Freshwater is essential for life because all organisms on the planet require fresh water resources to ensure their survival [1
]. According to the definition of Falkenmark [4
], water resources can be divided into green and blue water. Blue water is mainly water stored in rivers, lakes and shallow groundwater. At present, the water directly used by human beings in daily life is basically blue water. Green water refers to the water that originates from precipitation, is stored in unsaturated soil and is absorbed and undergoes transpiration by plants [4
]. Green water plays an irreplaceable role in global ecosystems and food production. Liu et al. [5
] estimated that 80% of global food production depends on green water, and the supply of water in grassland and forest ecosystems is mainly dependent on green water. In recent years, green/blue water research has triggered the rethinking of water resources concepts and evaluations by the scientific community, gradually affecting human thinking about water resources management, and it has become a hot research in the field of hydrology and water resources [6
]. First, green water is an important source of water for terrestrial ecosystems because it plays a key role in maintaining the coordination of terrestrial ecosystems and ecosystem health. Second, green water is the only resource that supports rainfed agriculture. In total, 83% of the cultivated land provides food security for 70% of the world’s population [8
]. Finally, green water plays an irreplaceable role in maintaining the function and service functions of the Earth’s terrestrial ecosystem [6
From the point of view of material circulation, the concept of green and blue water flow has been introduced in many studies [8
], and the green water flow is the actual evapotranspiration. From the perspective of the global water cycle, 65% of the total precipitation on the global scale is returned to the atmosphere through evapotranspiration of forests, grasslands, farmlands and so on, becoming green water flow [11
]. The surface runoff, the mid-soil flow, and the underground runoff are combined into the blue water flow [12
]. In this article, we focus on green and blue water flow. The concept of green and blue water flow closely relates the water cycle to the ecological process, which reflects the interaction between the ecosystem and hydrological processes [3
]. The concept of green and blue water flow not only broadens the scientific connotation of water resources research but also has provided new theories and ideas for water resources research and management [6
Currently, the evaluation of green and blue water flow currents is mainly concentrated on the global or basin scale with a focus on the evaluation of green and blue water flow and their spatial and temporal distribution [5
]. However, there is little research on green and blue water flow on the ecosystem scale, especially the combination of green and blue water flow and ecosystem service functions. Ecosystem service function is one of the main topics of the United Nations Millennium Ecosystem Assessment [18
]. It plays an important role in global ecosystem management and sustainable development research and is the basis for understanding the interaction between ecosystems and humans [21
]. Although freshwater demand for the production of these ecosystem services is often invisible, maintaining these functions consumes large amounts of green water flow [24
]. To date, researchers have performed many studies evaluating the supply and regulation functions of typical ecosystem services [25
], and many models and methods are used to study the function of ecosystem services [25
], such as the In-VEST (Integrated Valuation of Ecosystem Services and Tradeoffs) model is a classic model used to evaluate ecological services [27
]. However, there are few studies that combine the support function of the ecosystem services, especially the comprehensive evaluation of water resources consumption and ecosystem services as a whole [21
]. In short, the interrelationship between the hydrology, ecology and human activities is not clear. There is still insufficient scientific basis for applying regional ecosystem and water resources management.
The Heihe River Basin is location in arid regions of China, and researchers have paid much attention to the regional water cycle and ecosystem service functions based on blue water flow [24
]; However, the green water flow, the green water flow, which is important to river basin ecosystem service functions and to human beings, and the connections between water resources and ecosystem services are still need to strengthen understood. The main objectives of this study were to determine how to quantitatively assess the resources of green and blue water flow in typical ecosystems (cropland, forest, and grassland) and to determine how to analyse the relationship between green and blue water flow and ecosystem services in the typical ecosystems of the Heihe River Basin. For this study, we selected the Heihe River Basin, which is a typical inland river in an arid area as the research object. This study is a follow-up of our previous research. We calibrated and validated the Soil and Water Assessment Tool (SWAT) model for the simulation of the flows of green and blue water at the whole-basin level by comparing simulated and measured discharge, as shown in Zang et al [9
]. The calibration and validation performed were very satisfactory. The study consists of the following parts: (1) an analysis of the spatial and temporal distribution of green and blue water flow in typical ecosystems; (2) consideration of the consumption of green and blue water flow in typical ecosystems; and (3) a comprehensive evaluation of green and blue water flow and ecosystem services as a whole. Our research will provide new ideas for water resources and ecosystem services research and provide a theoretical basis for ecosystem and water resources assessment of inland river basins.
In this paper, we first applied the SWAT model to simulate the green and blue water flow in an arid river basin in China. Then, the temporal and spatial dynamic distribution of green and blue water flow for typical ecosystems (forest, cropland, and grassland) were analysed. A detailed investigation of the relationship between green and blue water flow and ecosystem service functions in typical ecosystems was conducted. In this study, the blue water flow and green water flow per unit area of the forest were the highest. In the grassland and farmland, the green water flow per unit area of farmland was higher than that of the grassland, whereas the blue water flow for the grassland was higher than that of the farmland from 1971 to 2010 in the Heihe River Basin. First, this is related to the natural distribution of the typical ecosystems in the Heihe River Basin. The forest ecosystems are mainly distributed in the upper reaches, the farmland is mainly distributed in the middle reaches, and grasslands are mainly distributed in the upstream and middle reaches (Figure 1
). Additionally, only a part of the low-density grasses are distributed downstream (Figure 1
). Second, this is due to climatic factors of the different reaches of the Heihe River Basin [24
]. Therefore, such precipitation and evapotranspiration conditions from the up to down reaches determine the distribution characteristics of the green and blue water flow in the area [41
]. Finally, and most importantly, these conditions are determined by the structural characteristics of the typical ecosystems themselves. In the upper reaches of the Heihe River, the forest is dominated by coniferous forests; forest canopy density and water consumption are relatively low compared with broad-leaf species. When precipitation through the forest forms a canopy interception, interflow and surface runoff, thus forming a blue water flow. Furthermore, farmland has a larger leaf area index and vegetation interception than grassland. The tall canopy then forms a shade, reduce surface runoff and reducing blue water flow [43
]. Therefore, this produce more green water flow. When the precipitation passing through the grassland quickly forms surface runoff and interflow, its causes more blue water flow [45
]. The interception for the farmland is relatively high due to the planting density of the farmland, which can influence the hydrological processes of green and blue water flow [46
This study is limited by several shortcomings. First, due to the characteristics of the SWAT model itself and the limited resolution of the land use data, the accuracy of the ecosystem division was affected. For example, forests can be divided into trees, shrubs, and sparse forests, or they can be divided into forest ages; however, these divisions are implemented in large-scale models. The same is true for farmlands and grasslands. This may impact the simulation results of the model. Because the ecosystem service functions use statistical data, the land use data used by the ecosystem is divided, which results in some deviation. Similar problems have arisen when researchers have used the in-VEST model [28
]. Researchers have been troubled by the question of how to solve the problem of large-scale simulation and small-scale verification. Therefore, the study of ecosystem service functions on a large scale and the precise division of ecosystems need to be further improved [47
]. Second, in this study, we did not adequately consider the effects of irrigation water use or land use change. Clearly, human activities, especially irrigation and land use change, have already significantly influenced the water cycle in the Heihe River Basin. However, the extent of hydrological responses to human intervention has not been quantitatively assessed. The current study provides the first step for quantifying the relationship between green and blue water flow and service functions in typical ecosystems. However, the impact of human activities on the temporal and spatial changes of green and blue water flow in typical ecosystems and the mechanism of human functions on ecosystem services need to be further explored. Lastly but most important, in our study, we just choose the representative ecosystem in our research domain which are the forest, cropland and grassland. In this study, we investigated three ecosystem services that are closely related to the selected ecosystems, i.e. crop yield, beef, and mutton. The representative ecosystem services are important for a case study, but other ecosystem services are also very important. Therefore, in further study, it is also interesting to analyse other ecosystem services thus provide different viewpoints for the different ecosystem services.
As shown in this study, less green and blue water flow consumption per hectare of farmland can produce more grain. The yield of beef and mutton per hectare of grassland in the Heihe River Basin was lower, but it consumed more blue water flow than the farmland. This is mainly because the basin is mostly middle and low-density grassland. Among the three ecosystems, the forest consumed the highest amount of green and blue water flow per hectare, indicating that a higher precipitation is needed to support their survival. This is also the reason why the forests in the Heihe River Basin are mainly distributed upstream. Regarding the relationship between green and blue water flow and service functions of the typical ecosystems, wheat yield was significantly correlated with green water flow, and corn yield was significantly correlated with blue water flow. This showed that corn production is more dependent on blue water flow; however, wheat production will produce more green water flow. There was a significant positive correlation between green water and wheat yield. According to Table 2
, this indicates that wheat production will increase as green water flow increases, and corn has the same trend with blue water flow. This showed that wheat yield is not significantly related to blue water and corn yield is not significantly related to green water (Table 1
). On the one hand, this is related to the physiological and ecological characteristics of corn and wheat itself. Meanwhile, the model simulation results of this research have been added to the irrigation scenario, and corn growth is more dependent on irrigation than wheat. The living wood growing stock was significantly correlated with the green water flow, which showed that the forest consumed more green water flow than blue water flow. Beef and mutton production were significantly associated with blue water flow; however, mutton production was more strongly correlated with green water flow. Thus, mutton production will consume more green water flow than beef. This may be related to the culture structure of cattle and sheep in the Heihe River Basin; however, the model simulation results and statistics do not fully reflect the relationship between them [49
]. This is a scientific question of the resource carrying capacity of the grassland in the basin, and we can examine this aspect in the future. In the future, it is necessary to further research the interaction mechanism between human activities and ecosystem service functions to determine the relationship between human activities and ecosystems.
In this study, we analysed the spatial and temporal distribution of green and blue water flow in typical ecosystems by applying the SWAT model. After the simulation of the water resources, a detailed investigation of the relationship between green and blue water flow and ecosystem service functions in the Heihe River Basin were conducted. The major findings are as follows:
The green and blue water flow per unit area in the forest ecosystem was greater than that in the farmland ecosystem; that in the cropland ecosystem was larger than that in the grassland ecosystem. However, the green and blue water flow in the grassland ecosystem was greater than that in the cropland ecosystem, and that in the cropland was larger than that in the forest ecosystem from 1971 to 2010 in the Heihe River Basin.
The corn yield per hectare of cropland was highest in the Heihe River Basin, followed by wheat, and the lowest yield was the oil yield from 2000 to 2010. The mutton yield in the grassland ecosystem was greater than the beef yield from 2000 to 2010, which shows that the beef production would consume more water flow. There was an obvious positive correlation between green or blue water flow with wheat and corn yield. The beef and mutton had a significant correlation with the blue water flow, whereas mutton had a stronger correlation with the green water flow. Overall, green water flow contributes more to ecosystem services in typical ecosystem of the arid and semi-arid regions.
Our results provide insights into the green and blue water flow distribution in typical ecosystems and service functions throughout the Heihe River Basin. These results provide a more comprehensive understanding of the spatial and temporal distributions in ecosystems and service functions of water resources in the Heihe River Basin. This could help policymakers, administrators, and researchers to manage these resources in the context of global and regional ecosystems. This information will also provide guidance for future studies of other inland river basins in China.