Inter-annual variations of Poyang Lake area during dry seasons: characteristics and implications

Variations in a lake area constitute an important indicator of the modifications of the lake hydrology. This paper explores the inter-annual variations of the Poyang Lake area during the dry seasons occurringwithin the 1961 to 2010 period and further quantifies the severity of dryness recently endured during the 2000s. A physically based hydrodynamic model of Poyang Lake established the relation between the lake area and lake level. The lake area was calculated using the observed lake water level. Results indicated the average lake area in the dry seasons was 1,015 km. There was a considerable inter-annual variation of theminimum lake area that varied from702.8 km to 1,259.7 km. Poyang Lake experienced themost severe dryness in the 2000s, resulting in an average lake area during 2001 to 2010 of 124 km less than that of the preceding period. During the dry seasons, the catchment of the river discharge is likely the primary cause of the changes in lake area. This study evaluated the inter-annual variations of the Poyang Lake over a period of 50 years. Our results may provide support for an integrated management of the lake-catchment system, securing the water supply. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/nh.2016.308 om https://iwaponline.com/hr/article-pdf/47/S1/40/366864/nh047s10040.pdf 2020 Mengfan Li Qi Zhang (corresponding author) Yunliang Li Jing Yao Zhiqiang Tan Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China E-mail: qzhang@niglas.ac.cn Mengfan Li University of Chinese Academy of Sciences, No. 19 Yuquan Road, Beijing 100049, China


INTRODUCTION
Lakes are valuable economic resources (fresh water resources, cultivation, fishing) for the human population and play an important role in regional ecological and environmental issues (Beeton ; Lehner & Döll ).
Lake areas are sensitive to climate change and human activity, and therefore provide an excellent indicator of climate/anthropogenic forcing and regional responses (Ma et al. ). Lake areas provide essential information on the characterization of hydrological dynamics (Hervé et al. ). Monitoring their inter-annual variation is critical to the understanding of the change in water balance (Huang et al. ). Consistent variations in lake area were shown to cause serious environmental problems, and to greatly affect the available freshwater resources. This leads to an adverse evolution of regional ecological environment and wetlands (Cui ; Fang et  China has a large number of lakes having a rich cultural, ecological, and economic history. However, over the last half century, nearly 13% of Chinese lakes have undergone a remarkable reduction in lake area (Ma et al. ). The Poyang Lake, located in the middle reach of the Yangtze River, is the largest freshwater lake of China and is recognized internationally as an important wetland. The lake is a valuable resource of freshwater and provides the largest winter habitat for more than 90% of Siberian migratory birds (Kanai et al. ; Xia et al. ; You et al. ; Han et al. ). During the 1990s, severe flooding occurred (1992, 1995, 1997, and 1998) To alleviate the environmental pressure caused by the rapid lake shrinkage, the local government proposed the Poyang Lake Hydraulic Project (PLHP) in March 2008.
The PLHP would contain a 2.8 km-wide dam across the northern lake channel. This hydraulic dam would operate during the dry seasons to control the water flow from the Poyang Lake to the Yangtze River, and maintain a reasonable lake water level (Wang et al. ). However, the proposed PLHP stirred considerable debate related to the impact on the ecology and environment (Li ; Ge et al. ; Wang et al. ). Thus, improving our knowledge of the variations in the area of Poyang Lake is critical to rationally managing the lake-river system.
Previous studies, using bathymetric data, revealed the annual average area of Poyang Lake was 5,160 km 2 in 1954 and decreased to ∼3,860 km 2 in 1992 (Min ). Several recent investigations have sought to define the variations in Poyang Lake's flooding area using remote sensing imagery.
However, the data were limited to the 2000s period. Lake, so that a complete knowledge of the degree of variations in lake area since the last decade can be obtained. We will establish a comparison relative to the early periods when the impact of human activity was small.
There exist a number of approaches to calculate lake areas.

)
to study the hydrodynamics of the Poyang Lake-River system. The advantages of hydrodynamic models to capture the spatial and temporal variations in lake level and lake area at high resolution enabled us to adopt the existing Poyang Lake hydrodynamic model (Li et al. ) to establish the relation between lake area and lake level for a defined period. The relation was then applied to calculate the lake area for the entire period of study, based on the observed lake level.
This study will analyze the long-term variations of the Poyang Lake area during the dry seasons through the lake area-lake level relation. The main objectives are to: (1) establish the time evolution of the Poyang Lake area during the dry seasons for the period starting in 1961 and ending in 2010; (2) analyze the inter-annual variations of the lake area and quantify the severity of the lake area shrinkage during the last decade; and (3) discuss the possible mechanism causing the lake to dry and the potential impacts on the wetland ecosystem.

Study area
Poyang Lake (

Data collection
The bathymetry of Poyang Lake was determined by the surveyed data obtained in 1998 with a resolution where x is the observed lake level at the Duchang station; y is the corresponding lake area.

Determination of dry seasons
A cumulative probability analysis of the water levels at Duchang station allowed detection of the dry seasons ( Figure 3). The water attaining a level having <25% probability to occur, e.g., 11.56 m, is adopted as the upper limit of the dry seasons. Thus, the dry seasons in this study were those when the water levels were less than 11.56 m.
The year 2002 was excluded, since the water level was consistently >11.56 m for the entire year. to explore the relative significance of their correlations with the variation of lake area (Figure 4(a), 4(c), and 4(e)). The correlation of the lake area with the catchment discharge is better with an R 2 value of 0.51 than that of the Yangtze River (R 2 ¼ 0.13). Figure 4 for the catchment discharges (Figure 4(d)). Moreover, only 15 years were characterized by negative river discharge anomalies (Figure 4(f)). These results indicate the catchment discharge had generally a greater impact on the lake area than the Yangtze River discharge during the dry seasons.

RESULTS
However, it should be acknowledged that the above correlation relationship is only statistically meaningful. We Using a simple correlation analysis, our work showed the lake area during dry seasons correlated significantly with the catchment inflows. This indicates any decrease in rainfall in the catchment could result in a decline of the lake area during the dry seasons. This also implies that optimized operation of the hundreds of reservoirs located in the catchment could help resolve the problem of dry conditions. Furthermore, the proposed construction of a hydraulic dam near the lake outlet may also be an adequate measure to maintain the lake water at an appropriate level. The dam would mitigate the impacts of unusual dryness on the wetland ecosystems and water supply for local residents and agricultural activity.
Application of the physically based hydrodynamic model in determining the lake area was successful. Poyang Lake presents a highly dynamic lake-floodplain system, and the model demonstrated the capability of capturing the temporal and spatial lake area variations at a satisfactory resolution. We could apply this model to similar lake systems around the globe as an alternative method to investigate hydrological changes when other data sources are limited.