Field observations are conducted to study certain ecohydrological processes (Yu et al., 2014). Ecohydrological variables include meteorological variables (e.g., rainfall, air temperature, wind speed, radiation, and evapotranspiration), hydrological variables (e.g., water level, water discharge, and flow velocity), environmental variables for different ecosystems (e.g., water, carbon, nitrogen, and phosphorus), and ecological variables (e.g., vegetation and its net primary productivity, plankton, benthos, and fish). Observation techniques and methods for various variables at different scales vary significantly, but the current attention mainly focuses on carbon flux, soil water transport, evapotranspiration, and soil water-related parameters observation at a point or a field scale. Remote sensing provides an effective method for achieving scale expansion from a point to an entire area (Montaldo et al., 2013). As a good tracer of water movement in different forms among soil, vegetation, atmosphere, and ocean, isotopes have become an important research tool in various disciplines, including meteorology, hydrology, and ecology. Therefore, in the future, we should focus on developing comprehensive monitoring technologies, including continuous flux and isotope measurements, field monitoring, remote sensing, and radar at multiple scales.

In addition to further research concerning the interaction mechanisms between ecological and hydrological variables, it is necessary to establish and improve a comprehensive ecohydrological observation network nationwide. Several long-term ecological monitoring programs exist globally, such as the Global Flux Network, the US National Ecological Observatory Network and Long-Term Ecological Research, and the European Flux Program. To date, China has mainly focused on monitoring network construction for individual variables, such as meteorology, hydrology, water quality, and ecology. The existing networks are ground meteorological observation networks and high-altitude meteorological observation networks that are operated by the China Meteorological Administration, the National Hydrological Station Network of the Ministry of Water Resources, the surface water quality monitoring network in the Ministry of Ecology and Environment, the Chinese Ecosystem Research Network, and the China Terrestrial Ecosystem Flux Observation Research Network. These observation networks have greatly promoted the development of ecology, hydrology, and other disciplines in China, but most are limited to single departments or disciplines, such as meteorology, water conservancy, ecology, and environment. Thus, long-term monitoring networks at a large scale and integration of data sharing must be improved. Meanwhile, ecohydrological observations in China should comprehensively consider regional divisions of physical geography, ecology, hydrology, and water resources, and should be expanded to include more indicators in the hydrology, ecology, climate, and soil fields. Moreover, the existing monitoring network systems should be optimized and improved at multiple spatiotemporal scales, and the relevant data sources of ecohydrology among different disciplines and departments should be shared. Together, these changes could provide a solid basis for the development of a comprehensive ecohydrology discipline in China.

Investigations of the interaction mechanisms of various variables in terrestrial and aquatic ecohydrological processes, coupling mechanisms between hydrological and ecological processes, and their responses to global changes, are not only the core themes in the comprehensive theory of ecohydrology, but also the foundations to support understanding water cycle evolution. Currently, studies on terrestrial ecohydrological processes have mainly focused on the response mechanisms among ecohydrological variables in farmlands and forests, most of which use the SPAC and Budyko water-heat balance theories to analyze the exchange mechanisms of energy and water between soil, vegetation, and atmospheric interfaces (Yang et al., 2016). However, there are more studies worthy of exploration regarding ecohydrological conversion mechanisms from field scales to watershed or global scales. Rivers and lakes are important parts of natural ecosystems and are the main pathways of material circulations, energy conversions, and information exchanges between terrestrial and aquatic ecosystems. Currently, most of the existing studies on ecohydrological processes exist at the stages of field experimental observation and semi-quantitative research (Zolezzi et al., 2009). Thus, it is important that future aquatic ecohydrological mechanism research comprehensively reveals the continuity characteristics and heterogeneities of ecohydrological processes via field monitoring, laboratory experiments, and numerical simulation approaches.

In addition to improving the ecohydrological mechanism and basic theoretical paradigm, it is necessary to further the exploration of coupling mechanisms and theoretical paradigms of terrestrial and aquatic ecohydrological processes, and clarify migration and transformation rules and evolution characteristics of biogenic (e.g., carbon and nitrogen), biological, environmental, and climatic factors at the soil, vegetation, atmosphere, and water interfaces. Meanwhile, the dynamic response mechanism of key ecohydrological variables to climate change and human activities should be further explored as it has become a research topic. In addition, it is important to develop a multi-scale and multi-variable theoretical system to comprehensively research terrestrial-aquatic ecohydrological processes in a changing environment.

In the Outline of 13th Five-Year Plan for Economic and Social Development in China (2016-2020), it is clearly stated that “the protection of the ecological environment should be strengthened with the improvement of environmental quality as the core, and with the solution of outstanding problems in the field of ecological environment as the focus”. Currently, the main research areas of terrestrial ecohydrological processes include water transport along the vertical gradient, physiological and ecological vegetation processes, and impact of ecosystems on hydrological processes and underlying surfaces. Conversely, the main research areas for aquatic ecosystems include relationships between biological habitats, ecosystem composition, and runoff processes. As vegetation, soil, and water are the key carriers of various life activities in both terrestrial and aquatic ecosystems, they are also the main reaction sites for biological, physical, and chemical processes. The contradiction between traditional socioeconomic development and ecological environment improvements grows increasingly worse, causing significant spatial variations in vegetation and biological communities, and remarkable ecosystem changes. This contradiction further aggravates the risk of ecological environment deterioration and restricts the construction of a national ecological civilization. Therefore, developing comprehensive research on terrestrial-aquatic ecohydrological processes is of great scientific significance and practical value (Xia et al., 2015). In addition to cross-disciplinary development, ecohydrological process studies incorporate ecological principles, ecological models, and spatiotemporal scales, leading them to the direction of comprehensive terrestrial-aquatic ecosystem research. To meet societal needs and achieve regional sustainable development, it is necessary to integrate multi-source observations (e.g., remote sensing and stations), build a comprehensive ecohydrological process model for multiple-scales (e.g., station, patch, and watershed), and assess and regulate stability of ecohydrological systems. Meanwhile, it is important for China to rationally allocate water resources and ensure functional integrity and species diversity of terrestrial and aquatic ecosystems.

Multiple aspects of socioeconomic development, including agricultural production, urban expansion, construction and regulation of water conservancy projects, have negatively affected ecohydrological processes, leading to imbalanced development of ecosystems. Comprehensive ecohydrological research is one way to solve many water issues, including resource shortages, environmental pollution, and ecosystem degradation, and is also one of the core topics of major global research programs. In 2004, the Earth System Science Partnership proposed the “Global Water System Plan”, which focuses on the role of human activities in global water system processes (i.e., physical processes, water quality and ecological processes, and human economic processes). In 2013, IAHS launched a scientific plan covering the next ten years (2013-2022), i.e., Panta Rhei-Everything Flows, with the theme of “Change in hydrology and society”, in which ecohydrology for the sustainable world is one of the six core themes. In 2017, an International Ecohydrology Conference was proposed to promote ecohydrological research oriented toward a circular economy, and to advocate for solutions that integrate the concept of a circular economy and follow natural ecological processes. Therefore, integrated research on ecohydrology and social science, and investigations regarding the impacts of human activities have important scientific significance and practical value for analyzing evolution characteristics of ecohydrological variables and formulating plans to restore and reconstruct ecohydrological systems.

Because of global change and economic integration, ecohydrological studies at the site or basin scale must be expanded to comprehensive studies at the global scale. The rapid development of global earth observation technology, data sharing mechanisms, and computer technology have provided strong basic supports for studying multiple ecohydrological processes in the world. For example, a large number of ecohydrological element products exist at the global scale, including global precipitation products (e.g., NCEP/NCAR, CRU TS, and CPC), global runoff databases (e.g., GRDC), global land cover data (e.g., GlobCover), global land use data (e.g., UMD and GLCC), and global evapotranspiration and soil datasets (e.g., GLDAS). Therefore, exploring the coupling mechanisms of energy, and hydrological, ecological processes in the terrestrial and aquatic systems, and economic and human processes at the global scale in order to realize a comprehensive simulation of global ecohydrological variables, and assess impacts of climate change and intensive human activities has become an ecohydrology research hotspot.

Multi-source information monitoring and comprehensive network construction are the cornerstones of multi-scale comprehensive ecohydrological research. Owing to the existing problems of current monitoring technologies and scale limitations, the comprehensive monitoring methods of ecohydrological variables and functions must be further developed, and the technology of large-scale lysimeters, eddy covariance, stable isotopes, and quantitative remote sensing must also be further improved regarding evaporation, soil moisture movement, carbon-water flux, plant transpiration, and soil evaporation processes. In the future, comprehensive monitoring of multi-source information combined with multiple monitoring technologies must be implemented, and comprehensive continuous measurement technologies for flux observation and isotopes should be improved. The integration of field observations at the site scale, remote sensing measurements, and other multi-scale observations should also be further developed. Moreover, real-time information collection, transmission, and remote monitoring technologies should be developed for automatic monitoring of ecohydrological variables. Furthermore, national technical standards and specifications should be formulated for long-term monitoring and systematic observation networks. Finally, a comprehensive monitoring network of ecohydrological variables for terrestrial-aquatic systems should be designed by integrating existing observation network systems, such as meteorology, hydrology, water quality, and ecosystems (Figure 1). Meanwhile, it is necessary to comprehensively consider regional divisions of physical geography, ecology, hydrology, and water resources in order to further improve monitoring network system, which would provide a solid foundation for future ecohydrological studies in China.

The spatiotemporal distributions of critical ecohydrological variables and their evolution characteristics are of increasing interest regarding ecohydrological mechanisms, and also can reveal the evolution mechanisms of ecohydrological processes and develop their basic theories. The specific ecohydrological characteristics of the spatiotemporal evolution of hydrological variables are presented as follows. Natural runoff variability is considered to be the driving force for landscape changes in the riparian zone, and spatiotemporal changes in the flow regimes and their interaction with groundwater can cause structural and functional changes in river ecosystems. Furthermore, variation analyses of flow regime metrics have become important for studying landscape patterns of aquatic ecohydrological systems (Vigiak et al., 2018). Moreover, evapotranspiration is a critical ecohydrological process, as its variation could disorder the ecohydrological regional pattern. Water quality is an essential indicator of the water environment, as its characteristics reflect water responses to the ecohydrological system. In addition, the ecohydrological characteristics of the spatiotemporal changes in ecological factors are presented as follows. Vegetation type and its structural shift directly affect the hydrological cycle (such as interception, runoff, and evapotranspiration), affecting the healthy operation of integrated ecological, hydrological, and socioeconomic systems at watershed or regional scales (Blanco- Gutierrez et al., 2013). As a paramount ecological indicator, the net primary productivity of vegetation precisely affects the spatiotemporal patterns of evapotranspiration and water use efficiency, and can be applied to regulate the impact and energy storage of ecological processes. Plankton is an important component of aquatic ecosystems, and its spatiotemporal distribution characteristics reflect ecosystem health. The habitat, species types, and distributions of fish or large invertebrates are also applied to evaluate river ecological health conditions, which can reveal potential problems and their causes in river ecosystems. By investigating the spatiotemporal patterns of these ecohydrological variables, a comprehensive analysis of ecohydrological evolution characteristics and driving mechanisms can be developed, acting as a foundation for the implementation of numerous ecohydrological studies.

A terrestrial ecohydrological model was developed based on a traditional rainfall-runoff relationship model, and coupled with other models, including the vegetation growth model and soil biogeochemical process model. It is advantageous for quantifying interactions between vegetation, nutrient, and hydrological processes, as well as the dynamic mechanism between hydrological and ecological processes. The existing models include the Rutter, Massman, and Philip models, which simulate canopy interception and forest soil water infiltration. The SVAT model is used to simulate ecohydrological processes for diverse ecological types. Watershed hydrological models (e.g., DHSVM, VIC, SWAT, and HEQM) consider dynamic vegetation processes, and MXW and IRM models examine the relationship between groundwater and vegetation communities. Hence, more attention should be paid to exploring the mechanisms of vegetation dynamics and water-nutrient cycles, and considering the effects of climate change and human activities in order to strengthen the physical mechanisms of terrestrial ecohydrological models. A river ecohydrological model was developed based on hydrological, hydraulic, and water quality models, and comprehensively considered the growth status of aquatic organisms and quantified the response mechanisms among various water quality variables, flow regimes, and aquatic biological communities. The spatial dimensions of this model range from one to three, which is similar to those of MIKE, EFDC, and HSPF. In China, some conceptual models for river ecohydrology that integrate river ecohydrological processes and take advantage of existing models have been proposed, such as a conceptual model based on the relationship between flow regime and fish demand, a holistic conceptual model for the structure and function of river ecosystems, and a conceptual model based on the relationships between flow and ecological responses (Dong et al., 2010; Wang et al., 2013; Xu et al., 2013). In the future, aquatic ecohydrological models should be developed to construct a comprehensive mathematical model that describes river hydrology-water quality processes and ecosystem changes affected by human activities using high-precision terrain data, nutrient observations, aquatic growth cycles, and human activities (e.g., dam regulation, water intake, and sewage).

In addition, integrated ecohydrological process models that consider multiple variables in a multi-scale terrestrial-aquatic ecosystem have emerged owing to the maturity of single ecohydrological process models, as an important research direction for ecohydrological model development. For instance, RHESSys was developed by coupling the distributed hydrological (TOPMODEL) and the forest carbon cycle models (Forest-BGC) at the watershed scale, and realized the two-way coupling of ecological and hydrological processes (Mackay and Band, 1997). Chinese scholars also developed the vegetation interface processes model (VIP) and the China AgroSys, which simultaneously simulated crucial ecohydrological processes, such as crop yield, evapotranspiration, runoff yield, and routing at the watershed or regional scale (Mo and Liu, 2001; Yu et al., 2007). In addition, Xia et al. (2017, 2018) explored the link and feedback mechanisms among hydrology, ecology, and human activities by considering the hydrological cycle, and developed integrated water system simulation platforms, such as the Yangtze River Simulator and Urban Water system model 5.0.

The scale mismatch of time and space in different ecohydrological processes has always been a challenge for comprehensive simulations and integrations of ecohydrological processes. Currently, it is still technically difficult to extend laws and conclusions from a small-scale to a watershed scale, which restricts ecohydrological simulations (Figure 2) (Gao and Yu, 2018). Theoretically, simulations at a smaller scale will be closer to the actual conditions. However, owing to the spatial heterogeneity at a small scale, it is difficult to generalize the obtained laws to a larger scale. Hydrological models generally simulate hydrological processes at sub-basins or grids on a daily or even hourly scale. For ecological models, the basic data observations are usually at the site or field scale, while the simulations of vegetation biochemical processes are at an hourly scale. To overcome this discrepancy, scholars usually adopted multi-scale nested methods. However, if a small-scale process simulation requires outputs from a large-scale process simulation, the outputs are usually regarded as constants, which affect the spatial resolution of final simulation results. It is also important for future ecohydrological models to achieve the dynamic calculation of exchange variables and scale conversion among modules using downscaling or upscaling approaches.

Furthermore, the uncertainty sources of individual processes and their cumulative impacts in the complex model are also critical issues in the ecohydrological model research (Zhang and Shao, 2018). Model uncertainty sources can be roughly divided into uncertainties associated with model inputs, model structure, parameters, and observations used for model calibration (Renard et al., 2010). Previous uncertainty studies are limited to uncertainty estimations of a single source and its impact on certain parameters and simulation results. However, estimating uncertainty sources in a complex system model is still in its initial stages. Zhang and Shao (2018) adopted the bootstrap sampling and a multi-objective optimization algorithm to estimate uncertainties and their impact in a complex system model (HEQM), and also proposed a balanced calibration method for multiple processes and multiple variables to handle the uncertainty propagation (Zhang et al., 2016) (Figure 3). However, additional studies are required to improve and develop theories and methods for estimating model uncertainty and establishing a scientific and reliable uncertainty evaluation platform.

The interaction mechanism and coupling of terrestrial vegetation ecological processes, river ecosystems, social systems, and hydrological processes are used as the core contents of the IGBP, UNESCO/IHP, Ten-Year Scientific Plan (2013-2022) of IAHS (i.e., Panta Rhei), and Future Earth plan. According to these major global scientific projects, further integration between the natural sciences (e.g., environmental sciences, atmospheric sciences, and others) and social sciences (e.g., socioeconomy and human activity) is a key development direction for ecohydrology. For example, climate change affects both the hydrological and material cycles of surface ecosystems, vegetation physiology, and ecology (Ouyang et al., 2016). Therefore, investigating the mechanism and feedback of climate change on related ecohydrological processes is also a pertinent issue that must be addressed in the future.

In addition, with rapid economic development, human activities (e.g., construction and regulation of water conservancy projects, massive application of pesticides and fertilizers, excessive discharge of pollutants, vegetation destruction, deforestation, and urbanization) have increasingly affected ecohydrological systems of river basins (Zhai et al., 2014). The solutions to these problems require the integration of multiple ecohydrology-related disciplines. The key scientific issues are as follows: the impact mechanism of water conservancy projects on river ecohydrological systems and rare species protection, nutrient cycle and pollution characteristics in the SPAC system, environmental effects of point and non-point pollution sources and their impacts on ecosystems, impacts of underlying surface changes caused by human activities on ecohydrological changes, urban ecohydrological evolution mechanism, and regulation characteristics of urban sponge measures. The Chinese government has proposed major strategies related to ecohydrology, such as the Beautiful China, the Yangtze River Protection, the ecological protection and high-quality development of the Yellow River Basin, the sponge cities, and the ecological construction of the coastal zone. Several major national science and technology projects have also been designed to meet both national and environmental needs, such as the national key research and development programs: “Typical Fragile Ecological Restoration and Protection” and “High-efficient Development and Utilization of Water Resources”, and the strategic priority research program (Class A) of the Chinese Academy of Sciences, “Ecological Civilization Construction Technology Project of Beautiful China”. The National Natural Science Foundation of China has also attached importance to developing the ecohydrology discipline, especially in arid and semi-arid areas where water use conflicts are prominent and the ecological environment is poor. Further, several major scientific foundations have been established, including the major research plan “Integrated Research on Ecohydrological Process of the Heihe River Basin,” and an innovative research group project “Ecohydrology in Arid Areas” (Cheng et al., 2014). Ecohydrological studies in the Heihe River Basin have not only developed an advanced monitoring system at the basin scale and an integrated ecohydrological process model, but also provided decision-making supports for sustainable development (Wang et al., 2001). Aiming at basin ecological environment protection, a major program of the National Natural Science Foundation of China “Reciprocal impact mechanism and countermeasures on the water cycle change in the Yangtze River Economic Belt and the green development of typical city groups in the middle and lower reaches” has also been proposed. In addition, the 13th Five-Year Plan proposed the development of integrated “ecology-hydrology-economy” research. These major national needs and disciplinary frontiers will further promote multi-disciplinary ecohydrological research in China, including the integration of natural and societal disciplines, and comprehensive investigations of the influence and feedback of economic development and human activities on ecohydrological processes. As a key component of ecological civilization construction in China, the water regulation capacity of the terrestrial ecosystem and its valuation are important for the sustainable management of ecosystems and natural capital (Zhang et al., 2021). They are also research hotspots in the disciplines of ecology, hydrology, ecological economics, and environmental economics.