influences of the two factors.

This study aims to identify and quantify the influences of the TGD on the water level of Poyang Lake. Firstly, the variations in the water level of Poyang Lake, and the starting time and duration of the dry season after the operation of the TGD were discussed. Secondly, the contributions of water storage of the TGD and geographical change to the variations in water level throughout the year and the dry season were quantified with a hydrodynamic model. This study is unique in that it quantifies the influences of TGD on water level variation of Poyang Lake for the first time, analyzes the discharge at Hukou and provides the basis for understanding the reason for the water level variation of Poyang Lake.

#### **2. Study Area and Data**

#### *2.1. Study Area*

Poyang Lake is in the north of Jiangxi Province, China (115◦49 –116◦46 E, 28◦24 –29◦46 N) (Figure 1). There is another lake, Dongting Lake, the second largest freshwater lake in China, located between the TGD and Poyang Lake. The basin area of Poyang Lake is 1.62 <sup>×</sup> 105 km2. Its water source

mainly from the Xiu River, Gan River, Fu River, Xin River, and Rao River and then flows into the main steam of the Yangtze River at Hukou from south to north.

**Figure 1.** Locations of Poyang Lake and sketch map of hydrodynamic model domain. TGD: Three Gorges Dam.

Poyang Lake is an important flood storage and detention basin and water source of the middle and lower reaches of the Yangtze River Basin. The water level of Poyang Lake shows strong seasonal variations and has a complicated relationship with the Yangtze River. During the flood season of Poyang Lake from April to July, the water level rises due to the increasing inflow from the lake catchment. The flood season of the Yangtze River is later than that of Poyang Lake. Due to the high water level of the Yangtze River from July to September, the water level of Poyang Lake is also high. When the water level of Poyang Lake is high, the water surface is wide. After October, the discharge and water level of the Yangtze River gradually decrease and the water level of Poyang Lake decreases. Due to the smaller inflow of the Poyang Lake catchment, Poyang Lake enters the dry season with a lower water level and then the water–land transition zone and independent seasonal bottomlands appear, the lake area is decreased, and separated river channels are formed. The water–land transition zone is constantly moving with the decrease in the water level and forms new food-rich habitats for migratory birds [23]. The large water level range, unique water situation and environmental conditions have created the high biodiversity and significantly affected wetland ecosystems [24].

#### *2.2. Data*

In previous studies [25,26], the influences of the TGD on the Yangtze River and lakes were usually explored in two stages: the period before the operation of the TGD from 1991 to 2002 and the period after the operation of the TGD in 2003. Thus, the observed daily water level data of Xingzi Station, a representative water level station in Poyang Lake, from 1991 to 2014 were used to analyze the variations of water level, daily discharge and water level of each hydrological station in the Yangtze River, Poyang Lake, and Dongting Lake catchments from 1991 to 2014 and daily inflow and outflow data of the TGD from 2003 to 2014 were used to simulate and analyze the influences of the TGD on the variation of the water level of Poyang Lake in this study. Geographical data in 2003 of the main stream, lakes, and main tributaries, which were interpolated with linear interpolation or curvilinear grid-based methods, were used to represent the pre-TGD geography. The newly measured geographic data in 2013 was

used to indicate the changed geography after the TGD operation. The missing geographical data of partial areas were calculated by the linear interpolation of irregular triangular mesh.

#### **3. Methodology**

In this study, a non-parametric method, Mann–Kendall (MK) [27,28] test was used to detect the trends of water level variation and influence of the TGD on Poyang Lake. A newly developed one-dimensional hydrodynamic model was used to interpret the reasons for water level variation of Poyang Lake.

#### *3.1. MK Test*

MK test has been widely used in hydrology and meteorology to examine the trends of time series. In MK test, the sample data are not required to follow a specific distribution or be disturbed by a small number of outliers. For the sample data (*x*1, *x*2, ..., *xn*), the statistical *S* can be calculated with Equation (1).

$$S = \sum\_{i=1}^{n-1} \sum\_{j=i+1}^{n} \text{sgn}(\mathbf{x}\_j - \mathbf{x}\_i). \tag{1}$$

The *sgn*( ) in this equation is a sign function which can be calculated with Equation (2).

$$\text{sgn}(\mathbf{x}\_{j} - \mathbf{x}\_{i}) = \begin{cases} 1 & \mathbf{x}\_{j} - \mathbf{x}\_{i} > 0 \\ 0 & \mathbf{x}\_{j} - \mathbf{x}\_{i} = 0 \\ -1 & \mathbf{x}\_{j} - \mathbf{x}\_{i} < 0 \end{cases}.\tag{2}$$

When the number of sample data *n* ≥ 10, the statistical *S* is approximate to the normal distribution with a mean of zero and variance shown in Equation (3)

$$Var(S) = \frac{n(n-1)(2n+5) - \sum\_{i=1}^{m} t\_i(t\_i - 1)(2t\_i + 5)}{18},\tag{3}$$

where *m* is the number of groups of *ti* tied observations, with the same value.

Then the standard normal distributed statistical *Z* can be calculated by Equation (4). In the two-sided trend test at a specified confidence level (α), if |*Z*| ≥ *Z*α/<sup>2</sup> (*Z*α/<sup>2</sup> = 1.96 at 0.05 confidence level), there is a significant upward trend in the sample data when *Z* > 0, and a significant downward trend when *Z* < 0.

$$Z = \begin{cases} \begin{array}{c} \frac{S-1}{\sqrt{Var(S)}} & S > 0\\ S & S = 0\\ \frac{S+1}{\sqrt{Var(S)}} & S > 0 \end{array} .\end{cases} \tag{4}$$

#### *3.2. Hydrodynamic Model*

The reasons for water level variation in lakes and river–lake systems are complicated. Usually, the reasons can be interpreted by hydrodynamic model simulation [29–31]. A developed one-dimensional hydrodynamic model, which covered 4772.8 km long channels (Figure 1), was used in this study. The model involves the main steam of the Yangtze River from Yichang to Datong, Poyang Lake, Dongting Lake, and numerous tributaries to the Yangtze River. The control boundaries of the hydrodynamic model include discharge boundaries (Yichang Hydrological Station and hydrological stations at each tributary of Poyang Lake and Dongting Lake) and water level boundaries (Datong Station). Saint-Venant equations are used in this hydrodynamic model to describe the governing equations (Equations (5) and (6)) and the water surface elevation and discharge are discretized into cross-sections through an implicit four-point finite-difference scheme and denoted as *Zj* and *Qj*, where *j* is the number of cross-sections:

$$\frac{\partial A}{\partial t} + \frac{\partial Q}{\partial \mathbf{x}} = q\_{\prime} \tag{5}$$

*Water* **2019**, *11*, 1519

$$\frac{\partial \mathcal{Q}}{\partial t} + \frac{\partial}{\partial \mathbf{x}} \left( \frac{\mathcal{Q}^2}{A} \right) + \mathcal{g}A \left( \frac{\partial Z}{\partial \mathbf{x}} + \mathbf{s}\_f + \mathbf{s}\_t \right) + L = \mathbf{0},\tag{6}$$

where *A* is the wetted cross-sectional area; *t* is time; *Q* is the flow discharge; *X* is the curvilinear discharge of the river channel; *q* is the lateral discharge per unit channel length; *Z* is the elevation of water surface; *Sf* is friction slope; *Se* is local bed slope; *L* is the momentum of lateral discharge and can be expressed as Equation (7) when *q* > 0 or Equation (8) when *q* < 0; *ub* is the magnitude of lateral velocity along the main streamline:

$$L = q(\mu\_b - Q/A),\tag{7}$$

$$L = -qQ/A.\tag{8}$$

Hydraulic conditions at river junctions are governed by mass and energy conservation equations:

$$\sum\_{k=1}^{m} \Delta Q\_l k = -A\_l \frac{\Delta Z\_l}{t},\tag{9}$$

where *m* is the total number of sub-channels linked to junction *l*; *k* is the number of the channel linked to junction *l*; *Al* is the storage area of junction *l*; *Zl* is the water surface elevation at *l* cross-sections.

Then the model is solved by Gauss–Jordan elimination method and the three-level solution method is used to reduce the required computational time and storage. More details can be found in the report by Huang et al. [32].

#### *3.3. Analysis of the Influence of the TGD*

A hydrodynamic model can provide the water level and discharge results under different hydrological conditions and distinguish and quantify the influences of different factors, such as water storage of the TGD and geographical change of the Yangtze River and Poyang Lake Basin. In this paper, the variation of water level before and after the TGH operation is denoted as Δ*Z*; the percentage of water level variation is denoted as *R*, *Rs* and *Rz* represent the percentage of the influences of water storage of the TGD and geographical change on water level variation of Poyang Lake. Three scenarios were set to quantify the influences of the TGD as follows.

Scenario 1. Geographical data in 2003 and measured discharge data from 2003 to 2014 were used in simulation, and the calculated water level at Xingzi Station is denoted as *Z*1.

Scenario 2. Natural discharge of the Yangtze River was calculated according to the inflow and outflow of the TGD from 2003 to 2014. With geographical data in 2003, the water level at Xingzi Station was calculated and denoted as *Z*2. The difference between *Z*<sup>1</sup> and *Z*<sup>2</sup> can be used to indicate the influence of water storage of the TGD without considering the influences of geographical change.

Scenario 3. Geographical data in 2013 and measured discharge data from 2003 to 2014 were used in the simulation. The calculated water level at Xingzi Station is denoted as *Z*3. The difference between *Z*<sup>1</sup> and *Z*<sup>3</sup> can reflect the influences of geographical change on water level.

Therefore, the contributions of water storage of the TGD and geographical change to water level variation can be calculated as follows:

$$R\_{\rm s} = \frac{Z\_1 - Z\_2}{\Delta Z} \times 100\% \,\tag{10}$$

$$R\_Z = \frac{Z\_3 - Z\_1}{\Delta Z} \times 100\%.\tag{11}$$

#### **4. Results**

#### *4.1. Variations of Water Level and the Dry Season*

Since the operation of the TGD in 2003, the water level of Poyang Lake was significantly changed, displaying the decreased annual average water level, the increased declining rate of water level from

September to December, and longer and earlier dry season. The annual water level and inner-annual 10 day mean water level at Xingzi Station from 1991 to 2014 are shown in Figure 2. The mean annual average water level from 2003 to 2014 was 1.32 m lower than that from 1991 to 2002. The statistical *Z* was −2.877, indicating a significant downward trend of water level. In addition, the inner-annual 10 day average water level was lowered and the declining percentage of 10 day average water level reached 10% in 21periods. Especially in late October and early November, the declining percentage of 10 day average water level reached 18.03% and 17.78%, respectively. Since October, the declining rate of water level had significantly increased, and the water level decreased by 3.36 m until early November. The decrease of the water level was increased by 56.96% compared to the value (2.14 m) in the period from 1991 to 2002. The water level had risen slightly in mid-November, displaying a different variation trend from that in the period from 1992 to 2002.

**Figure 2.** Water level at Xingzi Station: (**a**) annual water level from 1959 to 2014, (**b**) inner-annual 10 day mean water level during different periods.

When the water level at Xingzi Station was lower than 12 m, the surface area of Poyang Lake decreased sharply and impeded the local water supply. Therefore, water level decline lower than 12 m was usually regarded as a symbol of the dry season in the management of Poyang Lake. The duration and the first day of the dry season of Poyang Lake are shown in Figure 3. Duration of the dry season of Poyang Lake had increased significantly from 2003 to 2014 and the mean period was 57 days longer than that in the years from 1959 to 2002. The first day of the dry season was mostly in November or December before 2003 and the mean starting date of the dry season was in mid-November. Dry season was 32 days in advance on average since 2003, especially in 2006 and 2011. Both the mean period and first day of the dry season had indicated a significant upward trend, and the statistical *Z* values were 3.200 and 2.555, respectively.

**Figure 3.** Duration and starting date of the dry season of Poyang Lake.

#### *4.2. Quantitative Analysis of the Influences of Di*ff*erent Factors on Water Level Variation*

Based on the daily results of the hydrodynamic model simulation, the difference between Scenario 1 and Scenario 2 reflected the influence of water storage of the TGD on water level without considering the influences of geographical change, and the influence of geographical change on water level could be calculated based on the difference between Scenario 1 and Scenario 3.

Figure 4 shows the annual influences of water storage of the TGD and geographical change. With the operation of the TGD, the annual influence of water storage on the water level of Poyang Lake indicated a slight upward trend, and the corresponding statistical *Z* was 1.713. Especially after 2008, when the TGD was fully put into operation, the upward trend was relatively stable while the geographic change significantly lowered the annual water level of Poyang Lake, and the corresponding statistical *Z* was 3.270.

**Figure 4.** Annual influences of water storage of the TGD and geographical change on water level.

For the inner-annual influence, 10 day mean water level at Xingzi Station was calculated (Figure 5), and Table 1 shows the data during the period with significant influences. In general, the water level of Poyang Lake was decreased throughout the year under the influence of geographical change. Water level decrease was the most serious in early April and reached 0.37 m, whereas the water level in mid-August was just decreased by 0.10 m. The annual average decrease in the water level of Poyang Lake caused by geographical change was 0.26 m. As for the influence of water storage of the TGD on water level, the water level of Poyang Lake mainly reflected the significant decrease after the flood season. In the period from September to November, the water level of Poyang Lake was 0.51 m lower than that under natural conditions. The decrease in the water level was the most serious in October (0.95 m) and reached 0.34 m in September and 0.22 m in November. From mid-July to early August, the water level also decreased slightly and the average decrease was 0.12 m. From early December to early June of the next year, the water level was slightly increased by 0.21 m due to the water drainage of the TGD.

**Figure 5.** 10 day mean water level at Xingzi Station calculated based on hydrodynamic model simulation. **Table 1.** 10 day mean water level at Xingzi Station from the model during the time of significant influences.


The TGD mainly stores water in September and October every year and promotes the decrease in the runoff and water level at the Yangtze River, thus aggravating the declining trend of water level of Poyang Lake during the dry period. The decrease in water level and the advance of the dry period seriously affect the ecological system of Poyang Lake and more attention on this issue is needed. Therefore, this study mainly analyzed the contribution of water storage and geographical change to the water level of Poyang Lake in September and October (Table 2). The influence of water storage of the TGD on water level was mainly reflected in the period from late September to mid-October. Water storage of the TGD was the dominant factor in the decrease in water level in this period and showed a maximum contribution of 68.85% in late September. The contribution of geographical change to the decline in the water level of Poyang Lake was lower than 20%.


**Table 2.** Quantitative analysis of the factors of water level decrease in September and October.

#### **5. Discussion**

#### *5.1. Influences of the TGD on Poyang Lake*

The water storage of the TGD from 2003 to 2014 is shown in Figure 6. The TGD mainly stored water in September and October. Water storage of the TGD in September and October was respectively 48 <sup>×</sup> 10<sup>8</sup> m<sup>3</sup> and 61 <sup>×</sup> 108 m3 and accounted for 8.0% and 15.9% of the runoff, respectively. As a result, the discharge and water level of the Yangtze River was significantly reduced. Based on the flow capacity of the exit section at Poyang Lake and the relationship between the water level of Poyang Lake and the discharge of the Yangtze River, Fang et al. [33] indicated that the change of 1 m3/s in the discharge of Jiujiang Station corresponded to the change of 0.89–0.99 m3/s in the discharge of Hukou Station. The decrease in runoff and water level at the Yangtze River caused by water storage of the TGD in September and October accelerated the outflow at Hukou Station and the decrease in water level of Poyang Lake.

**Figure 6.** Annual average water storage of TGD (Notes: positive values represent increased drainage, negative values represent water storage).

After the operation of the TGD, the middle reach of Yangtze River was mainly subjected to deeper channel erosion, which resulted in the decrease in water level under the same discharge. The stage-discharge curve of Jiujiang Station in the study period is shown in Figure 7. The obvious erosion of the deeper channel changed the river–lake system and increased the hydraulic gradient between Poyang Lake and the Yangtze River. As a result, the outflow at Hukou increased. The water level at Xingzi Station is highly correlated with that at Jiujiang Station. The coefficient of determination was 0.9824 from 1991 to 2002 and 0.9911 from 2003 to 2014 (Figure 8). The water level at Xinzi Station, which had the same water level as that of the Yangtze River, was decreased.

**Figure 7.** Water level–discharge curve of Jiujiang Station.

**Figure 8.** Relationship between the 10 day water level measured at Xiangzi Station and Jiujiang Station.

Based on the simulation of three scenarios, Figure 9 shows the influences of water storage of the TGD and geographical change on the outflow at Hukou Station. The geographical change mainly increased the outflow of Hukou Station by 8.55 <sup>×</sup> 108 <sup>m</sup><sup>3</sup> from March to June during the flood season of Poyang Lake. The TGD started to store water in late June and water was stored mainly in September and October. Water storage of the TGD mainly increased the outflow at Hukou in July and September. The outflow from late June to early October was increased by 19.86 <sup>×</sup> 108 m3. Water storage of the TGD accelerated the declining rate of water level in the dry season of Poyang Lake and the water quantity in Poyang Lake decreased significantly in advance. Due to the significant decrease in the water quantity of Poyang Lake, the outflow of Hukou decreased gradually from mid-October.

**Figure 9.** Influences of TGD water storage and geographical change on the outflow of Hukou.

#### *5.2. Influences of Water Level Variation*

Due to the unique geographical features, Poyang Lake forms numerous lakes in low water conditions and capacious water surface in high water conditions. Figure 10 shows the relationship between the water level and water surface area of Poyang Lake. When the water level at Xingzi Station was below 15.5 m, every 0.5 m of the decrease in water level for led to the significant reduction in the water surface area, especially when the water level at Xingzi Station was between 11 m and 14 m. The mean water level of Poyang Lake in September and October from 2003 to 2014 is 13.74 m. Water storage of the TGD in September and October could cause the serious and earlier decrease in water level and water surface area. With the decline in water level, seasonal bottomlands were exposed in advance and the land–water transitional zone moved downward. Furthermore, the distribution of the habitats of migratory birds and other ecological systems were affected.

**Figure 10.** Relationship between water level and water surface area of Poyang Lake: (**a**) water surface area of Poyang Lake under different water levels, (**b**) water level–water surface area curve and the change rate of water surface area.

#### **6. Conclusions**

Since 2003, the water level of Poyang Lake has decreased, displaying a significant downward trend. The annual average water level decreased by 1.32 m and an especially sharp decline occurred in the period from September to November. The duration of the dry season was longer and the dry season occurred about one month in advance. The aggravation phenomenon of the dry season of Poyang Lake was significant and the habitats of migratory birds and wetland ecosystems were largely affected.

The operation of the TGD changed the hydrological regime and deeper channel geography in the middle reach of the Yangtze River. This study quantified the influences of the TGD in detail. The geographical change significantly lowered the annual water level of Poyang Lake. After flood season, due to water storage of the TGD in September and October, the sharp decrease in the discharge and water level in the Yangtze River accelerated the outflow at Hukou and enhanced the decrease in water level and water surface area of Poyang Lake. The water storage of the TGD was the main factor in

the decrease in water level during this period and the average decreases in water levels in September and October were respectively 0.34 m and 0.95 m. Especially in late September, early October, and mid-October, the contributions of water storage of the TGD to the decline in the water level of Poyang Lake respectively reached 68.85%, 59.04%, and 54.88%. The change in deeper channel geography accelerated the decrease in water level and the contributions of geographical change to the decrease in water level in September and October reached 11.02% and 12.76%, respectively. This study deeply explored the influences of the TGD on the variation in the water level of Poyang Lake. The increasing number of reservoirs in operation in the upper stream will generate adverse and irreversible influences on the water level and ecological environment of Poyang Lake, which should be widely concerned and well protected.

**Author Contributions:** S.Z. and G.W. developed the original ideas. D.W., Q.H. and G.H. developed the original ideas and completed the study of this paper. Y.L. and Y.Z. processed the raw data. D.W. drafted the manuscript and it was revised substantially by all authors.

**Funding:** This study was funded by the National Key Basic Research Program of China (Grant No: 2015CB452701) and the CAS "Light of West China" Program (Grant No: Y8R2230230).

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

#### **References**


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