Evolution Trend and Cause Analysis of Diversion Characteristics of Three Outlets Along Jingjiang River Under New Flow and Sediment Conditions

Abstract

Changes in the diversion characteristics of three outlets along Jingjiang River are of vital importance to the adjustment of river–lake relationships. This study analyzed the mechanism of periodic changes in the diversion ratio of the three outlets along the Jingjiang River after the storage of the Three Gorges Reservoir. It used the latest measured flow and sediment data. The analysis was conducted from the perspective of changes in the main stream regime at the three outlets along the Jingjiang River and the erosion and deposition trend of the floodway at the three outlets. On such a basis, the contribution ratio of three factors was analyzed quantitatively. These factors are Jingjiang River runoff reduction, reservoir regulation action, and diversion capacity drop. This analysis comprehensively considered the diversion capacity of the floodway at three outlets. It also considered the annual runoff volume and runoff process of the Jingjiang River mainstream. The purpose was to reveal the change laws of water resource quantity and response mechanism of Dongting Lake area under the new flow and sediment conditions. This will provide technical support for the sustainable management of water resources in the basin and the adaptive operation of reservoirs. The analysis results indicated that the diversion volume reduction at the three outlets along Jingjiang River is jointly caused by the regulation of the Three Gorges Reservoir and the runoff volume of the incoming flows of Jingjiang River. Seen from the proportion, the reservoir regulation action takes up 35% before the Three Gorges Reservoir is filled to 175 m, and less runoff of Jingjiang River takes up 65%; after the reservoir runs normally when filled to 175 m, the reservoir regulation action takes up 63%, the proportion of the diversion capacity drop of the three outlets causing diversion volume reduction takes up 2.5%, and less runoff of Jingjiang River takes up 34.5%.

1. Introduction

The three outlets along Jingjiang River are the link between Jingjiang reach and Dongting Lake and also the flow and sediment connecting the channel between the main stream of Yangtze River and Dongting Lake. The changes in the three outlets’ diversion are of great significance to the adjustment of the river–lake relationship, which directly relates to guaranteeing the flood control safety and water resources of the river and lake area [1,2]. Jingjiang River reach starts from Zhicheng Town to Chenglingji, with a total length of 347 km. It is divided into upper Jingjiang reach and lower Jingjiang reach with Ouchi Outlet as the boundary. Jianghan Plain lies to the north of Jingjiang River. To the south, it passes through Songzi Outlet, Taiping Outlet, Ouchi Outlet and Tiaoxian Outlet (blocked with a gate in 1959), and flows into Dongting Lake through floodways such as the Songzi River, Hudu River, Ouchi River and Huarong River. In addition, it integrates Xiang River, Zi River, Yuan River, and Li River at Chenglingji and then affluxes into the Yangtze River to form the complicated river–lake relationship [3,4]. The station network of the three outlets’ drainage system is shown in Figure 1. As shown in Figure 1, the diversion volume of Songzi Outlet is represented by Xinjiangkou Station and Shadaoguan Station, the diversion volume of Taiping Outlet is represented by Mituosi Station, and that of Ouchi Outlet is represented by Kangjiagang Station and Guanjiapu Station. The diversion ratio of each outlet refers to the proportion of corresponding diversion volume in the incoming flow of Zhicheng Station.

Over the years, the three outlets are always in a continuous evolution process due to the influences of natural geographical conditions, climate changes, and human activities [5,6]. Following the impoundment of the Three Gorges Reservoir, the sediment concentration of the released flow decreased substantially. Significant scour resumed in the downstream river channel [7,8]. Concurrently, the intra-annual process of diversion at the three outlets was redistributed due to the influence of runoff regulation by the Three Gorges Reservoir. It significantly influences the economic and social development of the lake area.

The issue of the Yangtze River–Dongting Lake relationship evolution and its influence on regional economic activities and ecological environmental protection has attracted much attention from society and academic circles [9,10,11]. According to literature research, scholars of the industry generally agreed with the idea that the diversion ratio of the three outlets had formed progressive decreasing change before storage of the Three Gorges Reservoir, but failed to reach consensus on the diversion characteristics evolution trend of the three outlets after storage of the Three Gorges Reservoir: Liu Kabo et al. [12], Gong Ping et al. [13], and Zhu Lingling et al. [14] thought that the floodway at the three outlets shrunk due to deposition and the diversion ratio of three outlets will keep declining, while Li Yitian [9] et al., Qu Geng et al. [15], and Guo Xiaohu et al. [16] considered that there is no obvious change in the diversion ratio of the three outlets along Jingjiang River through analysis of the measured data. The most direct effect caused by the diversion ratio reduction in the three outlets is that the capacity of Dongting Lake helping store the flood of Yangtze River is weakened [9,17], increasing the pressure of flood control of the section from Chenglingji to Wuhan reach. In addition, the silt volume entering Dongting Lake area is greatly reduced [3], causing the amount and rate of sediment deposition of Dongting Lake to decrease [15,18].

Currently, studies on the factors influencing the diversion ratio of the three outlets along Jingjiang River include the influences of erosion and deposition of the floodway at the three outlets, the river regime change near the outlets, the changes in erosion and deposition of riverway and the stage of main stream, shrinkage of Dongting Lake due to deposition, and the storage of the Three Gorges Reservoir [9,11,19]. Xijun Lai et al. thought that the flow of Jingjiang River main stream will be remarkably reduced after storage of the Three Gorges Reservoir, with the diversion flow of the three outlets reducing accordingly [20]; Fang Chunming [17] and Cao Wenhong et al. [21] considered that the diversion capacity of the three outlets did not show great changes after storage of the Three Gorges Reservoir, and the reduction in the actual diversion volume of the three outlets was caused by the runoff regulation of the Three Gorges Reservoir; Li Jingbao et al. [22] considered, through comparison of measured flow and sediment data, that the operation of the Three Gorges Reservoir reduced the diversion and sand division ratio of the three outlets; Wang Dong et al. [23] thought that the main factor influencing the diversion volume reduction in the three outlets along Jingjiang River is the continuous dry year of the main stream runoff, while the regulation action of the Three Gorges Reservoir is the secondary influencing factor. With further use of the Three Gorges Reservoir and upstream cascade reservoirs, incoming water and sediment conditions changed again, and a scouring trend appeared at the floodway at the three outlets and the Dongting Lake area [3], which made the factors affecting the three outlets’ diversion be adjusted, and led to certain differences between the actual changes in the diversion and sand division ratio of the three outlets and those of previous studies. Currently, research findings on the reduction process, mechanisms, and changing trends of diversion at the three outlets are fairly comprehensive. The historical process of changes in diversion at the three outlets is driven by multiple factors, including rainfall variation, river regime changes, and the operation of the Three Gorges Reservoir. However, studies systematically analyzing the influence of each factor, particularly quantifying their contribution ratios, remain insufficient. Therefore, further causal analysis of the diversion changes at the three outlets following the impoundment of the Three Gorges Reservoir is necessary. Based on measured hydrological and topographical data from the mainstem of the Yangtze River and the flood channels of the three outlets during 1956–2018, this paper analyzes the process and causes of the diversion changes at the three outlets. It employs the control variable method to quantify the contribution of each driving factor in different historical stages, aiming to provide references for water resource management in the middle and lower Yangtze River and for the regulation planning of the Jingjiang–Dongting Lake area.

2. Overview of Study Area

The Three Outlets of Jingjiang, namely Songzi Outlet, Taiping Outlet, and Ouchi Outlet, located on the south bank of the Jingjiang River, serve as critical channels through which the Yangtze River diverts its flow into Dongting Lake. These outlets are connected to the Songzi River, Taiping River (Hudu River), and Ouchi River, respectively, forming a vital link between the Yangtze River and Dongting Lake. As one of the primary sources of water replenishment for Dongting Lake, the Three Outlets of Jingjiang provide abundant water resources, playing an indispensable role in maintaining the water balance of Dongting Lake. However, the operation of the Three Gorges Reservoir has altered the water resource distribution pattern of the Three Outlets of Jingjiang, significantly impacting their diversion volumes and hydrological characteristics. On an annual basis, the total average diversion volume of the Three Outlets has decreased by 4.577 billion cubic meters, accounting for 9.25% of the total diversion volume under conditions without the Three Gorges Reservoir [24]. During the flood season, the Three Outlets of Jingjiang serve as crucial flood discharge channels for the middle reaches of the Yangtze River, diverting a portion of the floodwaters into Dongting Lake. This effectively alleviates flood pressure on the main stem of the Yangtze River, playing a significant role in ensuring flood control safety in the middle and lower reaches of the Yangtze River. The geographical and hydrological characteristics of the Three Outlets of Jingjiang are illustrated in Figure 1.

3. Evolution Trend of Diversion Characteristics of Three Outlets Along Jingjiang River

As shown in Figure 1, the three outlets along the Jingjiang River are Songzikou, Taipingkou, and Ouchikou. The diversion flow at Songzikou is represented by Xinjiangkou Station and Shadaoguan Station. The diversion flow at Taipingkou is represented by Mituosi Station. The diversion flow at Ouchikou is represented by Kangjiagang Station and Guanjiapu Station. The diversion ratio of each outlet is the proportion of its corresponding diversion flow relative to the inflow at Zhicheng Station. The diversion characteristics of the three outlets along Jingjiang River can be represented jointly by the diversion ratio and diversion volume. Diversion ratio is the ratio between the diversion volume of three outlets and the discharge of the main stream under the same discharge level of the main stream, which represents the diversion capacity of the floodway at three outlets. The impoundment of the Three Gorges Reservoir in 2002 exerted a profound influence on the hydrological regime of the middle and lower Yangtze River. This study statistically analyzes the runoff characteristics of the three outlets along the Jingjiang River before and after 2002.

To verify the statistical significance of hydrological series changes, this study adopts the non-parametric Mann–Kendall monotonic trend test method. This method has no strict requirements on data distribution and is suitable for trend analysis of hydro-meteorological variables. The statistic S is calculated as

$$\mathrm{S}={\sum }_{\mathrm{i}=1}^{\mathrm{n}-1}{\sum }_{\mathrm{j}=\mathrm{i}+1}^{\mathrm{n}}\mathrm{s}\mathrm{g}\mathrm{n}({\mathrm{x}}_{\mathrm{j}}-{\mathrm{x}}_{\mathrm{i}})$$

(1)

$$\mathrm{s}\mathrm{g}\mathrm{n}({\mathrm{x}}_{\mathrm{j}}-{\mathrm{x}}_{\mathrm{i}})=\left\{\begin{array}{c}1 (\text{when} {\mathrm{x}}_{\mathrm{j}}>{\mathrm{x}}_{\mathrm{i}})\\ 0 (\text{when} {\mathrm{x}}_{\mathrm{j}}={\mathrm{x}}_{\mathrm{i}})\\ -1 (\text{when} {\mathrm{x}}_{\mathrm{j}}<{\mathrm{x}}_{\mathrm{i}})\end{array}\right.$$

(2)

$$\mathrm{p}={\displaystyle \frac{\mathrm{S}-1}{\sqrt{\mathrm{v}\mathrm{a}\mathrm{r}\left(\mathrm{S}\right)}}}$$

(3)

where n denotes the total sample size of the hydrological series. ${\mathrm{x}}_{\mathrm{j}} \mathrm{a}\mathrm{n}\mathrm{d} {\mathrm{x}}_{\mathrm{i}}$ represent observations at the i-th and j-th time points, respectively. Sgn() is the sign function, determining the relative magnitude between two observations. S is the trend test statistic, where positive and negative values indicate increasing and decreasing trends, respectively. p is the standardized statistic used for significance determination. The significance of trends is determined by comparing the standardized statistic Z-value with significance level α (set at α = 0.05 in this study). All runoff and diversion ratio trends discussed in this paper passed the significance test at α = 0.05, ensuring the statistical reliability of our conclusions. Considering the length of this paper, the part of the significance test for the flow and diversion trends has been placed in the Supplementary Materials.

From 1956 to 2019, the diversion volume of the three outlets along Jingjiang River showed an obvious declining trend in long series (see Figure 2a), and the runoff of the three outlets along Jingjiang River in flood season decreased obviously after storage of the Three Gorges Reservoir (Figure 2b).

Compared with 1960~2002, the average runoff of the three outlets along Jingjiang River in 2003~2019 decreased by 37.69 billion m³, with a change amplitude of 43.8%; the average runoff of September and October over the years decreased by 15.36 billion m³, with a change amplitude of 57.7%. As can be seen from the change amplitude of each month, February shows the largest change amplitude, with runoff change amplitude varying from −68.9% to 229.9%. Except for February and March, the runoff of other months was less (

Table 1. The runoff of the three outlets along Jingjiang River in different periods and the changes.

Category	Runoff and Its Changes (Runoff: Billion m3; Percentage: %)
	January	February	March	April	May	June	July	August	September	October	November	December	September and October	Year
1960~2002	0.11	0.05	0.17	1	4.32	9.68	22.06	18.62	16.7	9.91	2.86	0.56	26.6	86.02
Annual distribution	0.1	0.1	0.2	1.2	5.0	11.3	25.6	21.6	19.4	11.5	3.3	0.6	30.9	100
2003~2019	0.21	0.17	0.26	0.76	3	6.07	14.07	11.11	8.17	3.08	1.19	0.25	11.25	48.33
Annual distribution	0.4	0.3	0.5	1.6	6.2	12.5	29.1	23.0	16.9	6.4	2.5	0.5	23.3	100
Runoff change	0.1	0.12	0.08	−0.23	−1.31	−3.61	−7.99	−7.52	−8.53	−6.83	−1.67	−0.3	−15.36	−37.69
Change percentage	89.8	229.9	47.9	−23.5	−30.4	−37.3	−36.2	−40.4	−51.1	−68.9	−58.3	−54.7	−57.7	−43.8

).

It is generally considered that, since the 1950s, the changes in the diversion capacity of the three outlets along Jingjiang River are mainly due to the following influencing factors: (1) the changes in the main stream regime of Jingjiang River affected the diversion capacity of the three outlets; (2) the shrinkage due to deposition of the floodway at the three outlets weakened the diversion capacity. The change in diversion volume needs to further consider (1) the changes in annual runoff of upstream incoming flow upward of Yichang; and (2) the changes in the duration of medium and high discharge level due to the process of changing the main stream runoff and stage of Jingjiang River caused by controlling reservoirs such as the Three Gorges Reservoir on the basis of clarifying the trend of diversion ratio changes. After storage of the Three Gorges Reservoir, the diversion characteristics of three outlets along Jingjiang River showed two periodic changes again (

Table 2. Changes in the diversion ratio of the three outlets along Jingjiang River in 1956~2019 (billion m³).

Period		Zhicheng	Songzi Outlet	Taiping Outlet	Ouchi Outlet	Total of Three Outlets	Diversion Ratio of Three Outlets (%)
Starting and Ending Year	No.
1956–1966	I	451.5	48.51	20.97	63.68	133.16	29%
1967–1972	II	430.2	44.54	18.58	39.02	102.14	24%
1973–1980	III	444.1	42.75	15.99	24.69	83.43	19%
1981–1998	IV	443.8	37.66	13.34	18.86	69.86	16%
1999–2002	V	445.4	34.49	12.56	15.48	62.53	13%
2003–2012	VI	409.3	29.24	9.21	10.87	49.32	12.8%
2013–2019	VII	434.6	29.59	6.6	10.0	46.18	10.6%

): the change trend of the diversion ratio of the three outlets along Jingjiang River in 2003~2012 was non-significant compared with that before storage, and the diversion ratio of the three outlets along Jingjiang River in 2013~2019 began to show a declining trend. This paper analyzed the mechanism of periodic changes in the diversion ratio of the three outlets along Jingjiang River after storage of the Three Gorges Reservoir from the perspective of the changes in the main stream regime at the three outlets along Jingjiang River and the erosion and deposition trends of the floodway at the three outlets.

4. Analysis of the Causes of the Changes in the Diversion Ratio of the Three Outlets Along Jingjiang River

The swing of the main channel in the Yangtze River main stream affects the directionality of diversion. The stage difference between the main stream and the outlet entrance affects the fundamental driving force of diversion. Changes in the stage relationship between the Yangtze River and its connected lakes dynamically regulate the magnitude and timing of the diversion force. Therefore, this study analyzes the impacts of the main stream river regime, the stage–discharge relationship, low water stages, and flow cutoff characteristics on the diversion at the three outlets.

4.1. Changes in Regime of Three Outlets Along Jingjiang River

4.1.1. Songzi Outlet

Lying in the reach from Zhicheng to Wanshi, Songzi Outlet includes the shoal of Lujia River, the shoal of Zhijiang, and the shoal of Jiangkou, and sits on the east terrace from Lujia River to Zhijiang. The riverway is mainly sand and pebble bed, and the anti-scouring capability of the riverbed of the reach upward of Lijiadu is strong, making the ratio drop of the section upward of Lijiadu obviously greater than that downward of Lijiadu. Due to the composition characteristics of the bed of the riverway, the diversion ratio of Songzi Outlet stays relatively stable from year to year. According to the research achievements of the Hydrological Bureau of the Yangtze River Water Resources Commission, Zhicheng is 3000 m³/s. The diversion ratio of Songzi Outlet was stable at 13.7% in 1956~1980, reduced to 10.5% in 1981 (after operation of Gezhouba Reservoir), and slightly increased to 11.2% in 2003~2008 due to the action of sand dredging and scouring by clear water at the riverway of Songzi Outlet. The changes in the main stream regime of Jingjiang River near Songzi Outlet were analyzed. After storage of the Three Gorges Reservoir, the scouring near Songzi Outlet is relatively obvious. It can be seen from Figure 3 that the deep channel of the right bank in 2009 was closer to the right bank compared with 2003, the ten-meter line advanced toward Songzi Outlet along the right bank since 2003, and the main stream position was close to Songzi Outlet. Through the comparison of the included angle of the main stream riverway and Songzi River diversion floodway in 2003~2009, the diversion angle in 2006 showed small changes compared with that in 2003, while the diversion angle of 2009 obviously reduced. All the above regime changes are beneficial for the increase in the diversion ratio of Songzi Outlet.

4.1.2. Taiping Outlet

Taiping Outlet lies in the reach from Wanshi to Shashi and includes the Taiping Outlet mid-channel bar. Jingjiang reach at Taiping Outlet began developing mid-channel bar in the 1990s, and it changed from a single riverway to a braided river. According to the mapping in 1996 and 2001 (south passage) and the mapping in 1998 and 2000 (north passage), the oscillation of the main flow appeared, with a maximum value of 600 m (near Jing 28). Moreover, affected by the reduction in upstream incoming sediment, taking the 30 m contour line as an example, scoured by the flood in 1998, the area reduced from 1.54 km² to 0.85 km² in 1996~1998; the crest level of the mid-channel bar dropped from 34.8 m to 32.4 m, with 2.4 m scoured; and the head of the mid-channel bar reduced by 560 m. The deposition of the mid-channel bar increased to a certain extent in 2000 and 2001, with the mid-channel bar body translating downstream. In 2001, the area was 0.93 km², the crest level was 32.8 m (the position was 240 m above Jing 34, moving downward for 1200 m compared with 1996), and the head also moved downward by 1960 m compared with that of 1996 (as shown in Figure 4a). Besides the changes in the Taiping Outlet mid-channel bar, the side bar of Taiping Outlet also underwent changes corresponding to the reduction in upstream incoming sediment, the head of side bar collapsed slightly, the deposition at the bottom half extended downward, and the crest level increased due to deposition.

Generally, Taiping Outlet mid-channel bar lies in the downstream of Taiping Outlet. Specifically after storage of the Three Gorges Reservoir, the Taiping Outlet mid-channel bar further moves downward due to scouring, and the swing of the main stream after reaching the mid-channel bar has a slight influence on the diversion capacity of Taiping Outlet. However, the size of the reservoir beach of Taiping Outlet can be regarded as an indication for changes in main stream erosion and deposition near Taiping Outlet. The form and area of the Taiping Outlet mid-channel bar evolves with the changes in upstream incoming water and sediment, which reflects the erosion and deposition state of the main stream riverway of Jingjiang River near Taiping Outlet. The stage of Taiping Outlet at the same discharge level rises or falls under the influences of mid-channel bar evolution and finally affects the diversion capacity (diversion ratio) of Taiping Outlet. After the Three Gorges Reservoir was put into operation, the area of the Taiping Outlet mid-channel bar underwent a process of increasing first and reducing suddenly in 2013 with a further reduction in sediment concentration of Yichang Station, as shown in Figure 4b. The area of the Taiping Outlet mid-channel bar reduced greatly after 2013, which indicated that the scouring amplitude of the main stream riverway near Taiping Outlet increased and the stage of the Jingjiang River main stream at the same discharge level decreased, causing stage reduction at Taiping Outlet and further reduction in diversion and sand division at Taiping Outlet, with an annual diversion ratio of 1.52% by 2019.

4.1.3. Ouchi Outlet

Ouchi Outlet lies in Tianxingzhou waterway at the end of the upper Jingjiang reach. The riverbed of Tianxingzhou waterway contains medium and fine sand, with great mobility of bed sand. In the dry season, the main stream transits from the deep channel close to the right bank to the deep channel close to the left bank through the Maolinkou mid-channel bar, and the position of the erosion ditch rises and falls along with the diversion of Ouchi Outlet and shows cyclic change from year to year. In a big flood year, the diversion volume of Ouchi Outlet is large, the right branch is developed, and the transition section is below the head of Zhengjiahe, which easily forms by crossing the ditch, with the position of the erosion ditch rising in small flood years. Affected slightly by the diversion at Ouchi Outlet, the time for flow return is long and the flow easily bends to form a braided river. In the case of medium flood years, the left branch can still develop into the main branch, and the top scouring point of the right branch moves downward under the action of the current flow, but the flow is not so smooth. In the case of big flood years, the diversion volume of Ouchi Outlet is large, the right branch development bends downward again, and the left branch deposits to finish the evolution of the main waterway from top down. Tianxingzhou waterway features an unfixed main channel and multiple float-off points. From the perspective of beneficial diversion of Ouchi Outlet, if the main stream of Tianxingzhou waterway is close to the right bank, it means the scouring development of the right branch and the swing of the main stream are close to Ouchi Outlet at the right bank, which is beneficial for improving the diversion capacity; if the right branch deposits in small floor years, the main stream changes to the left branch, which is unfavorable for diversion.

After the Three Gorges Reservoir was put into operation, Tianxingzhou separated from the mid-channel bar at the head, and the position is relatively stable (Figure 5). In 2002, the mid-channel bar at the head of Tianxingzhou was small in area (30 m elevation) and located in the middle of the riverway; in 2006, the head of Tianxingzhou and the mid-channel bar at the head moved backward and moved toward the right bank under the influences of current scour, with the area of the mid-channel bar at the head expanding to 1.536 million m²; in 2011, the right border of the mid-channel bar at the head continued to move backward due to scouring, and the left border widened and extended upstream due to deposition, merging with several scattered small mid-channel bars upstream to form a large mid-channel bar, so the mid-channel bar at the head expands greatly; in 2016, the mid-channel bar at the head witnessed small changes on the whole, extended slightly toward the upstream, and the mid-channel bar area increased to 2.773 million m², forming a very small new mid-channel bar in the upstream; in 2018, the upper mid-channel bar expanded due to deposition, the lower mid-channel bar shrank due to scouring, and the mid-channel bar area reduced from 2.773 million m² to 2.48 million m² compared with 2016. Changes in the characteristic values of the mid-channel bar at the head of Tianxingzhou in recent years are detailed in

Table 3. Changes in the characteristic values of the mid-channel bar at the head of Tianxingzhou (below 30 m elevation).

Time	The Largest Bar Length (km)	The Largest Bar Width (km)	Area (km2)	Top Elevation (m)
2002.10	1.69	0.74	1.07	33.6
2004.7	2.49	0.55	0.93	33.2
2006.6	2.73	0.89	1.54	33.9
2011.11	3.81	0.88	2.58	33.1
2016.10	5.59	0.89	2.77	34.7
2018.10	1.28	0.43	0.43	32.6
	4.26	0.81	2.48	33.6

.

The regime evolution process of Tianxingzhou waterway at Ouchi Outlet from 2002 to 2018 after the operation of the Three Gorges Reservoir was analyzed. The Tianxingzhou mid-channel bar gradually got close to Ouchi Outlet with the swing of the main stream, the main stream developed close to the left bank, and the right branch gradually deposited. Specifically around the year 2011, the area of the mid-channel bar increased obviously, which indicated that the right bank of the Jingjiang River main stream at Ouchi Outlet showed an obvious deposition trend and the main stream development was unfavorable for the diversion of Ouchi Outlet. Existing studies [25] selected two adjacent years, 1982 and 1983, as the typical years to analyze the influences of the changes in the main stream regime on the diversion capacity of the three outlets. In the two adjacent typical years, the changes in erosion and deposition at Ouchi Outlet were not so great, the main stream swung from the right branch to the left branch, and the changes in diversion volume in 1982 and 1983 were small, within 1%, reflecting to a certain extent that the changes in the main stream had a small influence on the diversion capacity.

To sum up, the main stream regime of the three outlets along Jingjiang River had certain impacts on the diversion capacity of the floodway at the three outlets (Supplementary Material S1), to varying modes and degrees [26,27]. After the operation of the Three Gorges Reservoir, the scouring of the Jingjiang River main stream continued. If Lijiadu shoal did not change greatly, the diversion and sand division ratio of Songzi Outlet will still keep relatively stable; as the area of the Taiping Outlet mid-channel bar served as an indication for the erosion and deposition status of the main stream riverway of Taiping Outlet, downward scouring and shrinkage showed that the Taiping Outlet waterway of the main stream continued to be scoured, where the reduced stage of the main stream at the same discharge level caused the diversion capacity of Taiping Outlet to decrease, tending to maintain seasonal diversion outlet; the left branch of Tianxingzhou waterway will be further scoured and the right branch will be deposited at Ouchi Outlet, which will further cut the diversion capacity of Ouchi Outlet, maintaining the seasonal diversion outlet. Generally speaking, the deposition of the three outlets and the stage reduction in the same discharge level caused by scouring of the main stream reach are the main modes of regime evolution affecting the diversion capacity of the three outlets. The slight swing of the main stream and the changes in diversion angle have a slight influence on the diversion capacity of the three outlets.

4.2. Changes in Stage–Discharge Relation of Jingjiang River Main Stream

After storage of the Three Gorges Reservoir, the downstream riverbed of the dam was scoured along the course, particularly fierce in Yizhi reach and Jingjiang reach close to the downstream of the dam. Scouring caused the riverbed downcutting and the discharge cross-section area to increase, with particular obvious scouring of the minor bed, which led to a decline in the low water stage along the course to varying degrees. It can be seen by analyzing the changes in the stage–discharge relation and stage of the same discharge level of Zhicheng and Shashi (as shown in Figure 6 and Figure 7) that, after the operation of the Three Gorges Reservoir, the scouring of Yichang–Shashi reach caused a decline of the low water stage, while the change trend of the medium and high stages was non-significant.

Han Jianqiao [28] used time series analysis to analyze the changes in the flood stage and low water stage before and after storage of the Three Gorges Reservoir, and found that the results were consistent with the qualitative analysis: the low water stage of each station in the midstream after storage of the Three Gorges Reservoir showed a declining trend, while the flood stage had no obvious declining trend. He also considered that the decline of the low water stage is because the stage decline caused by the riverbed downcutting in the dry season conditions was greater than the stage rise caused by the channel rate increase due to riverbed armoring and other factors in the same period; they were close at the flood stage, so the low water stage showed a declining trend, while the flood stage had no great change. The separation of variables was employed to separate the factors affecting the stage change at the same discharge. The results indicated that scouring is the main factor which affects the low water stage decline at the same discharge of the Yichang–Chenglingji reach. After 2009, the scouring intensity of Yizhi River decreased and its effect on promoting low water stage decline was weakened; the scouring intensity of Jingjiang River rose and its effect on promoting low water stage decline was enhanced. However, the main factor controlling the flood stage at the same discharge is the increase in the comprehensive channel rate of the riverbed. After 2009, the armoring of Yizhi reach was basically completed and its effect on the flood stage at the same discharge was weakened, while the armoring of the Jingjiang riverbed was still in progress, so the influence of the increasing comprehensive channel rate on the rise in the flood stage was increasing.

To sum up, after the operation of the Three Gorges Reservoir, the low water stage at the same discharge level of the main stream reach of Jingjiang River declined, while the change trend of the stage at medium and high flood discharge was non-significant. As the main factor of the regime change affecting the diversion capacity of the three outlets, the stage of the main stream at the same discharge level, esp. the medium and high stages, changed slightly, which is favorable to maintaining the diversion capacity and diversion volume of the three outlets; the influence of the decline of the low water stage at the same discharge level on the diversion capacity of the three outlets also involves analyzing the changes in the low water stage of the floodway at the three outlets apart from the main stream riverway. If the change amplitude of the two is inconsistent, it means that downcutting appeared at both the main stream and the floodway at the three outlets due to scouring, which has a slight influence on the diversion capacity; if the change amplitude of the low water stage of the main stream is greater than the floodway, the diversion capacity declines, otherwise, it rises.

4.3. Changes in Low Water Stage of Floodway at Three Outlets

4.3.1. Songzi Outlet

We analyzed the range of low water level variations at Zhicheng and Xinjiangkou stations from 2002 to 2016. Using 2002 as the baseline year, select flow rates below 10,000 m³/s at Zhicheng Station and corresponding flow rates below 450 m³/s at Songzikou. Calculate the changes in the low water level at Zhicheng and Xinjiangkou stations for identical flow levels. The results are presented in Figure 8. Generally speaking, the change amplitude of the low water stage of the Jingjiang River main stream around 10,000 m³/s of Zhicheng Station and 450 m³/s around Songzi Outlet is divided into two phases. From 2002 to 2012 around, the change amplitude of the low water stage of the main stream and Songzi Outlet floodway was basically consistent, whereas the decline amplitude of the low water stage for Xinjiangkou at the some discharge levels was even higher than the stage decline of Zhicheng Station; after 2012, the decline amplitude of the water stage of Songzi Outlet gradually weakened and even rose, while the low water stage of Zhicheng of the Jingjiang River main stream was still in a declining trend. Therefore, from 2002 to 2012 around, Zhicheng reach of the Jingjiang River main stream and Songzi Outlet floodway were scoured at the same time, with basically consistent scouring downcutting amplitude, and the low water stage change in the Jingjiang River main stream at the same discharge level in that period had little influence on the diversion capacity of Songzi Outlet; after 2012, the scouring downcutting amplitude of Zhicheng reach of the Jingjiang River main stream was obviously higher than that of Songzi Outlet floodway, and the low water stage decline of the Jingjiang River main stream at the same discharge level in that period caused the diversion capacity of Songzi Outlet to decrease.

4.3.2. Taiping Outlet

We analyzed the range of low water level variations at Shashi Station and Mituosi stations from 2002 to 2016. Using 2002 as the baseline year, select flow rates below 10,000 m³/s at Shashi Station and corresponding flow rates below 130 m³/s at Mituosi. Calculate the changes in the low water level at Shashi and Mituosi stations for identical flow levels. The results are presented in Figure 9. After the operation of the Three Gorges Reservoir, between the change amplitudes of the low water stage of the Jingjiang River main stream at Shashi Station below 10,000 m³/s and Mituosi Station below 130 m³/s, that of Shashi Station was always more obvious, and around 2012, the low water stage of both Shashi Station and Mituosi Station began to show a great declining trend. Generally speaking, the scouring downcutting amplitude of the Taiping Outlet floodway was much smaller than that of Shashi reach. In particular, around 2012, the low water stage declining trend of Shashi Station was more obvious, so for the diversion capacity of Taiping Outlet, the low water stage decline of the Jingjiang River main stream at the same discharge level caused the diversion capacity of Taiping Outlet to decrease, which was more obvious after 2012.

4.3.3. Ouchi Outlet

We analyzed the range of low water level variations at Shashi Station and Guanjiapu stations from 2002 to 2016. Using 2002 as the baseline year, select flow rates below 10,000 m³/s at Shashi Station and corresponding flow rates below 50 m³/s at Ouchi Outlet. Calculate the changes in low water level at Shashi and Guanjiapu stations for identical flow levels. The results are presented in Figure 10. Generally speaking, the change amplitude of the low water stage of the Jingjiang River main stream at Shashi Station below 10,000 m³/s and Ouchi Outlet below 50 m³/s showed two trends: the low water stage of Shashi Station at the same discharge level showed an obvious declining trend, while that of Ouchi Outlet had no significant change amplitude. As a result, the scouring downcutting of the riverway at Shashi reach of the Jingjiang River main stream at low water discharge was obvious, while the downcutting amplitude of the Ouchi Outlet floodway was far lower than the main stream. This is also the main reason for the sustained decrease in the diversion capacity and sustained increase in the cutoff duration of Ouchi Outlet after storage of the Three Gorges Reservoir.

To sum up, the low water erosion and deposition laws of the floodway at the three outlets exhibit differences after the declining amplitudes of the low water stage of the Jingjiang River main stream and the floodway at the three outlets at the same discharge level after the operation of the Three Gorges Reservoir was analyzed. At the Songzi Outlet, the scouring downcutting amplitude of the Jingjiang River main stream was basically consistent from 2002 to 2012, so the diversion capacity at the low water discharge level changed slightly; after 2012, the scouring trend was obviously weaker than the main stream, and the diversion capacity at the low water discharge level declined; the scouring downcutting amplitudes of the Taiping Outlet and Ouchi Outlet floodways were always weaker than the main stream, and the trend was more obvious after 2012; therefore, the diversion capabilities of Taiping Outlet and Ouchi Outlet at the low water discharge level gradually decreased, mainly embodied by diversion ratio reduction and cutoff duration extension.

4.4. Change Features of Cutoff Duration of Three Outlets

After the operation of the Three Gorges Reservoir, the cutoff duration of the three outlets also increased with the reduction in the diversion ratio. For example, the annual average cutoff duration of Shadaoguan—the east branch of Songzi River—was 178 days in 1981~2002, and increased to 188 days after storage (2003~2018), as shown in

Table 4. Average cutoff duration of three outlets in different periods over years.

Period	Annual Average Cutoff Duration over the Years				Corresponding Discharge of Zhicheng at the Time of Cutoff (m3/s)
	Shadaoguan	Mituosi	Ouchi (Guan)	Ouchi (Kang)	Shadaoguan	Mituosi	Ouchi (Guan)	Ouchi (Kang)
1956–1966	0	35	17	213	/	4290	3930	13,100
1967–1972	0	3	80	241	/	3470	4960	16,000
1973–1980	71	70	145	258	5330	5180	8050	18,900
1981–1998	167	152	161	251	8590	7680	8290	17,600
1999–2002	189	170	192	235	10,300	7650	10,300	16,500
2003–2018	188	140	180	272	9880	7220	9130	15,900

and Figure 11. In 2003~2012, the annual average cutoff durations of Shadaoguan, Mituosi, Ouchi (Guan), and Ouchi (Kang) were 188, 140, 180, and 272 days, respectively, which greatly reduced the discharge of the reservoir in the main storage period of the Three Gorges Reservoir (September and October), making the annual average cutoff durations of Shadaoguan, Mituosi, Ouchi (Guan), and Ouchi (Kang) in September and October increase from 6, 0, 4, and 25 days in 1999~2002 to 11, 2, 9, and 40 days in 2003~2018.

5. Analysis of the Causes of the Changes in the Diversion Volume of the Three Outlets Along Jingjiang River

The diversion volume of the three outlets along Jingjiang River is jointly decided by the diversion capacity of the floodway at the three outlets and the annual runoff (runoff volume and runoff process) of the Jingjiang River main stream. The diversion capacity of the floodway at the three outlets along Jingjiang River, i.e., the change characteristics of diversion ratio and the causes, was analyzed above. The diversion ratio during 2002–2012 was close to that during 1981–2002 and showed a declining trend after 2012 (Supplementary Material S2). So the causes for the changes in the diversion volume of the three outlets are divided into two periods: First, in 2002~2012, the diversion ratio of the three outlets remained unchanged, and the main factors affecting the diversion volume of the three outlets included (1) changes in the annual runoff of upstream incoming flow upward of Yichang; and (2) the changes in the duration of the medium and high discharge levels due to the process of changing the main stream runoff and stage of Jingjiang River caused by controlling reservoirs such as the Three Gorges Reservoir. Second, after 2012, the diversion ratio of the three outlets weakened. Through the above analysis, the main factor for such weakening was the low water stage decline at the same discharge level caused by scouring downcutting of the riverway of the Jingjiang River main stream, so the main factors affecting the diversion volume of the three outlets included (1) decreased diversion capacity; (2) changes in annual runoff of upstream incoming flow upward of Yichang; and (3) changes in the duration of the medium and high discharge levels due to the process of changing the main stream runoff and stage of Jingjiang River caused by controlling reservoirs such as the Three Gorges Reservoir.

5.1. Runoff Changes in Jingjiang River

After storage of the Three Gorges Reservoir, in 2003~2018, the annual average runoff of Yichang Station was less, 27.7 billion m³, as the upper reaches of Yangtze River suffered from the low water hydrologic cycle, and the runoff of Zhicheng Station and Shashi Station showed a certain decrease compared with that before storage as the upper Jingjiang reach was affected by reduced incoming flow of Yichang, as shown in

Table 5. Changes in annual average runoff of upper Jingjiang reach (unit: billion m³).

Period	Yichang	Zhicheng	Shashi
Before 1990	434.3	448.1	392
1991–2002	428.7	433.8	399.6
2003–2018	409.2	418.8	383.1
Change rate 1	−5.8%	−6.5%	−2.3%
Change rate 2	−4.5%	−3.5%	−4.1%

. The change characteristics of the diversion ratio of the three outlets along Jingjiang River was analyzed, as shown in Figure 12. The diversion capacity of three outlets improved gradually along with the increase in main stream discharge, so the more days of medium and high discharge in the annual runoff process of Jingjiang River main stream, the larger the annual diversion volume of the three outlets under the same diversion capacity will be, and vice versa. The reservoir inflow runoff process not affected by the regulation of the Three Gorges Reservoir was further analyzed. It can be seen from

Table 6. Annual average duration of inflow of Three Gorges Reservoir at different discharge levels in varying series years (unit: days).

Period	Inflow Runoff of the Three Gorges Reservoir
	10,000 < Q < 20,000 (m3/s)	20,000 < Q < 30,000 (m3/s)	30,000 < Q < 40,000 (m3/s)	40,000 < Q (m3/s)	Annual Average Runoff (Unit: Billion m3)
Before 1990	95	56	28	9	434.3
1991–2002	91	57	27	12	428.7
2003–2018	79	49	14	5	409.2

that, in 2002~2018, the duration of medium and high discharge in the inflow runoff process of the Three Gorges Reservoir showed a certain decrease compared with that before 2002. This was because several controlling cascade reservoirs were gradually constructed and put into operation, and it also reflected the weakening effect of the low water hydrologic cycle on the medium and high discharge of Jingjiang River. It can be seen by comparing the annual runoff process of Zhicheng Station (as shown in

Table 7. Annual average duration of Zhicheng Station of Jingjiang River at different discharge levels in varying series years (unit: days).

Period	Zhicheng					Three Outlets (Unit: Billion m3)
	10,000 < Q < 20,000 (m3/s)	20,000 < Q < 30,000 (m3/s)	30,000 < Q < 40,000 (m3/s)	40,000 < Q (m3/s)	Annual Average Runoff (Unit: 0.1 Billion m3)
Before 1990	84	57	28	15	4481	69.8
1991–2002	88	58	26	16	4338	62.5
2003–2018	105	40	16	8	4188	47.6

) that the medium and high discharge in the runoff process of the Jingjiang River main stream after the regulation of the Three Gorges Reservoir also showed a decrease in medium and high discharge duration compared with that before 2002, and due to the decrease in large discharge caused by the storage period of the Three Gorges Reservoir, the duration of high discharge may be reduced, causing medium and low discharge duration to increase, such as the large increase in the duration of Zhicheng Station at 10,000~20,000 m³/s compared with that before storage. Therefore, from a qualitative perspective, after the operation of the Three Gorges Reservoir, the runoff of Jingjiang River will lead to a decrease in the diversion volume of the three outlets under the same diversion capacity whether from the change trends of the annual runoff volume or annual runoff process.

5.2. Quantitative Analysis of Influencing Factors of Diversion Volume

The diversion volume of the three outlets along Jingjiang River exhibits a good fitting relationship with the discharge process of Zhicheng Station and Shashi Station of the Jingjiang River main stream, so the correlation between main stream discharge and diversion discharge can be used to calculate the diversion process of the three outlets under different main stream runoff processes in series years in which the change in diversion capacity (diversion ratio) of the three outlets is not so great, so as to quantitatively analyze the proportion of influences imposed by the runoff reduction in Jingjiang River, the regulation action of the reservoir, and the decline in the diversion capacity under the new flow and sediment conditions.

This paper conducts an attribution analysis of the diversion reduction in the three outlets under the new situation by comparing the flow characteristics of different levels of abundance and scarcity, the influence of reservoir regulation, and diversion capabilities (the approach is shown in Figure 13). The specific analysis process is as follows: Firstly, the change in the diversion ratio of the three outlets along Jingjiang River under the new flow and sediment conditions is divided into two phases: phase I: 2002~2012 (diversion capacity changed slightly compared with that before storage); phase II: 2013~2018 (diversion capacity showed a declining trend compared with that before storage). The diversion ratio of the three outlets along Jingjiang River in phase I is not great, so the proportion of influences imposed by the runoff reduction in Jingjiang River and the regulation action of the Three Gorges Reservoir on the diversion volume decrease in the three outlets can be quantitatively analyzed.

5.2.1. Phase I: 2002~2012

The diversion volume of the three outlets along Jingjiang River under natural conditions and without the regulation action of the Three Gorges Reservoir in 2003~2012 (calculated value) was calculated and compared with the measured diversion volume (measured value), as shown in

Table 8. Comparison between the calculated diversion volume of the three outlets along Jingjiang River under natural conditions and the measured value in 2002~2012 (unit: billion m³).

Year	Annual Runoff of Songzi Outlet		Annual Runoff of Taiping Outlet		Annual Runoff of Ouchi Outlet		Total Runoff of Three Outlets		Measured—Calculated
	Measured Value	Calculated Value	Measured Value	Calculated Value	Measured Value	Calculated Value	Measured Value	Calculated Value
2003	32.62	35.14	10.77	11.31	14	14.4	57.38	60.84	−3.45
2004	31	31.13	10.34	10.35	10.94	10.84	52.18	52.22	−0.04
2005	37.67	37.64	12.25	12.41	14.46	14.12	64.38	64.17	0.21
2006	11.92	14.68	3.43	4.28	2.91	3.36	18.26	22.32	−4.06
2007	31.79	32.55	9.97	10.35	12.6	13.42	54.36	56.32	−1.96
2008	31.32	33.94	9.87	10.69	11.69	12.96	52.87	57.59	−4.72
2009	26.34	29.29	8.67	9.71	9.47	10.85	44.49	49.84	−5.36
2010	32.19	36.15	10.7	12.06	13.71	16.03	56.6	64.23	−7.63
2011	18.65	23.95	4.79	6.14	4.47	6.13	27.91	36.22	−8.31
2012	25.5	32.75	6.55	8.4	6.11	8.38	38.16	49.53	−11.36
Average	27.9	30.72	8.73	9.57	10.04	11.05	46.66	51.33	−4.67

. In the table, during preliminary storage of the Three Gorges Reservoir, the measured diversion volume did not bear great difference from the calculated value. In 2003, due to the storage action of the reservoir from 70 m to 135 m for nearly 20 days from late May to early June, the influence on the high discharge of that period was significant, so the measured diversion volume of the three outlets bore great difference from the calculated value, but the proportion of difference in the actual total diversion was still non-significant; after the experimental high-stage operation of the reservoir, the influence of the regulation action of the Three Gorges Reservoir changing the downstream runoff process on the three outlets began to be obvious; in 2008~2011, the difference between the measured diversion volume of the three outlets and the calculated value began to increase obviously, and the influence of the reservoir changing the runoff process on the diversion of the three outlets was non-negligible. The annual average diversion volume of the three outlets along Jingjiang River in 2003~2012 reduced by 13.21 billion m³ compared with 1999~2002, of which a reduction of 4.67 billion m³ was due to the regulation of the Three Gorges Reservoir, taking up 35% of the total reduction. Therefore, after the storage of the Three Gorges Reservoir, under no significant change in the diversion capacity, the diversion volume reduction in the three outlets along Jingjiang River was jointly caused by the regulation of the Three Gorges Reservoir and the reduction in the incoming flow runoff of Jingjiang River. As can be seen from the proportion, the regulation of the reservoir took up 35%, while the less significant runoff of Jingjiang River took up 65%.

5.2.2. Phase II: 2013~2018

In order to calculate the influences of diversion ratio change on the diversion of the three outlets, two further conditions based on the diversion volume change in 2013~2018 were calculated: First, the diversion capacities of the three outlets in 2003~2012 and the actual runoff process of Jingjiang in 2013~2018 were used to calculate the diversion volume of the three outlets under the condition of unchanged diversion capacity, which were then compared with the measured diversion volume of the three outlets in 2013~2018, so as to analyze the influences imposed by the diversion capacity change in the three outlets on the diversion volume of the three outlets in phase II, as shown in

Table 9. Comparison between diversion volume of three outlets under unchanged diversion capacity and measured diversion volume in 2013~2018 (unit: billion m³).

Year	Annual Runoff of Songzi Outlet		Annual Runoff of Taiping Outlet		Annual Runoff of Ouchi Outlet		Total Runoff of Three Outlets		Measured— Calculated
	Measured Value	Calculated Value	Measured Value	Calculated Value	Measured Value	Calculated Value	Measured Value	Calculated Value
2013	20.59	20.79	5.29	5.34	4.93	4.98	30.81	31.12	−0.31
2014	25.16	25.53	6.46	6.56	6.03	6.12	37.64	38.21	−0.56
2015	21.63	22.01	5.55	5.65	5.18	5.28	32.36	32.95	−0.58
2016	23.36	23.57	6	6.05	5.6	5.65	34.95	35.27	−0.31
2017	24.19	24.38	6.21	6.26	5.8	5.84	36.19	36.48	−0.29
2018	25.92	26.23	6.66	6.74	6.21	6.29	38.78	39.25	−0.47
Average	23.47	23.75	6.03	6.1	5.63	5.69	35.12	35.55	−0.42

. Second, the diversion volume of the three outlets along Jingjiang River under natural conditions and without the regulation action of the Three Gorges Reservoir in 2013~2018 (calculated value) was calculated and compared with the measured diversion volume (measured value), as shown in

Table 10. Comparison between the calculated diversion volume of the three outlets along Jingjiang River under natural conditions and the measured value in 2013~2018 (unit: 0.1 billion m³).

Year	Annual Runoff of Songzi Outlet		Annual Runoff of Taiping Outlet		Annual Runoff of Ouchi Outlet		Total Runoff of Three Outlets		Measured—Calculated
	Measured Value	Calculated Value	Measured Value	Calculated Value	Measured Value	Calculated Value	Measured Value	Calculated Value
2013	20.59	26.44	5.29	6.78	4.93	6.77	30.81	39.98	−9.17
2014	25.16	32.3	6.46	8.28	6.03	8.27	37.64	48.85	−11.21
2015	21.63	27.77	5.55	7.12	5.18	7.11	32.36	42	−9.64
2016	23.36	29.99	6	7.69	5.6	7.68	34.95	45.36	−10.41
2017	24.19	31.06	6.21	7.96	5.8	7.95	36.19	46.97	−10.78
2018	25.92	33.28	6.66	8.53	6.21	8.52	38.78	50.33	−11.55
Average	23.47	30.14	6.03	7.73	5.63	7.71	35.12	45.58	−10.46

.

The annual average diversion volume of the three outlets along Jingjiang River in 2013~2018 reduced by 16.35 billion m³ compared with 1999~2002, of which a reduction of 10.4 billion m³ was due to the regulation of the Three Gorges Reservoir, taking up 63% of the total reduction; a reduction of 0.42 billion m³ was due to the decreased diversion capacity of the three outlets, taking up 2.5% of the total reduction; the runoff reduction in Jingjiang River main stream took up 34.5%. Consequently, the Three Gorges Reservoir could consider increasing its minimum discharge to augment water resources in Dongting Lake.

6. Conclusions

First, after the impoundment of the Three Gorges Reservoir, the diversion characteristics of the three outlets along Jingjiang River showed two periodic changes: the change trend of the diversion ratio of the three outlets along Jingjiang River in 2003~2012 was non-significant compared with that before storage; and the diversion ratio of the three outlets along Jingjiang River in 2013~2018 began to show a declining trend. There are three causes for periodic changes:

(1)

The evolution of the main stream channel near the outlets differentially affected their diversion capacity. While Lijiadu shoal (Songzi) remained stable, Taipingkou’s mid-channel bar shrinkage and Ouchikou’s Tianxingzhou branch scouring (left)/deposition (right) reduced their capacities. The dominant mode affecting the capacity was deposition within the outlets and stage reduction due to main stream scouring; main stream swing and diversion angle changes had minor influences.

(2)

The decline in the low water stage at identical discharges impacted the diversion capacity, unlike stable medium/high stages. The key factor is the relative low water stage change between the main stream and the outlet floodways. Consistent scouring with a similar amplitude had a minor impact. However, if the main stream stage decline is greater than that in the floodway, the diversion capacity reduces. Conversely, if the floodway stage decline is greater, the diversion capacity increases.

(3)

The low water scouring patterns varied significantly among the outlet floodways. At the Songzi Outlet, scouring downcutting matched the main stream amplitude until approximately 2012, resulting in stable diversion capacity; however, post-2012, its scouring trend weakened relative to the main stream, leading to a decline in capacity. In contrast, the floodways of Taipingkou and Ouchikou consistently exhibited weaker scouring downcutting than the main stream, a trend that intensified after 2012; this caused a progressive reduction in their low water diversion capacity, manifested as lower diversion ratios and extended periods of flow cutoff.

Finally, the diversion volume of the three outlets along Jingjiang River is jointly decided by the diversion capacity of the floodway at the three outlets and the annual runoff (runoff volume and runoff process) of the Jingjiang River main stream. After the storage of the Three Gorges Reservoir, under no significant change in the diversion capacity and non-significant regulation action of the reservoir (failing to reach the normal storage level of 175 m), the diversion volume reduction in the three outlets along Jingjiang River was jointly caused by the regulation of the Three Gorges Reservoir and the reduction in the incoming flow runoff of Jingjiang River. As can be seen from the proportion, the regulation of the reservoir took up 35%, while the less significant runoff of Jingjiang River took up 65%. Under reduced diversion capacity and significant regulation action of the reservoir (normal storage level of 175 m), a reduction of 10.4 billion m³ was due to the regulation of the Three Gorges Reservoir, taking up 63% of the total reduction; a reduction of 0.42 billion m³ was due to the decreased diversion capability of three outlets, taking up 2.5% of the total reduction; the runoff reduction in Jingjiang River main stream took up 34.5%.