Historical environmental changes in the Poyang Lake basin (Yangtze River,China) and impacts on agricultural activities

Abstract

Climate change and human activities have been an important part of studies regarding historical environmental changes in China over the past 2000 years. In this study, we focused on environmental changes, that is, natural disasters and human activities, in the Poyang Lake Basin over the past 2000 years, to analyze interactions between land use cover changes and human activities from the perspective of regional sustainable development. We collected historical records of climate and hydrology, floods and droughts, and rivers and lakes in the Poyang Lake area, and established time sequences for the floods and droughts, lake water level and lake area, amount of farming land, and population, in order to discuss interactions between changes in the environment and the climate, with emphasis on the impacts of extreme events on lake and river basin environment changes. The following results were obtained. First, climate changes in historical periods had wide-ranging and far-reaching impacts on agricultural production, especially disasters caused by climate change. Among the changes in the Poyang Lake basin environment, including river network systems, lake water levels, etc., changes in lake water volume are direct evidence of climate change, adaptation to climate change, and obvious phased characteristics. Second, in the process of changes to the lake and river network in the Poyang Lake Basin, social and economic development is accompanied by evolution of the lake. Increases and decreases in population, the scale of agricultural production, and lake environment changes have direct and significant interactions. Third, the Poyang Lake basin’s environmental changes during the historical period are mainly reflected in the pressure feedback mode of “population–agriculture” in the lake environment.

Keywords

agriculture activities climate extreme evens environment changes flood and drought Poyang Lake

Introduction

Environmental change is a comprehensive environmental evolution process in which certain environmental effects are triggered by the interaction of various environmental elements. This change is jointly affected by the natural environment and human activities and plays an important role in the development of human society. In the early stage of social development, human activities had a relatively weak impact on environmental changes, which manifested as adaptations and obedience to the environment. With technology improvements and increased exploitation of natural resources, the impacts of human activities on environmental changes increased (Novenko et al., 2019; Sadori et al., 2016).

In the study of environmental changes in river basins, hydrological environment, as one of the components that directly affects comprehensive environmental changes, is affected by factors such as climate and human activities, showing certain vulnerabilities and sensitivity (Solander et al., 2019; Sun et al., 2019). At the same time, occurrence of extreme events also interacts with changes to the hydrological environment and is directly controlled by precipitation (Jia et al., 2011; Zhang et al., 2016a). Over the past 2000 years, China’s climate has undergone phase changes with alternating cold and warm periods (Chu, 1973; Ge et al., 2002; Qin et al., 2005). In different periods, disasters caused by climate change have had a significant impact on the development of human society. For example, the occurrence of flood and drought disasters shows obvious waveband changes during the long-term drought and flood climate sequence changes (Hao et al., 2010; Tang and Hu, 2017; Zhang et al., 1997a). Another example is the impact of climate change on agriculture. In ancient China, social and economic stability was determined by the peasant economy during the traditional society. Therefore, the occurrence of extreme weather events was likely to delay planting, or delay or prevent the harvest, which would have seriously affected agricultural production and threatened social stability (Deelstra et al., 2011; Jørgensen and Termansen, 2016; Rosenzweig et al., 2001).

Over the past 40 years, significant progress has been made in the study of environmental changes in the Poyang Lake Basin. On the one hand, research has focused on changes in lake water levels, river networks, and lake connections under the influence of climate change and human activities (Huang and Zhu, 1983; Li et al., 2016; Tan and Zhang, 1982). On the other hand, the research has also focused on reconstructing the sequence of environmental elements during the historical period, and studying the relationship between social and economic activities and human social development under the influence of extreme weather events and natural disasters (Wan et al., 2018; Xiao and Yan, 2019; Zhan et al., 2013; Zhang et al., 2016b).

In the past 20 years, there have been many analyses and studies about the temporal characteristics of extreme weather and climate events around the world, and there are also different quantitative methods for defining extreme weather events (Alexander et al., 2006, 2009; Choi et al., 2009). For example, extreme weather events can be identified using anomalous records of climatic elements such as temperature and precipitation or indexes such as the number of days exceeding a certain threshold value. To be more precise, traditional climate research focuses on the uniform distribution of average weather conditions, while extreme climate research emphasizes small probability abnormal weather events, such as droughts, cold waves, heat waves, and heavy precipitation, that are far from the average value in the climate distribution (Ren et al., 2010).

Analysis of climate change (especially extreme climate events) and human activities has always been important to the study of historical environmental changes in the past 2000 years. Use of historical documents to study the relationship between Poyang Lake’s environmental changes, natural disasters, and human activities in the past 2000 years is also of great significance for understanding the mechanism of environmental changes, lake land remediation, and regional sustainable development. Although history, climatology, geology, and other disciplines have begun to predict the possibility of disasters from future climate and environmental changes by simulating historical data, they have always lacked knowledge and understanding of society’s risk adaptation and resistance capabilities. Previous studies mainly focused separately on evolution of the lake environment, disaster events, and agricultural socio-economic activities, with only a few studies of the lake environment as a comprehensive system. What are the main drivers of lake environmental changes? What are the interactions between changes in the lake environment and human activities? And what is the mode of interaction between climate change–natural disasters–human activities–agricultural production? These are important questions that need to be studied urgently.

Poyang Lake is the largest freshwater lake in China, including many rivers within the basin, and has been a direct and important participant in the socio-economic development of the middle and lower reaches of the Yangtze River. However, as an area of mega lake-oriented social economy, the socio-economic development of the Poyang Lake basin has been significantly slowed down in recent decades, related to the frequent occurrence of droughts and floods in the basin. Therefore, it is important to investigate the relationship between extreme climate events (droughts and floods) and agricultural production in the history of Poyang Lake basin environmental changes. This paper explores lake environmental changes and their impacts on history by studying flood and drought disasters and agricultural activities in the Poyang Lake basin. Through this research, we aim to improve understanding of Poyang Lake’s environmental changes and summarize the agricultural and socio-economic development under the influence of environmental changes in the middle and lower reaches of the Yangtze River. It is also helpful to understand interrelationships between environmental changes in river and lake systems and socio-economic development to solve existing problems. Therefore, we explore the correlation between humans and land concretely.

Material and methods

Meteorological and hydrological data from the historical periods have been delineated in previously published studies (Shi et al., 2015; Tang and Hu, 2017; Wan et al., 2018; Zhang et al., 1997b; Zhang et al., 2016b). In this research, we have synthesized past research results. By combining Hao et al.’s (2010) eastern drought and flood sequence and Li et al.’s (2016) δ¹³C total organic matter (δ¹³Corg) analysis of Poyang Lake sedimentary cores, we reconstructed the millennium-scale hydrological change sequence for the Poyang Lake Basin. Using historical records of drought and flood disasters since the Han Dynasty to record and reconstruct annual drought and flood grades, we obtained the drought and flood sequence for the eastern part of China over the past 2000 years, which roughly correlates to precipitation conditions in the Poyang Lake Basin. The δ¹³Corg value from the Poyang Lake sediment can be used as a substitute value in the reconstruction of regional precipitation. Negative δ¹³Corg values represent increases in regional precipitation, since the δ¹³Corg value is negatively correlated with regional precipitation. In addition, the Water Resources Department of Jiangxi Province compiled the “Collection of Flood and Drought Disasters in Jiangxi Through the Ages,” which includes records of flood and drought disasters from 381 to 1987 AD. The local records and related historical materials within the province were collected and unified, and the flood disasters in Jiangxi Province were divided into water, major floods, and mega-floods, while the droughts were divided into three different disaster levels: droughts, major droughts, and mega-droughts. Flood and drought disasters that occurred in the 12th–20th centuries were studied using Frequency analysis (Water resources department of Jiangxi Province, 1988).

Socio-economic and demographic data from historical periods were derived from ancient documents and published research (Liang, 2008). The discovery of these research data has been an important aid to understand environmental changes in the watershed and can effectively illustrate the organic link between environmental changes and socio-economic development in the watershed.

Study area

The Poyang Lake basin is located on the south bank of the middle and lower reaches of the Yangtze River, with a total area of 162,225 km² (Figure 1). The water system basin overlaps with Jiangxi Province, accounting for 97% of the province. The province basin includes Nanchang, Jinxian, Xinjian, Xingzi, Yongxiu, Duchang, Ruichang, Fengcheng, etc., with a total population of more than 40 million. At the end of 2016, 3082 thousand hectares of land was cultivated. With an area of 4125 km² during the lake’s high-water level, Poyang Lake is the largest freshwater lake in China. However, in the dry season, the lake area decreases to 500 km². The Poyang Lake basin has a subtropical humid monsoon climate with a mean annual temperature of about 17°C. Affected by the subtropical high, this area has prevailing south or southerly winds and has a higher temperature and more rain in summer; in winter and spring, affected by Siberia’s cold current, the area has lower temperatures, less rain, and prevailing northerly winds. The annual rainfall maximum is 1570 mm with seasonal and spatial variations. Therefore, Poyang Lake has seasonal characteristics with regard to floods and droughts. Since Poyang Lake converges with five rivers (Gan, Fu, Xin, Rao, and Xiu River) and their tributaries, and then it flows into the Yangtze river, it is an important part of the Yangtze river network.

Figure 1.

Location of the Poyang Lake Basin (climate and hydrological datasets were collected from 11 stations: Ganzhou, Ji’an, Nanchang, Fuzhou, Poyang County, Shangrao, Guixi, Yongxiu, Gao’an, Yichun, Xingzi, however, Xingzi, Nanchang, Yongxiu, Poyang, Fuzhou are within the map extent, while the others are outside the area of the map display).

Results

Hydroclimate changes, extreme climate events, and disasters

The historical climate change sequence of the Poyang Lake basin in the past 2000 years shows periodic changes to the climatic and hydrological conditions. Dry and wet changes occurred concurrently, and precipitation changes are consistent with changes in runoff. The precipitation condition directly affected lake water volume. In the past 2000 years, major droughts and floods in eastern China had noticeable phase changes. Extreme drought and flood events in the entire eastern region occurred in 100–150, 250–350, 750–850, 950–1000, 1050–1150, 1400–1450, 1550–1650, and 1800–1950. From the perspective of the entire eastern region, the periods with the most severe drought occurred in the 4th century, the second half of the 8th century, the second half of the 11th century to the first half of the 12th century, and the first half of the 17th century. The relatively frequent periods of major rainfall and flooding occurred in the first half of the 2nd century, the second half of the 3rd century, the first half of the 9th century, the second half of the 10th century, the first half of the 15th century, and the 18th to the first half of the 20th century. In some periods, major droughts or major rainfall and flooding occurred more frequently. In other years, both droughts and floods occur frequently. Simultaneously, there could be successive droughts or floods or both droughts and floods. The δ¹³Corg value shows that precipitation in the Poyang Lake Basin was highest during the period from 340 to 880 AD, and then dropped to its lowest levels with fluctuations, around 1110 AD. During the Medieval Warm Period (1110–1350 AD), precipitation was at a high level, but during the period from 1140 to 1220 AD, precipitation was low. In the Ming and Qing Dynasties, precipitation was relatively low during the Little Ice Age (1350–1620 AD), but suddenly increased from about 1550 to 1560 AD, and has continued to increase from 1610 AD to the present.

During the past 500 years (1470–2014) in the Poyang Lake Basin, precipitation can be separated into three descending stages, two ascending stages, and two fluctuation stages. Among these stages, there were mutation points around 1545, 1630, 1700, and 1880. The years 1470–1551 (average precipitation of 1587 mm), 1629–1718 (average precipitation of 1575 mm), and 1954–2014 (average precipitation of 1570 mm) are the decreasing periods when precipitation was reduced. The years 1603–1628 (average precipitation of 1682 mm) and 1719–1882 (average precipitation of 1654 mm) were the rising stages of increased precipitation. The volatility stages were in 1552–1602 and 1883–1953. In addition, the runoff sequence for the Poyang Lake Basin showed that compared to the runoff average for the past 1000 years in China, this area’s runoff has undergone seven stage changes. Runoff was higher than average in the periods of 1401–1450, 1513–1586, 1738–1758, 1832–1847, and lower in the periods of 1451–1512, 1587–1737, and 1759–1836 (Zhang et al., 2016b). During the Sui and Tang Dynasties, Poyang Lake runoff fluctuated around the mean value, alternating between dry and wet. During the Ming and Qing Dynasties, there were drastic changes to patterns of dryness and wetness. Extreme droughts and floods occurred frequently. Precipitation and runoff were in a dry state, separated from the wet state, and the dry and wet changes were evident.

Climate change in the historical period, especially disaster events caused by climate change, has a comprehensive and far-reaching impact on agricultural production. Droughts and floods in the Poyang Lake Basin affected agricultural production in the area, and when they were long-term and large-scale, severe famines happened. Tang et al. (2017) collected 1435 historical records of flood and drought disasters in the Poyang Lake Basin from 381 to 1949 and studied the characteristics of historical flood and drought disasters. Their results show that there were more floods in the humid period than during drought years, and the number of years with major floods is much greater than those with major droughts. The number of drought years during the dry period is slightly more than years with floods, but the number of major drought years is more significant than the number of major floods. From 960 to 1698, the Poyang Lake basin experienced three cycles of humidity and drought. From 1699 to 1949, there was a humid and rainy climate for more than 250 years (Figure 2).

Figure 2.

Historical flood and drought sequences recorded in the Poyang Lake Basin (Tang and Hu, 2017). (a) Historical flood and drought occurrence. (b–e) Flood and drought sequence of the Eastern Jin Dynasty to the Tang Dynasty, the Song Dynasty to Yuan Dynasty, the Ming Dynasty, and the Qing Dynasty.

The earliest record from Jiangxi’s collection of historical documents on flooding in the Poyang Lake area can be traced back to the fifth year of Emperor Wen (175 BC), which is recorded in the Book of Han: “Stormy weather in Wu damaged the city, government office, and people’s houses.” The “city” here refers to present-day Nanchang. A historical collection of flood and drought disasters in Jiangxi since the sixth year of Taiyuan in the Eastern Jin Dynasty (381 AD), has these record of extreme droughts: “In 884 AD, there was a severe drought in the south of the Yangtze River, and people were starving and turned to cannibalism.” “In 1434 AD, there was a severe drought in Jiangxi. There was no rain from April to August in the eight prefectural cities, including Linjiang, Ji’an, Ruizhou, Yuanzhou, Fuzhou, Nanchang, Nankang, and Ganzhou, where the crops dried up.” “In 1523 AD, droughts occurred in Guangxin, Linjiang, Ji’an, Yuanzhou, and other prefectural cities. There was great famine in Jianchangfu and Boyang, after the fern roots and bark of Chongren had been eaten up, people were still starving.” “In 1544 AD, a drought occurred in Jiangxi Province extended over a large scale, and people dying of starvation can be seen everywhere. It did not rain in the summer and autumn in nine prefecture, so people had no harvest, and a great famine occurred the following year.” “In 1835 AD, there was a severe drought, a plague of locusts, and famine in the province” and “In August, because the locusts blocked the sky and sunlight, people had to eat roots, bark, and Guanyin soil, and many people starved to death.” There are also records of extreme floods, “In 1404 AD, heavy rain and windy weather in Boyang caused a flood, the city was submerged more than 6 meters deep, houses were washed away, thousands of people drowned, the dike was destroyed more than 1500 meters, and people had to use boats.” “In 1647 AD, there was a great flood in the spring and summer of Yuanzhou, which led to a very severe famine, with people dying of starvation everywhere and turning to cannibalism. In Pingxiang, some people killed others and sold their flesh, and in some places in Fenyi and Gao’an, people died out, resulting in a 100-mile radius with no sign of humans. And “No grain was harvested in Ruichang and Boyang, people ate roots and dirt, and there were dead people everywhere.” “In 1869 AD, the province was flooded in the summer, and all the dikes along the shoreline were submerged, destroying fields and houses in ways that are difficult to calculate.”

The total number of recorded floods and droughts in the province during the historical period was 183, with 95 floods and 88 droughts, including 33 extreme floods and 37 extreme droughts. The Jiangxi Provincial Water Resources Department recorded 665 years of flood and drought disasters and recorded 393 years after the county’s local flood and drought disasters were classified as normal years. From the 12th to the 20th centuries, there were 88 floods and 80 droughts in the entire basin. On average, there were 3.2 floods for two consecutive years in each century, and 2.4 droughts for two consecutive years, so disasters occurred frequently. During the Yuan, Ming, and Qing Dynasties, the Poyang Lake area was already an important agricultural and grain production area. Floods and droughts caused a large amount of arable land to be unproductive. A catastrophic flood could cause 6 million mu of arable land to be unproductive for years, and a catastrophic drought was even more terrifying, since 10 million acres of arable land could dry up. The frequent floods and droughts in the Poyang Lake area during the historical period undoubtedly created huge losses in agricultural production.

Xu Guangqi said in Eliminating Locusts: “There are three causes of famine: floods, droughts, and locust plagues. The land has its height, and the rainfall has its density. There are remedies for floods and droughts, but a locust plague caused by drought can destroy trees and grass for thousands of miles, which is more serious than a flood or drought.” Along with the frequent flood and drought disasters in the Poyang Lake basin during the Ming and Qing dynasties, locust plagues also caused large losses to agricultural production. Judging the severity of locust infestations in a region by the number of the Eight Wax Temples and the General Liu Meng Temple, Chen (1983) concludes that the south of Central China was less infested with locusts. During the Qing Dynasty, the Poyang Lake region suffered from widespread locust infestations, especially in the northern part of Jiangxi Province. In terms of temporal distribution, locust plagues were recorded for 8 years in the early Qing Dynasty and 10 years in the middle and late Qing Dynasty. Although the temporal distribution of locust plagues was similar in both periods, it should be noted that locust plagues during the early Qing dynasty were limited to a small number of areas, whereas during the middle and late Qing dynasty, locust plagues occurred over a wide area. Records of Floods and Drought Disasters in Jiangxi include local chronicles of floods and droughts in Jiangxi’s various prefectures and counties. In 1835 AD (the 15th year of Daoguang), severe droughts occurred in various areas of Jiangxi, and locust plagues spread throughout the Poyang Lake basin. Many people were forced to leave their native land and died of starvation on the way. Yu Gan recorded the drought as “a major drought that occurred once in ten thousand years.” According to Chen’s (2019) rough statistics on the locust plague season in Jiangxi local chronicles during the Qing Dynasty, the number of locust plagues occurred 37 times in autumn, 12 times in summer, 8 times in spring, and 30 times that cannot be classified. The distribution of these locust plague season data can explain the fact that the Poyang Lake area in the Qing Dynasty during the “Ming and Qing Little Ice Age” had an abnormal climate, with frequent droughts in autumn, and locust ravages. It was undoubtedly worse for the fragile agricultural environment of the region.

Evolution of Poyang Lake and its river network

Change in water volume is direct evidence of climate change in the Poyang Lake Basin, which is comparable with climate change and shows obvious phased characteristics. The lake change process in the Poyang Lake basin can be divided into two lake periods: Penglize period and Poyang Lake period (Figure 3).

Figure 3.

Expansion and shrinkage of Lake Poyang Lakes over the past 2000 years (Liang, 2008; Tan, 1982). (a) The Qin Dynasty to Han Dynasty. (b) The Jin Dynasty. (c) The Tang Dynasty to Song Dynasty. (d) The Ming Dynasty to Qing Dynasty (1 jin = 0.5 kg; 1 dan = 20 L).

The first development stage of the Penglize period began in the Pleistocene. During the late Pleistocene, the Yangtze River’s main channel moved to the south, and a series of ancient Yangtze River sections were left behind in the north. By the mid-Holocene, due to the lower Yangtze quasi–geological tilting and subsidence zone, the Jiujiang, Susong, Huangmei, and Wangjiang depressions below Wuxue formed huge alluvial fans. In the regional climate, summer rains were concentrated and heavy, which made the river water create a lake in front of the alluvial fan and connected with the Yangtze River due to rainwater replenishment, creating the early form of Penglize. Shi Ji records: “Floating down the Yangtze River, from Xunyang (now southwest of Huangmei County, Hubei Province) out of Zongyang (now Zongyang County, Anhui Province), passed Penglai. (Volume 28)” Volume 29 Shui Jing Zhu records “Mianshui, merges with the river, and goes through Penglize to the east (Volume 29),” and “Penglize, in the northwest of Pengze County, Yuzhang (Volume 40)” Judging from the current geographical location, Penglize should be located in the current Yangtze River and its banks to the north. Shangshu Yugong records: “Penglize was full of water and formed,” Documents Tongkao said: “The place where water gathered is called Zhu,” Shuijing Zhu recorded: “There are nine rivers flowing into Penglize, including Huhan, etc.” The supplemental historical documents show that Penglize was not regarded as an independent lake during the Han Dynasty. Many rivers were flowing into it, laying a morphological foundation for the existence of the lake. Tan and Zhang (1982) verified that about 2000 years ago, that is, decades after Emperor Wu of the Han Dynasty, Penglize separated from the Yangtze River. The main channel of Jiujiang (now Jiujiang City) was affected by the flow of the Ganjiang River, and sediment piled up along the northern ridge line of the river. The ridge line piled up and merged with the sediment carried by the Jiujiang branch to form a natural embankment. This separated the ancient Penglize from the Yangtze River and gradually shrank the area. In the late Western Han Dynasty, the faulted ancient Ganjiang gradually expanded into a larger area. It inherited the name of Penglize in the north of Jiangbei and became the new Penglize. Ancient Penglize had disappeared, and the remaining water bodies and rivers that evolved were named “Leichi” or “Leishui” at the beginning of the Tang Dynasty. Today, they are called Longgan lake (at the junction of Susong County, Anhui Province, and Huangmei County, Hubei Province) and Daguan Lake (in Susong County, Anhui Province). The water areas of those two lakes are only 420 and 189 km², respectively.

The emergence of the new Penglize was the second stage of the development in the Penglize period. The new Penglize formed from the faulted waters in the late Western Han Dynasty to the Sui and Tang Dynasties. The lake was bounded by Yingzikou at the southern end. The water body was stable, and the area was about 65 km long and 3–14 km wide, roughly the same as the current North Poyang Lake.

Since the Holocene, neotectonic movement in the Poyang Lake area has been occurring, and there is an obvious fault depression. The plain landscape in the center of the area gradually became swampy during the long-term sinking process. In 421 AD, the abolition of Qiaoyang County can be regarded as the beginning of the Poyang Lake period, that is, from the end of the Tang Dynasty to the beginning of the Northern Song Dynasty, Penglize crossed the Yingzikou and expanded to Poyang County, and Poyang South Lake appeared. Chu (1973) concluded that from the Sui and Tang Dynasties period to the Northern Song Dynasty was the third warmest period of China’s climate (around 600–1000 AD). During this period, the climate was relatively warm. Plum blossoms and oranges were able to grow and bear fruit in Chang’an (now Xi’an, Shaanxi) (the lowest temperature plum blossoms can withstand is −14°C, for oranges, it is −8°C). Between 1931 and 1950, the annual absolute minimum temperature in Xi’an was below −8°C, and there were cases where the annual minimum temperature was below −14°C). The high temperature and rainy warm climate made expanded the lakes in the middle and lower reaches of the Yangtze River. In the early Tang Dynasty, Poyang Lake was about 50 km wide from east to west, 150 km from north to south, with the lake area exceeding 6000 km², reaching the peak of water body expansion for Poyang Lake. The high temperatures and rainy weather in the Tang and Song Dynasties increased runoff in the Yangtze River Basin. Because ancient Penglize, which originally had a certain capacity for water regulation and storage, but had shrunk and disappeared, the water level of the Yangtze River surged, the river channel was blocked, and the flood diversion capacity was insufficient. Under those hydrological conditions of, the water body expanded rapidly on the Qiaoyang Plain, which was subjected to continuous depression, forming the approximate range of Poyang Lake that is seen today.

Since the Song Dynasty, with the expansion of Poyang Lake, the unique polder agriculture to the south of the Yangtze River began to rise and develop in the Poyang Lake area, and showing the influence of human factors on changes to lake morphology. For example, there was expansion of branch lakes such as Junshan Lake and Qinglan Lake, and their river sand volumes changed. The expansion of the delta into the lake caused the water level and water area of Poyang Lake to shrink and expand repeatedly after the Song Dynasty, and the construction of ponds and dikes accelerated expansion of the lake branch. In the late Ming and early Qing dynasties, the lake branch on the right side of Zhangjiazhou (now Jiujiang City, Jiangxi) developed. The expanding water area and lake level caused frequent flooding in the lower reaches of Xiushui and other delta areas. Tangyin, Changshan (Jiujiang City), and other places had become islands in the lake. Junshan Lake and Qinglan Lake were formed during this period. Gu Zuyu, a scholar in the Qing Dynasty, described Poyang Lake, as where both distant and close rivers gathered, which is a true reflection of the scene of the Poyang Lake in the Ming and Qing Dynasties. During the Ming and Qing Dynasties, several reclamation climaxes in the Poyang Lake area caused the lake area to shrink. The lake water level was affected by factors such as the hijacking of the Yangtze River and the backflow of river water, which resulted in a large amount of sedimentation at the bottom of the lake. The water level gradually changed from deep in the north and shallow in the south to deep in the south and shallow in the north (Su, 1992). Water and sediment fluxes from rivers entering the lake in the Poyang Lake Basin from 1000 to 2000 AD can be used as proxy indicators. From 1000 to 1450 AD, the average annual runoff of the Gan, Fu, Xin, Rao, and Xiu Rivers were 62.13, 12.05, 17.15, 11.76, and 3.22 km³/year, and the sediment transport average was 7.35, 0.62, 1.17, 0.39, 0.13 Mt/year; average annual runoff in 1451–1850 was affected by the cold and dry effects of the ice period and decreased to 58.58, 11.26, 16.03, 10.91, and 3.01 km³/year. Under the cold and dry climate conditions during the same period, the amount of sediment transport decreased by 6.53, 0.48, 0.98, 0.30, and 0.10 Mt/year; in 1851–2000, the climate in the Poyang Lake basin turned to warm and humid, with Gan River runoff of 62.10 km³/year, while the other four rivers were 11.89, 16.92, 11.49, and 3.18 km³/year. Due to the strengthening of human activities such as polder and vegetation destruction, the amount of sediment increased after 1700. As of 1950, the amount of sediment had increased to 9.22, 1.01, 1.70, 0.62, and 0.21 Mt/year respectively; in the following 50 years, the amount of sand entering the lake continued to increase (Xn, 2015). In general, changes in water and sediment fluxes into the lake in the Poyang Lake Basin during the last 1000 years are closely linked to changes in climate, cold, warm, dry and wet, and human activities.

Modern Poyang Lake has continued to develop since the Ming and Qing Dynasties, and the lake’s water surface is still expanding. Due to the long-term development of farming in the Poyang Lake area and replenishment and construction of farmland water conservancy facilities such as embankments, ponds, and reservoirs in modern times, the expansion of the Poyang Lake water surface is not obvious. In the late 20th century, rapid urban development prompted a new change in the evolution of Poyang Lake Basin lakes, that is, the formation of the zigzag lake group. Summer flood season precipitation is the leading climatic reason that led to formation of the lake group.

Socioeconomic, population, and agricultural production

During the process of changes to the lake system and river network in the Poyang Lake Basin, social and economic development accompanied the evolution of the lake. Increases and decreases in population, the scale of agricultural production, and lake environment changes were direct and significantly interacting.

The history of development in the Poyang Lake area can be traced back to 10,000 years ago. The Xianrendong Neolithic site in Wannian County proves that the Poyang Lake area had the primary conditions for human survival as early as the early Neolithic. In 1976, two ancient cultural sites, Dawangling and Mopandun, were discovered in Shahe Street, Jiujiang County (Chaisang District, Jiujiang City). Among the ruins, apart from stone tools and bronzes, a small number of iron fragments were unearthed in the upper layer. One was shaped like a spade, and dates to sometime from the late Spring and Autumn Period to the early Warring States Period (Jiangxi Provincial Museum, 1978). The large number of cultural sites and the use of agricultural metal tools proves that the Poyang Lake area had been equipped with sophisticated agricultural location conditions during the Shang and Zhou Dynasties, and a lake-oriented agricultural society had gradually begun to take shape.

Population growth is an important indicator of economic development, providing necessary technical support and labor for land development in the Poyang Lake area. Table 1 shows population changes in the Poyang Lake area in various periods (Liang, 2008). Poyang Lake area was under the jurisdiction of Yuzhang County during the Han Dynasty. The county had a vast amount of territory with an area of about 165,915 km², accounting for 4.21% of the country’s area. In addition, there were eight counties of Yuzhang County around Penglize. According to Hanshu Geography, the population of Yuzhang County in AD 2 was 351,965 km², and the average population of the lake area accounted for 40% of the county’s population. Compared to the lake area, the southern area was more sparsely populated and had an adequate population capacity.

Table 1.

Historical population density in Jiangxi Province.

Period	Density
West Han	2.12
North Han	10.06
Tang (639a)	2.89
Tang (742a)	15.65
Tang (807a)	8.95
South Song (1162a)	68.35
South Song (1223a)	81.95
Yuan (1290a)	82.99
Ming (1393a)	59.25
Ming (1491a)	43.06
Ming (1578a)	38.41
Qing (1786–1791a)	107.52
Qing (1812a)	127.02
Qing (1830–1839a)	134.91
Qing (1840–1850a)	135.08
Qing (1851a)	135.12

Source: Liang (2008).

From the beginning of the Tang Dynasty, society tended to stabilize, and refugees from the north brought advanced agricultural technology to the south. While the land and water conservancy facilities in the Poyang Lake Basin were developed, the lake area population increased rapidly. The popularization and use of good agricultural planting models such as rice transplanting and multiple cropping of rice and wheat promoted a substantial increase in grain production in the Poyang Lake area. In the first year of Tang Tianbao (742 AD), the population of Jiangxi was 1,676,257, and the population density was 15.65, which was seven times that of the Western Han Dynasty. In the first year of Chongning in the Northern Song Dynasty (1102 AD), the population of Jiangxi was 4,459,547, an increase of 2.7 times compared with the Tang Dynasty. The denser population of the lake area, rapid development of agriculture, and continuous improvement in the economic level have promoted continuous expansion of the development area, which coupled with the continuous economic development of the south, gradually caused the economic center of gravity to migrate southward, while the superior agricultural location conditions attracted a large number of people from the north. During the same period, Poyang Lake expanded, completing its expansion to the south. At the same time, the agricultural environment developed along with the population carrying capacity of the basin. During the Tang Dynasty, Duchang County was added to the Poyang Lake District and Pengze County was removed. Five counties were added during the Northern Song Dynasty, including Xingzi, Xinjian, and Jinxian, reaching 11 counties. The entire Poyang Lake basin increased from 18 counties to 68 counties from the Western Han Dynasty to the Song Dynasty. The subdivision of the construction zone illustrates the rapid process of population growth in the Poyang Lake area.

The Yuan, Ming, and Qing Dynasties were the second stage of agricultural development in the Poyang Lake area. The development of lake agriculture was in a pressure-driven mode under the state of population proliferation, specifically manifested in the increased population density and saturation of arable land. In the 14th year of Zhiyuan in Yuan Dynasty (1277), the population of Raozhou Road was 4,036,570. This population was the largest population of any road in the country. Because of its superior agricultural location in the lake area, Poyang County, the leading county, had the densest population. Taking Linjiang Road and Ruizhou Road as examples, the population during the Yuan period was 791,740 and 722,302, respectively, with corresponding land areas of 5680 and 5360 km², and population densities of 139.39 and 134.76. At the same time, the population density in Jiangxi (including Raozhou Road) was 82.99, the regional population carrying capacity was close to the threshold, and the area of arable land became saturated. The Ming and Qing Dynasties were two periods of rapid population growth in Jiangxi (Table 2). In the 26th year of Hongwu in the Ming Dynasty (1393), according to statistics, the population of the Poyang Lake District was about 1.234 million, and the total population of Jiangxi was about 9 million, while the per capita area was about 4.8 mu. There was a decrease in the subsequent period due to land mergers, wars, and other reasons. By the middle of the Qing Dynasty, the implementation of Diding Yin promoted population growth. In the 25th year of Jiaqing (1820), Nanchang, Nankang, Jiujiang, and other areas had exceeded 200. According to statistics, in the first year of Xianfeng (1851), the population of the Poyang Lake area was about 6.242 million, and the total population of Jiangxi was more than 23.874 million, while the per capita area was only 1.94 mu. Rapid population growth intensified pressure on agricultural production and prompted the regional population’s demand to expand the area of arable land.

Table 2.

Fields in the Jiangxi Province during the Ming and Qing Dynasties.

Time	Population	Field (mu)	Per capita (mu)
Ming (1393a)	8,982,481	43,118,601	4.80
Ming (1502a)	6,895,293	40,235,247	6.14
Ming (1542a)	6,095,931	40,115,127	6.58
Ming (1578a)	5,859,026	40,115,127	6.85
Qing (1661a)	1,945,586	44,430,385	22.84
Qing (1685a)	2,126,407	45,161,071	21.24
Qing (1724a)	2,172,587	48,552,851	22.35
Qing (1753a)	5,055,251	48,571,128	9.61
Qing (1812a)	23,046,999	47,274,107	2.05
Qing (1851a)	23,874,604	46,218,727	1.94
Qing (1873a)	24,507,000	46,220,099	1.89
Qing (1887a)	24,559,000	47,343,012	1.92

Source: Liang (2008).

Continuing land development of mountainous and hilly areas since the Song Dynasty, the reclamation of terraces in Jiangxi region to the Ming and Qing Dynasties has been relatively sufficient. In a state of continuous population growth, the agricultural population began to reclaim beach land for continued survival, thereby expanding arable land. The basin entered the climax stage of embankment construction and lake reclamation, with 11 counties in Nanchang, Xinjian, and Poyang as the main areas. Nanchang County had more than 130 embankments in the Ming Dynasty. The newly built county dikes were concentrated near the Poyang Lake with more than 100 dikes built in the Ming Dynasty. By the 35th year of the Qianlong Emperor of the Qing Dynasty (1770), 85 dikes had been built. During the Song and Yuan Dynasties, Poyang County built 14 dikes, and in the Ming Dynasty, 12 dikes such as Kongmuwei and Nanhuwei were built one after another, with 15 new dikes added during the Tongzhi period of the Qing Dynasty. Large-scale reclamation was reasonable in the early stage, and polder construction promoted economic development in the lake area. However, due to rapid population increase in the area and sudden increased pressure on arable land, people continuously began to reclaim the seasonal beaches in the lake area. The lake’s water level continued to shrink, and the ability to regulate water storage gradually reduced. The number of polders increased sharply, and the threat of river disasters also increased.

The long-term tax and grain burdens was an important reason for the continuous increase in agricultural pressure on the Poyang Lake Basin during the Ming and Qing Dynasties and the deterioration of polder construction. Taking the Ming and Qing Dynasties as an example, in 1393, the court expropriated 2,585,256 dan from Jiangxi Province, accounting for 10.45% of the national total, ranking second in the provincial administration commission. In 1502 and 1578, 2,528,270 dan were collected, accounting for 11.41% and 11.47% of the national total, ranking first in the provincial administration commission. Although the Qing Dynasty’s taxation system implemented the policy of sharing man taxation into fields based on the One Lash Method, the actual amount of grain levied had been significantly reduced, with a fluctuation (increase or decrease in different time) of about 900,000 dan from 1661 to 1784. However, the levied amount of local silver remained at 2 million taels, which was the most in the country. These trends can be seen in the relevant data in Tables 3 and 4 (Liang, 2008).

Table 3.

Taxation of Jiangxi Province during the Qing Dynasty (1661–1784a).

Time	Food (dan)	Proportion (%)	Per capita (mu)
Qing (1661a)	938,753	14.49	2.11
Qing (1685a)	925,423	21.37	2.05
Qing (1753a)	899,632	10.70	1.88
Qing (1784a)	899,836	10.82	1.95

Source: Liang (2008).

Table 4.

Actual and estimated tax collection in Jiangxi Region in Qing Dynasty (1662–1903a).

Time	Tax (tael)
Qing (1662–1695a)	2,028,290
Qing (1784a)	1,901,917
Qing (1820a)	1,920,182
Qing (1841a)	2,292,360
Qing (1842a)	2,249,330
Qing (1845a)	2,237,133
Qing (1849a)	2,163,282
Qing (1893a)	1,291,288
Qing (1903a)	1,949,478

Source: Liang (2008).

After the middle of the 19th century, agricultural development in Jiangxi gradually declined. The war led to a sharp population decline, a large area of arable land was abandoned, and agricultural output fell sharply. In 1862, there were 2.83 million hectares of arable land in the basin, which had decreased to 2.15 million hectares in 1912, and to 2.08 million hectares by 1931. By 1937, the wasteland area in the basin accounted for 59.27% of the arable land, which was as much as half (Zheng, 2000). The wasteland caused by the war amounted to more than tens of millions of acres, and the amount in Nanchang County alone was as high as 300,000 acres. The degree of land desolation was extremely high. The war was the main reason for the retrogression of modern agricultural development and the harsh agricultural environment in the Poyang Lake Basin.

Discussion

Research on the effects of environmental changes in the basin can effectively reveal the interaction between natural environmental processes and human activities. According to statistics from years in which the peak values of warm and cold ENSO events occurred, when ENSO was a warm event, the annual total precipitation in most areas of the Poyang Lake Basin increased to varying degrees except for a small area in the north and south. Spatially, the increase in annual total precipitation was generally centered in the central part, and decreased to the north and south, with a maximum value of about 15%. In contrast, studies have shown that during warm ENSO years, annual precipitation in the north and south regions of the Qinling Mountains and Huaihe River Basin mostly decreased. Statistical tests show that for each specific meteorological station in the Poyang Lake Basin, annual total precipitation in the basin does not increase significantly during ENSO warm events. The main reason for changes in water level and area of Poyang Lake in history was changes to precipitation levels in the Poyang Lake Basin and the Yangtze River Basin in the spring and summer under the influence of the East Asian monsoon. An abnormal East Asian monsoon can increase or decrease summer rainfall in the Poyang Lake basin, and cause the lake level to rise or decrease, thus affecting the occurrence of drought and flood disasters in the Poyang Lake Basin. However, there is not much evidence of volcanic events which were closely related to drought and flood in the Poyang Lake Basin.

From the perspective of the basin, the Yangtze River is a giant system with many subsystems, with its tributaries, and related lakes as components. As the confluence of multiple tributaries of the Yangtze River, Poyang Lake is connected in series between the tributaries and the Yangtze River. Therefore, management of Poyang Lake plays a critical role in flood control and disaster reduction in Jiangxi Province and the entire lower reaches of the Yangtze River. The Poyang Lake basin system includes lake waters, rivers connecting the lakes, flood channels of rivers entering the lake, beaches and dikes surrounding them, with the existing dikes and natural bank slopes formed at its boundary. At the same time, changes in the lower reaches of the Yangtze River will also be fed back into the lake system through outlets. The environment in which the system is located includes the regional geological background, climate background, and human social background. The above interactions and connections constitute the current Poyang Lake environmental system.

In the early stage of Poyang Lake’s formation, anthropogenic effects were relatively small, while climate change played a leading role. Precipitation directly participated in soil erosion, sediment transport, and deposition in the Poyang Lake area. The erosion and deposition of sediment played a vital role in the evolution of Poyang Lake’s morphology. During the Tang and Song Dynasties, the warm and humid climate provided water source conditions for expanding lakes in the middle and lower reaches of the Yangtze River. As the last water storage area for the Yangtze River system to enter the Yangtze River’s lower reaches, Poyang Lake bears the pressure of receiving floodwaters and sediment from the upper reaches. In addition, the southern area of the Poyang Lake Basin is open and flat, with sufficient capacity to support the lake’s expansion, and the formation of Poyang South Lake has sufficient congenital conditions. Before the Yuan Dynasty, the Poyang Lake Basin population was mostly concentrated in the Poyang Lake area, while the population in the southern region was relatively small. Land development had a sizeable supplementary space. Expanded development on hills and mountains was an essential link in the agricultural development of Jiangxi before the Yuan Dynasty. In addition, the occurrence of disasters such as floods and droughts limits agricultural production in the basin area, so the impact was relatively weak. The impact of climate on the development of agriculture and society is rather positive.

With the continuous strengthening of human activities, artificially changing the lake boundary and the relationship between rivers and lakes intensified siltation, promoted the distribution of beaches in the lake area, and changes in the water level of the rivers and lakes, which were the direct factors for deterioration of the lake environmental system of Poyang Lake in modern times. After a long, stable period for the social environment in the Poyang Lake Basin, the agricultural infrastructure has developed well. A good agricultural environment will inevitably increase the basin’s population and entry of external population. Therefore, when the regional environmental population carrying capacity reaches a critical point, the agricultural population will lose basic survival resources and face a dire situation of no land to cultivate. The Ming and Qing Dynasties were a period of rapid population growth across the country. During the Qing Dynasty, reform of the tax system and enactment of the taxation system promoted rapid population growth in the Poyang Lake Basin and intensified the demand for arable land from the agricultural population in the basin. On the one hand, after the development of Poyang Lake during the Tang and Song Dynasties, the water surface of the lake had expanded greatly, and its seasonal surplus and shrinkage characteristics made the bare tidal flat grassland a natural target for farmland reclamation during the autumn and winter dry seasons. On the other hand, the Ming and Qing dynasties occurred during the cold period of China’s historical climate, with obvious variations in terms of dryness and wetness, when floods and other disasters occurred more frequently, and poor harvests occurred almost every year. Under such natural and social conditions, the agricultural population needed to replenish cultivated land to continually maintain their survival, and expansion of the polder scale was inevitable. Therefore, during the late Qing Dynasty, changes to the lakes of Poyang Lake were mostly characterized by flood disasters. The construction of levees restricted the normal evolution of lake morphology. The large number of lake branches was the most recognizable feature of the Lower Poyang Lake in this period.

After the Opium War, China was reduced to a semi-colonial and semi-feudal society, and the small peasant economy began to disintegrate. The economic depression caused by social unrest made the agricultural model of the Poyang Lake region unsustainable, and the original agricultural economic model was broken. However, the destruction of the agricultural economic model did not happen on the premise that advanced technology or productivity could be replaced, so the process of disasters could not be prevented. After the Qing Dynasty, the evolution of Poyang Lake still was expanding, but the form of lake agricultural society guided by Poyang Lake was on the verge of collapse. During the feudal period, agriculture was the fundamental driving force for national development in China, and the closed national environment made people always succumb to nature. Insufficient understanding of nature made it difficult for the development of agricultural farming to deepen. The fragility of a single agricultural economy inevitably led to agricultural production being restricted by natural and social factors.

Environmental changes in the Poyang Lake basin during the historical period were mainly reflected in the pressure feedback mode of “population–agriculture” in the lake environment. In the early stages of society, population growth promoted lake agriculture and the social economy. With the increasing rate of population growth, the agricultural environment reached the limit of its carrying capacity. Reclamation of land intensified, the area of the lakes shrunk, the environment of the basin gradually deteriorated, and the occurrence of disasters, such as floods and droughts, increased. On the one hand, the deterioration of the river basin environment caused many people to leave the area, and at the same time, agricultural production fell sharply when arable land was abandoned. On the other hand, the decline in agricultural production led to intensified reclamation of fields, accelerated destruction of rivers and lakes, a continued rise in the number of wastelands, and the river basin environment entered a vicious circle. However, changes due to environmental disasters in the Poyang Lake basin cannot play a decisive role in the stability of agricultural production. Whether it is the emergence of disasters, agricultural production, precipitation, etc., all of these factors are connected and interacted with each other due to human participation in jointly building a lake and river-oriented social and economic form. Therefore, we should not exaggerate the role of disasters. But it is undeniable that the decline of the agricultural economy in the Poyang Lake Basin during modern times has caused the development of Jiangxi to lag. That lag is related to the deterioration of the disaster environment in the Poyang Lake Basin during modern times.

Conclusion

Through the compilation of historical documents, we collected data on climate, hydrology, socio-economics and population of the Poyang Lake basin over the millennium, and deeply explored the process of environmental changes there, focusing on the impact of extreme climatic events (droughts and floods) on socio-economic activities such as agricultural production and population, and made the following conclusions. The historical climate change sequence of the Poyang Lake basin in the past 2000 years shows that there are obvious stages of climatic and hydrological changes in the Poyang Lake basin, with dry and wet changes occurring together. Precipitation changes were more consistent with runoff changes, and precipitation conditions directly affected lake water changes and showed obvious stage staging characteristics. During lake system and river network changes in the Poyang Lake basin, socio-economic development accompanied lake evolution, increases and decreases in population, and the scale of agricultural production. Changes in lake environment also have a direct and significant interaction process, and show the “population-agriculture” pressure feedback pattern. In general, disaster events (droughts and floods or secondary disasters caused by droughts and floods) caused by extreme climate change during the expansion of Poyang Lake have had a negative impact on agricultural production and population security. Due to climate control, precipitation amounts will lead to the increase or decrease of lake water and the scale of any negative effect will be different, but from the available research data, the increasing number of disaster events in Poyang Lake basin during the historical period had become an important reason for the socio-economic delay of the area in recent times. Of course, the basin also has an important connection with the unreasonable conduct of human activities.

This study discusses the process of environmental changes and effects on the Poyang Lake basin since the historical period from the perspective of natural and artificial causes, combined with the occurrence of disaster events in the basin, and examined the relationship between the socio-economic and disaster environments in the Poyang Lake basin in modern times. This research also included analysis of the internal mechanism that caused the gradual weakening of the society and economy in the Poyang Lake Basin and the difficulty of sustained force to gain a specific understanding.

The methodology adopted in the study of historical environment changes in the Poyang Lake Basin can be applied to similar cases and elsewhere in China. The relationship between lake environment and human activities is useful for analysis of lake environmental changes in history globally. However, this relationship needs to be studied in the context of the characteristics of each lake and watershed scale, and there may be some differences that need to be analyzed in the context of the actual situation in the region and the relevant literature. Meanwhile, there are still some problems unresolved in this research. For example, the characteristics of the environmental change process of Poyang Lake Basin and its environmental effects in different historical stages as well as their influence on the formation of regional culture are worthy of further in-depth study. These issues need to be studied in the next step.

Footnotes

Author note

Qilin He is now affiliated to State of Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University, P.R. China.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research was funded by the National Social Science Foundation of China (Grant No. 18BZS154), Key Platforms and Scientific Research Projects in Universities in Guangdong Province of China (Grant No. 2018KTSCX212), and Guangdong Rural Science and Technology Commissioner Project of China (163-2019-XMZC-0009-02-0065).

ORCID iDs

Maofeng He

Hongfei Zhao

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