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Journal of Geographical Sciences >

2018 , Vol. 28 >Issue 8: 1059 - 1071

DOI: https://doi.org/10.1007/s11442-018-1541-6

研究论文

Applying energy theory to understand the relationship between the Yangtze River and Poyang Lake

  • CHEN Mofei ,
  • DENG Jinyun , * ,
  • FAN Shaoying ,
  • LI Yitian
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  • State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China
*Corresponding author: Deng Jinyun, Associate Professor, E-mail:

Author: Chen Mofei, PhD Candidate, E-mail:

Received date: 2018-01-12

  Accepted date: 2018-03-02

  Online published: 2018-08-10

Supported by

State Key Program of National Science Foundation of China, No.41331174;Science and Technology Planning Project of Jiangxi Province, No.20051BBG70044

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Journal of Geographical Sciences, All Rights Reserved
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Abstract

The complex relationship between the Yangtze River and Poyang Lake controls the exchange of water and sediment between the two, and exerts effects on water resources, flooding, shipping, and the ecological environment. The theory of energy is applied in this paper to investigate the physical mechanisms that determine the nature of the contact between the Yangtze River and Poyang Lake and to establish an energy difference (Fe) index to quantify the interactions between the two systems. Data show that Fe values for this interaction have increased since the 1950s, indicating a weakening in the river effect while the lake effect has been enhanced. Enclosure of the Three Gorges Reservoir (TGR) has also significantly influenced the relationship between the river and the lake by further reducing the impacts of the Yangtze River. The river effect also increases slightly during the dry season, and decreases significantly at the end of the flooding period, while interactions between the two to some extent influence the development of droughts and floods within the lake area. Data show that when the flow of the five rivers within this area is significant and a blocking effect due to the Yangtze River is also clearly apparent, floods occur easily; in contrast, when the opposite is true and the flow of the five rivers is small, and the Yangtze River can accommodate the flow, droughts occur frequently. Construction and enclosure of the TGR also means that the lake area is prone to droughts during September and October.

Key words： Poyang Lake; Yangtze River; river-lake relationship; Three Gorges Reservoir; characterization index

Cite this article

CHEN Mofei , DENG Jinyun , FAN Shaoying , LI Yitian . Applying energy theory to understand the relationship between the Yangtze River and Poyang Lake[J]. Journal of Geographical Sciences, 2018 , 28(8) : 1059 -1071 . DOI: 10.1007/s11442-018-1541-6

1 Introduction

The Yangtze River is the longest river in China, and Poyang Lake is the largest freshwater lake. Interactions between the two systems have significantly influenced the natural evolution of the lake (Liu and Ni, 2015), dictating hydrodynamics, sediment transport, and the morphodynamics of this system, which is critical to regional water resources, flood control, irrigation, and the ecological environment (Nakayama and Watanabe, 2008; Wang et al., 2017; Zhang et al., 2017). Interactions between rivers and lakes encompass a range of physical processes, including the exchange of water between the two, the natural evolution of riverbeds and lake basins, and the interchange of material and energy (Wan et al., 2014).
Previous studies that have addressed the interactions between rivers and lakes have mainly focused on water interactions by addressing the exchange coefficient, as well as the storage and discharge capacity of lakes (Zhao et al., 2011; Lai et al., 2014; Wang et al., 2014). Investigations have also considered the ecological characteristics of these systems by addressing the nature of impacts due to the exchange of water, sand, and dissolved substances on lake quality and ecosystems (Carmack et al., 1979; Kneis et al., 2006; Elisa et al., 2010; Jones et al., 2017).
Interactions between the Yangtze River and Poyang Lake can be divided into river and lake effects. Thus, the role of the river mainly includes blocking outflows and backward flowing (river effect) (Guo et al., 2011), while the influence of the lake is felt in terms of regulating floods and the water supply to the lower reaches of the Yangtze River (lake effect) (Zhao et al., 2011; Fang et al., 2012). Characterizing river-lake interactions and their changing mechanisms, and predicting their future development, have attracted substantial research attention.
A number of studies have applied characterization indices to quantify river-lake interactions. In one study, Fang et al. (2012) developed an approach based on a judgement of backflow conditions at Hukou Station and a method to calculate flood reservation, while Lai et al. (2014) evaluated the discharge capacity of Poyang Lake at different times by investigating hydrological data over years between January and March, 1955-2011. In order to determine the intensity of river-lake interactions, Hu et al. (2007) proposed a series of five different conditions; if any of these conditions are met, then the influence of the Yangtze River can be said to be strong, but if none are met, then Poyang Lake is playing a major role. A formula for the water exchange coefficient (Ip) between rivers and lakes based on the water balance equation (which quantifies these interactions) was proposed by Zhao et al. (2011), while Dai et al. (2015) investigated water level variations within Poyang Lake and utilized the ratio of water level difference to distance between stations (i.e., Duchang-Kangshan, Hukou-Xingzi), to characterize river-lake interactions.
It is generally the case that previous studies in this area have been developed from the perspective of utilizing water interactions to investigate the discharge capacity of Poyang Lake or the backflow and blocking effects of the Yangtze River (Lai et al., 2014; Fang et al., 2012; Dai et al., 2015). To date, no systematic analysis of river-lake interactions has yet been carried out, and the theoretical basis upon which indices can be proposed to quantify river-lake interactions still remains unclear. Indeed, some quantitative results even appear to contradict one another (Hu et al., 2007; Zhao et al., 2011). Thus, applying the theory of energy, the physical mechanisms that underlie the interaction between the Yangtze River and Poyang Lake are evaluated in this study and a new quantitative index is developed. The results of this study will facilitate the future effective management of rivers and lakes.

2 Study area

The total length of the Yangtze River is about 6300 km; the main stream of this major watercourse flows through 11 provincial-level areas (i.e., Qinghai, Tibet, Sichuan, Yunnan, Chongqing, Hubei, Hunan, Jiangxi, Anhui, Jiangsu, and Shanghai), before discharging into the East China Sea at Chongming Island. As a key component of this system, Poyang Lake provides important water storage within the middle and lower reaches of the Yangtze River; this waterbody is located in the north of Jiangxi Province (Figure 1) at 115°31’E- 117°06’E, 28°11’N-29°51’N, and is a typical open and seasonal lake with a basin area of 162,225 km² that exhibits a lake phase at a high water level and a river phase at a low water level. The Ganjiang, Fuhe, Xinjiang, Raohe, and Xiushui rivers all flow into Poyang Lake from the southern, eastern, and western sides respectively. Thus, once regulated by the lake, water flow is then injected into the Yangtze River at Hukou (Du et al., 2015). Measurements of water flow and lake sedimentary conditions are recorded at Waizhou, Lijiadu, Meigang, Hushan, Dufengkeng, and Wanjiabu stations (referred as the five rivers and six stations throughout this paper), while Hukou Station (Figure 2) provides the control at the lake exit. In addition, the largest water control project in the world, the Three Gorges Reservoir (TGR), is also located at Yichang in Hubei Province, just 955 km from Hukou Staion; this reservoir has a normal water level of 175 m and a corresponding flood control capacity of 221.5 × 108 m3 (Han et al., 2017a; Han et al., 2017b). The TGR commenced operations in June 2003 and a 175 m pilot storage trial was initiated in 2008; this trial had a profound effect on erosion and deposition within the middle and lower reaches of the Yangtze River, as well as on the relationship between this watercourse and the lake (Fang et al., 2012; Dai and Liu, 2013; Wang et al., 2014; Wang et al., 2017).
Figure 1 Map showing the location of the study area discussed in this paper
Figure 2 Map showing the location of Poyang Lake and associated hydrological stations

3 Data and methods

3.1 Data

The data analyzed in this study were all collected from hydrological stations within the basin, and were collated by the Jiangxi Provincial Institute of Hydraulic Research and the Jiangxi Bureau of Hydrology. Measurements include water level and flow discharge measured at Hukou Station between 1953 and 2015, as well as those at “the five rivers and six stations”, flow discharge for the period between 1952 and 2015 measured at Hankou Station on the Yangtze River, flow discharge measured at Jiujiang Station in 2011, and water level measured at Xingzi Station in 2011. Notable missing data include flow discharge for December 1966 at Hukou Station, as well as for the period between September 1987 and December 1987 at Dufengkeng Station.

3.2 Theory and method

3.2.1 Interaction between the Yangtze River and Poyang Lake
Previous research has shown that the exchange of water between rivers and lakes is a critical component of interaction between the two systems (Ye et al., 2012). In this case, once water has been regulated by Poyang Lake, the flow from five rivers enters into the Yangtze River at Hukou and is subject to complicated backwater effects and water exchange processes. Thus, from an energy perspective, there are clear interactions between the Yangtze River and the five tributary rivers; variation in the river-lake relationship is actually the result of these energy changes. In other words, if the inflow discharge of the Yangtze River and its five tributary counterparts remain constants, flow will gradually approach, and then maintain, an equilibrium state that is reflected at Hukou Station. However, if the balance between the rivers becomes unstable, the river-lake interaction will also change (Table 1), as outlined below.
Table 1 Theories and forms of expression of the river-lake relationship
Effect Theory Indication
Yangtze River Blocking effect Flow discharge and energy of the Yangtze River increases Increase in ES hinders outflow
Poyang Lake Recharging effect Flow discharges and energies of the five rivers increase Increase in ED favors outflow
Measurements show that when the flow of the Yangtze River increases, its energy also increases, leading to a water level rise at Hukou Staion. This also causes a corresponding increase in potential energy (ES), and a reduction in the water slope between the lake area and Hukou Staion which hinders outflow at the mouth. This means that the role of the Yangtze River is characterized by blocking outflow; however, if the flow of this river increases still further, ES at Hukou Staion also increases rapidly and further enhances the blocking capability of the Yangtze River, and water pours back into Poyang Lake.
In the second possible situation, when the flows of the five rivers increase, energy is also enhanced and the water surface gradient between the lake area and Hukou Station rises, favoring outflow. This also causes a rapid growth in kinetic energy (ED) at Hukuou Station, which provides the supply to the river reaches below this junction and causes recharging of the Yangtze River from Poyang Lake. An increase in discharge from Poyang Lake also causes a rise in water level at Hukou Station in order to return to the original equilibrium state; this also demonstrates the blocking of Poyang Lake by the Yangtze River to some extent, although in this case, compared to recharging, the effect is relatively minor.
A sketch of the interaction between the Yangtze River and Poyang Lake is presented in Figure 3. In this formulation, EC denotes the energy of the Yangtze River, while EW refers to that of the five rivers; EC = EW, and an equilibrium state therefore exists between the Yangtze River and Poyang Lake. Hydraulic theory also suggests the presence of a relationship between different sections, as shown in Equations (1) to (3); in these expressions, Zi denotes the average water level of section i, vi denotes the average flow velocity of section i, hwi-j denotes the head loss between sections i to j, C refers to the Chezy coefficient, and R and J denote the hydraulic radius and slope, respectively. Thus, if EC increases, then z2 and v2 increase between section 1-1 and section 2-2 and the backwater effect of the Yangtze River on Poyang Lake is indicated by an increase in z2(z3). Similarly, an increase in z3 reduces the water surface gradient between section 4-4 and section 3-3, which causes an indirect decrease in v3 (according to the Chezy formula 4), as well as an increase in z3 and a decrease in v3 within section 3-3. Therefore, if EW increases, z4 and v4 will increase; as the period featuring strong Poyang Lake effects corresponds to the time when the water level (z2) of the Yangtze River is relatively low, an increase in z4 improves the water surface gradient between section 4-4 and section 3-3, and z3 increases according to the Chezy formula. This means that since Poyang Lake is located between the Yangtze River and the five tributaries, it is functioning as “a weir”; thus, any increase in v3 will lead to an indirect increase of z3(z2). The weir formula shows that v3 is proportional to z31/2, rendering any increase in v3 primarily and any increase in z3 secondarily. An overall increase in EC leads to a rise in the water level at Hukou Station, indirectly causing a decrease in flow velocity and an increase of ES; similarly, an increase in EW leads to the increased flow rate at Hukou Station, indirectly causing a rise in water level and ED.
Figure 3 Sketch to show the nature of interactions between the Yangtze River and Poyang Lake
The equations applied in this analysis are as follows:
${{z}_{1}}+\frac{{{a}_{1}}v_{1}^{2}}{2g}={{z}_{2}}+\frac{{{a}_{2}}v_{2}^{2}}{2g}+{{h}_{w1-2}}$ (1)
${{z}_{4}}+\frac{{{a}_{4}}v_{4}^{2}}{2g}={{z}_{3}}+\frac{{{a}_{3}}v_{3}^{2}}{2g}+{{h}_{w3-4}}$ (2)
${{z}_{2}}={{z}_{3}}$ (3)
$v=C\sqrt{RJ}$ (4)
3.2.2 A characterization index for river-lake interactions based on energy theory
As noted above, river-lake interactions mainly take the form of energy exchange between the Yangtze River and its five feeder rivers, with the junction at Hukou Station acting as the contact point between the two. The flow and water level at this interface are therefore the result of energy interactions.
Measurements show that the Yangtze River mainly causes an increase in the water flow ES at Hukou Station, while Poyang Lake mainly causes an increase in the ED of outflow at the junction. Therefore, EC and EW can be replaced in this context by ES and ED, calculated using Equation (5) and Equation (6), respectively. However, because energy is always positive, ED cannot represent situations where water flows backward (i.e., when flow discharge is negative and the velocity direction is oriented back towards the interior of the lake); in this case, energy is computed using Equation (7) and Equation (8) rather than Equation (5) and Equation (6) so that flow direction can also be incorporated.
The equations used for this section of the analysis are as follows:
${{E}_{S}}=mgh$ (5)
${{E}_{D}}=\frac{1}{2}m{{v}^{2}}$ (6)
${{e}_{s}}=\sqrt{mgh}$ (7)
${{e}_{d}}=\frac{\sqrt{m}v}{\sqrt{2}}$ (8)
In order to quantify the energy flow of the Yangtze River and the five tributary rivers, Fe is defined to illustrate the relationship between ES and ED at Hukou Station. Equation (9) shows that the interval difference between these two variables ranges between minus one and one for the same quantity of water, es∝h1/2. Thus, neglecting bed erosion and deposition, ed∝v∝Q/h and h∝z. In these expressions, Q represents the flow discharge at Hukou Station, z represents the water level, and h denotes water depth. Equation (9) can be transformed into Equation (10), in which f1 and f2 are empirical coefficients and f3 denotes the correction value, as follows:
${{F}_{e}}={{e}_{d}}-{{e}_{s}}$ (9)
${{F}_{e}}={{f}_{1}}\frac{Q}{z}-{{f}_{2}}{{z}^{\frac{1}{2}}}\text{+}{{f}_{3}}$ (10)
Data show that when flow discharge at Hukou Station increases and water level falls, ed and Fe increase, and the effect of Poyang Lake is enhanced. Similarly, when the flow discharge at Hukou Station decreases and the water level rises, es increases, Fe decreases, and the effect of the Yangtze River is enhanced. The empirical constants f1, f2, and f3 are determined by three distinct conditions in each case, including the initial combination of maximum flow discharge and minimum water level at Hukou Station over a long-term hydrological sequence. This first case assumes that the influence of Poyang Lake is most significant and so ed will be maximal and Fe is 1. The second case assumes the combination of maximum water level and minimum flow discharge at Hukou Station over a long-term hydrological sequence and considers that the influence of the Yangtze River is the most significant, which means that es is maximal and Fe is -1. The third case assumes that river-lake interactions are at a constant state and so Fe is zero after the multi-year process; the empirical values of f1, f2, and f3 in this case are 0.00027, 0.18, and 0.54, respectively.

4 Results

4.1 Interannual changes

Energy difference (Fe) was calculated using average flow discharge and water level measurements made at Hukou Station between 1953 and 2014. These data are presented in Figure 4 and Table 2 and show an overall increase in Fe variation over time (Figure 4); this result suggests that the effect of the Yangtze River has grown weaker while the effect of Poyang Lake has continuously increased over time. Decadal-level results show that the mean Fe value in the 1980s was ‒0.014, the smallest recorded over the course of this study. Thus, alongside changes in the inflow of the Yangtze River and five feeder rivers (Figure 5), inflow discharge from the former has been larger over the time period of this study and has exerted a stronger influence than its five counterparts. In contrast, mean Fe was 0.003 in the 1970s, suggesting that Poyang Lake played a major role at this time. This can be explained by the fact that abundant rainfall provided substantial flow to the five feeder rivers in the 1970s and therefore indirectly enhanced the role of Poyang Lake (Hu et al., 2007).
Figure 4 Graph showing Fe variation between 1953 and 2015
Figure 5 Graph showing variation in annual average discharge between 1953 and 2015
Table 2 Fe values over the time period of this study
Date Fe Date Fe Date Fe Date Fe Date Fe
1953 0.009 1967 ‒0.011 1980 ‒0.022 1993 ‒0.012 2006 0.062
1954 ‒0.025 1968 ‒0.024 1981 ‒0.009 1994 0.014 2007 0.009
1955 ‒0.009 1969 0.006 1982 ‒0.027 1995 0.013 2008 0.010
1956 0.002 1970 0.005 1983 ‒0.030 1996 ‒0.017 2009 0.010
1957 0.004 1971 ‒0.004 1984 ‒0.010 1997 0.026 2010 0.025
1958 0.005 1972 0.019 1985 ‒0.015 1998 ‒0.006 2011 0.030
1959 0.025 1973 0.005 1986 0.005 1999 0.009 2012 0.020
1960 0.018 1974 ‒0.018 1987 ‒0.009 2000 ‒0.010 2013 0.029
1961 0.016 1975 0.000 1988 0.010 2001 0.015 2014 0.009
1962 0.012 1976 0.010 1989 ‒0.029 2002 0.006 2015 0.028
1963 ‒0.028 1977 ‒0.002 1990 ‒0.029 2003 ‒0.014
1964 ‒0.049 1978 0.014 1991 ‒0.032 2004 ‒0.008
1965 ‒0.025 1979 0.002 1992 0.024 2005 ‒0.005
Records show that the intensity of river-lake interactions has also fundamentally influenced the frequency of drought and flood-related disasters within the area of Poyang Lake across all the decades of this study. Guo et al. (2012a) considered flooding during the summer of the 1990s as one example; these workers compared the flow discharge of the Yangtze River and five feeder rivers and determined that values for the latter were relatively large at this time. Indeed, data show that flow discharges from the five rivers during the 1990s were 1.14 times higher than the multi-year average, while those of the Yangtze River throughout this period were 1.02 times higher than the corresponding value. At the same time, however, the average Fe value for this period was -0.001, which indicates that when the flow discharge of the five rivers is large, the role of the Yangtze River is correspondingly relatively strong, and Poyang Lake will be prone to flooding. Drought disasters are known to have occurred within the lake area in both the 1960s and 2000s, with drought that occurred during the latter period the more serious of the two (Zhang et al., 2015). These events were largely the result of relatively small flow discharges from the five rivers at this time, just 0.87 and 0.88 times its multi-year average in the 1960s and 2000s, respectively. The average Fe value in the 2000s was 0.009, however, the largest recorded throughout this study; this indicates that more water is supplied to the Yangtze River when the flow of the five rivers is insufficient, and further illustrates an enhanced role for Poyang Lake.

4.2 Average monthly Fe changes

The average monthly Fe values calculated in this study are shown in Figure 6; these data show that values are larger between February and April, before falling between July and September. This change can be explained by the fact that the period between February and April is not flood season, flow discharge is relatively small, and the influence of the Yangtze River is minor. Switching into the flooding season in July (Figure 7a) means that the flow discharge of the main stream increases and Poyang Lake begins to experience a strong backwater effect, including sometimes the phenomenon of backward water flow.
Figure 6 Graph showing average monthly Fe over time
Figure 7 Averaged monthly discharges measured at Hankou Station and the other six stations on the five tributaries of Poyang Lake
The total number of days when water flows backwards alongside the induced water volume for each month between 1953 and 2015 are presented in Table 3. The highest daily values were seen in September alongside the highest water volume values in July; this indicates the strong effect of the Yangtze River, while corresponding Fe values are small in July and September.
Table 3 Backward flow conditions at Hukou Station between 1953 and 2015
Month Days Volume
(108 m3)		 Month Days Volume
(108 m3)		 Month Days Volume
(108 m3)
Jan 0 0 May 0 0 Sep 251 454
Feb 0 0 June 13 15 Oct 59 80
Mar 0 0 July 204 540 Nov 14 13
Apr 0 0 Aug 186 336 Dec 2 0.4
River-lake interactions also influence occurrences of droughts and flooding within Poyang Lake area on an annual basis. The data assembled in this study show that most floods within the lake basin occurred between June and August, whereas droughts tend to happen around October (Zhang et al., 2015). The flow discharge of the five rivers also tends to increase gradually between June and July; this means that the role played by the Yangtze River is also rapidly enhanced over this period, and remains at a high level between July and August (Figure 6), which facilitates flooding. In contrast, if flow discharge from the five rivers gradually decreases, then the role of the Yangtze River is weakened and the water supply from Poyang Lake increases. All these factors combine to make drought more frequent within the area of the lake.

4.3 Influence of TGR operation

Data show that the average Fe value was ‒0.004 before the enclosure of the TGR (between 1953 and 2002), and then subsequently increased to 0.018 (between 2004 and 2015). This result clearly shows that operation of the TGR has reduced the influence of the Yangtze River, at least to some extent.
Changes in average monthly flow discharge before, and after, the enclosure of the TGR are shown in Table 4, while the data presented in Figure 8 highlight variations in annual Fe calculated using the average monthly water level and flow discharge measured at Hukou Station. Most notably, these data show that Fe values decrease slightly between January and March. This can be explained by the fact that discharge from the TGR during dry seasons has increased the flow at Hukou Station by more than 20% between January and March. It is clear that the advent of the TGR has slightly weakened the influence of Poyang Lake.
Figure 8 Variations in Fe before, and after, TGR construction and enclosure
Table 4 Comparison of average monthly discharge data for Hankou Station both before (1953-2002) and after (between 1953-2002) construction of the TGR
Month 1953 and
2002		 1953-2002 Variation
rate (%)		 Month 1953-2002 2004-2015 Variation
rate (%)
Jan 8,320 10,252 23 July 43,482 37,022 ‒15
Feb 8,400 10,389 24 Aug 38,137 33,883 ‒11
Mar 10,737 13,202 23 Sep 34,891 30,003 ‒14
Apr 16,401 16,296 ‒1 Oct 27,438 19,588 ‒29
May 24,585 23,148 ‒6 Nov 17,827 16,079 ‒10
June 30,009 30,753 2 Dec 11,045 11,561 5
Measured Fe values between April and June increase slightly when Poyang Lake is in flood, reflected by an increase in the flow of the five tributary rivers and a decrease in the flow of the Yangtze River, both of which enhance the role of Poyang Lake. Data also reveal a significant decrease in Fe between July and December, largely due to the peak scheduling operation during the flood season and the subsequent water capture of the TGR. These both act to cause drastic reductions in main stream inflow of the Yangtze River and also weaken its supporting effect. The results collated in this study are also consistent with those presented previously (Guo et al., 2012b; Hu and Wang, 2014); specifically, the variation in flow rate in October reaches a maximum of -29%, in concert with the largest recorded annual Fe increase.
Construction and enclosure of the TGR exerted a marked effect on droughts and floods within the area of Poyang Lake. Data show that when flow of the five rivers remains constant between July and August, the backwater effect of the Yangtze River reduces the probability of flooding. Similarly, between September and October, as the role of Poyang Lake is enhanced, its storage capacity and the likelihood of droughts are also reduced (Zhang et al., 2014, 2016).

4.4 Rationalizing the characterization indicator

Previous studies that have rationalized the interaction index between Poyang Lake and the Yangtze River have utilized runoff from the latter as well as the five tributary rivers. Then, computing the departure based on the dimensionless runoffs by using Equation (11), a further analysis was performed to compare two runoffs alongside the calculated index. These results, however, suggest that runoff comparisons cannot accurately represent the relative energies of the Yangtze River and Poyang Lake. The measurement data presented in this study demonstrate that though the flows of the five tributary rivers were relatively large during the 1990s, the effect of the Yangtze River was comparatively strong. Similarly, during the 2000s, the flows of the five tributary rivers were small, although those of Poyang Lake remained strong. Improvements need to be made to the rationality assessments of this index in future work.
Utilizing average monthly values for 2011, runoff and energy departure values between the Yangtze River and the five tributary rivers were calculated (Table 5). The energy of the Yangtze River can be computed via flow and water-level data from Jiujiang Station, while calculations for this variable for the five rivers can be based on the sum of the inflow of these tributaries as well as Xingzi water levels. Thus, if an anomalous flow rate is used as a reference value (Table 5), calculated results for January, April, June, November, and December are inconsistent with Fe values, especially those for April and June. The results calculated show that the flow of the Yangtze River is relatively larger than that of the five tributary rivers; this means that the Yangtze River should be playing a dominant role in river-lake interactions. In practice, however, between April and June the flow of the five rivers has rapidly increased because they remain in flood, whereas the Yangtze River remains within the dry season, and so the influence of Poyang Lake is more marked. It is therefore unreasonable to perform the analysis detailed in this paper using just a comparison between the relative flow discharges of the Yangtze River and the five tributary rivers.
Table 5 Results of a reasonableness analysis for Fe using hydrological data from 2011
Month IFJ IFW Fe IQJ IQW Month IFJ IFW Fe IQJ IQW
Jan 0.32 0.39 0.066 0.10* 0.08* July 0.29 0.09 ‒0.049 0.73 0.28
Feb 0.31 0.43 0.077 0.04 0.06 Aug 0.52 0.14 ‒0.065 0.63 0.10
Mar 0.30 0.46 0.091 0.06 0.09 Sep 0.54 0.24 ‒0.020 0.41 0.07
Apr 0.35 0.44 0.066 0.10* 0.08* Oct 0.30 0.31 0.026 0.23 0.10
May 0.29 0.46 0.084 0.10 0.12 Nov 0.48* 0.32* 0.019 0.26* 0.07*
June 0.33 0.34 0.023 0.60* 0.37* Dec 0.34 0.41 0.069 0.05* 0.05*

Abbreviations: IFJ, and IFW denote energy departures between the main stream of the Yangtze River and the five tributary rivers, respectively; IQJ and IQW denote flow departures between the main stream of the Yangtze River and the five tributary rivers, respectively; * refers to results when relative size does not match Fe values.

The analysis of computed energy departure values presented in this study exhibits a high degree of consistency, other than for November. Thus, if IFJ is larger than IFW, Fe at Hukou Station will also be negative, and vice versa; this indicates that Fe values calculated on the basis of water level and flow at Hukou Station can be used to accurately demonstrate the relative influences of the Yangtze River and Poyang Lake, as follows:
${{I}_{Ji}}=\frac{{{J}_{i}}-{{J}_{\min }}}{{{J}_{\max }}-{{J}_{\min }}}$ (11)

5 Discussion and conclusions

This paper has applied energy theory to investigate the physical mechanisms underlying the interaction between the Yangtze River and Poyang Lake and presents a series of characterization index calculations. A number of clear conclusions can be presented on the basis of this analysis.
(1) The interaction between the Yangtze River and Poyang Lake are, in actuality, a reflection of the relationship of energy between the former and the five tributary rivers. The main role of the Yangtze River is to block Poyang Lake, which leads to an increase in ES at Hukou Station. At the same time, Poyang Lake mainly provides water to the Yangtze River, causing a concomitant growth in ED at Hukou Station.
(2) Data show that it would not be unreasonable to perform the analysis in this paper via a comparison of respective flow discharges into the Yangtze River and the five tributary rivers as flow discharge cannot represent the overall energy of this system. The concept of Fe proposed in this paper can be utilized to demonstrate the relationship between the ES and ED at Hukou Station, and therefore also indirectly reflect the energies of these systems. The concept of Fe is therefore more appropriate for quantifying the river-lake interaction.
(3) Overall, values of Fe have increased since the 1950s and are indicative of an enhanced role for Poyang Lake and a weakened role for the Yangtze River. Data show that the influence of Poyang Lake was the greatest in the 2000s, whereas that of the Yangtze River peaked in the 1980s. In addition, development of the TGR has further demoted the role of the Yangtze River; over the course of a single year, the effect of the Yangtze River has increased slightly over the course of the dry season but has become significantly weakened by the end of the flooding period.
(4) River-lake interactions have also influenced the probability of flooding and drought within the area of the lake. When the flow of the five rivers is large and the role of the Yangtze River is indicative, the lake area is prone to floods. In contrast, when the flow from the five tributary rivers is small and Poyang Lake plays a major role, this area is prone to drought. The enclosure of the TGR has also further weakened the influence of the Yangtze River, making the area of the lake more prone to drought between September and October.
(5) The energy-based index proposed in this study is to characterize the interaction between the Yangtze River and Poyang Lake. It can also be used to investigate these consequences, especially given the fact that similar conditions are shared between the two, and that the water volume of the main stream is much larger than that of the tributary lake.

The authors have declared that no competing interests exist.

References

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Carmack E C, Gray C B J, Pharo C Het al., 1979. Importance of lake-river interaction on seasonal patterns in the general circulation of Kamloops Lake, British Columbia.Limnology & Oceanography, 24(4): 634-644.The physical effects of a large river (Thompson River) entering a deep, intermontane lake (Kamloops Lake, British Columbia) suggest that, depending upon its temperature relative to that of lake water, river water moves through the lake as a surface overflow, an intermediate depth interflow, or a near-bottom underflow. Circulation is further influenced by the earth's rotation so that the incoming river flows preferentially along the right-hand shoreline of the lake. Convective overturn in autumn and spring is influenced by cabbeling, which occurs whenever two parcels of water, one warmer than$4^\circC$and one colder, combine to form a mixture whose temperature is at or near$4\circC$.

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Dai X, Wan R R, Yang G S, 2015. Non-stationary water level fluctuation in China’s Poyang Lake and its interactions with Yangtze River.Journal of Geographical Sciences, 25(3): 274-288.

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Dai Z J, Liu J T, 2013. Impacts of large dams on downstream fluvial sedimentation: An example of the Three Gorges Dam (TGD) on the Changjiang (Yangtze River).Journal of Hydrology, 480: 10-18.Under the influence of climate and human activities, fluvial systems have natural ability to make adjustments so that the river hydrology, sediment movement, and channel morphology are in dynamic equilibrium. Taking the Changjiang (Yangtze River) for example. In the early stages after the Three Gorges Dam (TGD) began operational ten years ago, the suspended sediment content (SSC) and fluxes in the middle and lower reaches of the river decreased noticeably. At present, they appear to be in a stable state on the decadal scale. Although the river runoff has not shown any trends, the water level in the river decreased appreciably in time. In the meantime, channel down cutting along the thalweg almost existed throughout the river course. The riverbed has turned from depositional before the dam construction to erosional afterwards. In other words, the riverbed had turned from being sediment sinks to sediment sources. In the main channel of the Changjiang between Yichang and Nanjing, a distance of 1300km, the riverbed sedimentation mode displays strong, intermediate, and weak erosion depending on the closeness to the TGD.

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Du Y L, Zhou H D, Peng W Qet al., 2015. Modeling the impacts of the change of river-lake relationship on the hydrodynamic and water quality revolution in Poyang Lake.Acta Scientiae Circumstantiae, 35(5): 1274-1284. (in Chinese)The hydrological regime condition of Poyang Lake was mainly influenced by watershed runoff and the Yangtze River. In recent 10 years,the relationship of basin,Yangtze river and Poyang lake had been changed greatly. This change not only caused an altered hydrological rhythm,but also affected the water environment in the Lake. The average daily hydrological processes in the period of 2003 —2012 had been compared with those in1956—2002. Results show that seasonal allocations of basin inflow and lake outflow were altered,the highest water level were decreased,and the time of limnetic facies of Poyang Lake were shortened. Furthermore,a 2 dimensional numerical model of hydrodynamic and water quality was applied to study the impacts of the changing river-lake relationship,and the model was validated by the measured data in 2010. The results reveal that water quality became worse due to the drought season coming earlier 21 days during Aug to Nov,and 13 days lagging of flood season during Apr to Jun. The concentration of TN in Poyang lake increased by 10. 6% and 12. 4% during the periods of Apr to Jun,and Aug to Oct,respectively. TP concentration increased by 11. 7%and 13. 6% during the above two respective periods. In the recent 10 years,the separation of dish-shaped lakes at south and west of Poyang Lake from the main lake occurred earlier in the period of Aug to Oct than that in 1956—2002. which increased the risk of eutrophication.

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Elisa M, Gara J I, Wolanski E, 2010. A review of the water crisis in Tanzania’s protected areas, with emphasis on the Katuma River-Lake Rukwa ecosystem.Ecohydrology & Hydrobiology, 10(2-4): 153-165.Several cases of the developing water crisis in semi-arid regions of Tanzania are described. Some cases have transboundary causes. These include Lake Victoria and the riparian population as a result of hydroelectricity developments in Uganda, the Serengeti ecosystem threatened by deforestation of the Mau forest and irrigation in Kenya, and several national parks threatened by irrigation projects within Tanzania. Some of these developments are given national priorities like in case of the Great Ruaha River. Other irrigation projects are driven by the local population to combat poverty. Most of these developments are in breach of state laws because there is no consideration of minimal environmental flows and all have profound negative impacts on people and wildlife downstream. The paper describes the previously unreported case of irrigation in the upper Katuma River that flows into Lake Rukwa, which lead to poverty increase, environmental degradation and a decrease in ecosystem services provision downstream. Governance at the watershed scale in a framework compatible with ecohydrology principles is needed. Such solutions are proposed.

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Fang C M, Cao W H, Mao J Xet al., 2012. Relationship between Poyang Lake and Yangtze River and the influence of Three Gorges Reservoir.Journal of Hydraulic Engineering, 43(2): 175-181. (in Chinese)

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Guo H, Hu Q, Zhang Q, 2011. Changes in hydrological interactions of the Yangtze River and the Poyang Lake in China during 1957-2008.Acta Geographica Sinica, 66(5): 609-618. (in Chinese)Interaction of the Yangtze River and the Poyang Lake strongly affects the Poyang Lake water resources and flood and drought potentials in the lake basin. This interaction and its variation are examined in this study for the period 1957-2008. In addition, changes in the relationship between the Yangtze River and Poyang Lake after the operation of the Three-Gorge Dam (TGD) are evaluated using the TGD operation data available for 2004-2006. Major findings are shown as follows. 1) At interannual to decadal timescales the variation in the Poyang Lake water level is primarily determined by fluctuations of the climate and the inflow from the five major tributaries in the lake basin. By influencing the lake level these fluctuations also affect the lake-river interaction. 2) At the seasonal timescale, the Poyang Lake has the largest outflow to the Yangtze River and exerts a strong pressure on the mainstream during April-June, when the lake receives annual peak inflow from its tributaries. The Yangtze River's blocking and/or reversed flow to the Poyang Lake are the strongest during July-September. 3) The operation of the TGD has changed little in terms of seasonal variations of these lake and river forcings. In most of the seasons, the influence of the TGD has resulted in less than 10% of the variation in the Yangtze River flow. However, by damming or releasing water, the TGD can result in seasonal changes in the Yangtze River flow and river-lake interactions. These subtle effects from the TGD operation are quantified and compared to the climate effect on the river-lake interactions. These comparisons show enhancement in some seasons while weakening in the other seasons of the climate effect on river-lake interactions and flood or drought potential in the lake basin by the TGD operation.

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Guo H, Hu Q, Zhang Qet al., 2012a. Annual variations in climatic and hydrological processes and related flood and drought occurrences in the Poyang Lake basin.Acta Geographica Sinica, 64(5): 699-709. (in Chinese)Observational data from 1960-2008 were analyzed to obtain the mean annual climate and hydrological variations in the Poyang Lake basin, China. These variations were explained by the surface water and energy budgets, and the characteristics of these variations and the budgets were further used to deduce the rhythms of flood and drought developments in the lake basin. Some conclusions can be drawn as follows. (1) The precipitation in the lake basin increases at a large rate from January to June, reaching the annual maximum in late June. Amplifying monthly precipitation, particularly from April to June, coupled with weak surface evaporation and transpiration (ET), saturates the soils and produces a large amount of surface runoff which raises the lake level. In July, rainfall decreases sharply with clear sky and high ET, which reduces surface runoff and reverses the hydrological processes developed in the first half of the year. These drying processes continue through October, making the basin dry again. (2) When June and July have above average monthly precipitation the lake basin becomes supersaturated, and floods develop. In such conditions, if July rainfall in the middle and upper reaches of the Yangtze River basin is above average, the Yangtze River blocking (or reversed flow) effect can trigger, intensify, and prolong Poyang Lake floods. Thus, Poyang Lake floods generally occur in July and August. (3) Because the lake basin is drying after July with strong ET, if there is below average rainfall in July and the months afterwards, ET would be greater than rainfall, causing drying. Drought could occur in fall. This fall drought can persist through the winter and into the following spring and result in severe spring drought in the lake basin. Thus, the droughts occur in the fall and spring months in the lake basin. (4) The Yangtze River effects on the floods and droughts in the lake basin are most significant during the "river-lake coupling period" from July to August. Strong effects can lead to floods in the lake basin. Lack of river effects would help lower the lake level and dry the basin, leading to drought in fall.

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Guo H, Hu Q, Zhang Qet al., 2012b. Effects of the Three Gorges Dam on Yangtze River flow and river interaction with Poyang Lake, China: 2003-2008.Journal of Hydrology, 416(2): 19-27.The Three Gorges Dam (TGD) has been in operation since 2003. Over the operation period from 2003–2008, data have been collected for preliminary evaluations of actual effects of the TGD on the Yangtze River flow and river interactions with downstream lakes and tributaries. These effects are examined in this study, after the climate influence was minimized by comparing hydrological changes between years of similar climate conditions before and after the operation of the TGD. Major results show that the TGD operation has affected the Yangtze River discharge and water level. The significance of these effects varies seasonally and with different locations along the river. The seasonal variation follows the TGD’s seasonal impounding and releasing of water. The magnitude of the effects is dependent on the impounding/releasing rate and the seasonal flow of the river. The most significant effects are confined in the river reach near the TGD and are as great as five times those of sections downstream. The weakening and diminishing of effect of the TGD is primarily because of “dilutions” to the effect by inflows to the Yangtze River from downstream tributaries. Changes in the Yangtze River discharge caused by the TGD have further altered the interrelationship between the river and Poyang Lake, disturbing the lake basin hydrological processes and water resources. A major consequence of such changes has been a weakening in the river forcing on the lake, allowing more lake flow to the river from July–March. This effect of the TGD may partially fulfill the TGD’s mission to mitigate flood risks in the lake basin, especially during the peak wet season of the Yangtze River basin from July–September. In the 6 years since the TGD operation began the annual average number of severe outflow events of rates of 823000 m 3 s 611 from the lake in July–September has increased by 74. It has also resulted in reduction of water storage in Poyang Lake. Results of this study point to strong needs for working strategies to balance the TGD impacts on flood control and water resources as well as their societal and ecological consequences in the Poyang Lake basin. Meanwhile, in the context of studies of impacts of large dams this study shows an example of extending the previous studies in the dam–river setting to a new dam–river–lake construct.

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Han J Q, Sun Z H, Li Y Tet al., 2017a. Combined effects of multiple large-scale hydraulic engineering on water stages in the middle Yangtze River.Geomorphology, 298: 31-40.

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Han J Q, Zhang W, Fan Yet al., 2017b. Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam.Journal of Geographical Sciences, 27(10): 1268-1278.Elucidating the influence of dams on fluvial processes can inform river protection and basin management.However,relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches.Using hydrological and topographical data,we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD).Our conclusions are as follows.(1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period:G1 reaches,characterized by convex point bar erosion and concave channel deposition (CECD),and G2 reaches,characterized by convex point bar deposition and concave channel erosion (CDCE).Both reach types exhibited CECD features during the post-dam period.(2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars.However,changes in the river regime,river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches.(3) Flood discharges ranging from 20,000 to 25,000 m3/s result in greater erosion of convex point bars.The point bars become scoured if the durations of these flows,which are close to bankfull discharge,exceed 20 days.In addition,the reduction in bedload causes the decreasing of point bar siltation in the water-falling period.(4) During the post-dam period,flood abatement,the increased duration of discharges ranging from 20,000 to 25,000 m3/s,and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features.Our results are relevant to other meandering reaches,where they can inform estimates of riverbed change,river management strategies and river protection.

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Hu C H, Wang Y G, 2014. Sediment problems and relationship between river and lakes since the operation of the Three Gorges project.Journal of Yangtze River Scientific Research Institute, 31(5): 107-116. (in Chinese)Sediment problem is one of the key factors which affect the dispatching operation and life of the Three Gorges Project( TGP). Many approaches were employed to research the sediment problems of the TGP during its demonstration,planning,design,construction and operation,and many important results were obtained. It was confirmed that the previous sediment research results are reasonable and reliable by analyzing the field data of the actual testing operation of TGP. The 10-year operation of TGP shows that the basic situation of sediment problems is good,and the local problems were or are in control through proper treatment. However,these sediment problems can probably accumulate,develop and transform with time,so they should be paid continuous attention. The influence of the TGP operation on the relationship between rivers and lakes was also analyzed in this paper. It was pointed out that intense and large-scale erosion happens in the downstream channel of the dam since TGP operation,and the water levels at the outlets of Dongting Lake and Poyang Lake have decreased,which has some influence on the water and sediment diversions and results in sharp contradiction of water resource supply and demand in dry season for both lakes. It is effective and feasible to solve the existing problems through constructing sluices and other measures in the Dongting Lake and the Poyang Lake.

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Hu Q, Feng S, Guo Het al., 2007. Interactions of the Yangtze River flow and hydrologic process of the Poyang Lake, China.Journal of Hydrology, 347: 90-100.Recently available hydrological data from Hukou station at the junction of the Poyang Lake with the Yangtze River along with other data from stations in the Poyang Lake basin have allowed further examination and understanding of the basin effect (basin discharge generated by rainfall) and the Yangtze River blocking effect on variations of the Poyang Lake level and floods at annual to decadal scales. Major results show that the basin effect has played a primary role influencing the level of Poyang Lake and development of severe floods, while the Yangtze River played a complementary role of blocking outflows from the lake. In most cases, only when the basin effect weakened did the river effect become large, a relationship indicating that the river’s blocking effect diminishes when the lake level is high from receiving large amount of basin discharge, albeit a few exceptions to this relationship occurred when river flow also was elevated from receiving large rainfall discharges in upstream areas. Moreover, the basin effect has become stronger in the period 1960–2003 in accordance with the increase of warm season rainfall in the Poyang Lake basin. In particular, large increases of the basin’s rainfall in the 1990s corresponded to the most severe floods (in 1998, 1995, and 1992) of the last 4 decades. The strong increase of warm season rainfall in the Poyang Lake basin in the 1990s is consistent with the recent southward shift of major warm season rain bands in eastern China. Results of this study provide a utility for improving predictions of the Poyang Lake level and floods, which affect a population of about 10 million.

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Jones F C, Plewes R, Murison Let al., 2017. Random forests as cumulative effects models: A case study of lakes and rivers in Muskoka, Canada.Journal of Environmental Management, 201: 407-424.Abstract Cumulative effects assessment (CEA) - a type of environmental appraisal - lacks effective methods for modeling cumulative effects, evaluating indicators of ecosystem condition, and exploring the likely outcomes of development scenarios. Random forests are an extension of classification and regression trees, which model response variables by recursive partitioning. Random forests were used to model a series of candidate ecological indicators that described lakes and rivers from a case study watershed (The Muskoka River Watershed, Canada). Suitability of the candidate indicators for use in cumulative effects assessment and watershed monitoring was assessed according to how well they could be predicted from natural habitat features and how sensitive they were to human land-use. The best models explained 75% of the variation in a multivariate descriptor of lake benthic-macroinvertebrate community structure, and 76% of the variation in the conductivity of river water. Similar results were obtained by cross-validation. Several candidate indicators detected a simulated doubling of urban land-use in their catchments, and a few were able to detect a simulated doubling of agricultural land-use. The paper demonstrates that random forests can be used to describe the combined and singular effects of multiple stressors and natural environmental factors, and furthermore, that random forests can be used to evaluate the performance of monitoring indicators. The numerical methods presented are applicable to any ecosystem and indicator type, and therefore represent a step forward for CEA. Crown Copyright 2017. Published by Elsevier Ltd. All rights reserved.

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Kneis D, Knoesche R, Bronstert A, 2006. Analysis and simulation of nutrient retention and management for a lowland river-lake system.Hydrology & Earth System Sciences, 26(1): 121-124.In the context of the European Water Framework Directive, we studied the possible impact of reduced emissions on phosphorus and nitrogen concentrations in a lowland river-lake system (Havel River, Germany). As a prerequisite, we quantified the retention of nutrients in the river from mass balances and deduced its seasonal variation. We detected that about 30% of the total nitrogen input is retained within the surveyed river section. In contrast, phosphorus release from sediments was shown to cause a considerable increase in present P concentrations. Average net phosphorus release rates of about 20 mg P m 2 d 1 in late summer were estimated for the Havel Lakes. Based on the observed patterns of N retention and P release we parametrized a newly developed water quality simulation program (TRAM), which allows alternative model approaches of different complexity to be implemented and tested. To account for the future trend of internal P loading, the phosphorus excess in lake sediments was estimated from core samples and included in the model as a state variable. For analyzing scenarios of reduced nutrient emissisions, the water quality simulation program was linked to mesoscale hydrological catchment models for the first time. From scenario simulations we conclude that internal P loading is likely to counteract efforts of emission control for decades. Even by significant reductions in external P loads, a persistent phosphorus limitation of primary production can hardly be established in the analyzed time frame of 13 years. Though in the short run a continued reduction in nitrogen loads appears to be the more promising approach of eutrophication management, we recommend enhanced efforts to diminish both N and P emissions.

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Lai X J, Huang Q, Zhang Y Het al., 2014. Discharge capacity analysis on Poyang Lake.Journal of Lake Science, 26(4): 529-534. (in Chinese)

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Liu Z G, Ni Z K, 2015. The rules and the effects of varying river-lake relationships on the evolution of Poyang Lake.Acta Scientiae Circumstantiae, 35(5): 1265-1273. (in Chinese)Poyang Lake is the largest river-connected lake along the Yangtze River basin,with a unique river-lake relationship. Formation and evolution of the modern Poyang Lake is the result of long-term interaction and development of multi-factors,which has undergone several different phases. These factors played the distinct roles in the different phases of the evolution,in the process of which which the natrual effect comes slowly,while anthropogenic effect appears faster and more directly. With the social-economic development around the lake area,human dependence highlights the importance,vulnerability and susceptibility of Poyang Lake ecosystem. This dependence surely will detrimentally take the effect on the course of lake evolution,especially to the shallow lakes in alluvial plains. It's worth noting that the significant impacts of the alteration of river-lake and human-lake relationships will bring more variables and uncertainties on the evolution of Poyang Lake in the future. To guarantee the Poyang Lake's eco-safety,it's particularly important and urgent to understand and predict the changing tendences of river-lake relationship scientifically and to propose countmeasures.

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Nakayama T, Watanabe M, 2008. Role of flood storage ability of lakes in the Changjiang River catchment.Global & Planetary Change, 63(1): 9-22.Almost every year, China is affected by severe flooding, which causes considerable economic loss and serious damage to towns and farms, in particular, at the middle and lower regions of Changjiang (Yangtze) River. In addition to abnormal weather, previous research has concluded that artificial effects have also been significant causes of major flood disasters along this catchment. These have included (i) extreme deforestation and soil erosion in the upper reaches, (ii) shrinking of lake water volumes and their reduced connection with the Changjiang River due to reclamation of lakes that retarded water in the middle reaches, and (iii) restriction of channel capacity following levee construction. The present research focused on the flood storage ability of Dongting and Poyang Lake in the middle region and simulated the water/heat dynamics for the 1998 large-scale flood, which was one of the largest floods in the 20th century, in the entire Changjiang River catchment. The authors expanded the process-based model for application to broader catchments in order to evaluate large-scale flooding in the Changjiang River (NICE-FLD), and compared the severe flood-period of 1998 with the usual flood-period of 1987 1988. The model reproduced excellently the river discharge, soil moisture, evapotranspiration, groundwater level, and other factors. Furthermore, the authors conducted the back-casting simulation by recreating the areas of both lakes in the 1950s and by inputting the meteorological data in the 1998 severe flood in order to evaluate the role of flood storage ability of lakes. The simulation shows that the peak flow and groundwater level around the junction and downstream from it decrease as qualitatively described by the previous researches. This indicates that the increase of lake storage capacity moderates the peak value of lake water level during the flood periods and decreases the groundwater level surrounding the lakes. This result is very important for minimizing flood damage as far as possible and making the better decision on sustainable development in the catchment.

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Wan R R, Yang G S, Wang X Let al., 2014. Progress of research on the relationship between the Yangtze River and its connected lakes in the middle reaches.Journal of Lake Science, 26(1): 1-8. (in Chinese)

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Wang D, Li Y T, Deng J Yet al., 2014. Preliminary analysis of changes in hydraulic elements of Dongting Lake in storage period of Three Gorges Reservoir.Journal of Hydroelectric Engineering, 33(2): 26-32. (in Chinese)

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Wang J D, Sheng Y W, Wada Y, 2017. Little impact of the Three Gorges Dam on recent decadal lake decline across China’s Yangtze Plain.Water Resources Research, 53: 3854-3877.The ubiquitous lakes across China's Yangtze Plain (YP) are indispensable freshwater resources sustaining ecosystems and socioeconomics for nearly half a billion people. Our recent survey revealed a widespread net decline in the total YP lake inundation area during 2000-2011 (a cumulative decrease of 10%), yet its mechanism remained contentious. Here we uncover the impacts of climate variability and anthropogenic activities including (i) Yangtze flow and sediment alterations by the Three Gorges Dam (TGD) and (ii) human water consumption in agricultural, industrial, and domestic sectors throughout the downstream Yangtze Basin. Results suggest that climate variability is the dominant driver of this decadal lake decline, whereas studied human activities, despite varying seasonal impacts that peak in fall, contribute marginal fraction ( 10-20% or less) to the interannual lake area decrease. Given that the TGD impacts on the total YP lake area and its seasonal variation are both under 藴5%, we also dismiss the speculation that the TGD might be responsible for evident downstream climate change by altering lake surface extent and thus open water evaporation. Nevertheless, anthropogenic impacts exhibited a strengthening trend during the past decade. Although the TGD has reached its full-capacity water regulation, the negative impacts of human water consumption and TGD-induced net channel erosion, which are already comparable to that of TGD's flow regulation, may continue to grow as crucial anthropogenic factors to future YP lake conservation.

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Ye X C, Li X H, Zhang Q, 2012. Temporal variation of backflow frequency from the Yangtze River to Poyang Lake and its influencing factors.Journal of Southwest University (Natural Science Edition), 34(11): 69-75. (in Chinese)

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Zhang D, Chen P, Zhang Qet al., 2017. Copula-based probability of concurrent hydrological drought in the Poyang Lake-catchment-river system (China) from 1960 to 2013.Journal of Hydrology, 553: 773-784.Investigation of concurrent hydrological drought events is helpful for understanding the inherent mechanism of hydrological extremes and designing corresponding adaptation strategy. This study investigates concurrent hydrological drought in the Poyang lake-catchment-river system from 1960-2013 based on copula functions. The standard water level index (SWI) and the standard runoff index (SRI) are employed to identify hydrological drought in the lake-catchment-river system. The appropriate marginal distributions and copulas are selected by the corrected Akaike Information Criterion and Bayesian copulas selection method. The probability of hydrological drought in Poyang Lake in any given year is 16.6% (return period of 6 years), and droughts occurred six times from 2003-2013. Additionally, the joint probability of concurrent drought events between the lake and catchment is 10.1% (return period of 9.9 years). Since 2003, concurrent drought has intensified in spring due to frequent hydrological drought in the catchment. The joint probability of concurrent drought between the lake and the Yangtze River is 11.5% (return period of 8.7 years). This simultaneous occurrence intensified in spring, summer and autumn from 2003-2013 due to the weakened blocking effect of the Yangtze River. Notably, although the lake drought intensified in winter during the past decade, hydrological drought in the catchment and the Yangtze River did not intensify simultaneously. Thus, this winter intensification might be caused by human activities in the lake region. The results of this study demonstrate that the Poyang lake-catchment-river system has been drying since 2003 based on a statistical approach. An adaptation strategy should be urgently established to mitigate the worsening situation in the Poyang lake-catchment-river system.

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Zhang Q, Ye X C, Werner A Det al., 2014. An investigation of enhanced recessions in Poyang Lake: Comparison of Yangtze River and local catchment.Journal of Hydrology, 517: 425-434.Changes in lake hydrological regimes and the associated impacts on water supplies and ecosystems are internationally recognized issues. During the past decade, the persistent dryness of Poyang Lake (the largest freshwater lake in China) has caused water supply and irrigation crises for the 12.4 million inhabitants of the region. There is conjecture as to whether this dryness is caused by climate variability and/or human activities. This study examines long-term datasets of catchment inflow and Lake outflow, and employs a physically-based hydrodynamic model to explore catchment and Yangtze River controls on the Lake’s hydrology. Lake water levels fell to their lowest during 2001–2010 relative to previous decades. The average Lake size and volume reduced by 154km2 and 11×108m3 during the same period, compared to those for the preceding period (1970–2000). Model simulations demonstrated that the drainage effect of the Yangtze River was the primary causal factor. Modeling also revealed that, compared to climate variability impacts on the Lake catchment, modifications to Yangtze River flows from the Three Gorges Dam have had a much greater impact on the seasonal (September–October) dryness of the Lake. Yangtze River effects are attenuated in the Lake with distance from the River, but nonetheless propagate some 100km to the Lake’s upstream limit. Proposals to build additional dams in the upper Yangtze River and its tributaries are expected to impose significant challenges for the management of Poyang Lake. Hydraulic engineering to modify the flow regime between the Lake and the Yangtze River would somewhat resolve the seasonal dryness of the Lake, but will likely introduce other issues in terms of water quality and aquatic ecosystem health, requiring considerable further research.

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Zhang Z X, Chen X, Xu C Yet al., 2015. Examining the influence of river-lake interaction on the drought and water resources in the Poyang Lake basin.Journal of Hydrology, 522: 510-521.In recent years, the Poyang Lake basin is in a prolonged drought which has placed immense pressure on the water resources utilization. In this paper, we explore the spatial and temporal distributions of extreme droughts in the Poyang Lake basin by using the methods of SPI (Standardized Precipitation Index) and EOF (Empirical Orthogonal Function) for the period of 1956 2009, which are influenced by regional precipitation anomalies and river ake interaction due to water impounding of the Three Gorges Dam (TGD). The results show that: (1) the Poyang Lake basin experienced six extreme droughts during the past 60years, which lead to decreases in streamflow from five tributary rivers down to the Poyang Lake. The droughts in the 1960s and the 2000s were the most serious ones. However, there was an increasing trend of streamflow in the upper and middle Yangtze in the 1960s, and a decreasing trend appeared in the 2000s. The decline of streamflow in the upper Yangtze reaches has lowered the water level of lower Yangtze River which has caused more outflow from the Poyang Lake to the Yangtze River; (2) the operation of the Three Gorges Dam (TGD) has altered the seasonal pattern of flow regimes in the Poyang Lake and significantly reduced the water level in the lower Yangtze River during the TGD impounding period from late September to early November; and (3) the conjunction of extreme droughts in the Poyang Lake and the upper Yangtze reaches coincided with the impounding of the TGD is the main cause of the low water level in the Poyang Lake. Although the impact of the recent droughts in the Poyang Lake and upper Yangtze reaches has played a crucial role in the low water level of Poyang Lake, more attention should be paid to its sensitivity to the influence of the large dam-induced changes in the interaction between river and lake, particularly during impounding periods.

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Zhang Z X, Huang Y H, Xu C Yet al., 2016. Analysis of Poyang Lake water balance and its indication of river-lake interaction.SpringerPlus, 5(1): 1555.In recent years, water shortage is becoming one of the most serious problems in the Poyang Lake. In this paper, the long-term water balance items of the Poyang Lake have been analyzed to reveal the coupling effects of Three Gorges Dam (TGD) and droughts on the water balance of Poyang Lake. The results indicate that: (1) the water balance items of Poyang Lake vary greatly, e.g. lake precipitation and inflow decrease during the past several decades while evaporation and water consumption increase significantly; (2) the water balance of Poyang Lake has been affected by the operation of TGD. Negative lake water balance in recent years leads to a serious water shortage problem in the Poyang Lake. Moreover, the operation of TGD also changed the river ake relationship in the lower Yangtze River basin; (3) the coupling effects of drought and TGD on the lake water balance has been analyzed by using composite analysis method and it can be found that the operation of TGD has significantly altered the lake water balance. But it is not the only factor that affects the lake water balance, and the droughts might cause their relations to be much more complicated.

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Zhao J K, Li J F, Dai Z Jet al., 2011. Analysis of water exchange between river and lakes in the middle and lower Yangtze River in low flow years.Journal of Natural Resources, 26(9): 1613-1624. (in Chinese)This paper focuses on analyzing water exchange between river and lakes along the middle and lower Yangtze River in typical low flow years of 1978 and 2006,by using water level and discharge data at Yichang,Hankou,Datong,Chenglingji and Hukou hydrologic stations.The empirical formula for explaining water interchange coefficients of river and lakes are obtained and by this,degree of water interchange between river and lakes can be showed in a quantitative way.Result indicates: the data in 1978 can stand for conditions before the construction of the Three Gorges Reservoir(TGR) and reflect water interchange process between lakes and main streams in typical low flow years.While the running of TGR in 2006 shows that lakes have exerted positive effects on runoff regulation of main streams in typical low flow year.What's more,water exchange coefficients at Dongting and Poyang lakes in 1978 are 0.57 and 0.56,and this shows that Dongting Lake and Poyang Lake water exchange is stable.However,the coefficients are 0.89 and 0.51 in 2006 and this shows water exchange in 2006 is stronger than that in 1978.The quantities of water supplied by Dongting and Poyang lakes to the Yangtze River are 1990 108m3 and 947 108m3,respectively accounting for 29% and 14% of the runoff in the corresponding period at Datong Station in 1978.While the quantities of water in 2006 are 1962 108m3 and 1564 108m3,respectively,accounting for 28% and 23% of the runoff in the corresponding period at Datong Station.Especially the Poyang Lake,its recharge is 5% more than the normal years.Thus,the discharge at Datong Station is more than 10000 m3/s all the year round in 2006.

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Zhou Y Q, Erik J, Li J Bet al., 2016. Impacts of the Three Gorges Reservoir on sedimentation regimes in the downstream-linked two largest Chinese freshwater lakes.Scientific Reports, 6: 35396.We studied the impacts of Three Gorges Reservoir (TGR) on the sedimentation regimes in the downstream-linked two largest Chinese freshwater lakes, Lake Dongting and Lake Poyang. Our results indicate that up to 1.7365×65109t sediment was retained in TGR from June 2003 to December 2014. This resulted in a 145.965×65106t yr611decline in the suspended sediment load at Zhicheng and a 16.865×65106t yr611lower sediment flow from Yangtze River to Lake Dongting, which partially explains the 13.465×65106t yr611lower sedimentation in Lake Dongting during the post-TGR period. Furthermore, TGR resulted in a 0.565±650.365m reduction of the multi-year mean water level at the Lake Poyang outlet Hukou, accelerating the suspended sediment export discharge from the lake. The reduced sedimentation in Lake Poyang during the post-TGR period was estimated to 6.365×65106t yr611. We estimate that a monthly mean concentration of sediment flow from TGR below 0.6065kg m613will lead to erosion in Lake Dongting and Lake Poyang. Better regulation of TGR may extend the life expectancy of the two vanishing large lakes.

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