## 2 Materials  
Chongqing is a megacity located in southwestern China, with a population of 31.9 million and a GDP of RMB 3014 billion (about USD 412.9 billion). The Chongqing metro network is one of the largest mountainous URT systems in the world, providing services to the 14 districts of Chongqing. By the end of 2023, it had opened Lines 1, 2, 3, 4, 5, 6, 9, 10, and 18, as well as the Ring Line, International Expo Line, and Jiangtao Line, totaling 12 lines and 256 stations (Figure 1). The total operating mileage reached up to 538 km, with a peak daily passenger volume of 5.162 million, accounting for 54.28% of public transportation travel in the city [30], which makes a significant contribution to alleviate traffic congestions and promote green travel.  
Figure 2 shows the daily ridership of each metro line, which is retrieved from the metro company. Lines 3 and 6 have the highest ridership, reaching 607,000 passengers per day, making them the backbone of the URT network. Following them, Line 1, which connects the east and west sides, and the Ring Line, which encircles the core urban area, exhibit relatively high passenger flows, underscoring their importance. In contrast, Lines 4, 18, the International Expo Line, and the Jiangtao Line have the lowest ridership, each under 70,000 passengers, mainly due to serving suburban areas with the lower population density.  
Figure 1: A screenshot of the Chongqing metro network.  
Lines 2, 5, 9, and 10 have moderate ridership, with around 300,000 passengers per day. Overall, passenger volumes across the Chongqing metro lines show a clear hierarchical pattern, closely related to their geographic locations, service areas, and regional development, reflecting the varying capacities of each line to meet urban travel demands.  
As a mountainous city, Chongqing is prone to have landslides and floods under rainstorms. Here, landslide risk is determined based on slope and cumulative rainfall, while flood risk is assessed using elevation and the cumulative rainfall. The rainfall data of Chongqing in 2022 is retrieved from the Earth Resource Data Cloud ([[http://www.gis5g.com](http://www.gis5g.com)]([http://www.gis5g.com](http://www.gis5g.com)), accessed on 15 September 2024), whereas the elevation and slope data are obtained from the Geospatial Data Cloud ([[https://www.gscloud.cn](https://www.gscloud.cn)]([https://www.gscloud.cn](https://www.gscloud.cn)), accessed on 15 September 2024). The resampling tool in ArcGIS (v.10.8.0) is adopted to adjust the spatial resolution of all raster data to be 1 km \(\times\) 1 km and then converts them to vector data for clustering. Figure 3 displays spatial distributions of these three indicators.  
Figure 3: Spatial distributions of risk factors for extreme rainfalls in Chongqing urban area.  
Figure 2: The average daily ridership of Chongqing metro lines (2023).  
Here, both landslide risk and flood risk are classified into five levels, i.e., higher, high, medium, low, and lower, with the K-means clustering method. Figure 4 presents the clustering results. As shown in Figure 4a, stations with high landslide risk are predominantly located in the northern part of the city. This region experiences heavy rainfall and features steep slopes due to the presence of several mountains, including Tongluo Mountain, Longwangdong Mountain, and Zhongliang Mountain. Similarly, Figure 4b highlights that high flood risk stations are also concentrated in the northern area, characterized by abundant rainfall and low-lying terrain.