GEOGRAPHY

Satellite observations reveal hot spots for changes in river waters around the world


On March 22, the research group of Song Chunqiao, a researcher at the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, published a research paper entitled “Satellites reveal hotspots of global river extent change” online in the journal Nature Communications, reporting that the use of long-time series satellite remote sensing, global lake reservoir datasets and multi-source meteorological data to reveal the types of decadal changes in global river waters. Research findings on hotspots and their main drivers (climate change/anthropogenic impacts).

Rivers are one of the most active ecosystems and water cycle components on the earth’s surface, which are of great significance to human socio-economic development, the sustainability of the ecological environment of the river basin and the stability of regional climate. In the context of increased disturbance of hydrological systems by global changes (global warming, melting of glacial frozen soil, floods) and human activities (reservoir construction, aquaculture, etc.), the hydrological situation of rivers has undergone large-scale and significant changes. Monitoring this signal of change in rivers on a global scale and understanding the drivers behind it is challenging.

Based on the latest SWOT Satellite River Database (SWORD) and the Global Surface Water (GSW) dataset, the study comprehensively investigated the evolution of river waters with a total length of 2,097,799 km and a total area of 769,390 km2 globally in the early 21st century (2000-2018) relative to 1984-1999. The study compiled the global dataset of new reservoirs since 2000, and through the establishment of massive human-interpreted samples and machine learning methods, for the first time, three types of global river water changes were distinguished: dam-driven river expansion (Type-R), river channel morphological evolution (Type-M), and dry and wet hydrological signal dominant type ((Type-H) (Figure 1). Focusing on the dominant types of hydrological signals, the spatial pattern and hotspot areas of global river water expansion/contraction were reported, and the main influencing factors of river water changes were analyzed by combining long-term meteorological data, night lighting data, and published literature.

The results show that about one-fifth of the world’s rivers have undergone significant geomorphological evolution (such as river channel migration, braided water system swing, etc.) (Type-M). About 25% of these types of evolution occur around the high mountains of Asia (Brahmaputra, Indus, Ganges, Ayewady, Amu Dary) and in the middle and upper reaches of the Amazon in South America, where up to 40-80% of intra-channel evolution occurs. Such changes in river morphology occur under specific hydrological and geoenvironmental conditions, such as curved, multitributary channels, and are related to geological activity, runoff intensity, slope, riparian erosion intensity and sedimentation rate, reflecting the unstable characteristics of river courses. In addition to the characteristics of rivers themselves, climate change and human activities may lead to the increase of river instability, such as in the Brahmaputra River, Ganges, Indus River and other river basins, the seasonal runoff caused by glacial snow meltwater and engineering impacts such as water transfer and storage, river course instability is very prominent. The expansion of river water area (Type-R) caused by reservoir construction is particularly significant: at the six-level basin scale, the new reservoir construction has led to an overall increase of 30.5% in the river water area, most notably in developing countries and regions in Asia, South America and central and western Africa, with Brazil, China and India being the three countries with the largest impact on river water area by new reservoirs, contributing 21.7%, 18.5% and 10.5% respectively. Compared with other types (Type-H) river water expansion signal strength, the river water expansion effect of dam construction cannot be ignored, and its increased river water area accounts for 31.9% of the global river water expansion.

Fig. 1 Distribution of different river water change types in the world (Type-M: river morphological evolution; Type-H: Wet and dry hydrological signal dominant; Type-R: Dam-driven river expansion). (a) Global distribution of different river change types. (b) Area statistics on changes in different types of rivers on six continents. (c) Statistics on different types of change in 25 major basins around the world. (d-f) Examples of water frequency change (OCI) patterns of three types of change. Red, green, and blue line drawings indicate the maximum statistical ranges of rivers of Type-M, Type-H, and Type-R types, respectively.

In addition to the evolution of river channels and the expansion of rivers driven by damming, the research focuses on the analysis of river water extent changes dominated by hydrological signals (Type-H). The results of the study (Figure 2) found that at the global scale, the percentage of significant (moderate) increase in the area of river waters reached 9.0% (8.6%), higher than the significant (moderate) decrease of 4.8% (7.4%). By quantifying the net change of river area in each basin unit, this study reveals the change characteristics of the eight hot spots with the most significant global increase and decrease (expressed as positive and negative hotspot areas) and their relationship with major climate factors (precipitation, temperature and evapotranspiration). The positive hotspots are all located in Asia, including eastern Siberia, the Qinghai-Tibet Plateau, north-central Siberia and east-central Asia, mainly because high latitudes or altitudes are more sensitive to climate change; Negative hotspots are distributed in the central Great Plains of North America, central and eastern South America, western Siberia and northern India, mainly dominated by arid or semi-arid climates. The study also discusses the reasons for the relative expansion of river water area in the Yellow River Basin since the 21st century in China, which may be related to the unified water flow control of the Yellow River and a series of water-saving measures since the 21st century.

More than half (70.2%) of the world’s rivers are relatively stable relative to the proportion of river water expansion, with the highest proportion distributed in North America (82.1%), followed by Europe (79.5%) and South America (70.5%). The rivers of developed regions such as northwest Europe (such as Finland and Sweden) and North America (such as Canada and the United States) are relatively more stable than those in developing regions such as Asia (such as Myanmar and China) and South America (such as Bolivia and Peru), and there is a certain correlation between their stability and nighttime light intensity, which may be related to the level of socio-economic development: on the one hand, residential agglomerations are generally far from areas with river height changes (such as river sources, floodplains, etc.); On the other hand, the river embankment project developed earlier in developed areas has stabilized the scope of river waters.

Fig. 2 Characteristics of global river dry and wet hydrological signals. The figure shows the characteristics of river water changes from 2000 to 2018 relative to 1984-1999 (PI-PD-PGS: the ratio of increase-decrease-relative stability of river waters), and the main hotspots of water expansion and shrinkage are shown below.

In general, based on long-time series satellite observations, this study reveals the changing characteristics and dominant driving mechanisms of global river water areas in the early 21st century, which can provide a scientific basis for the formulation of future river priority protection and restoration plans in the United Nations 2030 Agenda for Sustainable Development, and also calls for international action to strengthen the long-term tracking, monitoring and protection of river water ecosystems.

Researcher Song Chunqiao of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences is the corresponding author of the paper, and graduate student Wu Qianhun and Associate Professor Ke Linghong of Hohai University are the first authors of the paper. Co-authors include Professor Jida Wang from Kansas State University, Professor Tamlin M. Pavelsky from the University of North Carolina at Chapel Hill, Professor George H. Allen from Virginia Institute of Technology, Professor Yongwei Sheng from UCLA, Professor Jin Wu from the University of Hong Kong, Researcher Zhu Yunqiang from the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Professor Yongbin from Hohai University, and Wensong Zhang from Nanjing University, and Duan Xuejun, researcher Wang Lei, associate researcher Liu Kai, assistant researcher Chen Tan, and doctoral student Fan Chenyu from Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. The research was supported by the National Key R&D Program, the Pilot A Program of the Chinese Academy of Sciences, the General Project of the National Natural Science Foundation of China, and the Institute Independent Deployment Project. (Source: Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences)

Related paper information:https://www.nature.com/articles/s41467-023-37061-3

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