The earth is a system consisting of lithosphere, hydrosphere, pedosphere, biosphere, and atmosphere, and the water cycle process is the link between these spheres. Maintaining a healthy water cycle is key to achieving sustainable development. The water cycle is not an independent natural process, and solutions to water problems associated with the water cycle typically involve all spheres of the earth system and all aspects of the economy and society. In this study, we discuss the water cycle research from the perspective of the earth system, taking the three topics of forest-water relationship, water cycle change prediction, and comprehensive water cycle research as examples, for the reference of water science colleagues.

The water cycle in a river basin has been increasingly influenced by intense human activities along with the progress of human civilization and continuously growing water intake, revealing itself to complex natural-artificial dualistic characteristics. This study elaborates on the origin and development, scientific issues, and framework system of the natural-artificial dualistic water cycle theory in a river basin, as well as the scientific connotation of five dimensional attributes of water resources from the new perspective of dualistic water cycle, i.e. resources, environment, ecology, society, and economy. After that, key technologies based on the dualistic water cycle theory are proposed, and the frontier directions and development trends of research on water resources are summarized. This study intends to promote the research on water cycle in the river basin, hydrography, and the subject of hydrology and water resources.

The national water network project is a major strategic deployment in the 14th Five-Year Plan (2021-2025) and the Outline of the 2035 Vision Goals. However, under the influence of global changes, it is facing new opportunities and challenges. Based on the analysis of the background of climate, geography and economic and social development and the characteristics of unbalanced distribution of soil and water resources, this paper has completed three research works. It studies the relationship between supply and demand of water resources in the changing environment. Then it analyzes the planning and layout of the national water network and the effectiveness of the typical water network. Finally, some problems and countermeasures in the planning and construction of the national water network are discussed. Research shows that the impact of global change is more and more prominent. As the country enters the stage of high-quality development, people have higher requirements on the quality and guarantee of water supply. Moreover, the construction of national and regional water networks is an important measure to cope with the impact of environmental change and improve national water security. The construction of the national water network can significantly improve the guarantee of water supply and irrigation, flood control and drainage capacity, and enhance the quality of natural river and lake water environment, but it still has room for optimization and improvement. We need to coordinate regional "water-soil-air-gas" and "man-land" relations, strengthen scientific and technological innovation in the construction of the national water network, improve the comprehensive benefits of ecological environment and economic development of the project, and support the construction of national ecological civilization and high-quality development.

Global warming and urbanization aggravates the disaster risk of regional rainstorm and flood, and seriously threatens the security and development of regional economy and society. The "7·20" rainstorm and flood in Zhengzhou is the urban waterlogging disaster with the most casualties, the heaviest economic losses and the greatest disaster impact in a single city in China in the past 10 years. In this paper, we systematically analyzes the characteristics and causes of the "7·20" rainstorm and flood disaster. Focusing on the main problems of rainstorm and flood disaster prevention, the shortcomings of meteorological and hydrological monitoring and early warning are clarified. Finally, the key strengthening directions of weather forecast and early warning, hydrological monitoring and early warning, and new technology application in the prevention of urban rainstorm and flood are pointed out, in order to provide support for urban flood disaster prevention and intelligent management.

Subsurface stormflow is a special form of subsurface flow in the vadose zone of steep slopes in mountainous and hilly regions, and it is also one of the most important runoff generation mechanisms in the process of flash flood formation. However, the systematic summary on the scientific connotation, mechanisms, and disaster-causing characteristics of the subsurface stormflow in current research is very rare and this limits the development of the theoretical system of runoff generation, routing, and modeling methods of flash floods. Herein, we clarify the scientific connotation and definition of subsurface stormflow based on the division of the development periods of the subsurface stormflow research around the world as well as the analysis of physical processes for the runoff generation. Meanwhile, three physical conditions for the occurrence of subsurface stormflow, together with their influences on the nonlinear response process, are highlighted. The influences of the subsurface stormflow on the dynamic development of flash floods, landslides, and debris flow hazards are analyzed and summarized. Finally, three key research directions are proposed: water transport processes and hydrological connectivity mechanism of the macropores in the vadose zone, processes and mechanisms of water exchange at the geotechnical channel interfaces inside the slopes, and the formation mechanism for flash flood from multiple runoff generation processes with complex slope topography conditions. This work can provide new research perspectives for the formation mechanism and hydrological theoretical framework of flash floods in complex mountainous regions, and better support the national scientific and technological needs for flash flood forecast as well as disaster prevention and mitigation.

Terrestrial recycling of green water, which is created through a continuous cycle of evapotranspiration, atmospheric transport, and terrestrial precipitation, serves as bridges connecting hydrosphere, atmosphere, biosphere, and anthroposphere. Limited knowledge of terrestrial recycling may underestimate the impact of human activities on water and ecology. This study focuses on the structure of China's green water recycling (CGWR) and quantifies the contributions of natural and human ecosystems. Results indicate a high green water recycling ratio (50.4%) and a large recycled amount (2.75 trillion m³/a) over China, with significant south-to-north transport structures over eastern China, southwest-to-northeast transport structures from southwestern China to northern China, and high self-recycling structures in western China. The grassland-dominated Qinghai-Tibet Plateau provides the largest amount of green water (about 800 billion m³/a) and serves as the most critical region for CGWR. The forest-dominated southern hilly region and Yunnan-Guizhou-Sichuan region play a crucial role in supplying green water to a wide area of China. The cropland-dominated Middle-Lower Yangtze Plain and the grassland-dominated Loess Plateau are the key hubs for south-to-north and southwest-to-northeast transport of green water. The structure of CGWR is driven by large-scale geosystems such as ecosystems and atmospheric circulation systems, and has strategic implications in the following three aspects. First, CGWR serves as the "domestic cycle" of water, based on which a general domestic-level water governance framework can be established. Second, CGWR is a representative picture of the water-related processes in the Mountain-River-Forest-Farmland-Lake-Grassland-Desert (MRFFLGD) system and serves as a critical pivot to actualize the conservation and improvement of the MRFFLGD system. Finally, CGWR, together with inter-basin water resources transfer and virtual water transfer, forms a complete picture of China's inter-basin water transfer. The scientific coordination and integration of the three processes present a new approach for achieving spatial equilibrium between China's water resources and economic development.

Designing and building up the natural resources surveying and monitoring technological system (referred to as the "Technological System") is an important measure to establish and improve the natural resources management, promote the intensive and economical utilization of natural resources, and harmonize the coexistence between man and nature. It is a geo-science related research project with many complicated factors and challenging tasks. Following the mandates and requirement of the unified natural resources surveying and monitoring, the paper firstly analyzes the basic problems in designing and building up the "Technological System" from the holistic characteristics of the surveying and monitoring process and the systematic characteristics of its engineering implementation. A comprehensive overall framework of "Technological System" is resulted and discussed. Then the development direction of "Technological System" is proposed and analyzed, including the collaborative data sensing, automatic information processing, elaborative natural resource scene modeling and management, as well as intelligent knowledge service. Then the article discusses the digital-intelligent realization path of the "Technological System" to meet the advanced and practical requirement. It has led to the formulation of the future research agenda with five major research and development tasks, including the development of space-sir-ground-ocean-web collaborative sensing network system, construction of automatic information processing platform, spatio-temporal scene modeling of natural resources, development of spatio-temporal knowledge service of natural resources, and reconstruction of engineering technical flow. In the future, it is necessary to strengthen the strategic needs analysis, enhance key technologies innovation and promote interdisciplinary collaboration in order to effectively build up the "Technological System" and promote its operational application.

Water-related issues have long been one of the most important topics in China, and these issues are related to a series of major national requirements, including social stability, people's livelihood and welfare, and economic development. Hydro-geography is an interdisciplinary subject of hydrology and geography that adopts the principles, perspective and methodologies of geography. The theory of the terrestrial surface system has an important enlightening effect on the systematic research of hydro-geography. First, this study discusses the cross-relationship and research content in hydro-geography, as well as the rise of research on global water systems and the ecohydrology. After summarizing the main advocates of terrestrial surface system theory and their viewpoints, this study explores the relationship between the comprehensive and regional aspects of geographic research, while proposing that the "three characteristics" (i.e., comprehensive, regional and orderly characteristics of processes) should be the research thought for hydro-geographical system. Furthermore, based on the expansion of the basic theories of the subject, this study presents several cases of systems science and technology that can deepen the spatiotemporal dynamic process mechanism (orderliness) of hydro-geography. Finally, several key development and research directions of hydro-geographic research in China are discussed, mainly including: research on the impact of climate change and impacts of human activities on the water cycle process; research on the dynamic evolution mechanism of the four major balances in ecohydrology; research on renewability of water resources; the process of evapotranspiration of plants (crops) and water-saving control; hydro-geographic zoning, mapping technology and intelligence technology applications.

Ecohydrology is a key discipline developed in recent decades, which can give aid in the protection and restoration of complex ecological systems (e.g., mountain, river, forest, farmland, lake), ultimately promoting the ecological civilization construction and the green development of China. In this paper, the progress and existing challenges of ecohydrological discipline are elaborated, and the future development directions are proposed according to the international scientific frontiers and national demands on ecological civilization construction. The main directions are to develop new ecohydrological monitoring methods and improve comprehensive observation network of ecohydrological systems; to perfect the ecohydrological mechanisms and their basic theories; to promote the integrations of multi-scales and multi-elements by considering both terrestrial and aquatic ecosystems; to promote the multidisciplinary integrations between ecohydrology and social sciences. Furthermore, specific future research interests in China are proposed as follows: multi-source information fusion and comprehensive monitoring system construction, spatio-temporal patterns of key ecohydrological elements and their variation characteristics; integrated models of ecological, hydrological and economic processes and their uncertainty estimation; interdisciplinary studies including physical and social sciences. The application prospective in China is further explicated in a variety of ecosystems (e.g., forest, grassland, river and lake, wetland, farmland and urban area). This paper is expected to provide a reference to support the development strategy of the ecohydrological discipline in China, and to give a theoretical foundation and technical support for the implementation of national ecological civilization construction.