3.1. Study Area
The Chengdu–Chongqing urban agglomeration is located in southwestern China, which is centered between Chengdu and Chongqing. The detailed location of the research area is shown in
Figure 1, and the city scale distribution map is shown in
Figure 2. According to the new city scale classification standard (urban resident population), cities are divided into five types, they are small city (under 500,000), medium-sized city (from 500,000 to 1 million), big city (from 1 million to 5 million), megapolis (from 5 million to 10 million), and super city (more than 10 million), respectively. It is the largest urban agglomeration area in the Western region. It is also an important platform for the development of the Western region and the strategic support of the Yangtze River Economic Belt. The Chengdu–Chongqing urban agglomeration has a total area of 185,000 km² with complex topography, which is high in the West and low in the East. It is generally dominated by hills, plains, and basins, and is one of the five major urban agglomerations in China, the others being the Yangtze River Delta, the Pearl River Delta, the Beijing–Tianjin–Hebei agglomeration, and the middle reaches of the Yangtze River. Currently, the urbanization of the Chengdu–Chongqing urban agglomeration is still at a relatively low level, without an obvious city structure and appropriate city hierarchical structure, which is the lack of the small city and super city, presenting the phenomenon of “big in the middle and small at the ends”. According to existing research, Chengdu and Chongqing have outstanding high-quality economic and social development, with obvious fault characteristics and obvious regional differences in ecological environment quality. As an important urbanization region in China, the Chengdu–Chongqing urban agglomeration has obvious geographical advantages, connecting the East to the West and the North to the South. Further, it is highly developed economically, has an increasingly perfect urban system, and has close economic, social, and cultural links. The two leading cities, Chengdu and Chongqing, are not far apart and have advantages in terms of integration over other urban agglomerations. To cultivate and develop the Chongqing urban agglomeration, to give full play to its unique advantages of connecting the southwest and northwest, and domestic and foreign countries, and to promote the interaction between the “One Belt And One Road” and the Yangtze River Economic Belt strategy, it is conducive to accelerate the development of the Central and Western regions, expanding new spaces for national economic growth, ensuring national security, and optimizing the layout of the country. Based on the promotion of the above-mentioned policies, the Chengdu–Chongqing urban agglomeration is booming.
3.2. Data Source
Based on the “Chengdu–Chongqing Urban Agglomeration Development Plan” issued by the National Development and Reform Commission and the Ministry of Housing and Urban–Rural Development, this paper takes 14 prefecture-level cities in Sichuan Province, Chengdu, the capital of Sichuan Province, and Chongqing, the municipality directly under the central government, as the research objects. The research period is set from 2009 to 2018. Considering that this paper focuses on the development of the whole urban agglomeration led by Chengdu and Chongqing, as well as the size of Chongqing and the lack of data, all districts and counties in Chongqing are combined and treated as an evaluation unit. Considering the desirability and scientific aspects of the original data, in this paper, the ecological environment and social-economic statistics are derived from the China City Statistical Yearbook, Sichuan Statistical Yearbook, Chongqing Statistical Yearbook, and the statistical bulletins of various cities and government work reports. The missing values are predicted and supplemented by linear regression. Quantitative analysis, spatial analysis, and spatial visualization processing of the data are undertaken with the help of SPSS, ArcGIS, Stata, and other software.
3.4. Evaluation of CCD of the Social Economy and Ecological Environment under the High-Quality Concept
- (1)
Construction of the indicator system
Based on the principles of being systematic, integral, scientific, and operational, this paper combines the five concepts of innovation, coordination, openness, sharing, and green of the high-quality development concept to establish the two sub-systems of the social economy and ecological environment. As shown in
Table 1, the ecological environmental system is decomposed into three dimensions of ecological environment pressure, eco-environment level, and ecological environment protection based on the PSR environmental quality assessment model, with a total of eight indicators. The social economy system includes four dimensions: innovation, coordination, openness, and sharing, with a total of 11 indicators.
Due to the different properties of each evaluation index, it is necessary to standardize the data to eliminate the influence of dimension. In this paper, the commonly used min-max standardization is adopted, which is a linear normalization method, and all the original data are processed to 0–1 value.
If there are r years, n cities, and m indexes, the ositive indicator is as follows:
and the negative indicator is:
where,
represents the original and normalized values of the j-th index in the i-th city in the θ-th year, and
and
represent the maximum and minimum values, respectively.
- (2)
CCD model
In this paper, the CCD model is adopted to explore the coupling coordination level between the social economy and ecological environment. First, the improved entropy method is used to determine the index weight. As panel data are used in this paper to realize the comparison between different years, this paper uses Yang Li’s method [
23] for reference, which improves the method of entropy, and time variables are added to make the analysis results more comprehensive and reasonable. The evaluation index of the two systems of the social-economic system and ecological environment is then calculated. Finally, the coupling degree and CCD are calculated.
Step 1. Calculate the information entropy of each index:
where,
,
k > 0, and
k = 1/ln(rn).
Step 2. Determine the index weight:
where,
.
Step 3. Calculate the evaluation value of the two systems:
where,
represents the economy development, and
represents the eco-environment system.
Step 5. Coupling coordination degree:
where, representing the comprehensive development, C is the coupling degree between
and
, α,β are undetermined coefficients, and
and
indicate the contribution coefficients of
and
, respectively. We assume that α,β is equally important of the two systems, so the values of α,β can be set as 0.5 D, representing the coupling coordination degree between the two systems. The coupling degree reflects the intensity of mutual influence between the two systems, and the coupling coordination degree model can further reflect if the development of the system is coordinated to some extent.
- (3)
Spatial autocorrelation
In order to further explore the spatial relationship of CCD, we use Moran’s I to conduct spatial autocorrelation analysis. Moran’s I index takes values in the range [−1,1]. A positive value of the Moran’s I indicates that the variables are positively autocorrelated on the spatial units, and the closer the value to 1, the closer the relationship between the spatial units. That is, high-value spatial units are adjacent to high-value spatial units and low-value spatial units are adjacent to low-value spatial units. If Moran’s I index is 0, there is no spatial relationship between the cells, i.e., high-value and low-value spatial cells are completely randomly distributed. Further, a negative value of Moran’s I indicates that the variables are negatively autocorrelated on spatial units, and the closer its value is to −1, the greater the difference between spatial units and the more dispersed the distribution, i.e., high-value spatial units are adjacent to low-value spatial units. Local Moran’s I test specifically reflects the degree of local spatial agglomeration in each region, including “high-high”, “low-low”, “low-high”, “high-low” agglomeration patterns and “no significant spatial correlation”, and the LISA significance test reflects its spatial distribution pattern and significance degree.
Among them, , . denotes the observation of the ith region, is the sample variance, n is the total number of regions, and is the (i,j) element of the spatial weight matrix.
- (4)
Regression model
In order to explore the degree of influence and significance of various indicators on the CCD, the CCD is considered as an explained variable and 19 indicators are considered as explanatory variables to conduct a regression analysis of panel data from 2009 to 2018. Firstly, the data are assumed to have individual effects, and the xtreg command is used for regression. The xtreg command performs simple clustering and difference on the data, which is more suitable for the processing of panel data [
24]. Whether the fixed effect model or random effect model is used is determined by the Hausman test. All these operations are undertaken using Stata 15.1.