The hydrological situation of Poyang Lake has undergone changes due to the influence of climate change and human activities. Its typical throughput and seasonal variation characteristics have attracted much attention. Research on the long-term hydrological variation of Poyang Lake needs to be enriched. Using remote sensing images from 1986 to 2020 and combining them with the Normalized Difference Vegetation Index (NDVI), this study extracts the water area of Poyang Lake and analyzes its variation characteristics. The results show that the water area of Poyang Lake has a consistent periodic variation with meteorological factors. Both the water area and meteorological factors have a main period of 18 m (months). Water area change points occurred in 2003, 2010, and 2016. In different research periods, there is a significant correlation between the water area and meteorological factor changes. Furthermore, the correlation between water vapor pressure and precipitation with the water area change is the strongest.
The change of the Poyang Lake water area in long time series was studied.
The change of long-term meteorological factors was studied.
The response relationship between the water area and meteorological factors was studied.
Wavelet analysis and M-K mutation test were applied.
Multi-source long time series remote sensing image application was applied.
Inland water bodies such as rivers and lakes are an important part of global water resources. However, with the increase in pollution caused by industrial production, the problem of inland water pollution and spatial variation of water quality caused by climate change and human activities have become a major concern for scholars and governments. Rivers and lakes provide important water sources and habitats for aquatic organisms, regulate flow, and improve multiple ecological functions and regional microclimate. Changes in hydrology can reflect the combined effects of climate and human activities on the water cycle and the ecosystem (Yan et al. 2020). Poyang Lake is the largest freshwater lake in China. Since the 21st century, climate change and frequent human activities have caused changes in the water regime of Poyang Lake (Liu et al. 2020). The existing research shows that since 2003, the low water levels and extreme low water levels of Poyang Lake have occurred earlier and the duration of low water levels has been prolonged, which is closely related to the completion of the Three Gorges Dam (Yang & Liao 2017).
Zheng et al. used Landsat data, multi-spectral image vegetation classification, and principal components to analyze the impact of the completion of the Three Gorges Dam on the hydrological handover at the Poyang Lake estuary. The results showed that after the impoundment of the Three Gorges Dam, the dry season of Poyang Lake was prolonged, the flood season was shortened (Lyu et al. 2018), and the hydrological changes in the basin were obvious (Zheng et al. 2020). Sun & Ma used satellite altimetry data and MODIS data to invert the water level data of Poyang Lake and used the area and water level changes to simulate its water volume change, the linear relationship between the area and water level, and the water balance relationship to calculate the water volume interaction between Poyang Lake and the Yangtze River. The results showed that the Yangtze River mainstream has a certain backing effect on the water intrusion of Poyang Lake in the rainy season with increased precipitation (Sun & Ma 2020). Tian et al. extracted the water area of Poyang Lake from Landsat-8 images and Sentinel-1 images. A prediction model based on 45-day cumulative precipitation data was established. According to the verification of real data, it showed that the prediction model had a good effect on precipitation in the southwest of Poyang Lake, which verified the correlation between hydrological changes and precipitation changes in Poyang Lake (Tian et al. 2020). Liu et al. (2020), based on remote sensing data and DEM data, extracted the Normalized Difference Water Index (NDWI) of Poyang Lake, estimated the water volume information by using the dynamic storage capacity method, studied the long-term dynamic changes of the water volume of Poyang Lake from 1989 to 2018, and analyzed its driving factors and ecological effects. Zhang et al. (2019) used the Landsat image and the GaoFen (GF) image NDWI to extract the water body of Poyang Lake, combined with the time series data of the Hukou hydrological station in the Poyang Lake area, and established the relationship model between the water area and the water level in different periods of the year. The model can monitor the continuous change of lake water level and has important guidance and reference significance for drawing volume curves (Zhang et al. 2019). Wu et al. and Tian et al. used remote sensing images of Poyang Lake in the dry season and the flood season from 1980 to 2010 to extract its water area and explored the main driving factors affecting the change of the Poyang Lake water area. The results showed that the water area in the flood season and the dry season showed a decreasing trend. Human activities such as the construction of water conservancy projects in the upper reaches of the Yangtze River and the development of urbanization in the basin were the main driving factors for the reduction of water area in Poyang Lake during the flood season. Land use change caused by human activities was the main reason for the significant reduction of water area in Poyang Lake in the dry season (Tian et al. 2020; Wu et al. 2021). Liu et al. used the NDWI/Normalized Difference Vegetation Index (NDVI) joint method to extract the Poyang Lake water area of the Coastal Zone Imager (CZI) multi-spectral image of the HY-1C/D satellite and analyzed the time series of the Poyang Lake area. Combined with the water level data, the quantitative relationship between the water area and the water level of Poyang Lake was analyzed, and the flood risk area of Poyang Lake was identified. The results show that the water area of the Poyang Lake area has a high correlation with the water level, and the flood-prone areas are mainly located in the southeast and west regions of the lake (Liu et al. 2022). All of these studies have revealed the hydrological changes in the Poyang Lake water area and its influencing factors. There are many factors affecting the hydrological changes in Poyang Lake, mainly including meteorological factors, water from the Yangtze River, water from five rivers, and human activities. In order to analyze the long-term series of Poyang Lake water area regime changes and its response to meteorology, this paper used Advanced Very High Resolution Radiometer (AVHRR) images and Moderate-Resolution Imaging Spectroradiometer (MODIS) images to construct time series data of the Poyang Lake water area from 1986 to 2020 and combined the Mann–Kendall (M–K) mutation test, wavelet analysis, correlation analysis, and other methods to quantitatively analyze the water regime changes and its response to meteorology. This provides new technical support and policy recommendations for Poyang Lake water regime change research, regional ecological protection, and regional development and construction.
Poyang Lake is in the northern part of Jiangxi Province, which is located on the south bank of the middle and lower reaches of the Yangtze River (Guo et al. 2021). It receives water from the Ganjiang River, Fuhe River, Xin River, Rao River, and Xiushui River and connects to the Yangtze River through the lake emissary (Guo et al. 2021; Yang & Liao 2017). Poyang Lake is bounded by Songmen Mountain, which divides the lake into two parts: the north, which is the channel into the river; and the south, which is the main lake body, and covers an area of 3,150 km2 (Yang & Liao 2017). The region has a typical subtropical monsoon climate, and the water area changes significantly with the seasons (Tian et al. 2021). The wet season is from June to September, and the dry season is from December to February. As the largest freshwater lake in China and one of the top 10 ecological function protected areas in China (Liu et al. 2011), Poyang Lake's hydrological regime change has always been a focus of attention for domestic and foreign researchers. Poyang Lake plays an important role in regulating the water level of the Yangtze River, conserving local water sources, and maintaining the ecological balance around it. Therefore, the study of the long-term changes in the Poyang Lake water area and its influencing factors is crucial for the ecological protection and ecological health of the Poyang Lake area.
In this paper, AVHRR LAC 1KM 1B dataset and Terra-MODIS dataset are used to extract and calculate the water area of the Poyang Lake area (data can be downloaded from www.avl, class.noaa.gov, and https://earthdata.nasa.gov). AVHRR LAC is the regional land cover data that has coverage time from 1985 to the present, with a time resolution of 1 day and a spatial resolution of 1.1 km.
Water body extraction method
BandNIR represents the near infrared band; BandRed represents the red band.
The water body of the AVHRR image is extracted by using a combination of visual interpretation experience, superimposed image judgment, and reference to other scholars’ research experience. Through repeated experiments to compare the extraction results, the water range of the Poyang Lake area in the AVHRR image can be determined. The water body of the MODIS image is obtained by comparing the water body pixel range with the ISODATA unsupervised classification. The water area extraction results obtained from the two datasets are compared with the existing research results and found to have a consistent trend in the change of water area in the Poyang Lake area.
M–K mutation test
The M–K trend test is a non-parametric statistical test method widely used in hydrological time series analysis. The M–K test can be used to determine whether there is a mutation in the time series data which, if there is, can determine the time of mutation. This method is commonly used in hydrological analysis, and in this specific case, the study uses it to determine the overall trend and point of change in the water area of the Poyang Lake area. The principle is as follows:
Wavelet analysis is a technique used to analyze time series data. It can identify different cycles and patterns within the data and reveal trends at various time scales. In this study, the Morlet wavelet analysis is used to examine the variation trend of the water area in Poyang Lake and the variation period of meteorological factors.
Correlation analysis is a method used to measure the relationship between two or more variables. In this paper, the Pearson correlation coefficient and the Spearman correlation coefficient are used to analyze the possible correlation between changes in the Poyang Lake area and meteorological factors.
In the formula, r is the correlation coefficient value, and are different values corresponding to x and y; and are the average values of x and y, respectively, and is the number of variables.
In this paper, 520 high-definition cloudless images from 1986 to 2003 were selected. The MODIS image is a three-level reflectivity 8-day composite product. MOD09Q1 has a time resolution of 8 days and a spatial resolution of 0.25 km (Xu et al. 2019). This paper selects 820 MODIS images from 2000 to 2020. The NDVI is calculated for both types of images, and the water body of the Poyang Lake area is extracted by thresholding. Finally, both of these images were used to reflect the change in the Poyang Lake region water area from 1986 to 2020. The images from 2001 to 2003 from the same year of the two datasets were selected for comparison. Because the resolution of the AVHRR image is lower than that of the MODIS image, the area extracted by MODIS is slightly larger than that extracted by AVHRR, but the changing trend of the two is roughly consistent, indicating that both images can accurately reflect the water area of the Poyang Lake area.
Variation of Poyang Lake water area
From the normal distribution curve of the multi-year water area, it can be seen that the water area in 1987, 1993, 1995, 2001, 2005, 2008, 2013, and 2015 generally showed a symmetrical normal distribution state. During these years, the water area changed smoothly, and there was no extremely dry or flooded hydrological situation. In 1989, 1992, 2007, 2010, 2011, 2014, and 2019, it was a serious non-normal distribution. In these years, extreme hydrological phenomena may occur. For example, in 2011, it was an obvious non-normal distribution, and for the whole year, the water area was low, which was an extremely dry year. In 2010 also, it was a non-normal distribution, and the annual water area was generally high, which was a wet year. Therefore, from 2010 to 2011, Poyang Lake experienced two years of drought and flooding.
Mutation test of the Poyang Lake water area
Mutation test of the meteorological factor
Wavelet analysis of the Poyang Lake water area
Wavelet analysis of the meteorological factor
Poyang Lake water area and meteorological factor correlation analysis
According to the seasonal variation characteristics of the Poyang Lake water area, the correlation between the change in the Poyang Lake water area and meteorological factors was divided into five time periods: the whole year (January–December), the wet season (May–September), the dry season (November–March of the next year), before the wet season (January–April), and after the wet season (October–December). A different correlation coefficient was used for the different research periods. The Pearson correlation coefficient was used for the variables for the year-round study period and the Spearman correlation coefficient was used for the variables for the other study periods. From the perspective of the annual research period, there is a strong significant correlation between the water area and temperature and water vapor pressure, a moderate correlation between the water area and precipitation, and a weak correlation between the water area and relative humidity. From the perspective of the wet season, the strongest correlation is the water vapor pressure, which has a significant moderate correlation at the 0.05 level. From the perspective of the dry season, the strongest correlation is precipitation, which has a stronger correlation at 0.05 level than that in the wet season, which is related to the seasonal variation of the water area of Poyang Lake with precipitation. From the point of view before the wet season, there is a significant correlation between water vapor pressure and precipitation and the water area, which is related to the expansion of the Poyang Lake area before the wet season. After the wet season, the correlation coefficient of water vapor pressure, precipitation, and relative humidity increased, indicating that the decrease in the Poyang Lake water area after the wet season was related to the decrease in precipitation, water vapor pressure, and relative humidity (Table 1).
|Period .||Coefficient .||Temperature .||Water vapor pressure .||Precipitation .||Relative humidity .|
|Period .||Coefficient .||Temperature .||Water vapor pressure .||Precipitation .||Relative humidity .|
The two-tailed significance test * represents a significant correlation at the 0.05 level and ** represents a significant correlation at the 0.01 level.
0.8–1.0 strong correlation, 0.6–0.8 strong correlation, 0.4–0.6 moderate correlation, 0.2–0.4 weak correlation, 0.0–0.2 very weak correlation or no correlation.
The water quantity of Poyang Lake is mainly affected by three aspects: the first is the supporting effect of the Yangtze River on Poyang Lake, the second is the interaction effect of the five rivers on Poyang Lake, both of which are related to the runoff into the lake. The third is the recharge effect of precipitation on Poyang Lake. Precipitation is the main factor affecting runoff, and it is also the main factor affecting the change of the water area in most research periods. The correlation between precipitation before and after the wet season has increased. The precipitation increased before the water area of Poyang Lake reached the ample flow value, and the water area began to expand before the wet season. The change in water area during this period is mainly affected by precipitation. During the wet season, the water quantity of Poyang Lake is not only affected by precipitation, but also by water from the Yangtze River and the five rivers. The amount of lake water increases and the water area increases, so the response of the influencing factors of precipitation weakened. After the wet season, precipitation decreases, and runoff also decreases. The five rivers and the Yangtze River no longer have a replenishment effect, and the water area is mainly affected by precipitation. Therefore, the response factors of precipitation increase again. From the response relationship between meteorological factors, the response of relative humidity and precipitation to the change in water area has a consistency of change in the coefficient. The same temperature and vapor pressure also have these characteristics.
As an important part of the water cycle, the dynamic change characteristics of lakes are affected by meteorological factors, river–lake interaction, and human activities, and the hydrological situation is complex. In this paper, the long time series multi-source remote sensing image data are used to quantitatively and qualitatively analyze the water area change of Poyang Lake in the past 35 years. It provides a research basis for consolidating and improving the ecological environment quality of Poyang Lake, continuously improving the regulation and storage capacity of Poyang Lake, and comprehensively building a healthy, stable, and complete ecosystem around the lake basin. The results show that the use of the NOAA AVHRR image and the MODIS image can realize the long time series change monitoring of the Poyang Lake water area. The main research conclusions are the following:
The water area change of Poyang Lake has an obvious seasonal wet and dry change process. The wet season is from May to September every year, and the water area usually reaches its peak in July every year. The length of the wet season is closely related to precipitation, so the interannual variation of the wet season has a small fluctuation. The minimum water area month of the year usually occurs in December, and the maximum water area can be four times the minimum. During the study period of this paper, the water area of Poyang Lake had a trend of expansion in the dry season. From 1986 to 2000, the wet and dry seasons changed steadily. In 2000, the dry season began to extend. From 2000 to 2020, the dry season was significantly extended. This is related to the increase in human activities and reservoir impoundment.
The temporal variation of the Poyang Lake water area has a main period of 18 m time scale, as well as secondary periods of 30, 68, and 96 m. The periodic variation of the water area is consistent with the main cycle of precipitation, water vapor pressure, temperature, and relative humidity, indicating that there is a responsible relationship between the water area and meteorological factors. There is also a similarity and response relationship between the mutation point of water area changing trend and precipitation.
During the study period of this paper, there is a significant correlation between the water area of Poyang Lake and precipitation, water vapor pressure, temperature, and relative humidity. Water vapor pressure and precipitation are the main factors affecting the change in the water area. The correlation of various factors in different research periods has changed, either increased or decreased, which indicates that the change in the water area is not only affected by various meteorological factors but also by human activities and the interaction between rivers and lakes.
With the continuous development of social economy in the Poyang Lake area and the further development of water conservancy and hydropower in the upper reaches of the Yangtze River, Poyang Lake is experiencing new changes. Therefore, it is necessary to conduct more in-depth research to explore the causes and consequences of water regime changes in Poyang Lake. For example, Poyang Lake quantifies the changes in the relationship between rivers and lakes, considers the changes in climate factors, the impact of large-scale water conservancy projects on the water regime changes, and identifies the primary and secondary contradictions to carry out the targeted research, so as to provide new research ideas for protecting the ecosystem and economic development policies in the Poyang Lake area.
The work was supported by the National Natural Science Foundation of China (41101322) and the Science and Technology Research Project of the Education Department of Jiangxi Province (GJJ160617).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.