Water resources are essential for maintaining the sustainable development of eco-socio-economic systems [1
]. However, due to climate change, economic growth, population increase, and improper water resource management, many environmental problems, such as serious water pollution, the deterioration of the water environment, and an increased contradiction between water supply and demand, have become increasingly severe, creating strategic problems worldwide [3
]. The United Nations (UN) estimates that more than one-third of the population on the planet will face a freshwater crisis by 2030 [8
], and residents in regions with relatively abundant water resources will still have to spend time and energy to obtain water resources, which not only increases their living costs but also impacts health [9
]. Therefore, whether people living in different regions can easily and fairly obtain sufficient water resources to meet their water demand has become a hot topic for academics and policymakers.
Accessibility is a broadly accepted concept in various scientific fields such as urban planning, transportation planning, and geography [11
], and was initially used to measure potential interaction opportunities in transportation networks [12
]. As research progressed, accessibility was defined as the difficulty in reaching a destination from a given location [13
]. There are two main manifestations of accessibility: The number of opportunities or benefits that can be obtained within a given time or distance, and the amount of spatial resistance that needs to be overcome to reach a destination [14
]. Therefore, the measurement of accessibility depends primarily on the spatial distribution of potential destinations and the spatial resistance that needs to be overcome to reach each destination [16
]. The former is mainly measured by quantity or quality, reflecting the attractiveness of the destination, and the latter is mainly measured by indicators such as time, distance, or cumulative cost, reflecting the convenience of reaching the destination [17
The quantity, quality, and accessibility of water resources are the basic requirements for ensuring water resource security [19
]. However, the existing water resource evaluations mostly focus on the assessment of water quality and water quantity [21
], but less attention has been paid to water resource accessibility [10
]. Water resource accessibility refers to the difficulty of obtaining water resources from water sources [30
], which is the fundamental factor determining the quantity, quality, and the efficiency of the water supply. Water resource accessibility is essential for human well-being, economic development, and ecological maintenance [31
] and includes both spatial accessibility and time accessibility [10
], often measured by indicators such as water intake distance (Euclidean distance, cost distance, path distance, etc.) and water collection time (shortest distance time, shortest path time, self-report time, etc.) [32
]. However, these indicators ignore the impact of water quantity and various spatial resistance factors. Therefore, some development space remains within the existing quantitative research for the examination of water resource accessibility, and the quantitative methods need to be further improved.
Southwest China is the source and upstream of many rivers, and is also an important ecological barrier zone. It plays a key role in maintaining the ecological and socio-economic security of East China, South China, and even Southeast Asia [35
]. The region has abundant rainfall and a large amount of water resources. However, due to the uneven water distribution in time and space, coupled with the limited infrastructure and the influence of complex topography, the use of water resources is difficult and costly, resulting in serious seasonal, regional, and engineering water shortages [36
]. With the rapid population and economy growth, the demand for water resources in Southwest China continues to increase, and the misalignment between supply and demand is becoming increasingly acute. Therefore, we used a water resource accessibility index model considering five factors—runoff, slope, relative height difference, water intake distance and land use resistance—to evaluate the grid-scale water resource accessibility in Southwest China, using the ArcGIS platform (Environmental Systems Research Institute, Redlands, California, America) to construct the spatial pattern of water resource accessibility. Then, we analyzed the coupling coordination degree between water resource accessibility and eco-socio-economic water demand elements. The aims are to improve the water resource accessibility evaluation method, identify the areas with relatively low water resource accessibility and the key regions that the coupling coordination degree between water resource accessibility and eco-socio-economic water demand elements is relatively low, which is important for improving the determination of regional water resource security levels, strengthening regional water resource management allocation, and effectively implementing water conservancy facilities planning and urban development planning.
4.1. Evaluation Method of Water Resource Accessibility
Water resource accessibility refers to the difficulty in obtaining water resources from water sources and is affected by multiple factors such as water quality, water quantity, distance, elevation, slope, land use, capital, infrastructure, and technology. Among them, the quantity and quality of water resources are decisive factors for the availability of water resources, whereas the distance, altitude, slope, land use, and others are factors affecting the convenience of obtaining water resources. The existing quantitative analyses of water resource accessibility often involved a single indicator or a few factors. For example, Jeff et al. [9
] only considered the linear distance to the water source. Smiley [56
] selected the four elements of water quality, water cost, water reliability, and water intake burden, and measured water resource accessibility through questionnaires and statistical analysis. Yu et al. [28
] selected four factors including the slope, relative height difference, distance, and runoff to comprehensively analyze the accessibility of river water resources in the Hanjiang River Basin. Li et al. [29
] constructed a grid-scale water accessibility evaluation model based on the length, runoff and viewshed value. Li et al. [57
] constructed a water accessibility index by selecting indicators such as distance, altitude, ditch density, road density, and culvert number to study the water resource accessibility of freshwater wetland. In this study, we evaluated water resource accessibility by considering the five factors of runoff, slope, relative height difference, water intake distance, and land use resistance, and analyzed the spatial pattern of the water resource accessibility in different grid units. Among them, water quantity represents the attractiveness of the water source to the water users, and the other factors reflect the spatial resistance.
However, the water resource accessibility evaluation in this study still has some room for improvement. First, our evaluation only considered the impact of water quantity and ignored the water quality. Thus, in future research, the quality of water resources should be measured in terms of water quality requirements for different water demand elements. Second, we assumed that water users obtain water from the closest water source, but in practice, multiple water sources provide water for users. Therefore, it is necessary to weight the multiple water sources within a certain range in future research. Third, we ignored the impact of socio-economic factors, such as water supply facilities, water treatment technology, irrigation technology, and water fees, on water resource accessibility, which affects the accuracy of the evaluation results to a certain extent; thus, future research needs to comprehensively measure multiple factors.
4.2. Water Resource Accessibility and Regional Eco-Socio-Economic Development
Water resources are an important basis for supporting the development of eco-socio-economic systems, whereas social and economic development provides the necessary funds and conditions for ensuring the sustainable development and use of water resources [58
], which affect and restrict each other. The limited water resource accessibility of spatial units not only threatens the supply of drinking water and irrigation water, but also threatens the sustainable and healthy development of the ecosystem [20
]. Therefore, spatially accessible water resources are essential for an adequate freshwater supply [28
In Southwest China, the topography and geomorphology are particularly complex, the spatial-temporal distribution of water resources is uneven, and the water supply facilities lack expansion and improvement potential, resulting in serious seasonal, regional, and engineering water shortages. The use of water resources is difficult and costly, which seriously restricts regionally sustainable eco-socio-economic development. Therefore, water resource management in Southwest China must fully consider the characteristics and formation of water resources in the region and should adopt different water resource development and use models. The analysis of the coupling coordination degree between water resource accessibility and eco-socio-economic water demand elements can be used to effectively identify the areas where the coupling coordination degree between the two is relatively low. This can provide a decision-making basis for strengthening regional water resource management and allocation, for effectively implementing water conservancy facilities planning, and for urban development planning, thus ensuring the coordinated and sustainable development of various systems.
In West Sichuan and Northwest Yunnan, there are many extremely high mountains, resulting in significant altitude differences and steep slopes, and the water sources are far away from water users, so the water resource accessibility is low and agricultural irrigation is difficult. Therefore, these areas require more investment to improve irrigation conditions and increase irrigation efficiency to meet crop water requirements. In the Sichuan Basin, Central Yunnan, Northwest Guizhou, and Southeast Guangxi, the population density is relatively high; it is necessary to continuously strengthen water resource protection and infrastructure construction in densely populated areas to ensure a safe and adequate supply of drinking water. The economic development level of most counties (districts) in Southwest China is low, somewhat lagging behind the water resource accessibility level, whereas the water resource accessibility in West Sichuan and Northwest Yunnan is relatively low, which restricts economic development to some extent. Therefore, according to the spatial pattern of water resource accessibility, rationally adjusting the industrial structure and developing a circular economy is necessary, which contribute to promoting rational and rapid economic development and ensuring the coordinated development of water resource accessibility and social and economic elements. In Northwest Yunnan and West and Northeast Sichuan, relatively poor natural conditions and scarce water resources result in relatively low water resource accessibility, which considerably limits vegetation growth. Therefore, these regions should be the focus for ecological conservation and restoration.
This paper applied a water resource accessibility index model, considering five spatial factors of runoff, slope, relative height difference, water intake distance and land use resistance, which enabled the quantitative analysis of the spatial distribution characteristics of water resource accessibility on a grid-scale in Southwest China. The results show that due to the large spatial distribution differences of different spatial elements, the spatial differences in water resource accessibility in Southwest China are relatively significant, and the overall trend is a decrease from Southeast to Northwest.
Due to the differences in hydrological conditions, topography, and economic development level, the coupling coordination degree between water resource accessibility and eco-socio-economic water demand elements in Southwest China has obvious regional differences, and the overall distribution characteristics are higher in the Southeast and lower in the Northwest. The proportion of counties (districts) with moderate coordination or higher was only 6.23%, mainly concentrated in the Northeast part of Guangxi. The counties (districts) with near incoordination, low incoordination, and moderate incoordination accounted for 13.00%, mainly concentrated in West Sichuan and Northwest Yunnan. The coupling coordination degree between the two is relatively low overall.
The water resource accessibility and the eco-socio-economic system in Southwest China have not achieved coordinated or sustainable development. The insufficient water resource support capacity in the region has restricted the development of the region to a certain extent, and the rapidly increasing population and economic development have increased water supply stress to a certain extent. Therefore, it is necessary to continuously coordinate the relationship between water resource management and regional development.