Analysis of Dual-Core Type City and Its Hinterland Contact Pattern under the Background of High-Speed Rail Networking — Urban Agglomeration in Zhejiang Province as an Example

Under the background of high-speed rail networking, this paper uses the passenger trains, the type and direction of the railway via Hang Yong and its extension road line, to construct adsorption and dependency index among cities, depict the contact pattern between the Hang Yong dualcore and its hinterland, measure the “net effect” that two center cities (Hangzhou, Ningbo) have on their hinterland, and estimate population agglomeration potential and future possible population flows of Zhejiang Province and the main sample cities. The result shows that, compared with Ningbo, Hangzhou has stronger radiation force to the vast majority of sample cities, and the sample cities affected more by Ningbo mainly concentrates in Ningbo-Taizhou-Wenzhou along; in addition, the sample cities such as Hangzhou, Ningbo and so on show better population agglomeration, then the population “scramble” phenomenon between cities has begun to appear.


Introduction
The contact strength between cities is gradually enhanced by the increasingly perfect high-speed rail (HSR) network.In China, original orbit of urban population agglomeration is much likely to be rewritten by HSR.
Since 2008, at a distance of 300 -750 km between cities, HSR is gradually becoming the best mean of transportation, and the twins linkage development era within a radius of 500 km has been opened.At present, almost every Chinese coastal province is dual-core [1].In the group of dual-core cities which has opened HSR in the same province, Hangzhou (Hang) and Ningbo (Yong) of Zhejiang Province are fairly representative.With the opening of the Yong-tai-wen Railway, Wen-Fu Railway, Hang Yong specialized passenger railway, Shanghai-Hangzhou intercity high-speed railway and Hangzhou high-speed rail, Zhejiang has become one of the most intensive provinces in China high-speed rail network.Hangzhou metropolitan area was selected as China first comprehensive reform experiment site of metropolitan area economic transformation and upgrading in 2014, therefore the practice of Zhejiang shoulders great times responsibility that leads Chinese cities linkage and regional integration.Take Zhejiang province as an example, to discuss the linkage relationship between the Hang Yong dual-core and its hinterlands in Yangtze River Delta polycentric urban system pattern, measuring Hang Yong dual city and its hinterland contact pattern under the background of high-speed rail networking are helpful to understand the evolution trend of the future contact pattern between China coastal regions as well as other urban agglomerations and provide observation sample for the linkage development between China central city and its hinterland.
A more extensive exploration on regional spatial contact patterns measurement has been made by academia.Relative research can be summed up in the following two aspects: one is describing the regional economic contact pattern based on population, GDP and other related socio-economic indicators [2]- [4].The other is measuring the regional spatial contact pattern based on the traffic accessibility, cargo flow, passenger flow and information flow [5]- [13].However, the possibility of cities contact and attracting scope [14] are offered by analysis based on the transportation network accessibility, but the actual flow occurred among cities can't be represented by the index.Although analysis based on social network can judge the city's close contact degree and the overall pattern, it can't offer urban linkage directions, dependency or radiation relationship.Except that, proximity effect and distance attenuation characteristics have been largely ignored.The study of Chen Jianjun etc. has made the beneficial attempt, but the relative importance of the category of the trains is still unable to be identified by the adoption of commuter frequency indicator between any two cities [13].Throughout, the urgent needs are to reconstruct methodology, and to find new way of data sources, especially assessment method whose form is simple but connotation is rich [15].Four effects caused by high-speed rail, which are space-time, border breakthrough, regional enhancement, and elements integration, so far lack much data to support, and the study of the influence high-speed rail has on urban system spatial pattern is mainly in form of qualitative description, synthesize and technology-based tool [16].In addition, the domestic related research tends to be "heavy grade, light contact", which lacks systematic study on directed contact between cities [17] [18].The research based on relationship data's influence on the networking contact between cities is still rare, and research on the relationship of interaction and linkage between cities is still in the initial exploratory stage [19].
In view of this, urban agglomeration in Zhejiang Province is taken as an example to depict the space contact pattern between the Hang Yong dual-core and its hinterland, measure the "net effect" that two center cities (Hangzhou, Ningbo) have on their respective hinterland, and fit and contrast the population of Zhejiang Province and other main central cities in Yangtze River Delta, according to the passenger trains, the type and direction of railway to construct adsorption and dependency index among cities.This research is based on the following considerations: First, the flow of people implies most information of various elements flows.The quantity and structure of population are ultimate decision variables of urban development, whose flow direction determines physical capital, information and goods flow directions, and population scale and direction in the flow can be well characterized by the railway passenger trains and running direction.Second, because of the difficulties in obtaining the data, current research on the accurate measurement of urban association, which is based on the comparability, is very rare.There is no substantial change in the function of railway transportation since the 19th century, and the space-time effect based on the connection of railway passenger transportation to evaluate the high-speed networking can undoubtedly establish an analytical framework in which the urban contact pattern is more accurate.

Method Design and Data Collection
The passenger trains, the type and direction of the railway via Hang Yong and its extension road line are used in the paper to measure the cities adsorption index and dependency index among cities, and the associated features between central cities and their hinterlands are analyzed.

Delimit the Distribution Area of Sample Cities
Based on railway passenger links, to depict the correlation features of Hang Yong twins and urban agglomeration in Yangtze River Delta, and the geographical scope of the sample cities: Shanghai and all prefecture-level cities in Zhejiang provinces, Jiangsu provinces and Anhui provinces as well as those counties in their railway lines, part cities of Hunan, Jiangxi, Fujian via by the railway extension line, which passes Hang Yong dual cities, as well as a part of the reference cities beyond Hang Yong railway and its extension line, including Shanghai, Nanjing, Hangzhou, Ningbo, Hefei, Changsha, Nanchang, and other cities.Then based on the main railway and its extension line via Hang Yong twins, to determine the sample city unit and site.Sample cities are mainly located in Hang Yong railway line and within the distance of 500 km radius, and parts of sample cities are distributed in 500 -1000 km railway extension line.What's more, we introduce a small amount of reference cities.By screening, there are 93 sample cities and 106 sites included in the analysis and measurement at last.Among them, there is one municipality, three deputy provincial cities, 49 prefecture-level cities (including a small area of municipalities), and 40 county-level cities (districts and counties).Sample cities and its provincial domain refer to Table 1.
At present, the main railway lines operated in Yangtze River Delta region include Beijing-Shanghai Railway, Nanjing-Qidong Railway, Shanghai-Hangzhou Railway, Zhejiang-Jiangxi Railway, Xin Chang Railway, the Longhai Railway and other ordinary railway lines, as well as the Shanghai-Nanjing high-speed railway, Shanghai-Hangzhou high-speed railway, Nanjing-Hangzhou high-speed railway, Beijing-Shanghai high-speed railway, Hening part of Shanghai-Chengdu high-speed railway, NingAn intercity railway, railway Ningbo-Taizhou-Wenzhou high-speed railway and other high-speed railway lines.Railways under construction include Shanghai-Nantong railway, Lianyungang-Yancheng railway, Jiangsu coastal railway, Lian-Huai-Yang-Zhen railway, Hangzhou-Huangshan high-speed railway and Shang-He-Hang high-speed railway, etc. High-speed rail lines in Yangtze River Delta region, which have been opened and are about to open by the end of 2016 and the site details are shown in Table 2.

Index Construction
According to method of Zhou Jingxiang [20] and the railway passenger trains, type and the corresponding weights to construct city's adsorption (Adsorption) and dependency (Dependency) index.Calculation method of city's adsorption index is 3 8 where X ij is the quantity of trains, j is the weight of trains' type, according to the importance of different traintypes, the weight is decreasing in turn.I is the ways passenger trains go through the city which include "originator", "route" and "end to", and its weight (η i ) is assigned 0.35, 0.3 and 0.35.Calculation method of city's dependence index: where A → B means the dependency relationship of city A to city B, and vice versa.X ijA→B is the quantity of trains which is "originator" and "end to" from city A to Band vice versa; X ijA˅B is the train from A to B or via A end to B. The assignment of weight and the meaning of other letters are the same as the way of adsorption index.

Data Collection
The actual city passenger train data used in this paper is all from the website: Where to go.Where to go provides sites, lines, trains and other information of railway passenger transportation query time between April 20, 2015-May 20, 2015.City site information includes all trains and types of the "originator", "route" and "end to" in railway stations.After choosing departure and arrival, information of the line shows all trains and types of the "originator", "route" and "end to".When making data statistics of city sites and passenger trains between cities, according to the type of trains (G, C, D, Z, T, K, L, O, etc.) to classify, to distinguish the importance of different types of trains in urban linkages.Empirical part involves urban household population, the resident population, GDP, total fixed asset investment as well as its growth rate of the whole society and other variables, whose data is derived from "National Economy and Social Development Statistics Bulletin" and "Government Work Report" of the city over the years.What is more, other data that can't be obtained is filled by "China city statistical Yearbook" and "regional economic China statistical Yearbook".

Calculation Results
After 2008, high-speed rail lines are opened one after another in the Yangtze River Delta region, which increases the uncertainty of the city population size and flow direction.Transportation system reform caused by highspeed rail is changing the spatial connection and development pattern among cities.

Adsorption Capacity and Spatial Distribution Pattern of Hinterland Cities
According to the adsorption capacity of sample cities calculated by Formula (1), the cities whose absorption index greater than 20 are Shanghai, Nanjing, Hangzhou, Suzhou, Wuxi and Changzhou, and three cities of the top are Shanghai, Nanjing and Hangzhou, whose city absorption index in order are 62.73, 40.78, and 32.29.Shanghai's is nearly 20 more than Nanjing's which ranked second, showing a very strong adsorption capacity of the population.Cities whose absorption index ranges from 10 to 20 include: Xuzhou, Jinshan, Hefei, Zhenjiang, Kunshan, Jiaxing, Jinhua, Ningbo, Yiwu, Bengbu; absorption index ranges from 5 to 10: Danyang, Shaoxing, Wenzhou, Quzhou, Huzhou, Yuyao, Taizhou, Haining, Yixing, Zhuji; absorption index less than 5: Chuzhou Shangyu, Tongxiang, Changxing, Huainan, Wenling, Liyang, Deqing, Quanjiao, Jiashan, Fuyang, Yushan and Songjiang etc.Take Zhoushan as an example of reference points of the city although located in the axis of development (Shaoxing, Huzhou) or have special industries (Zhoushan's Fishery), the absorption index is 0 due to traffic inconvenience.Table 3 shows the top 30 sample cities and their rank features, and Figure 2 shows their spatial distribution.

The Dependency Pattern of Hinterland Cities on the Central City
According to Formula (2), first measure the dependent pattern between each two cities of the five central cities:  3 and Table 4, draw according to the city's latitude and longitude.It is not difficult to find that there are significant spatial differences in the strength and direction of contact between each two cities of five cities.As shown in Table 4.
Next measure the dependency pattern of sample cities to regional center (Shanghai).Among them, the dependencyof Nanjing to Shanghai is the strongest, followed by four cities: Suzhou, Wuxi, Changzhou and Hangzhou, and their respective dependency indexes to Shanghai in the order are884.55,769.05, 671.4,616.15 and 525.1.The dependency index of Ningbo to Shanghai is 171.1, ranking 14 th .These cities, which are not in the Yangtze River Delta and with a longer space distance away from Shanghai, such as Nanchang, Yingtan, Shangrao, Changsha, to Shanghai's dependency index also enter the top 30.The cities of the top 30 of dependency index gather in Nanjing to Shanghai, Shanghai to Hangzhou, but are relatively sparse between Hang Yongtwins, as is shown in Figure 3. Comparing Figure 2 with Figure 3, it is not difficult to find that there is great overlapin the dependency index between the sample cities and the cities of the top 30 of dependency index.The latter's concentration degree between Shanghai-Hangzhou, Shanghai-Nanjing and Hang Yong is higher than the former.The top 30 cities of dependency index are shown in Table 5.

Comparison on the Adsorption Capacity of Hang Yong Twins to Hinterland Cities
Given that city's absorption index meets with additivity principle, we subtract Hang Yong twins' adsorption index to hinterland cities, then get the "net effect" that the twins have on their respective hinterland cities.To ma-  3 and Table 4, draw according to the city's latitude and longitude.jority sample cities, Hangzhou's radiation is more significant than Ningbo's, and Ningbo's radiation, only to Shaoxing, Shangyu, Yuyao, Ninghai, Linhai, Taizhou, Shen Fang, Chizhou, Cangnan, Fuzhou, Wenling, Yueqing, Yongjia, Xiapu, Fuding, Ningde, Lin'an, Rui'an and other 19 cities, is relatively strong.These cities, except for Shaoxing, Yuyao and Shangyu, mainly gather in Yong Tai Wen Railway along.Except the cities mentioned above, Ningbo's radiation to the other sample cities is less significant than Hangzhou's.The "net effect" which Hang Yong has on their respective hinterland cities is shown in Table 6.

Urban Population Agglomeration Potential under the Center-Periphery Pattern
Spatial interaction model shows that, the larger the scale of the urban population, the more potential inter-city    Note: "+"indicates "too much"; "−" indicates "inadequate", thus there still remains a huge space to absorb the population.
Changzhou in Jiangsu province, their fitting population scale is greater than their actual resident population scale in 2014.To some extent, these cities' population scale is "small", and there is still some room for its growth; while the resident population of Wenzhou and Taizhou, which take on a greater population absorption ability, has decreased, and the population "scramble" phenomenon between cities has begun to appear.

Figure 1
is a flow block diagram of study method.

Figure 1 .
Figure 1.A flow block diagram of study method.

Figure 2 .
Figure 2. The spatial distribution of the top 30 cities of Adsorption index.Note: Figure 2 and Figure 3 respectively combined with data of Table3 and Table 4, draw according to the city's latitude and longitude.

Figure 3 .
Figure 3.The spatial distribution of the top 30 cities toward Shanghai Dependency index.Note: Figure 2 and Figure 3 respectively combined with data of Table3 and Table 4, draw according to the city's latitude and longitude.

Figure 4 .
Figure 4.The relationship between Dependence index and the city's resident population (2014).Note: The figures are drawn according to the statistical bulletin of the city over the years and the author's calculation, the abscissa in Figure 4 represents the Dependency index of part cities in Zhejiang and the Yangtze River Delta, toward Shanghai.

Figure 5 .
Figure 5.The resident population of major cities in Zhejiang province (2006-2014).Note: The figures are drawn according to the statistical bulletin of the city over the years and the author's calculation, the abscissa in Figure 4 represents the Dependency index of part cities in Zhejiang and the Yangtze River Delta, toward Shanghai.

Table 1 .
Sample cities and its belonging province area.
Note: According to history and publicly reported data sorting summary.Table2is the same.

Table 2 .
The Yangtze River Delta high-speed rail line and the site overview.

Table 3 .
Absorption index and its rank of sample cities in Zhejiang province.
Note: According the results by author's calculation, then sort out this table.Shanghai, Hangzhou, Ningbo, Nanjing and Hefei.The dependency index of Hangzhou to Shanghai is 525.1, and Shanghai to Hangzhou is 340.2.The dependency index of Shanghai to Nanjing is 634.3, and Nanjing to Shanghai is 884.55.Likewise, Hangzhou to Ningbo is 239.2, and Ningbo to Hangzhou is 179.3,Hangzhou is most closely related to Ningbo in the five cities, and the following city is Shanghai, whose dependency index to Ningbo is 108.95, and converse is 171.1.

Table 4 .
Passenger trains contact and dependency index between core cities in the Yangtze river delta.
Note: According the results by author's calculation, then sort out this table.

Table 5 .
Dependency index and its rank of hinterland cities to Shanghai (At the top of 30).

Table 6 .
Comparison on the net adsorption capacity of Hang Yong twins to hinterland cities.

Table 7 .
The fitting population and deviation of Zhejiang province prefecture-level cities above and major cities in the Yangtze river delta.