Evaluate Urban Resilience in Light of Water Resources and Identify Critical Driving Factors-a Case Study from the Yangtze River Economic Belt's Lower Reaches

14 The acceleration of urbanization created a series of problems such as water shortage crisis and 15 serious water pollution which brought the vulnerability of urban water resources, thus affected the 16 economic and social development of city and increased risk of external impact on the urban system. 17 Therefore, improving urban resilience (UR) from the viewpoint of water resources plays an essential 18 role in resisting external shocks. Our paper based on 21 cities along the Yangtze River Economic 19 Belt’s (YREB) lower reaches relevant data from 2010 to 2018, constructs a new UR evaluation 20 framework of “Economy-Society-Infrastructure-Water Resources”. Then the Projection Pursuit 21 Evaluation Model of Real Coding based on Accelerating Genetic Algorithm (RAGA-PP) is used to 22 measure the UR scientifically, Geographic Detectors (GD) method detected and analyzed the UR 23 driving factors. The results present that: (1) The general level of UR in study area was low and 24 characterized by attenuation from central cities to marginal cities, among them Shanghai is the 25 highest (0.651), while Chizhou is the lowest (0.178). The resilience of cities along the YREB’s 26 lower reaches shows a downward trend. (2) Compared with the UR without considering water 27 resources, the UR from the perspective of water resources is relatively higher. Which means that 28 cities along the YREB’s lower reaches have enhanced their ability to deal with external pressure 29 and impact from the perspective of water resources. (3) Water resources and Infrastructure were two 30 factors affected the spatial diversity of UR along the YREB’s lower reaches in 2018 and their 31 average explanatory power of factors increased by 30.610% and 139.035% respectively; the 32 explanatory power of Economy (5.270%) and Society (1.342%) were relatively smaller. With these 33 findings our paper provides relevant suggestions to enhance UR along the YREB’s lower reaches 34 from the perspective of water resources. 35

Its area is about 350,300 square kilometers, accounting for 17.1% of the YREB (as shown in Fig. 1). As 125 the economic center of the YREB, the GDP of the cities along the lower reaches in 2018 was 21.15 126 trillion-yuan, and the proportion is 52% of YREB's total GDP. Moreover, cities along the lower reaches 127 have always been traditionally densely populated areas in China. In 2018, there were about 225 million 128 people in the lower reaches, accounting for 37.6% of the total population of the YREB. It indicates that 129 the lower reaches are facing huge regional population and economic pressure. In 2019, the coverage of influence caused by the drought was expanding and its extent continued to 136 increase. Therefore, it is critical to analyze the resilience level of cities along the YREB's lower reaches 137 and explore its driving factors to improve their ability withstand risks, which is of very important strategic 138 for the high-quality evolution of cities in the YREB. 139

Construction of Indicator System
141 UR means the cities' ability to pull through, adapt and develop when facing any sustained chronic 142 pressure or sudden disaster (Shamsuddin, 2020). It's usually the result of the combined effect of many 143 factors, such as economic development, social security, resources scarce, infrastructure improvement and 144 so on. Base on this the UR index evaluation system can be composed of Economy, Society, Infrastructure 145 and Water Resources. First of all, economic resilience is the fundament of UR. It is the ability of cities 146 to maintain a stable life and production after an economic shock. The change of GDP not only has a 147 crucial impact on various urban economic activities and residents' lives, but also provides economic 148 security and financial support for urban society, infrastructure and water resources subsystems. Secondly, 149 social resilience often repairs to communities or groups' power of dealing with exoteric pressure which 150 is brought by social and political changes (Adger, 2000). A perfect social service system can ensure the 151 infrastructure conditions of residents' life, promote the development of water resources system, and 152 provide labor and technical support for economic development. Thirdly, infrastructure resilience is urban 153 infrastructure systems' capability of recovering quickly and effectively from external shocks (Childers et 154 al., 2015). A high level of urban infrastructure services can form a highly resilient system, which supports 155 the city's economy, society and water resources. Finally, there is a complex relation both urban water 156 resources and socio-economic comprehensive development , a good water resources 157 system can not only provide a good resource environment for the economy and society, but also promote 158 each other with the infrastructure subsystem and the development of UR. Based on this, our paper 159 established an IS based on "Economy-Society-Infrastructure-Water Resources". The relationship 160 between the four dimensions is shown in Fig. 2

Fig. 2 Relationship of IS 163
Urban economic level is the core indicator of UR. At present, scholars mostly use GDP to measure 164 it, but a single indicator is not enough and the information covered by multi-indicator evaluation often 165 more comprehensive (Davies, 2011). Economic resilience used to be self-adaptability and ability to resist 166 risks. Among them, self-adaptability is the ability to integrate social resources, maintain life and 167 production stability after a region suffers an economic shock which can be expressed in terms of per 168 capita GDP (Xin et al., 2019). While the ability to withstand risks aims at the capability of the regional 169 economic system resists interference and absorbs shocks after facing economic shocks or disturbances, 170 which is reflected in the proportion of secondary industries (SI) and tertiary industries (TI) in GDP (Tan 171 et al., 2017). 172 The level of social resilience is nearly associated with the basic lives of the people. The society evaluation index of UR designed by Institute of University of California Berkeley emphasizes that 174 community organizations can realize pre disaster response, mutual assistance in disaster process and post 175 disaster self-recovery by enhancing social integration. Therefore, when calculate UR in society 176 dimension should divided into three aspect: regional economic capacity, community participation 177 capacity and socio demographic capacity (Li, 2021). Based on this, our paper selects three index to 178 characterize social resilience: regional economic capacity, community participation capacity and social 179 population capacity, which includes the average salary of employees, the collection of books in 100 180 public libraries and the number of college students per 10 thousand people. 181 Urban infrastructure is primarily reflected in the city's ability to provide a shelter and evacuation 182 for men in crisis (Goncalves and Ribeiro, 2020). Therefore, our paper measures the resilience of urban 183 infrastructure from four aspects: per capita road area, road passenger volume, sickbeds per 10 thousand 184 people and the proportion of education expenditure in financial expenditure. Among them, the per capita 185 road area and road passenger volume reflect the traffic accessibility of the city. The number of sickbeds 186 per 10000 people and the proportion of education expenditure in financial expenditure reflect the 187 emergency medical services and talent export that health institutions and educational institutions can 188 provide in the event of a disaster. 189 UR of water resources used to be mainly reflected in the low pollutant treatment rate and high water 190 pressure (Yang et al., 2021b). It will increase the load of water resources system, resulting in the 191 degradation of urban water resources system function and reduce UR. Therefore, our paper measures the 192 resilience of urban water resources from four aspects: domestic sewage centralized treatment rate, 193 harmless treatment rate of domestic waste, industrial wastewater discharge of 10 thousand GDP and per 194 capita domestic water consumption. IS is presented in Table 1. 195 For negative indicators: 224 (2) 225 In the above formula, m is the number of cities; n is the number of indexes; * Above formula shows, E (z) is the mean of comprehensive eigenvalue ; R is the window radius of density, it is usually taken 242 if not the value is 0. 244

(4) Optimized projection index function 245
Diverse projection directions present various statistics structure features, and the best projection 246 direction means that it is possible to reveal some feature structure of data with high-dimension. Then, the 247 optimal projection direction will be evaluated through dealing with the problem that make the function 248 of projection index reach maximum, that is: 249 This is a nonlinear optimization problem, which is very sophisticated with according to the formulas (1~8) , our paper processes data using Matlab 2020b, and selects the population 255 size is Along with the advancement of geographic information system, GD has been developed. It is a kind 263 of method which can be applied to detect spatial heterogeneity and disclose its drivers (Wang and Xu,264 2017), whose key idea is if the research variables have an essential impact on the studied variables, they 265 should have similar spatial distribution. In our study, due to the spatial stratification heterogeneity 266 between the study areas and the spatial difference between the action intensity of factors, GD can be used 267 to quantitatively analyze the single factor driving force and detect the main driving factors. In the study, 268 statistics q is measured so as to study and analyze Y's spatial differentiation and the degree to which X 269 interprets Y's spatial differentiation. The formula is as follows. carried out, which is depicted in Table 2. 281    Table 3 for the economy and society, but also promotes the construction of related water conservancy 360 infrastructure, so as to strengthen the overall ability of the city to resist external shocks (Huang et al., 361 2021). In addition, the resilience of cities along the YREB's lower reaches appears a downward trend on 362 the whole. The reasons may be that as the economy and society developed the industrial evolution of the 363 YREB's lower reaches has increased, the industrialization level has improved, even this area had good 364 water saving technology but its increasing speed lags behind compare with economy and society, which 365 makes the cities unable to respond in time in the face of external disturbances (Cao et al., 2018). It could 366 also see that in the area with sufficient water resources, the efficient utilization of water resources can 367 improve the ability of cities to resist external shocks. However, due to the limited study area, the impact 368 of water resources carrying capacity or utilization efficiency on UR level in drought areas has not been 369 considered. 370

371
The change of UR development level along the YREB's lower reaches is shown in Fig. 7 Fig. 7 Resilience grade structure of cities along the YREB's lower reaches 374 In Fig. 7, the resilience of cities along the YREB's lower reaches presents different hierarchical 375 structures. The hierarchical structures of UR in Shanghai and Zhejiang are a single core and a positive 376 triangle; the change of UR hierarchical structure in Jiangsu is "positive triangle-ellipse-ellipse-ellipse", 377 while the UR in Anhui is "inverted triangle-ellipse-pyramid-positive triangle". From above mapping, we has the following characteristic. The resilience of cities along the YREB's lower reaches is uneven, and 380 the number of low resilient cities and moderate to low resilient cities is increasing. Due to the "siphon 381 effect" of high resilience cities along of the YREB's lower reaches, the surrounding cities cannot profit 382 from high resilience cities. At the same time, rich resources and environment are also attracted and 383 gathered by high resilience cities, which causes certain damage to the ability of low resilience cities to 384 resist external shocks . It makes the difference of resilience level between cities larger 385 and larger. Therefore, the resilience level of cities along the river has significant spatial heterogeneity. 386

387
According to the exploration of urban driving factors by GD method, our paper draws the following 388 heat map (Fig. 8)  Hangzhou have always been moderate to high resilience, while Shanghai has decreased from high 438 resilience to moderate to high resilience. The UR of each province decays from the central city to the 439 edge city. 440 (2) Compared with cities without water resource, cities' resilience with consider water resource are 441 significantly higher. This shows that the existence of water resources promotes the resilient development 442 of cities along the YREB's lower reaches. 443 (3) Compared with 2010, the spatial diversity of UR along the YREB's lower reaches in 2018 was 444 mainly affected by water resources and infrastructure subsystems, the average explanatory power of 445 factors increased by 30.610% and 139.035% respectively; the explanatory powers of economic and social 446 dimension are relatively smaller, with an increase of 5.270% and 1.342% respectively. 447

448
Based on above analysis, if cities along the YREB's lower reaches increase UR, following suggestions 449 should be put forward. 450 Government should concentrate on improving the level of residents' water-saving and current 451 situation of urban water resources management. Since explanatory power of water resources increases 452 gradually, cities along the YREB's lower reaches related department should pay attention to the water 453 resource utilization efficiency of urban residents, accurately grasp that of each city, actively improve 454 urban water-saving technology, promote the construction and development of water-saving cities. At the 455 same time, the water conservancy department should improve the resilience of urban water resources, 456 focus on making the imbalance both supply and demand of water resources and the carrying capacity of 457 water environment get better, and pay attention to improving the function of regional water resources 458 regulation. 459 The resilience of cities along the YREB's lower reaches is uneven. In order to actively respond to 460 the challenges brought by external shocks, the urban governments along the YREB's lower reaches need 461 to speed up the proceeding of regional development integration and intensify the radiation and driving 462 role of central cities such as Shanghai, Nanjing, Hangzhou and Hefei, so as to drive the surrounding cities' 463 resilience enhancing. Furthermore, in order to promote the transformation from competition to 464 cooperation between cities, the urban governments should also give full play to local advantages and 465