Spatiotemporal Characteristics and Interactive Effect between Urbanization and Sustainable Urban Development: Evidence from Yangtze River Delta

: Complex dynamic interactions exist between urbanization and sustainable urban development, and clarifying the collaborative relationship and interactive response mechanism between the two systems is key to ensuring high-quality urban development. Taking Yangtze River Delta as a typical case, dynamic coordinated and interactive response relationships between urbanization and sustainable urban development were measured using a multi-index comprehensive evaluation method, grey correlation analysis


Introduction
Urbanization and sustainable urban development are intricately linked, with a complex interplay between the two systems, representing a cutting-edge topic in urban scientific research [1,2].Urbanization is a multifaceted process involving critical transformation in economic upgrading, social development and production modes, as well as numerous other factors [3].Previous studies have primarily delineated the dimensions of urbanization related to population, economy, space, and other facets.At its core lies population urbanization (PU), propelled by economic urbanization (EU) and manifested through spatial urbanization (SU) encompassing population and economic activities [4].The concept of "sustainable development" was formally introduced by World Commission on Environment and Development in "Our Common Future", published in 1987, aiming to fulfillment the needs of present generations without compromising the ability of future generations.The rapid increase in urbanization has significantly impacted the functioning of urban system [5], resulting in many ecological challenges [6,7] such as a substantial decline in biodiversity, soil erosion and land desertification [8,9].Factors of production including population, capital, and information are increasingly concentrated in urban areas, placing complex pressures on land resource management, urban infrastructure development, employment opportunities for residents, and energy supply [10].The current urban development model requires immediate revision in order to align with the principles of sustainable development [11,12].Consequently, the interplay between urbanization and sustainable urban development has emerged as a focal point within the realms of natural and social sciences.
To date, research has primarily focused on urbanization in national and international contexts with regard to the accompanying changes in economy, society, and technology.Following the industrial revolution, western urbanization occurred within the framework of market economic systems.Urbanization has undergone a transition from concentration to dispersion.In recent decades, certain Western developed countries, such as the United States, have entered a phase of counter urbanization [13,14].Owing to the stable privatization of land and the relatively small discrepancy between population and urban land in Western countries, scholars have paid less attention to measuring various elements of urbanization [15,16].China's urbanization commenced relatively late, and has been accompanied by rapid economic and social transformations in recent decades.This has led to the emergence of various challenges, including disparities between urban and rural residents, intensified land utilization, and uneven economic development [17,18].With the deepening of urbanization process, domestic scholars have shifted their focus towards examining the developmental prerequisites of new-type urbanization.They have conducted research on the interactive coupling relationship among population, land, economy, and society from diverse perspectives and scales [1,19].Drawing on this new-type of urbanization, some scholars have measured and classified the coordination level of urbanization by constructing an evaluation index system and by empirically analyzing the evolutionary characteristics of the coordinated development pattern in different cities [20].Based on cointegration theory and the regression model, other scholars have evaluated the action relationship and coupling degree of each subsystem, analyzed the influencing factors and driving mechanisms underlying it, and then proposed an overall optimization path [21][22][23].
Research on sustainable development mainly focuses on proposing valuable results for evaluating its performance [24][25][26] and exploring its influencing factors [27,28].However, it is important to recognize that sustainable development cannot be comprehensively understood through the analysis of only one component, whether economic or social [29].Hence, in order to comprehensively grasp the concept of sustainable development, it is essential to consider a range of indicators [11,30,31] and integrated assessment tools [32], such as principal component analysis [33,34] or Gram-Schmidt orthogonalization [35].Due to the intricate interconnections between ecological and human systems, a universally accepted framework for evaluating sustainable development at the national or regional level has yet to be established.Thus, comprehensive index assessment is currently the most appropriate method because of its ability to reflect and incorporate socio-economic and environmental conditions [36,37].Numerous studies yield a set of findings in line with the actual circumstances, thereby aiding governmental formulation of evidencebased strategies.
Limited research has been currently underway on the interaction mechanism between urbanization and sustainable urban development.Utilizing remote sensing images and urban lighting data, Bren D'Amour et al. quantitatively projected that global cultivated land area will decrease by 1.8-2.4% by 2030 due to urban expansion, with 25% of this reduction expected in China [38].Additionally, an interaction effect has been observed between urban development and species evolution [39].Moreover, integrating the natural environment with urban social issues has emerged as a crucial aspect of achieving sustainable urban development.To date, the comprehensive findings have yielded a profound comprehension of pertinent theoretical knowledge and methodologies; however, the majority of studies have assessed urbanization or sustainable urban development without taking into account the underlying interactive response mechanism from a synergistic perspective.Hence, the equilibrium between the preservation of high-quality urban development and collective advancement has been disregarded.Previous research has predominantly examined a singular time or location when assessing regional characteristics, neglecting to incorporate an evaluation from a spatiotemporal perspective.Furthermore, limited attention has been given to studies on urbanization and sustainable urban development.In comparison to national-level macro analysis and single-city micro examination, there is a scarcity of regional studies focusing on provinces or urban agglomerations.
Summarizing existing research findings, our research contributions are mainly reflected in the following two aspects.First, we construct a new theoretical outline to explain the relationship between urbanization and sustainable urban development, which will provides useful reference for the coordinated development of the social and economic sectors in Yangtze River Delta (YRD).Second, it accurately evaluates the level of sustainable development and urbanization in various cities in YRD, and summarized the characteristics of regional differences.Therefore, this is of great significance for promoting academic progress and guiding social practice.The other parts of this research are as follows.After the introduction and theoretical outline, the study utilizes the YRD as a case study and establishes a comprehensive evaluation index system.Entropy method is employed in order to assess the degree of urbanization and sustainable urban development, while comparing their evolution through time series change and spatial evolution dimensions.Subsequently, panel-data vector autoregressive (PVAR) model is chosen to explore interactive response mechanism.Finally, the sixth section summarizes the conclusion and limitations, aiming to provide useful reference for sustainable urbanization in YRD.

Theoretical Outline
Referring to existing research results [40,41], urbanization and sustainable urban development are two aspects by which to measure urban systems, and their coordinated evolution is key to the healthy operation of these systems (Figure 1).Urbanization systems comprise the population, space, economy, and other elements, and their development entails a process of population agglomeration, spatial extension, and economic growth.Sustainable urban development includes economic development, social progress, ecological environmental development, and infrastructure construction.Considering the current spatial imbalance between urbanization and sustainable urban development, urbanization process generally has three stages of development, including primary stage of quality lag, transitional stage of quality catch-up, and mature stage of high-quality development.
Land 2024, 13, x FOR PEER REVIEW 4 of 18 population and spatial factors requires industrial restructuring to enhance the output effect of sustainable urban development.
(iii) Mature stage of high-quality development: with the deceleration in urban-scale expansion, the improvement in sustainable urban development exceeds the speed of urbanization.Economic development, social progress, ecological environment, and urban construction are efficient, and the urban system enters the mature stage of high-quality development.Urban high-quality development will inevitably lead to spillover effects and further promote the upgradation of population, spatial, and economic factors.Thus, such spillover effects pave the way for a virtuous circle of sustainable urban development.Therefore, the direction of urban system evolution comprises a shift from urbanization to sustainable urban development and relative stability.

Study Area and Data
The YRD, one of China's largest economic zones, has the fastest economic development and has the highest potential for development among all regions in China.It has 26 neighboring cities centered around Shanghai and is also a key area in China's urbanization process.The spatial scope covers four provinces and city, including Shanghai, Jiangsu, Zhejiang, and Anhui (Figure 2).The YRD has a total area of 108,749 km 2 and a population of 96.05 million, accounting for 2.22% of the national land area and comprising 10.07% of the permanent population.Since the 2000, the YRD economy has developed  (i) Primary stage of quality lag: when the speed of urbanization is quicker than the improvement in sustainable development, problems arise because of a decline in economic level and quality of life, a lack of infrastructure, and degradation of the ecological environment.Urban development relies on scale expansion in this stage, while sustainable urban development lags behind.
(ii) Transitional stage of quality catch-up: with the impact of urban environmental regulations, labor quality improvements, and technological progress, the level of sustainable urban development gradually improves and continues to catch up with the urbanization speed.Thus, urban development enters a transitional stage.Here, a large amount of talent and a large rural labor force are transferred to the cities, with a clear population-scale effect.To effectively absorb this talent, the government increases investment in urban housing, transportation, infrastructure, and ecological greening to enhance sustainable urban development.In addition, the efficient agglomeration of population and spatial factors requires industrial restructuring to enhance the output effect of sustainable urban development.
(iii) Mature stage of high-quality development: with the deceleration in urban-scale expansion, the improvement in sustainable urban development exceeds the speed of urbanization.Economic development, social progress, ecological environment, and urban construction are efficient, and the urban system enters the mature stage of high-quality development.Urban high-quality development will inevitably lead to spillover effects and further promote the upgradation of population, spatial, and economic factors.Thus, such spillover effects pave the way for a virtuous circle of sustainable urban development.Therefore, the direction of urban system evolution comprises a shift from urbanization to sustainable urban development and relative stability.

Study Area and Data
The YRD, one of China's largest economic zones, has the fastest economic development and has the highest potential for development among all regions in China.It has 26 neighboring cities centered around Shanghai and is also a key area in China's urbanization process.The spatial scope covers four provinces and city, including Shanghai, Jiangsu, Zhejiang, and Anhui (Figure 2).The YRD has a total area of 108,749 km 2 and a population of 96.05 million, accounting for 2.22% of the national land area and comprising 10.07% of the permanent population.Since the 2000, the YRD economy has developed rapidly.The total area of the Yangtze River Delta region is 108,749 km 2 , with a population of 96.05 million.It carries 10.07% of the permanent population, accounting for 2.22% of the national land area.In 2020, the GDP reached 17.86 trillion yuan, accounting for 19.98% of China's total GDP.From 2000 to 2020, the average annual growth rate was 9.27%, higher than the national average of 8.44%.Meanwhile, the urbanization rate in YRD has continued to rise from 40.35% in 2000 to 67.38% in 2020.However, the region is also facing the challenges of low-carbon economic growth and ecological environmental problems.According to China's New-Type Urbanization Plan, the YRD will play an important role in China's sustainable urbanization.Therefore, it would be necessary to explore the interactive response mechanisms between urbanization and sustainable urban development in the YRD.
The research selected panel data from 26 cities in YRD from 2000 to 2020.Basic data were obtained from the China City Statistical Yearbook (2000-2020), and the statistical yearbooks of Shanghai, Jiangsu, Zhejiang, and Anhui .All indicators are converted by CPI index, fixed assets investment price index, etc. to eliminate price impact.In response to the situation of missing data for individual years, the research has carried out moving average processing.

Methodology 4.1. Multi-Index Evaluation Method 4.1.1. Construction of Indicator System
Based on the urban development concept of the harmonious coexistence of human land systems and meeting human needs, and following the principles of comprehensively reflecting the element strength and structural function of urban systems, this study constructed a measurement system of urbanization and sustainable urban development.Among them, urbanization was divided into three aspects: PU, SU, and EU, from a comprehensive of measuring the synergistic symbiosis of population, land, and industry.Three typical indicators were chosen to characterize the level of urbanization.More specifically, choose proportion of urban population to characterize the demographic characteristics of urbanization separately.Proportion of urban construction land area mainly characterizes the characteristics of urbanization in terms of spatial expansion.Proportion of output value of the secondary and tertiary industries to GDP was used to represent the economic characteristics of urbanization, mainly due to the developed economy in YRD.Overall, the indicator system was constructed to select specific indicators more accurately.The comprehensive level of urbanization can be expressed by three typical indicators, and the specific meanings of indicators can also be shown in Table 1.Sustainable urban development performance can be comprehensively determined based on the economic development level (EDL), social progress level (SPL), ecological environment quality (EEQ), and urban construction quality (UCQ).Specifically, EDL comprises six indicators from the aspects of comprehensive economic level, industrial structure, and developmental power and effect to reflect the economic function of the sustainable urban development system.Four indicators were selected to measure the SPL from work return, consumption ability, cultural resources, and medical conditions to reflect the service function of the sustainable urban development system.The EEQ comprised three indicators selected from the aspects of natural environmental conditions and artificial environmental governance to reflect the environmental function of the sustainable urban development system.Regarding innovation ability and hardware infrastructure, four indicators were selected to represent the UCQ, reflecting the governance function of the sustainable urban development system (Table 2).Similarly, according to the top-down method, four thematic indexes were determined initially so the targeted indicators could be selected more accurately.A more detailed explanation of the index system for estimating urban sustainable development level can be found in previous studies [35,42].In this study, entropy method was chosen to determine indicator weights in the evaluation system.Entropy constitutes a universal objective law in nature, and this principle applies to all natural phenomena [43,44].When applied to social systems, information entropy reflects the degree of disorder and can measure well the uncertainty of the indicator system state [45].Specifically, the higher the information entropy, the smaller the indicator difference, and the more balanced the system structure will be.Conversely, the lower the information entropy, the more unbalanced the system structure and the greater the difference.Therefore, the weight can be calculated in line with the entropy value.It can eliminate human subjective factors in the process of determining weight.

Panel-Data Vector Autoregressive Model
Sims [46] proposed a vector autoregressive (VAR) model, which was used to investigate the dynamic relationship of endogenous variables [47].However, the VAR model cannot run panel data, and the individual heterogeneity of evaluation units is not within its scope of consideration.Holtz Eakin et al. overcame the shortcomings of VAR model and developed the PVAR model.PVAR model combines the advantages of both temporal and panel data, which can examine the dynamic relationships of endogenous variables based on panel data [48].The model can effectively explore the correlation between economic growth and CO 2 emissions, and accurately explains the interactive effect between urban smart development and ecological efficiency [49,50].In addition, there are also good practical applications in the financial field, it explore the impact of financial frictions on the investment business of foreign trade companies [51].The abovementioned theoretical outline indicates that the interaction mechanism between urbanization and sustainable urban development is complicated, and may have an endogenous causal relationship.The above theoretical framework indicates that the relationship between urbanization and urban sustainable development is complex, and there may be endogenous causal relationships between evaluation units.To accurately measure the interactive response between urbanization and sustainable urban development, we chose the PVAR model based on the research findings of Kuang et al. [52] and Grossmann et al. [53].The calculation formula for this model is: In the equation, y it is a column vector containing two endogenous variables: the logarithm of urbanization (ln UI) and the logarithm of sustainable urban development (ln USD).β 0 and β j denote intercept term vector and coefficient vector of the equation regression, respectively.α i is vector of fixed effects, which represents individual difference.µ t is time-effect vector, which can denote the temporal changes of variables.ε it is a randominterference term.i = 1, 2, . .., N, and represents the cities in the YRD urban agglomeration.t = 1, 2, . .., T, and stands for year.n is the lag period.

Spatiotemporal Characteristics
Using the natural discontinuity method in ArcGIS10.2, the evaluation results of urbanization and urban sustainable development level were divided into four levels (Figures 3 and 4). Figure 3 shows the changing trend in the urbanization level in the YRD from 2000 to 2020.The findings show that the urbanization level of each city continuously improved, and had significant spatial differences.Typically, the urbanization level of core cities such as Shanghai, Nanjing, Hangzhou, and Suzhou, was the highest.Industrialization and marketization have significantly promoted the urbanization development.By 2020, the urbanization level of central cities became more prominent, with significant polarization characteristics.However, Yancheng and Taizhou in Jiangsu, Chuzhou and Xuancheng in Anhui, and other peripheral cities showed a low level of economic development owing to a lack of agglomeration capacity for factors such as labor, resources, and funding.Compared to the urbanization development of central cities, they also showed a gap.This is because resource endowment and location conditions directly affect the performance of these cities.In future, initiatives should be implemented to undertake industrial spillovers of central cities and accept their development radiation.It is also necessary to coordinate development, infrastructure interconnection, and public service integration towards the "new normal" to improve the level of sustainable urbanization.The overall pattern of sustainable development follows a "core-periphery" circular distribution, wherein primary cities in the YRD form the center of urban agglomeration, surrounded by other cities.
The level of sustainable urban development in the YRD increased annually from 2000 to 2020 (Figure 4).This development entailed the characteristics of coupling and synergy with the spatiotemporal evolution of urbanization.From a regional perspective, taking Shanghai as the center, there is a clear stepwise descent with respect to neighboring cities; this forms a core-periphery circular spatial pattern.Shanghai and its surrounding cities seized the opportunity to open to the outside world, strengthened their economy, innovation, education, and infrastructure, and thus greatly improved their level of sustainable urbanization.In 2000, cities along the Yangtze River relied on the convenient water transportation advantages of the "golden waterway", and the process of urbanization and industrialization was constantly advancing.This promoted high-quality economic development and also improved the service capacity of urban infrastructure, which led to a high level of sustainable development.By taking advantage of foreign direct investment, setting up various types of development zones, and vigorously developing an export-oriented economy, the level of sustainable urbanization in coastal cities was constantly improving.In 2005 and 2010, industrialization in cities such as Xuancheng, Tongling, and Anqing was slow, which restricted their economic development and urban-quality improvement and showed a low level of sustainable development.After 2015, Shanghai, Hangzhou, and southern Jiangsu responded effectively to the global financial crisis.They improved their level of sustainable development by upgrading their industrial structure and transferring several industries along the Yangtze River to upstream cities and adjacent areas.Generally, the level of sustainable urbanization has been affected by the SDL, EDL, EEQ, and UCQ, showing an imbalanced spatiotemporal pattern.sustainable urbanization.The overall pattern of sustainable development follows a "core-periphery" circular distribution, wherein primary cities in the YRD form the center of urban agglomeration, surrounded by other cities.The level of sustainable urban development in the YRD increased annually from 2000 to 2020 (Figure 4).This development entailed the characteristics of coupling and synergy with the spatiotemporal evolution of urbanization.From a regional perspective, taking Shanghai as the center, there is a clear stepwise descent with respect to neighbor-

Stationary and Causality Tests
The stationarity of panel data is a prerequisite for using PVAR model analysis [54].It is necessary to ensure that there is no unit root, so as to carry out the research on PVAR model regression analysis, impulse response analysis, and variance decomposition.Perform logarithmic clustering on the raw data, and then conduct unit root tests on urbanization (lnUR) and sustainable urban development (lnSUD).The adopted panel unit root tests mainly included Levine-Lin-Chu (LLC), Pesaran and Shin (IPS), and Fisher-Augmented Dickey-Fuller (Fisher-ADF) [54].To overcome the potential limitations of a single testing method, methods such as LLC, IPS, and Fisher-ADF were selected to test the stationarity of lnUR and lnSUD.The results show that the two variables pass the significance test and are stationary series (Table 3).To reveal whether urbanization and sustainable urban development constitute causality, a Granger causality test was performed for lnUR and lnSUD.The results show that there was bidirectional Granger causal relationship between urbanization and sustainable development, which verifies the synergistic correlation characteristics between the two systems (Table 4).

PVAR Model Regression Analysis
Before conducting regression analysis on the PVAR model, this research referred to the existing literature to select Akaike information criterion (AIC), Bayesian information criterion (BIC), and Hannan and Quinn's information criterion (HQIC) to determine the optimal number of lag periods [53,55].As shown in Table 5, AIC, BIC, and HQIC statistics are the lowest when the lag period is four.Therefore, "lag 4" was the optimal period for establishing the PVAR model.Considering the occurrence of individual variables during model operation [48], forward-mean differencing was selected to eliminate individual effects [56].This research used the two lagged periods as instrumental variables to solve the endogenous problems.On this basis, GMM estimation was performed on the panel data (Table 6).The lag coefficients of the first three periods of urbanization and sustainable urban development had a positive impact on the period.The coefficients were 0.837, 0.127, and 0.200, and 0.749, 0.157, and 0.11, indicating that both constructs had a positive-progressive effect and self-enhancing impact on the time scale.The improvement in sustainable urban development level followed an inertial growth trend to a certain extent.It is necessary to take a medium-and long-term development perspective and actively implement highquality economic development and environmental governance countermeasures to ensure a progressive effect.However, the fourth lag period shows a certain degree of negative impact (coefficient = 0.062).This is because it was in the early stage of urbanization, with extensive space utilization and excessive population agglomeration, which restricts the healthy development process of urbanization.Along with the rapid urbanization, this relationship should be coordinated with rapid economic growth and population migration.
The regression coefficients of sustainable urban development in the first three periods of the model lag were −0.036, −0.154, and 0.119, respectively, and the direction of action was first negative and then positive.In the early stage of sustainable urban development, this had a convergent hindering effect on urbanization.In later stages, it played a divergent role, which was conducive to promoting the evolution of urbanization toward a highquality model.The coefficients in the first, second, and fourth lag periods of urbanization were 0.127, −0.051, and 0.068, denoting that the promoting influence of urbanization on sustainable urban development also had an obvious fluctuation effect.Urbanization has a significant impact on promoting sustainable development.The first and fourth lag periods of urbanization significantly promoted sustainable urban development.In contrast, the second lag period had a strong convergence inhibitory effect on sustainable urban development.The results show that the impact of urbanization on sustainable development had nonlinear characteristics.

Impulse Response Analysis
The study used Monte Carlo simulation method to the dynamic impulse process between urbanization and sustainable urban development in the YRD, and analyzed their dynamic interactions and response trends in the next 20 years.In Figure 5, the x-axis represents the lag period, which is set to 20, and the Y-axis represents the order of the impulse response.Red color denotes the pulse response value, while blue color denotes the estimated values at the 5th and 95th percentiles, respectively.

Variance Decomposition
Variance decomposition reflects the extent to which one variable accumulated over time is affected by the impact of another variable.It assesses the relative cumulative contribution rate of each variable and then measures the importance of each variable in explaining changes in other variables [52,57].The results shown in Table 7 indicate that, in terms of contribution rate, urbanization has greater impact on sustainable development than vice versa.In phase 1, the contribution rate of urbanization to sustainable urban development was 0.23%, while that of sustainable urban development to urbanization was 0.00%.Over time, the contribution rates gradually increased to 25.15% and 15.54% in the 20th period.The impact of urbanization and sustainable urban development on themselves decreases, and the weakening trend of the impact of sustainable urban development on itself is prominent.Urbanization and sustainable urban development have certain path-dependent characteristics and follow an inertial development trend.At the initial stage of implementing the sustainable development promotion policies, the contribution to their own development was not <95%.In phase 5, the contribution of the interaction between urbanization and sustainable urban development was approximately 5%.In addition, explanatory power gradually increased over time.In subsequent periods, the impact of urbanization on sustainable urban development gradually increased, while the impact of sustainable urban development on urbanization decreased.Figure 5a shows a significant positive pulse response to urbanization when subjected to a standard deviation shock.Over time, the positive continuous response tends to decrease, which confirms the inertia and path dependence effects of urbanization evolution.Figure 5b reflects the response of urbanization to a standard deviation change of one in sustainable urban development.In the 20 periods, urbanization had a significant positive response.Fluctuation was exhibited in the first three periods, with urbanization first increasing and then decreasing.Subsequently, sustainable urban development has a relatively stable positive impact on urbanization, showing that the improvement of sustainable urbanization plays a fluctuating role in promoting urbanization.In addition, this shows that the adverse effects of sustainable urban development on urbanization can be mitigated by improving the EDL and promoting urban development quality.Figure 5c shows the impact of sustainable urban development on urbanization after one standard deviation shock.Sustainable urban development also exhibits a positive impulse response, and the first three periods show a V-shaped fluctuation track.However, the lower limit of the 95% confidence interval decreases to a negative value in phase 3, while the impulse response after phase 4 appears to follow a steady positive trend.Generally, the impact of sustainable development on urbanization indicates a positive impulse response and will continue to exert a significant positive enhancement effect in the future.This indicates that the long-term promotion of policies such as local PU, intensive spatial development, and industrial transformation will ensure that urbanization plays an important role in enhancing sustainable urbanization.Figure 5d shows the impulse effect that sustainable urban development has on itself.The response peaked at 0.067 in phase 1 and yielded a positive response, after which the positive response decreased annually.While confirming the results of the above GMM estimation, this demonstrates that the enhancement mechanism of sustainable urban development gradually weakened over time.
Overall, urbanization and sustainable urban development in the YRD had strong self-enhancement effects and cumulative responses.On the basis of promoting sustainable development policies and smart growth measures, urbanization could elevate the level of sustainable development.And sustainable urban development also played a significant positive role in promoting urbanization.Thus, they are synergistic and improve together.

Variance Decomposition
Variance decomposition reflects the extent to which one variable accumulated over time is affected by the impact of another variable.It assesses the relative cumulative contribution rate of each variable and then measures the importance of each variable in explaining changes in other variables [52,57].The results shown in Table 7 indicate that, in terms of contribution rate, urbanization has greater impact on sustainable development than vice versa.In phase 1, the contribution rate of urbanization to sustainable urban development was 0.23%, while that of sustainable urban development to urbanization was 0.00%.Over time, the contribution rates gradually increased to 25.15% and 15.54% in the 20th period.The impact of urbanization and sustainable urban development on themselves decreases, and the weakening trend of the impact of sustainable urban development on itself is prominent.Urbanization and sustainable urban development have certain pathdependent characteristics and follow an inertial development trend.At the initial stage of implementing the sustainable development promotion policies, the contribution to their own development was not <95%.In phase 5, the contribution of the interaction between urbanization and sustainable urban development was approximately 5%.In addition, explanatory power gradually increased over time.In subsequent periods, the impact of urbanization on sustainable urban development gradually increased, while the impact of sustainable urban development on urbanization decreased.Construct a new theoretical outline to analyze the collaborative evolution process between urbanization and sustainable urban development.According to the current spatial imbalance between urbanization and sustainable urban development in China, urban development process can be divided into the initial stage of lagging quality, the transitional stage of catching up with quality, and the mature stage of high-quality development.These theoretical reasoning processes have been confirmed in empirical analysis typical of the YRD.
From Figure 5, it can be seen that there is a dynamic pulse process between urbanization and sustainable urban development in the YRD.The promotion of urban sustainable development has a fluctuating effect on urbanization, indicating that by improving the level of socio-economic development and promoting the quality of urban development, the driving force of sustainable development on urbanization can be enhanced.Furthermore, according to GMM estimates, the enhancement mechanism of urban sustainable development on itself will gradually weaken over time.Overall, both urbanization and sustainable development in the YRD have strong self enhancement mechanisms and cumulative response effects.Based on the promotion of national urban development policies and the principle of efficient development, urbanization has become a booster for the improvement of sustainable development level, and sustainable development also has a significant positive promoting effect on urbanization.The two systems interact with each other and improve together.
In addition, both urbanization and sustainable development have certain path dependence characteristics and inertia development trends.Especially in the early stages of implementing policies to enhance urbanization and sustainable development, the contribution of urbanization and sustainable development to their own development is not less than 95%.And within 5 years, the contribution of the interaction between urbanization and sustainable urban development is relatively high, and the degree of explanation continues to improve.

Advantages and Limitations of the Research
In this study, GMM estimation was chosen to explore the causal relationship and interactive effect at the macro level, and then effectively manage the endogeneity problem of variable data.Through impulse response analysis and variance decomposition, it examines the evolving dynamic interactive responses between urbanization and sustainable urban development, offering insights for regional policy formulation and planning practice.The specific advantages are reflected in two aspects: firstly, based on a deep summary of existing research results, theoretical outline is constructed to explain the interactive relationship between urbanization and sustainable development, providing scientific reference for subsequent case studies and policy formulation.Secondly, taking Yangtze River Delta as an example, scientific indicator system is constructed to accurately evaluate the sustainable development and urbanization level of each city, explore the overall characteristics of regional differences, explain the interaction mechanism between the two systems, and provide case support for verifying and adjusting the theoretical analysis framework.
However, there are still some limitations to the research.One of these is that it only focuses on the synergy and interactive response mechanism, while the impact of other factors such as urban development planning, spatial structure adjustment, and system transformation.The research scope did not encompass these additional factors.Due to scale heterogeneity, regional differences may influence the degree and direction of interaction response given the varying scale conditions.Furthermore, the study solely focused on prefecture-level analysis, indicating the necessity for further investigation at various administrative levels.Due to limitations in data collection, it is challenging to identify indicators that comprehensively encompass all facets of urbanization and sustainable urban development systems.Additionally, this research concentrated on historical periods without incorporating future projections.A more comprehensive exploration of the dynamic interrelationships and interactive impacts between urbanization and sustainable urban development should be conducted from a long-term perspective.These considerations can serve as a foundation for devising diverse and effective strategies to advance high-quality urban development.

Conclusions and Policy Implications
This study used multi-index evaluation method to measure the level of urbanization and sustainable urban development in YRD, and then analyzed the spatiotemporal characteristics and interactive response mechanism between them.The primary conclusions are as follows: From 2000 to 2020, the level of urbanization in the YRD showed continuous annual improvement.Core cities such as Shanghai, Nanjing and Hangzhou exhibited high levels of urbanization and significant polarization characteristics.Conversely, peripheral cities like Yancheng and Xuancheng experienced limited attraction to the factors such as labor, resources, and funding due to their lower level of economic development, thus widening the urbanization gap with central cities.The increasing trend of sustainable urban development was also notable, along with the spatiotemporal evolution trend of urbanization.
There was evidence of bidirectional Granger causal relationship between urbanization and sustainable urban development.The regression results of PVAR model indicate that urbanization and sustainable urban development exhibit positive self-enhancement effects and follow an inertial growth trend.Furthermore, the impulse response analysis reveals a small initial positive fluctuation response of urbanization to sustainable urban development in the first three stages, with a significant long-term enhancement effect.Similarly, sustainable urban development initially plays a fluctuating role in promoting urbanization, but exhibits a stable positive promoting effect in subsequent stages.The variance decomposition results demonstrate path-dependent characteristics between urbanization and sustainable urban development, with urbanization exerting a greater impact on the change in sustainable urban development than vice versa.
The policy recommendations of the research are as follows: Firstly, given the significant spatial differences in urbanization and urban sustainable development in YRD, policymakers fully contemplate regional differences and avoid "one size fits all" to policy implementation.Therefore, when formulating corresponding policy measures, cities need to renew their development paths by increasing their understanding of the significance of urbanization and urban sustainable development.Secondly, policy makers should comprehensively evaluate the different stages of urbanization and sustainable urban development; fully understand the development stages and functional positioning of each city.Exploring innovative models and mechanisms for sustainable urban development is crucial for decision-makers in the process of rapid urbanization.Especially in constructing a coordinated development strategy between urbanization and sustainable development, it is crucial to grasp different population sizes and explore the optimal development path of cities. Thirdly, implement the "Yangtze River Delta Integration" to achieve complementary advantages in regional development.With the continuous expansion of urbanization effects and the sustained existence of externalities in urban space, it has occasion to encourage the circular mode of urban-rural interface, improve related mechanisms and common regional development, and improve the level of sustainable urban development.

Figure 1 .
Figure 1.Interaction framework of urbanization and sustainable urban development.

Figure 1 .
Figure 1.Interaction framework of urbanization and sustainable urban development.

Land 2024 ,
13,  x FOR PEER REVIEW 5 of 18 converted by CPI index, fixed assets investment price index, etc. to eliminate price impact.In response to the situation of missing data for individual years, the research has carried out moving average processing.

Figure 2 .
Figure 2. Location of the study area.

4 . Methodology 4 . 1 .
Multi-Index Evaluation Method 4.1.1.Construction of Indicator System Based on the urban development concept of the harmonious coexistence of human land systems and meeting human needs, and following the principles of comprehen-

Figure 2 .
Figure 2. Location of the study area.

Figure 3 .
Figure 3. Spatiotemporal patterns of urbanization in the YRD.Figure 3. Spatiotemporal patterns of urbanization in the YRD.

Figure 3 .
Figure 3. Spatiotemporal patterns of urbanization in the YRD.Figure 3. Spatiotemporal patterns of urbanization in the YRD.

Figure 4 .
Figure 4. Spatiotemporal patterns of sustainable urban development in the YRD.

Figure 4 .
Figure 4. Spatiotemporal patterns of sustainable urban development in the YRD.

Table 1 .
Indicator system for estimating urbanization level.

Table 2 .
Indicator system for estimating sustainable urban development level.

Table 3 .
Stationary test of variables.

Table 4 .
Test for Granger causality.

Table 5 .
Results of multicriteria joint judgments.

Table 6 .
Estimated results of the PVAR model based on the GMM method.

Table 7 .
Estimated variance decomposition results of PVAR model.

Table 7 .
Estimated variance decomposition results of PVAR model.