1. Introduction
Water resources are considered essential elements for promotion of urban development [
1]. Since the 1950s, the process of urbanization around the world has accelerated. Rapid urbanization has caused an excessive waste of water resources, water pollution, water ecological imbalance, and other environmental issues, which threatens the security of water resource system and negatively affects urbanization to some extent [
2,
3,
4]. The global urban population has soared from 50 million in 1950 to 4.2 billion in 2019, accounting for 55% of the world’s total population [
5]. Developing countries, such as China and India, have experienced a particularly evident urbanization expansion policy [
6]. Due to the limited resources and environment, the urbanization rate increased from 17% in 1951 to 35% in 2017 in India [
7]. According to the China Statistical Yearbook, since the reform and opening-up of 1978, Chinese urbanization has presented a rapid upward trend, from 17.9% in 1978 to 59.58% in 2018 [
8]. It is predicted that the global urbanization rate will reach 68% in 2050 [
5]. It is believed that this process will threaten the environment and resources, leading to an imbalance and depletion of water resources [
9]. These problems will eventually become serious obstacles to sustainable urban development [
10]. In the framework of the future earth plan and 2030 agenda for sustainable development, scholars have suggested that urbanization should be mindful of the eco-environment and emphasize that urbanization must accommodate the resources capacity [
11]. In this context, the coordination and interaction between urbanization and the environment have become a useful research issue.
WRCC is the ability of water resources to support the coordinated development of the population, society, economy and environment in a region [
12]. The rational way that human’s use water resources makes it possible to push toward the development of the economy and society [
13]. However, water resources are significantly unevenly distributed in China, where water is abundant in the south and scarce in the south. According to the research of Chinese scholars [
14], cities in northern China are confronted with water problems, including less rainfall, water eutrophication and water environmental pollution, which have caused a water shortage [
15]. It is noteworthy that the imbalanced development of WRCC and urbanization has become the principal contradiction that must be solved in order to achieve sustainability in the northern regions. In consideration of this, it is necessary to coordinate the relationship between urbanization and WRCC and design a scientific management framework for guiding the sustainable development of cities in the future.
The previous studies on Chinese urban development have focused on the Yangtze River economic belt region [
16] and the Jing-Jin-Ji region [
17] whereas the YRB region did not stand out. After the 1980s, after the rapid economic and social development of the YRB, various provinces were shown to be excessively exploiting their natural resources [
18]. However, the economic gap among the provinces in the YRB is growing, so it is challenging to transform the mode of production in traditional industries [
19]; the ecological environment deteriorated rapidly. These phenomena have seriously restricted the coordinated development of the economy, society, and the ecological environment [
20]. The analysis of WRCC is a prerequisite for the planning of sustainable urban development; however, there is a lack of empirical studies on the relationship between WRCC and urbanization.
The research on the coupling relationship between urbanization and WRCC is based on the urbanization–environment perspective. Many scholars have used the Environmental Kuznets curve (EKC) hypothesis to examine the relationship between urbanization and eco-environment [
21]. Grossman and Krueger proposed a curve reflecting the relationship between the economic level and environment after exploring the relationship between urban development and air pollution in North America in 1991. It is called the EKC hypothesis [
22]. The EKC hypothesis reveals an inverted “U”-shape evolution law between the eco-environment and economic level [
23]. Some scholars have verified the realistic relationship between economic growth and environmental pollution shown in EKC theory [
24,
25]. Other scholars have pointed out that the EKC curve has different shapes due to the different backgrounds of economic levels, industrial structures, and environmental protection policies in different countries [
26]. In addition, the EKC theory assumes that urban development is independent upon the eco-environment, but the interaction and restriction exist among economic and social development and the eco-environment. While the EKC theory has some defects [
27,
28], it can still be applied to the study of urbanization development and environment [
29].
In the 1970s, Haken, a famous German physicist, first proposed the concept of “coupling” [
30], and elaborated it in his concept of “advanced synergetics”. Coupling reflects the interaction between two or more systems [
31,
32]. According to the synergetics theory, the coordination state reflects the trend of the system from disorder toward order. CCD shows the degree of mutual influence and coordination between systems in the development process. It describes the coordination status of various elements or subsystems within a system. Because of its applicability, it has been widely applied in different disciplines, such as physics [
33,
34] and biology [
35]. Coupling relationship between urbanization and the eco-environment presents the sum of the interactions among all the elements of the two systems [
36]. In recent years, the CCD model has been used to investigate the relationship between the eco-environment and sustainable development [
37]. For instance, Ariken et al. [
38] analyzed the relationship between urbanization and the eco-environment by taking advantage of the coupling coordination theory based on multi-source remote sensing data. Kurniawan et al. [
39] utilized CCD as a tool for examining the relationship between the social-ecological status and sustainable development of small island tourism.
In the CCD model, the contribution coefficient shows the impact of the target system on the overall coordination states [
40]. However, the contribution coefficients were defined subjectively in the traditional CCD model. A large number of scholars have believed that the contributions of the two systems were equal and determined the value of the contribution coefficient to be 0.5 [
40,
41,
42,
43]. However, some scholars, such as Li et al. (2012), believed that the subjectivity had negative impact on the evaluation of the coordination state [
44]. Some scholars, such as He et al. [
45], Shen [
46], found that the value of the CCD varied significantly when the contribution coefficient was found to have subjective disparate values. It cannot show the actual relationship between urbanization and WRCC systems. Based on the synergy theory, Shen developed an improved CCD model by proposing an alternative method for defining contribution system [
46]. The synergy theory reveals that the underdeveloped system should be developed to improve the coordination state of two systems [
31]. The high CCD would be obtained only if the performance gap between the system is small [
47]. The improved CCD model has been verified in practice [
48,
49]. The high performance of urbanization and WRCC systems is conducive to sustainable development [
50]. Thus, the improved CCD model provides an effective way to investigate the interaction relationship between WRCC and urbanization systems.
The purpose of this paper is to explore the relationship between urbanization and the regional WRCC in the YRB. Firstly, based on the PESS framework and PSR model, we establish two index systems for the urbanization and WRCC, respectively. Secondly, at the basis of the two index systems, we investigate the coupling coordination states between urbanization and WRCC in the YRB using the improved CCD model. The spatio-temporal analysis is conducted to obtain the characteristics of the spatial gradients, temporal scales and evolution law. This analysis provides empirical support for the high-quality development of urbanization and WRCC in the YRB. Finally, the obstacle factor model enables us to ascertain and analyze the obstacle factors that restrain the coordinated development of the urbanization and WRCC and discuss the mechanism of the influence on the two-coupling system. The flow diagram of the structure is shown in
Figure 1. The WRCC, a crucial factor of the urbanization process, is inextricably linked with the economic and social development in the YRB [
51]. This study provides theoretical support and policy recommendations for urban development and management.
This paper’s structure is as follows: the research materials and methods are shown in
Section 2. The results analysis and discussion, including trend analysis, spatial–temporal characteristics analysis, and obstacle analysis are provided in
Section 3. In
Section 4, the conclusions and suggestions are provided.
4. Conclusions
The ecological protection and high-quality development of the YRB is a Chinese national strategy. It is essential for analyzing the relationship between urbanization and WRCC in the YRB. This study developed a evaluation model of the urbanization and WRCC systems by applying the PESS and PSR models. Then, an improved CCD model was established to evaluate the coordination state between the urbanization and WRCC quantitatively from 2008 to 2017. The evolution trend and characteristics of the CCD between the two systems were analyzed from the spatial–temporal dimension. The main factors that affect the coordination state were clarified and analyzed using the obstacle model. This study’s results can enrich the content of the EKC theory’s and develop the coupling framework of urbanization and WRCC. In contrast, the results reveal the spatiotemporal evolution law of CCD and provide an empirical and effective basis for high-quality development in the YRB. The main contributions are as follows:
Firstly, the research indicates that the urbanization performance level experienced a rapidly rising process in the YRB. The gap between the seven provinces’ performance levels widened, because the urbanization grew at different rates. In the WRCC system, the WRCC system’s performance presented a fluctuating downward trend from 2008 to 2017 in the YRB.
Secondly, the study confirms that, during 2008–2017, the coordination state between the urbanization and WRCC systems was improved to some extent, but there are differences in the coordination state of the different provinces in the YRB. In terms of the spatial–temporal dimension, the tendency of the coordination state in the western region of the YRB, such as Gansu, Ningxia, and Inner Mongolia, presents an inverted U-shape. The trend of the coordination state in the eastern region, such as Shandong and Henan, was still upwards. The coordination state of each province was gradually improved as time went by, and the spatial difference in the seven provinces’ coordination state grew.
Finally, based on the obstacle factors analysis, we discovered the main impact subsystems and key indicators for the two systems’ coordination state. The pressure subsystem had a great impact on the two systems’ coordination state in the YRB, while the social urbanization and response subsystems had little impact. The major indicators restricting the coordination state of the urbanization system and WRCC system include X3 (Urban population density), X13 (Proportion of built-up areas to urban areas), Y1 (Water consumption per unit of GDP), Y3 (Water consumption of industrial output), Y7 (Total volume of water resources), Y8 (Per capita water occupancy volume), and Y11 (Industrial water pollution control investment).
The results show that the local governments in the YRB should pay more attention to the urban space, urban population, and economic development. City planning should be carried out scientifically and reasonably. We should upgrade industries, develop green industries, and increase personal income and the employment rate. To alleviate the pressure of the WRCC, intensive industrial and agricultural production modes should be adopted to reduce water consumption. On the other hand, the government can increase its investment in environmental protection to control industrial sewage discharge. In addition, the construction of a water resources allocation project might be implemented, and the utilization efficiency of water resources should be improved. Considering the difference in resource endowments in the YRB, the local government should formulate sustainable development plans according to their own coordination status and obstacle factors.
This paper’s index system was designed for the urbanization system and the WRCC system of the provinces in the YRB and may therefore not be applicable to other regions, but it can provide other countries or regions with an effective approach or reference to study a similar problem.