Evaluating the corresponding relationship between the characteristics of resource utilization and the level of urbanization: a case study in Chengdu-Chongqing Economic Circle, China

The Chengdu-Chongqing Economic Circle is an important center for promoting economic growth in the western region. Clarifying the driving force and restrictive factors of the urbanization development in Chengdu-Chongqing area is conducive to the further development of the region. Firstly, this study uses geospatial information to describe the resource consumption characteristics of the Chengdu-Chongqing Economic Circle. Then, Moran index has been used to test the spatial agglomeration relationship. Finally, the Chengdu-Chongqing Economic Circle is classified according to the characteristics of natural resource utilization and spatial relations. The results show that (1) the relationship between the comprehensive utilization of natural resources and urbanization in the Chengdu-Chongqing Economic Circle is high in the east and low in the west, and the two places jointly drive the development of the north and the south. (2) From 2015 to 2020, the comprehensive utilization capacity of natural resources in the core area on the west side decrease, and the core area on the east side increase. Urbanization in the north and south is slow, and the direct utilization capacity of natural resources needs to be improved. (3) The city with the best coordination relationship between comprehensive utilization of natural resources and urbanization is on the periphery of the core cities of the Chengdu-Chongqing Economic Circle, and the protection and management measures of the core cities for natural resources do not match their urbanization level.


Introduction
Rapid urbanization is an important trend of current world development. The main features are economic development, population migration, urban spatial expansion, etc. (C. Li et al. 2013a, b). At present, the more developed areas such as North America and Europe have a relatively high degree of urbanization, and the underdeveloped regions will become the main driving force for the growth of world cities (Madlener and Sunak 2011). Today, more than 50% of the world's population lives in urban areas. According to United Nations projections, by 2050, the global population will increase to 9.3 billion. Among them, the urban population will increase to 6.25 billion, and 75% of the world's population will live in cities and their surrounding areas. In addition, 83% of the world's urban population will live in underdeveloped areas (UN 2012).
The rapid development of urbanization also consumes a lot of natural resources. There is constant interaction between people and the land, and the land changes from a natural state to a state used by humans (Y. Liu 2018). At the same time, the concentrated use of water resources has also led to an increase in water pressure. This promotes climate and environmental changes (R. I. McDonald et al. 2011;Robert I. McDonald et al. 2014). Cities occupy only 2% of the world's area, but consume 75% of the world's resources and are accompanied by a large amount of emissions. The International Energy Agency predicts that by 2030, the proportion of urban energy demand will increase to 73% and carbon dioxide emissions will increase to 76% (IEA 2008). Moreover, comparing large cities in developed and developing countries, excluding traditional energy consumption, it can be found that the per capita energy consumption in developing countries is lower than that in developed countries (Madlener and Sunak, 2011). Therefore, analyzing the consumption of urban natural resources can also evaluate the urbanization process.
Natural resources such as water resources, forest resources, land resources and mineral resources are the necessary basis for the existence and development of ecosystems (Nathaniel et al. 2021), and human activities and social development are based on ecosystems (T. Peng and Deng 2021), so natural resources are important conditions for urbanization development. Human activities and socio-economic development will consume large amounts of natural resources. The higher the level of urbanization, the more concentrated the population, the more natural resources are consumed, and the efficiency of natural resource utilization will also increase, which will promote the development of urbanization. At the same time, factors such as the content of natural resources and storage conditions limit the development of urbanization, so urbanization and natural resources should be mutually constrained. However, too fast or unfettered urbanization will cause serious resource and environmental problems , so in order to achieve sustainable development, we must evaluate whether the reserve and consumption of natural resources matches the level of urbanization development. Due to the diversity of the relationship between urbanization and natural resources, different response characteristics will occur under the influence of different factors such as time, space, their own conditions, and related policies. Although we do not yet know the specific relationship between the consumption of natural resources, the amount of resource storage, conservation measures, and the level of urbanization, there is at least a balance between them, and cities in this state of balance can adapt the consumption of natural resources to the level of urbanization development, while taking into account the maintenance and protection of natural resources. Most of the current research is to assess the current changes in the natural environment (Ding and Peng 2018), or the impact of changes in the natural environment on urbanization (C. Li et al. 2013a, b;J. Li et al. 2013a, b;J. Peng et al. 2015). There are few studies on the relationship between the process of urbanization and the change of resources and environment. Therefore, we need to discuss and study the relationship between natural resources and urbanization.
The Chengdu-Chongqing Economic Circle, officially identified as China's fourth-largest urban agglomeration in 2020, has undergone rapid urbanization in the past few decades. China's Statistical Yearbook shows that urbanization levels in Sichuan Province and Chongqing City rose by 13.46% and 9.68% respectively from 2015 to 2020. The rapid development of urbanization has caused a large number of changes in the natural landscape, and the consumption of natural resources has gradually increased, so this study takes the Chengdu-Chongqing economic Circle as the research area, explores the actual level of natural resource consumption and urbanization development, analyzes the spatial relationship between natural resources and urbanization from the perspective of pressure, state, and response direction, and finally classifies the cities in the Chengdu-Chongqing economic Circle. The remainder of this article is organized as follows. The second part of this article introduces basic data, data processing methods, and models. The third part is the analysis result of the model. The fourth part discusses and analyzes the results. The fifth part is the summary. References are at the end of the article.

Study area and data sources
According to the "Urban Group Development Plan" issued by the National Development and Reform Commission, the subjects of this survey have been determined. It has jurisdiction over 142 districts, including 113 cities in Sichuan Province and 29 cities in Chongqing, as shown in Fig. 1. The Chengdu-Chongqing urban agglomeration has a vast area, with a total area of 185,000 square kilometers. Its geographic location is between 101° 55′-109° 14′ 51″ east longitude and 27° 39′-32° 19′ 18″ north latitude. It is located in the Sichuan Basin east of the Yangtze River in China. There are many mountains and rivers in this area, the terrain is undulating, and the water resources and forest resources are abundant. The Chengdu-Chongqing area has a subtropical humid monsoon climate, with annual precipitation of 1107.8 mm and 1406.4 mm, respectively. The annual average temperature is 16.4 and 19.2 °C, respectively. It has a long history, rich natural, and cultural landscape.
The data used in this article includes statistical data and spatial data. The statistics are mainly derived from the 2016 and 2021 yearbooks, 2016 and 2021 environmental bulletins corresponding to the cities in the Chengdu-Chongqing Economic Circle. The missing data was supplemented using spatial interpolation of ArcGIS. The specific types and sources of spatial data are shown in Table 1.

Index system
The rapid development of Chengdu-Chongqing area has brought increasing pressure on the ecological environment. Economic growth is accompanied by a large amount of resource consumption and pollutant discharge. With the rapid development of urbanization, the urban ecological environment is gradually deteriorating. In order to measure the interaction between the level of urbanization and natural resources in Chengdu-Chongqing area, this study develops a PSRU model. The PSR model is widely used in resources (H. Liu and Men 2018;Wang et al. 2013), ecological carrying capacity (Wei et al. 2014), and ecological safety assessment (Fan and Fang 2020;Sun et al. 2018) and so on. The model is constructed from three directions: human consumption patterns, institutional conditions, and strategies to promote sustainable development, making the model systematic and causal. Then, establish an index system to evaluate the degree of urbanization, and record it as the U subsystem. The research on urbanization mostly starts from economy, population, society, ecological environment, and public services, while the selection of ecological environment indicators mostly starts from natural conditions and environmental governance. Therefore, the PSRU model can simultaneously reflect the comprehensive utilization of natural resources and the level of urbanization in the Chengdu-Chongqing Economic Circle Fang et al. 2019;Verma and Raghubanshi 2018). The specific model is shown in Fig. 2. The P subsystem reflects the burden of urban development on environmental resources. The S subsystem reflects the total amount of urban ecological resources. The R subsystem reflects the measures taken by various regions in response to ecological deterioration. U subsystems consist of population agglomeration, economic development, and spatial expansion. See Table 2 for specific indicators.
This study proposes a number of indicators to show the relationship between integrated utilization of natural resources and urbanization. In contrast to previous studies, which have mostly considered the level of existing resources in the natural environment, this study takes into account the consumption of actual resources and the resources actually produced by the natural environment. This study uses nighttime light coefficients and NPP to reflect the consumption and current levels of natural resources. Nighttime light data collects the radiation signals generated by human activities such as lights and fires, which can effectively reflect the level of resource consumption (Lv et al. 2020;Zhang and Seto 2011). NPP is the organic matter produced by green plants in unit area and unit time, is an important indicator to evaluate the sustainable development of terrestrial ecosystems (Z. J. Liu et al. 2021;Sutton et al. 2016),  and natural environmental resources are the basis for ecosystem development, so the existing level of NPP evaluation of natural resources has been added to this research index system.

Mapping the spatial image of urbanization and ecological resources
The Chengdu-Chongqing Economic Circle has different resource conditions and levels of urbanization. In order to show the spatial pattern of natural resources and urbanization, and to study how they change in time, this section uses ArcGIS to combine the indicators of each subsystem and plot spatial images of the development level of P subsystem, S subsystem, R subsystem, and U subsystem in 2015 and 2020.

Standardization
The Chengdu-Chongqing Economic Circle includes 142 districts, and there are big differences in the level of urban development and the level of natural resource utilization. In order to compare regions with a large gap in development level, it is necessary to standardize the data first, and then quantify the consumption of ecological resources and the level of urban development. The standardized formula is as follows: where X' ij is the standard value, X ij is the original value, max (X j ) is the maximum value of the jth index of all samples, and min (X j ) is the minimum value of the jth index of all samples.

Entropy method
Entropy method is often used to study the ecological and environmental problems (Fu et al. 2020;He et al. 2021).
According to the characteristics of the entropy method, the weight of the subsystem can be determined by the degree of deviation of each index, which can better retain the characteristics of the data and reduce the loss of information. (1) The relationship between the PSRU system The amount of water supply (P 2 ) Billion cubic meters Gross Regional Product (P 3 ) Ten thousand yuan State NPP (S 1 ) Proportion of cultivated area (S 2 ) % Proportion of forest area (S 3 ) % Proportion of water area (S 4 ) % Response Energy saving and environmental protection expenditure (R 1 ) Ten thousand yuan Pollution prevention expenditure (R 2 ) Ten thousand yuan Environmental protection management expenditure (R 3 ) Ten thousand yuan Urbanization GDP density (U 1 ) Ten thousand yuan of per sq. km The population density (U 2 ) Person of per sq. km Proportion of urban area (U 3 ) % Therefore, this study uses the entropy method to process the data in the PSRU system. Import the standardized data into the entropy formula; the calculation formulas are shown as follows: X' ij represents the standardized value of the ith index and the jth area. e i and W i are the entropy and comprehensive weight corresponding to the index. U j represents the comprehensive development index of the area.

Spatial correlation analysis
Spatial autocorrelation emphasizes spatial correlation. Spatial correlation includes positive correlation and negative correlation, both of which indicate significant correlation. The difference is that positive spatial correlation means the aggregation of spatial distribution positions, and negative spatial correlation means that spatial distribution positions are more discrete. Moran index is adopted to analyze global correlation. After determining the global correlation exists, in order to clarify the specific aggregation state, the local Moran index can be used. The global and local correlation Moran index equations are as follows: (2) I is the global Moran index, and Ii is the local Moran index of the ith area. N is the total number of spatial units. Y i and Y j represent the attribute values of unit i and unit j, respectively. W ij is the spatial weight value.

Spatial pattern of PSRU system
Figures 3, 4, 5 and 6 show significant spatial variations in the PSRU system from 2015 to 2020. Fig. 3 shows that the overall growth of the pressure coefficient of resource consumption in Chengdu-Chongqing area and the pressure of resource use is obviously concentrated from the surrounding areas to the large cities. It can be found that the P subsystem has three main agglomeration centers, which are Chengdu in Sichuan Province, Jiangjin District in Chongqing, and central urban area of Chongqing.
As shown in Fig. 4, the overall development coefficient of the S subsystem has decreased, and this downward trend is more obvious in Chongqing. At the same time, the level of natural resources in Chengdu and the central urban area of Chongqing is decreasing, and this impact has also spread to surrounding cities. Compared with other cities, the level of natural resources in Chengdu and the central urban areas of Chongqing is significantly lower. In addition, we can also find that the resource coefficient in the western part of the Chengdu-Chongqing region has increased significantly.
As shown in Fig. 5, from 2015 to 2020, there was no significant overall improvement in the comprehensive coefficient of the R subsystem, and most cities did not make corresponding changes to the current situation of large consumption of natural resources. However, some cities have added environmental protection measures, such as Yibin in Sichuan Province and the central urban area of Chongqing.
As shown in Fig. 6, the level of urbanization in the Chengdu-Chongqing area has been significantly improved, and the center of development is still Chengdu, and the central urban area of Chongqing, which has played a leading role in promoting the urbanization process in the surrounding areas. In addition, after 5 years of development, the cities in the north of the Chengdu-Chongqing Economic Circle have developed significantly, while the southern region still needs to be upgraded.

Calculation of PSRU
Equations (1) through (9) are used to calculate the entropy value of the PSRU system in the Chengdu-Chongqing area.
The PSRU system weights are shown in Table 3. The entropy value of the PSRU system in the Chengdu-Chongqing Economic Circle in 2015 and 2020 will be used as the basis for spatial correlation analysis.

Global autocorrelation
The global Moran index of PSRU is shown in Table 4 and   Table 5. Using GeoDa to construct spatial weights, 999 random tests are performed. Univariate correlation reflects whether the subsystem itself is related, and bivariate correlation tests whether there is a correlation between the PSR subsystem and the U subsystem. The global Moran index  test results for 2015 and 2020 show that the subsystems of PSRU are all P < 0.01 and the Moran index is positive, which means that each subsystem has a significant positive correlation, and the overall space has an agglomeration effect. From the results of the bivariate Moran index test, it can be found that there is a significant relationship between the PSR subsystem and the U subsystem, where there is a negative correlation between the S subsystem and the U subsystem (I < 0), and the PR subsystem and the U subsystem are positively correlated (I > 0). After proving the significant existence of the global Moran index, the local Moran index can be used to test the clustering mode of urban agglomerations.

Bivariate spatial autocorrelation analysis
In order to clarify the specific agglomeration effect of Chengdu-Chongqing economic Circle, we carry out local spatial correlation analysis. Spatial distribution images are shown in Fig. 7 through Fig. 9, and the study subjections can be divided into five categories according to color, namely cities with no significant relationship (P > 0.5) and high-high (H-H), low-low (L-L), low-high (L-H), low-high (L-H), and high-low (H-L) types with significant relationships (P < 0.5). Taking the spatial correlation of P subsystem and U subsystem as an example, H-H represents that the cities in the region have both high natural resource use pressure and high urbanization level. L-L represents a low coefficient of stress and a low level of urbanization in the region's cities. L-H represents a higher level of urbanization near units with low resource use pressures, while H-L is the opposite.
From Fig. 7, it can be found that the areas with H-H types in the spatial relationship between P subsystems and U subsystems in 2015 are mainly concentrated near the central urban areas of Chengdu and Chongqing, and  L-L types are concentrated in the north and south of the Chengdu-Chongqing Economic Circle. L-H is distributed around the H-H area, and H-L-type cities are located around L-L. Compared with 2015, although the spatial distribution relationship between the P subsystem and the U subsystem is roughly unchanged, it can be found that the number of H-H types and L-L types of cities has declined, and some cities have changed to H-L or L-H types. The spatial relationship between the S subsystem and the U subsystem is shown in Fig. 8. H-H types of cities are the fewest, always on the outskirts of the two core cities of the Chengdu-Chongqing Economic Circle. L-L types of cities are concentrated in Nanchong and Suining, and the number of cities has dropped significantly within 5 years. L-H types of cities are concentrated in the center of Chengdu, and L-H types are relatively scattered in the central urban area of Chongqing. And in 2020, most of the cities in the central urban area of Chongqing that originally belonged to the L-H type have changed to the H-H type, which also shows that the level of natural resources in the area has been significantly improved. The H-L type is distributed in the southwest and northeast of the Chengdu-Chongqing Economic Circle.
As shown in Fig. 9, the spatial relationship between the R subsystem and the U subsystem is roughly the same in 2015 and 2020, the H-H type is developed outwards to the central urban areas of Chengdu and Chongqing, the L-L type is located in the southwest and north of the Chengdu-Chongqing Economic Circle, L-H is located in the outskirts of the central urban area of Chengdu, and the H-L type is mostly in the northeast of the Chengdu-Chongqing Economic Circle.

Classification of cities according to spatial agglomeration effects
From the perspective of bivariate spatial correlation, the spatial relationship between resource consumption and urbanization in the Chengdu-Chongqing Economic Circle has been basically classified. In order to further study the spatial relationship between the comprehensive utilization of resources and urbanization in the Chengdu-Chongqing Economic Circle, we have subdivided the urban agglomeration again. Firstly, the areas with significant correlation were divided into high urbanization areas (HU) and low urbanization areas (LU) according to the agglomeration type of urbanization level, and then the cluster areas of each subsystem were superimposed by the method of spatial superposition; finally, the high-value agglomeration area of each subsystem was set as a high advantage area (HAA), the mixed-value area of high-low value was set as a low advantage area (LAA), and the low-value cluster area was set as an disadvantaged area (UA) (see Fig. 10).
The areas with high levels of urbanization are located in Chengdu and the central urban areas of Chongqing, which are also the core areas for the development of the Chengdu-Chongqing Economic Circle. Among them, HUHAA type cities have achieved a balance between high consumption of natural resources, high actual level of resources, and attention to environmental protection; such cities have achieved a balance between comprehensive utilization of natural resources and urbanization development in social development, but the number of such cities is small and distributed in the periphery of the core development area. The integrated use of natural resources is weaker in HULAA areas than in HUHAA areas, where cities do not take into account the use, maintenance, and protection of natural resources. In addition, the HUUA region has a less efficient use of natural resources. The areas with low urbanization level are concentrated on the north and south sides of the Chengdu-Chongqing Economic Circle, and the north and south are located in Fig. 9 Spatial correlation of R subsystem and U subsystem in 2015 and 2020 the mountains on the edge of the Sichuan Basin, the natural environment is more complex, and natural resources are difficult to use, so the urbanization development is slow. In the northeast of the Chengdu-Chongqing Economic Circle, there are individual cities belonging to the LUHAA type, which can effectively use natural resources and can positively promote the urbanization of backward areas. The LULAA type has a certain utilization of resources, but it needs to be improved, and the LUUA type of city has the weakest ability to use natural resources, and this type is concentrated in the Nanchong city area.
From 2015 to 2020, the overall resource utilization efficiency of Chongqing's central urban area has improved significantly, and the cities that have been upgraded from HULAA type to HUHAA type include Banan District, Qijiang District, and Fuling District; Banan District is located in the central urban area of Chongqing, and Qijiang District and Fuling District are located in the new district of Chongqing. As a city with slower development in the central urban area, Banan District focused on the development of emerging industries during the 13th five-year plan period, optimized the layout of traditional industrial parks, achieved remarkable results in pollution protection and control, and improved the comprehensive utilization efficiency of resources from the perspective of response. Qijiang District and Fuling District, as the industrial concentrated development areas in Chongqing, have undertaken the industrial transfer and division of labor in the central urban area in the past five years, realized the industrial cluster, and positively improved the level of energy resource utilization from the direction of pressure. Cities upgraded from LUUA type to LULAA type are mainly concentrated in Nanchong in the north of the Chengdu-Chongqing Economic Circle. Although the level of urbanization in Nanchong is still relatively low, the area pays attention to scientific and technological innovation, improves the industrial system, and establishes an important logistics distribution center in western China, which significantly improves the ability to use natural resources, which also provides impetus for the further development of the city.
The western core area of the Chengdu-Chongqing Economic Circle showed an overall downward trend between 2015 and 2020. Cities that changed from HUHAA type to HULAA type are Mianzhu, Shifang, and Pengzhou. The phosphorus chemical industry is the pillar industry of Mianzhu and Shifang, and the petrochemical industry is the pillar industry of Pengzhou, but both will produce a large amount of polluting waste in the process of development, such as phosphogypsum waste and dust. The natural resource consumption level of these cities was originally high, but the pollution prevention and control expenditure of the three places fell by 41.73%, 43.35%, and 67.09%, respectively, which is not conducive to achieving sustainable development. Cities that have changed from HULAA type to HUUA type include Zhongjiang County, Danling County, and Pujiang County, all of which were originally agriculture-oriented cities, but in recent years, these three cities have begun to focus on the development of secondary industries, and are now in a period of industrial transformation. Although the completion of a large number of industrial parks has promoted the development of urbanization, it has also led to a

Recommend for urban sustainability
We can find that the capacity of the core area on the east side of the Chengdu-Chongqing Economic Circle for the comprehensive utilization of natural resources is higher than that of the core area on the west side, and the relationship between the comprehensive utilization capacity of resources and urbanization shows the characteristics of high in the east and low in the west, which jointly drives the development of the north and the south. As one of the cores of the Chengdu-Chongqing economic Circle, Chengdu needs to adjust the pattern of resource utilization in order to catch up with the central city of Chongqing and achieve sustainable development. For such cities with a high level of urbanization, the full use of natural resources has been realized, and it is more necessary to expand the depth and breadth of the industry, pay attention to the critical period of industrial transformation, strengthen environmental governance, and attach importance to the management and protection of natural resources. At this stage, Chengdu has implemented prevention and control measures such as car restriction travel, but in addition to the measures implemented by the core cities, the core peripheral cities, as areas where the relationship between resource consumption and urban development tends to be balanced, need to enhance their own attractiveness, provide industrial migration for the core areas, share the pressure of resource use, and enable the core cities to allocate more human and financial resources to adjust the pattern of natural resource use.
For cities with low levels of urbanization in the north and south of the Chengdu-Chongqing Economic Circle, the primary demand is to improve the ability to use natural resources and improve the level of urbanization. Due to the complex mountainous conditions on the north and south sides, it is necessary to improve infrastructure such as transportation to create conditions for the development of natural resources and the improvement of industrial structure. Therefore, the north and south sides of the Chengdu-Chongqing Economic Circle should give priority to the construction of infrastructure, redevelop characteristic industries, enhance the city's attraction capacity, and achieve sustainable development.

Conclusions
Natural resources are an important foundation for sustaining the environment and powering human activities (T. Peng and Deng 2021). Therefore, this study discusses from the two directions of time and space, and through the methods of geospatial information quantification and spatial correlation analysis, the spatial relationship between the comprehensive use capacity of natural resources and the level of urbanization in the Chengdu-Chongqing Economic Circle is discussed, which is conducive to planning the development direction of the Chengdu-Chongqing Economic Circle. The specific conclusions of the study are as follows: (1) The spatial relationship between the comprehensive utilization of natural resources and urbanization in the Chengdu-Chongqing Economic Circle shows the characteristics of high in the east and low in the west, which jointly drives the development of the north and the south. The comprehensive capacity to utilize, maintain, and protect natural resources has declined in the core area of Chengdu (the western core area of the Chengdu-Chongqing Economic Circle), as opposed to the central city of Chongqing (the eastern core area of the Chengdu-Chongqing Economic Circle). The use of natural resources on the north and south sides is in a period of development, of which the development of Nanchong City is the most obvious.
(2) Cities with balanced use of natural resources, resource levels, environmental protection measures, and urbanization levels are located on the outskirts of the core area of the development of the Chengdu-Chongqing Economic Circle, which not only has a high level of comprehensive utilization of resources, but also plays a role in absorbing the transfer of core urban industries, accommodating the population of core cities, and reducing the pressure on the use of natural resources in core cities. (3) The main reasons for the reduction in the efficiency of comprehensive utilization of natural resources in areas with high urbanization levels are the transformation of industrial structure and the reduction of investment in environmental protection, which require relevant departments to strengthen management measures and improve the structure of resource utilization. While areas with slow urbanization are currently in a period of improving the ability to utilize natural resources, the government needs to improve the infrastructure construction and establish a complete industrial structure in these areas to lay the foundation for the effective development of natural resources and promote urbanization.
Author contribution Writing, checking, and correcting: Ying Zhou. Collect data and analyze: Zhuolu Li, Yuan Chen. Literature review and contact editor: Wei Wei.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests
The authors declare no competing interests.