Damage Inflicted by Extreme Drought on Poyang Lake Delta Wetland and the Establishment of Countermeasures

Abstract

Due to the joint influence of climate change and human activities, the hydrological rhythm of Poyang Lake has changed in recent years, leading to an increasingly severe drought problem during autumn and winter in this region. Notably, the extreme drought that occurred in 2022 had profound impacts on shipping, water supply and the ecological environment of the wetlands in the Poyang Lake Delta, sparking widespread concern. Based on the historical hydrometeorological data of Poyang Lake, we used statistical models (such as Chow test, correlation analysis, etc.) to analyze the cause of the extreme drought in the Poyang Lake Delta from the perspectives of natural factors and human activity. Through correlation analysis, we found that the water level, discharge, and drought duration of the Poyang Lake Delta were all significantly affected by climate change, particularly rainfall in the Poyang Lake basin. Furthermore, combining the results of Chow test and correlation analysis, we also found that the operation of the Three Gorges Reservoir had a notable impact on the water level of the Poyang Lake Delta. Based on remote sensing images, ecological and environmental sampling monitoring, the damage inflicted by the extreme drought event on the Poyang Lake Delta was analyzed. The results show that the inundated area of the delta wetlands in the extreme-drought year (2022) decreased by 45.75% compared with that in a normal year (2017). In addition, the ecological environment of the wetlands deteriorated significantly. The water quality parameters (TN, TP, NH₄⁺-N) increased by 50.2%, 240% and 64.7%, respectively. The concentrations of TN and TP were 3.8 mg/L and 0.17 mg/L, respectively, while the context values in the delta were 1.2 mg/L and 0.075 mg/L. The density and biomass of algae increased by 87.2% and 557.9%, respectively. In contrast, the density and biomass of benthos decreased by 59.9% and 78.5%, respectively. The control strategy for the Poyang Lake Delta under extreme drought was studied through an experiment on the operation of hydraulic controllers. The results show that under extreme drought conditions, the newly built hydraulic controllers could raise the water level of the delta from 9.1 ± 0.7 m to 14.2 ± 1.8 m, thus effectively solving the water cut-off problem in the four branches of the delta. Furthermore, by adjusting the distributive ratio of the main, north, middle and south branches of the delta to 50%, 4%, 24% and 22% through newly built hydraulic controllers, the water area can be increased by 56%.

1. Introduction

As one of the most severe natural disasters in the world, drought is a prevalent topic in the literature. Under the background of global warming, global drought is becoming increasingly serious. From 1950 to 2008, the area of land affected by global drought increased by 1.74 percent every 10 years, and Africa, Eurasia, Australia and South America all showed drought trends. The average annual economic loss caused by drought worldwide was USD 17.33 billion from 1980 to 2009, and this increased to USD 23.125 billion from 2010 to 2017, far exceeding the rate of loss from other meteorological disasters [1]. This increasing drought trend will become more significant in the coming decades, and even extreme drought events will occur [2]. For example, extreme drought events in the Amazon basin in 2005 and 2010 resulted in the death of many trees, resulting in carbon emissions of 2.2 × 10⁹ t [3]. In 2016, South Africa was affected by extreme drought that put 14 million people at risk of food insecurity [4]. Drought is also one of the most prevalent natural disasters in China. Drought accounts for about one-third of its natural disasters each year, and loss caused by droughts accounts for more than 15% of all losses. Since the 1970s, the average annual area of crops affected by drought is 2.09 × 10⁷ hm², with the highest being 4.05 × 10⁷ hm². The average area of land affected by drought is 8.87 × 10⁶ hm², and the highest is 2.68 × 10⁷ hm². Each year, the annual reduction in grain output ranges from a few million to 30 million tons, and the direct economic losses caused by drought are as high as CNY 44 billion [5]. Poyang Lake is located on the south bank of the middle and lower reaches of the Yangtze River. As the largest freshwater lake in China, it is an important strategic water source for the middle and lower reaches of the Yangtze River and an important ecological barrier for the Yangtze River [6]. In recent years, due to the continuous adjustment of the relationship between the rivers and lakes in the middle reaches of the Yangtze River, the hydrological rhythm of Poyang Lake has changed. The lake has begun to enter the dry season earlier, the duration of which has been extended, and the water level in the dry season has dropped rapidly, hitting a record low [7]. In 2022, the Yangtze River Basin experienced a rare event of “returning to dry in flood season”. On 23 September 2022, the water level of Poyang Lake dropped to 7.1 m, a record low since records began in 1951. The area of Poyang Lake shrunk to 244 km², and the water storage capacity was only 7.8 × 10⁸ m³. The whole lake had almost dried up. By the end of October 2022, the number of people in need of assistance due to the extreme drought event in Jiangxi Province had reached more than 407,000, including more than 19,000 people experiencing difficulty obtaining drinking water; 5.306 million people in 1504 townships (towns) across the province were affected, and serious damage was inflicted on the ecological environment [8]. Therefore, it is urgent to study the causes, damages and countermeasures of extreme drought events in Poyang Lake.

In recent years, numerous studies have focused on drought conditions in Poyang Lake and Dongting Lake, which are the two largest river-connecting lakes in the middle reaches of the Yangtze River. Li et al. [9] analyzed the correlation between drought and large-scale climate indices, and found that the changes in large-scale climate indices significantly influenced the drought in Poyang Lake, especially in autumn and winter. Zhang et al. [10] analyzed the joint probability of concurrent drought events between Poyang Lake, the catchment and Yangtze River based on a statistical approach. Zhang et al. [11] analyzed the spatial and temporal distribution of drought in Poyang Lake, exploring the influence of river–lake interaction with water impounding of the Three Gorges Dam. Guo et al. [12] quantified and evaluated the effects of the Three Gorges Dam operation and climate change on drought potential in Poyang Lake. Zhai et al. [13] found that the water conservancy projects largely aggravated the droughts in Dongting Lake, as the frequency of droughts increased by 6.81% following the construction of the Three Gorges Dam. Liu et al. [14] pointed out that the drought in Dongting Lake was intensified after the impoundment of TGD as the dry period was extended from 123 days/year before the operation of TGD (1981–2002) to 141 days/year (2003–2016) on average. Overall, these previous studies have promoted the understanding of the cause of drought in Poyang Lake and Dongting Lake, by analyzing the hydrological and meteorological data.

The Ganjiang River is the largest river flowing into Poyang Lake. Its runoff and sediment discharge account for 62% and 66.7% of the total inflow of the five rivers flowing into this lake (the Ganjiang, Fuhe, Xinjiang, Raohe and Xiuhe Rivers), forming the Poyang Lake Delta [15]. In recent years, due to the combined impact of climate change and human activities, the water and sediment conditions of the rivers in the Poyang Lake Delta have changed. The annual lowest water level decreased, and the low water period was advanced and extended. During the low water period, some rivers were cut off, the water surface area decreased, and large areas of beaches were exposed for extended periods of time (Figure 1), resulting in a series of problems, such as poor navigation conditions, difficulties in utilizing water resources, reduced water supply security, and degradation of the ecological environment.

Due to the complex multistage bifurcation structure of the rivers in the Poyang Lake Delta, previous studies have focused little on the damage of droughts in this region and have not proposed countermeasures to deal with drought. This lack of research is not conducive to mitigating drought in the delta region of Poyang Lake, which is an important water source and wetland protection area in Nanchang, the provincial capital city of Jiangxi Province.

Compared with previous studies about Poyang Lake droughts, this study has the following three novelties: (1) this study focuses on the latest extreme drought event in the Poyang Lake Delta in 2022 and analyzes the causes of the extreme drought event based on the latest hydrological and meteorological data; (2) By using remote sensing observation and on-site monitoring, this study provides comprehensive analysis of the damage inflicted by the extreme drought on the water supply, navigation and ecological environment in the delta region; (3) Through physical model experiments, this paper explores how to manage extreme drought events in the delta region by regulating hydraulic controllers. Our research results could provide guidance for the protection and restoration of the aquatic ecological environment of Poyang Lake under the threat of drought, and serve as a reference for the development of engineering countermeasures in the Lake Delta region during drought conditions.

2. Materials and Methods

2.1. Study Area

The Poyang Lake Basin is located in the humid monsoon-climate zone in Southeast China, on the South Bank of the middle and lower reaches of the Yangtze River, and connected to the Yangtze River through Poyang Lake. It is the largest freshwater lake in China. Of this area, 156,700 km² falls within Jiangxi Province, accounting for 96.6% of the entire Poyang Lake water system. The lake basin is low and flat and surrounded by hills. It is connected to the water from the Ganjiang, Fuhe, Xinjiang, Raohe and Xiushui Rivers, and flows into the Yangtze River through the lake outlet after regulation and storage, forming a complete water system with Poyang Lake as the focal point. It is a flowing, fluctuating and seasonal lake.

Poyang Lake is shaped similarly to a gourd and is divided into north and south parts, with Songmen Mountain at the boundary. The south, i.e., the main lake area, is broad and shallow. The north is narrow, long and deep, and is the waterway leading into the Yangtze River. The maximum length (north–south direction) of the lake is 173 km, the average width from east to west is 16.9 km, and the widest part is about 74 km. The width of the narrowest part of the waterway area is about 2.8 km, and the total length of the lake shoreline is about 1200 km. The lake basin is inclined from east to west and from south to north, and the elevation is generally reduced from 10 m to about −1 m at the mouth of the lake.

Poyang Lake waterway is divided into a west waterway, an east waterway and a river waterway leading to the Yangtze River. The Ganjiang River is divided into four branches below Nanchang city. The main branch joins Xiushui River in Wucheng town, forming the west waterway. The south, middle and north branches of the Ganjiang River, Fuhe River, Xinjiang River and Raohe River successively converge into the main lake area, forming the east waterway. The east and west waterways converge to form a waterway into the Yangtze River at the mouth of the lake. There are three types of beaches, including sand, mud and grass beaches, with a total area of 3130 km². The number of sand beaches is small and their elevation is low, and they are distributed on both sides of the main channel. There are more mud beaches than sand beaches, and their elevation is between that of sand beaches and grass beaches. A grass beach is a mud beach with grass; their elevation is mostly 12–15 m, and they are mainly distributed in the east, south and west of the delta.

The Poyang Lake Delta, also known as the rump of the Ganjiang River (Figure 2), is the primary hub connecting Nanchang City and Poyang Lake. The river channel consists of the Ganjiang River trunk and multi-level branches. The Ganjiang River enters the Poyang Lake Delta from the east of Nanchang city. Its main stream is divided into the east and west rivers at the beginning of Qiujiazhou and Yangzi in Nanchang. The east river is further divided into the middle and south branches at Jiaojitou, while the west river is divided into the north and main branches at Qiaoshe. The main branch serves as the primary channel connecting the Ganjiang River to the lake and ultimately the Yangtze River. To enhance navigation conditions, water supply and the water environment in the delta wetland area, four hydraulic controllers will be constructed to regulate water levels and flows. The spatial distribution of the four hydraulic controllers is shown in Figure 2. The hydraulic controllers in the main branch and the south branch consist of sluice gates, navigation locks and fishways. The hydraulic controller in the middle branch consists of sluice gates and fishways, while the hydraulic controller in the north branch consists of sluice gates.

2.2. Data and Analysis

The data studied in this paper include hydrometeorological data, remote sensing images and ecological environment data of Poyang Lake Delta. All data were provided by Jiangxi Academy of Water Science and Engineering.

2.2.1. Hydrometeorological Data

Each branch of the Poyang Lake Delta is equipped with hydrological monitoring stations.

Table 1. Basic information on each station in the Poyang Lake Delta.

River Name	Station	Longitude	Latitude	Data
Ganjiang River	Waizhou	115.84	28.64	Discharge and Water level
Main branch	Changyi	116.03	29.00	Water level
North branch	Jiangbu	116.03	28.83	Water level
Middle branch	Louqian	116.07	28.80	Water level
Poyang Lake	Xingzi	116.05	29.45	Water level
South branch	Chucha	116.09	28.77	Water level
Yangtze River	Jiujiang	116.02	29.75	Discharge and Water level

gives basic information on each hydrological station in the Poyang Lake Delta. To analyze the causes of extreme drought in the Poyang Lake Delta, this paper collected water level and flow data from stations in the delta region from 1951 to 2022, water level of Poyang Lake (Xingzi station) from 1951 to 2022, and flow data of Jiujiang station in 2022. In addition, this paper collected the monthly rainfall data of the Poyang Lake Basin from 1951 to 2022. All the meteorological data were collected from the China Meteorological Science Data Sharing Service System (http://data.cma.cn/, accessed on 1 June 2024).

2.2.2. Remote Sensing Data

In this study, optical remote sensing images of the Poyang Lake Delta region were obtained from Landsat 8. Since the spatial resolution of Landsat 8 is 15 m, which is far less than the river width in the delta region (ranging from 600 m to 1000 m), the Landsat 8 data is suitable for analyzing variations of the submerged area of the delta. The images were taken during a normal-water year (on 1 November 2017) and an extreme-drought year (on 14 October 2022). In this paper, the normalized difference water index (NDWI) model [16] was used to extract the delta river water from the remote sensing images. The NDWI has been developed to delineate open water features and enhance their presence in remotely-sensed digital imagery, which is widely used in the study of Poyang Lake wetland [17].

2.3. Research Method

2.3.1. Statistical Modelling

The Chow test is a structural change point testing method [18], which is used to statistically detect structural breaks of hydrological data in time series. Chow test was implemented by the R package function “strucchange” in this paper. The correlation analysis was carried out to establish relations between drought characteristic variables and their explanatory variables.

2.3.2. Physical Model of Ganjiang River Rump

In this paper, a physical model of Ganjiang River rump was established. The research scope of the model ranged from Waizhou hydrological station to the Ganjiang River Estuary, with a total length of about 54 km and a width of 36 km. The model was based on a measured topographic map from 2013. The plane scale was 1:300, the vertical scale was 1:80, the rate of change was 3.75, the length was 180 m and the width was 125 m.

Table 2. Physical model experimental conditions.

Case	Case 1	Case 2
Discharge of Waizhou (m3/s)	452	500

shows the setup of the physical model experiments. Case 1 refers to the dry season before the construction of the hydraulic controllers, and Case 2 refers to the dry season after the operation of the hydraulic controllers.

Table 3. Location of water level measuring points.

Name	Station	Distance to Waizhou (km)
Main branch	X1	0
	X2	1.01
	X3	4.19
	X4	6.38
	X5	8.46
	X6	12.85
	X7	16.34
	X8	24.04
	X9	28.43
	X10	31.4
	X11	36.14
	X12	42.18
	X13	46.39
	X14	50.23
	X15	54.79
	X16	58.82
South branch	N1	12.57
	N2	17.17
	N3	20.28
	N4	23.43
	N5	28.96
	N6	36.61
	N7	44.43
Middle branch	Z1	17.06
	Z2	20.15
	Z3	24.21
	Z4	28.65
	Z5	30.32
	Z6	31.07
	Z7	37.64
	Z8	41.11
	Z9	44.57
North branch	B1	31.64
	B2	36.28
	B3	40.64
	B4	41.7
	B5	45.15
	B6	49.35
	B7	53.43
	B8	56.81
	B9	60.77

and Figure 3 show the location of water level measuring points in the delta river during the model experiment. The resistance similarity of the model was verified by using the measured water surface profile data during the flood (Q = 9910 m³/s) and dry seasons (Q = 932 m³/s), and the validation results met the requirements of the river model experiments (Figure 4) [19]. Using the verified model, the improvement effect of engineering regulation on drought in the Poyang Lake Delta was studied.

2.3.3. Field Monitoring

Two rounds of on-site monitoring were carried out in December 2017 (normal-water year) and February 2023 (after extreme drought). A total of 13 monitoring points were arranged in the study area, with 6, 1, 3 and 3 points, respectively, arranged at the main, north, middle and south branches. The specific distribution of monitoring points is shown in Figure 5. The monitoring parameters included environmental (such as total nitrogen, ammonia nitrogen and total phosphorus) and ecological parameters (such as phytoplankton and benthos).

3. Results

3.1. Cause of Extreme Drought Events in Poyang Lake Delta

3.1.1. The Drought Regime in Poyang Lake Delta

Chow test result of the lowest water level in the Poyang Lake Delta during the dry season (Z_min) from 1951 to 2022 shows that the Z_min presented structural variation in 2003 (p < 0.05). Figure 6 shows the linear fitting results of the water level time series before and after the change point (2003): Before the change point (1951–2002), the minimum water level (Z_min) in the delta during the dry season decreased slowly year by year, and the slope of the trend line was −0.01. After the change point (2003–2022), the minimum water level (Z_min) in the delta decreased rapidly, and the slope of the trend line was −0.2, 20 times that before the change point. It can be seen that the drought situation in the Poyang Lake Delta accelerated after 2003.

In order to analyze the drought situation in the Poyang Lake Delta during the dry season from 2003 to 2022, the drought period in the Poyang Lake Delta was extracted by taking 15.5 m as the water level threshold [19] of the delta region drought. The aim of the construction of regulation gates in the delta was to raise the water level to 15.5 m in dry seasons, which was the critical water level for the water supply, navigation and water environment. Figure 7 shows the distribution of drought in the Poyang Lake Delta from 2003 to 2022, in which Z_ave represents the annual average daily water level in the delta region. The result shows that the drought in the delta region mainly occurs from October to March.

3.1.2. Natural Factors Causing Extreme Drought

Compared with human activities, natural factors (e.g., climate change) are more important causes of extreme drought in the Poyang Lake Delta [9]. With respect to climate change, rainfall and temperature play dominant roles in changing basin hydrology and streamflow. In particular, the runoff and rainfall in Poyang Lake Basin have been found to be significantly correlated [20]. Xu et al. [21] evaluated the impact of climate changes on runoff in Poyang Lake Basin, pointing out that the proportion of impact from temperature and rainfall was 3:10.

In the summer of 2022, the extremely high temperature frequency and the number of high-temperature days in central and eastern China reached their highest levels since 1979, and precipitation was at its lowest [22]. Figure 8 shows the monthly rainfall of the Poyang Lake Basin in the extreme-drought year (2022) and the annual average (1951–2021). The rainfall data was calculated by collecting data from meteorological stations distributed throughout the entire Poyang Lake Basin, including Ganjiang Basin, Fuhe Basin, Xinjiang Basin, et al., which ensures that the calculated rainfall accurately reflects the overall rainfall regime in the Poyang Lake Basin. In 2022, the temporal distribution of rainfall in the whole basin of Poyang Lake was extremely uneven. The rainfall in the first half of the year (January to June) was high, while the rainfall in the second half (July to December) was low, especially in July, August, September and October, during which it was far lower than historical levels in the same period.

The temporal distribution of rainfall in the Poyang Lake Basin was uneven, especially the sharp decline in rainfall in summer, which changed the hydrological conditions of the Poyang Lake Delta. The temporal distribution of discharge in the delta was extremely uneven. In the extreme-drought year (2022), the average discharge of Ganjiang River (Waizhou station) was 2128.5 m³/s, which was similar to that from 1951 to 2021 (2175.2 m³/s). Further analysis of the coefficient of variation (CV) of the Ganjiang River discharge shows that the coefficient of variation in the extreme-drought year was 0.95, while that from 1951 to 2021 was only 0.61, indicating that the temporal distribution of the Ganjiang River discharge in the extreme-drought year was more uneven (Figure 9). Compared with the 1951 to 2021, the Ganjiang River discharge in the extreme-drought year decreased significantly in July. From July to December, the Ganjiang River discharge in the extreme-drought year was lower than that from 1951 to 2021, and the minimum discharge was 314 m³/s. In extreme-drought years, the discharge and water level of Poyang Lake delt were both significantly correlated to rainfall in the Poyang Lake basin, with correlation coefficients of 0.808 (p < 0.01) and 0.875 (p < 0.01), respectively.

3.1.3. Human Activity Causing Extreme Drought

Three Gorges Reservoir is located in the upper reaches of the main stream of the Yangtze River and the upper reaches of the junction of Poyang Lake and the Yangtze River. Its operation can affect the flow rate in the middle and lower reaches of the Yangtze River. In particular, the impoundment of the Three Gorges Reservoir at the end of the flood season will cause the water level at the mouth of Poyang Lake to drop, which will “empty” the Poyang Lake and exacerbate the drought in Poyang Lake [23,24]. This is also the main reason why the structural change of the lowest water level in the Poyang Lake Delta during the dry season occurred in 2003 (Figure 6). Jiujiang station is the nearest hydrological station located on the main stream of the Yangtze River to Poyang Lake, and its discharge variation can reflect the impact of the operation of Three Gerges Reservoir on Poyang Lake Delta. In the extreme drought year, the water level of Poyang Lake Delta was significantly correlated with the discharge of Jiujiang station, with a correlation coefficient of 0.959 (p < 0.01), indicating that the impoundment of the Three Gorges Reservoir is also one of the important causes of extreme drought in the delta. In addition, due to the construction of cascade reservoirs in the upper reaches of the Poyang Lake Delta and the development of water and soil conservation measures, the sediment flux in the delta river has continued to decline in recent years [25]. Under the combined influence of the reduction in sediment flux and artificial sand mining, the riverbed of the delta river exhibits uneven undercutting, and the low water level decreases significantly [26]. The riverbed undercutting degree of the main branch of the delta is higher than that of other tributaries, which causes the main branch to divert the main runoff of the delta in the dry season, resulting in a higher risk of flow cut-off for other branches.

In order to quantitatively analyze the impact of upstream flow and downstream water level on drought in Poyang Lake Delta in recent years (2003–2022), the minimum water level in dry season (Z_min) of Poyang Lake Delta and drought duration (D, the number of days when the water level in dry season is lower than 15.5 m) of the delta were taken as the characteristic variables to describe the drought. The correlation analysis was carried out with the upstream flow (Q_input, the average daily flow at Waizhou station from October to next March) and the minimum water level in the dry season (Z_lakemin) of Poyang Lake (Xingzi Station) as the explanatory variables. Figure 10 shows the correlation between drought characteristic variables (Z_min, D) and explanatory variables (Q_input, Z_lakemin). The result shows that the drought duration has a significant negative correlation with the upstream flow during the drought period and the lowest water level of Poyang Lake in the dry season, indicating that the decrease in upstream flow and the decline of Poyang Lake water level in the dry season jointly promote the increase in drought days in the delta region.

3.2. Damage Inflicted by Extreme Drought Event on Delta Wetlands

3.2.1. Human Society

Figure 11 shows satellite remote sensing images of the dry season in the Poyang Lake Delta region in the normal year (2017) and the extreme-drought year (2022). The results show that in the normal year, the delta river water area in the dry season was approximately 102.47 km², whereas, under extreme drought conditions, the delta river water area decreased to about 55.59 km², representing a reduction of 45.75% compared to the normal year. The reduction in submerged area in extreme drought conditions narrowed the river channel in the delta and exposed the beaches, causing difficulties for navigation, living and water supply for production.

3.2.2. Ecology and Environment

Table 4. Water environment parameters before and after extreme drought in Poyang Lake Delta.

Sampling Time	TN (mg/L)	TP (mg/L)	NH4+-N (mg/L)
2017.12	2.53	0.05	0.51
2023.02	3.80	0.17	0.84

and

Table 5. Water ecological parameters before and after extreme drought in Poyang Lake Delta.

Sampling Time	Algae			Benthos
	Number of Species	Density (Cells/cm2)	Biomass (mg/cm2)	Number of Species	Density (ind/m2)	Biomass (g/cm2)
2017.12	50	1.49 × 104	0.0076	11	132.31	283.7
2023.02	41	2.79 × 104	0.05	19	53.11	60.87

show the environmental and ecological parameters of Poyang Lake Delta in the normal year (December 2017) and after extreme drought (February 2023). The results show that compared to the normal year, the nutrient concentration in the delta increased after extreme drought. TN, TP and NH₄⁺-N increased by 50.2%, 240% and 64.7%, respectively. The concentrations of TN and TP were 3.8 mg/L and 0.17 mg/L, respectively, while the context values in the delta were 1.2 mg/L and 0.075 mg/L [27]. The density and biomass of algae increased by 87.2% and 557.9%, respectively, while the density and biomass of benthos decreased by 59.9% and 78.5%, respectively. The deterioration in water quality after the extreme drought was primarily due to the significant reduction in water volume and the increase in nutrient concentration caused by the extreme drought. Nitrogen and phosphorus are the main nutrients required by algae. The increase in nutrient concentration promoted the growth of algae and increased the risk of cyanobacterial bloom in the delta region. The increase in nitrogen concentration in the water body reduced the density and biomass of pollution-sensitive species (such as Ephemeroptera and Trichoptera) among benthic animals. Additionally, following the extreme drought conditions in 2022, the long-term exposure of the delta beach has had a long-term impact on the wetland benthos from which it is difficult to recover.

3.3. Countermeasures

The extreme drought event in 2022 has had many adverse effects on navigation, water supply and the wetland water ecological environment in the Poyang Lake Delta. Raising the water level and increasing water volume during the drought period is of great importance to reduce the damage caused by drought. In this paper, the improvement in the hydraulic conditions in the delta through hydraulic controller regulation is studied through a physical model experiment. To simulate extreme drought conditions, the discharge of Ganjiang River (waizhou station) was set to 452 m³/s (before the construction of the controllers) and 500 m³/s (under the regulation of the controllers). Figure 12 and Figure 13, respectively show the water level of the delta river before and after the construction of the controllers in the dry season. Figure 12 shows that when the flow of Ganjiang River was 452 m³/s, under the condition of no controller regulation, except for the main branch, all branches were cut off, and the water level of the main branch was only 9.1 ± 0.7 m. Since the delta river was not regulated by the controller, there was a gradual decline of the water level from section X1 to X16, resulting in a minor range of water level variation. When the discharge of Ganjiang River was 500 m³/s, under the control of the four controllers in the delta, all rivers in the delta could flow, and the overall water level of the delta was maintained at 14.2 ± 1.8 m (Figure 13). The water levels of the four branches underwent abrupt changes along the way, which was because the gate could significantly raise the water level upstream of it, resulting in a large water level difference before and after the gate.

Further analysis of the relationship between the elevation of the beaches and the water level of branches in the delta shows that the elevation of the beaches of the middle branch was about 12 m, and the water level of the middle branch under the regulation of the controllers reached 14.0 ± 1.9 m, indicating that the regulation of the controllers could effectively improve the horizontal hydrological connectivity of the river channel in the Poyang Lake Delta region.

4. Discussion

4.1. Impact of Hydraulic Controller Operation on Delta Wetland

Under extreme drought conditions, the regulation and control of the delta hydraulic controllers can effectively raise the water level in the region, because the hydraulic controllers located in the four branches are in gate-controlled states. Lu et al. [28] pointed out that the function of the hydraulic controller in the Poyang Lake Delta is to change the ratio of flow distribution and water levels in this region. It could improve the spatial distribution of water resources in the delta, especially by enhancing the inflow from the northern and southern branches. Under the influence of gate control, the four branches of the delta exhibited significant backwater in front of the gates, and there was a water level difference in front of and behind the gates (Figure 13). To raise the water level of the delta under extreme drought conditions, in the physical experiment, each gate was set to have a single-hole gate that was partially open, and the openings of the main, north, middle and south branches measured 0.64, 0.28, 0.48 and 0.4, respectively. The gate not only controls the water level, but also alters the diversion ratio of each branch, resulting in diversion ratios of 50%, 4%, 24% and 22%. It can be seen that under extreme drought conditions, the operation of the delta hydraulic controllers is of great significance for raising the overall water level, ensuring the flow of each branch and maintaining the longitudinal hydrological connectivity of the delta river.

The elevation of the water level in the delta region helps to increase the water surface area in the region.

Table 6. Delta water area under different water levels of Ganjiang River.

Water Level (m)	Water Area (km2)	Increase Ratio
10	38.63	0
15	58.67	52%
15.5	60.27	56%
16	61.87	60%

shows the water area of the delta from Waizhou station to the front of each gate under different water levels of Ganjiang River (Waizhou station) in the physical experiments. The results of the physical experiments show that the delta water area could be increased by 56% by raising the water level of Ganjiang River (Waizhou station) from 10 m to 15.5 m under extreme drought conditions through the operation of the delta hydraulic controllers. Raising the water level and increasing the water volume of Poyang Lake Delta contributes to enhancing the self-purification ability and degradation ability in this region, which is significant to water environment protection and maintenance of healthy aquatic ecosystems [29].

4.2. Impact of Operation of Poyang Lake Hydraulic Controllers on Delta Wetland

The operation of Poyang Lake hydraulic controllers is of great importance in alleviating the adverse effects of extreme drought. Their operation in the dry season can increase the inundation area of the lake region by 100–400 km², which is especially significant for improving the water level of the northern lake area. It can increase the water level of the northern lake area by 4–5 m during the recession period. However, its impact on the western and southern lake areas is small [30]. In addition, the operation of the Poyang Lake hydraulic controllers reduces the water flow mobility of the waterway and of the northern and eastern lake areas during the dry season, but has little impact on the water flow mobility of the central and southern lake areas [31]. It can be seen that the operation of the Poyang Lake hydraulic controllers has a significant impact on the overall hydrological regime and hydrodynamic conditions of Poyang Lake, but the impact on the delta wetland is relatively small. The rational operation of the delta water hydraulic controllers is an effective way to deal with the extreme drought in the region.

5. Conclusions

In response to the extreme drought event that occurred in the Poyang Lake Delta in 2022, this paper analyzes its causes from the perspectives of natural factors and human impacts, reveals the damage inflicted by extreme drought on the delta wetlands, and discusses countermeasures against extreme drought from the perspective of hydraulic controller regulation:

(1)

The occurrence of the extreme drought event in the Poyang Lake Delta was due to the joint influence of climate change (the uneven temporal distribution of rainfall) and human activities (e.g., Three Gorges Reservoir). In the extreme-drought year, the rainfall in Poyang Lake Basin began to decrease significantly in July 2022, far below the historical levels for the same period, leading to a significant reduction in the discharge of the Ganjiang River into the Poyang Lake Delta, with the minimum discharge being only 314 m³/s. The discharge and water level of Poyang Lake delt were both significantly correlated to rainfall in the Poyang Lake basin, with correlation coefficients of 0.808 (p < 0.01) and 0.875 (p < 0.01), respectively. In addition, the water level of Poyang Lake Delta was significantly correlated with the discharge of Jiujiang station, with a correlation coefficient of 0.959 (p < 0.01), indicating that the impoundment of the Three Gorges Reservoir is also one of the important causes of extreme drought in the delta.

(2)

The extreme drought event reduced the water level and volume in the Poyang Lake Delta region, causing many adverse effects on navigation, water supply and the wetland ecological environment. Compared with the normal year (2017), the extreme drought in 2022 reduced the inundation area of the Poyang Lake Delta in the dry season by 45.75%. TN, TP and NH4+-N increased by 50.2%, 240% and 64.7%, respectively, resulting in significant water quality deterioration. The density and biomass of algae increased significantly by 87.2% and 557.9%, respectively, thus increasing the risk of cyanobacterial bloom. The density and biomass of benthos decreased significantly by 59.9% and 78.5%, respectively. The key to reducing the adverse effects of extreme drought on the delta wetland is to improve the water level of the river channel and increase the submerged area in the Poyang Lake Delta.

(3)

The regulation and control of the proposed hydraulic controllers can effectively raise the water level, ensure the flow of each branch, increase the water area and reduce the damage inflicted by extreme drought on the delta wetland. Before the construction of the controllers, the water level of the Poyang Lake Delta under extreme drought conditions was 9.1 ± 0.7 m, and only the main branch flowed, while the other branches were cutoff. The operation of the controllers can raise the water level of the delta to 14.2 ± 1.8 m, adjust the diversion ratio of the main, north, middle and south branches to 50%, 4%, 24% and 22%, respectively, and increase the water area of the delta by 56%.

Limitations and future work: The construction of hydraulic controllers can disrupt the natural flow of river water bodies in the Poyang Lake Delta, potentially triggering the outbreak of algae blooms. Consequently, establishing a rational water project scheduling scheme that promotes orderly water flow is an effective measure to inhibit algae blooms. Furthermore, this is an area that requires further research in the Poyang Lake Delta in the future.