# Recent Spatiotemporal Changes of Subhourly Extreme Rainfall Events in Seoul
## 5 Discussion  
The present study analyzed whether the following two hypotheses are true: (1) temporal characteristics of extreme rainfall in Seoul have changed; (2) spatial patterns of extreme rainfall in Seoul have been altered. Regarding the first hypothesis, \"temporal characteristics of extreme rainfall in Seoul have changed\", the analysis results of the current study indicated that temporal changes indeed occurred. Firstly, the mean AMP series in Seoul showed a decreasing trend, and for the AMP with a duration of 5-6 h, a statistically significant decreasing trend was confirmed through the MK test with a 5% significance level. The reduction in the AMP values across Seoul will consequently lead to a decrease in the quantile values used for the design criteria of hydraulic infrastructure. The decrease  
Figure 7: Time series of coefficients of variation (CVs) for annual maximum precipitation for 360 min in Seoul. The black dashed line indicates the linear slope of the CV. in rainfall quantiles implies a lower value of design criteria for hydraulics infrastructure within Seoul. Several factors, including impervious areas, drainage systems, and rainfall intensity, influence urban flooding. If other factors are constant, it can be inferred that climate change will lead to a reduction in urban flooding damage in the Seoul area based on the aforementioned results. The hypothesis \"spatial patterns of extreme rainfall in Seoul have been altered\" was addressed by indicating that the spatial characteristics of extreme rainfall were indeed changing. The spatial patterns of AMP in Seoul were changing based on the duration of AMP, with this phenomenon being particularly noticeable for subhourly durations. The analysis results indicated an increasing frequency of localized heavy rainfall events in Seoul, highlighting an intensification of localized extreme rainfall in the city. While the overall AMP across Seoul exhibited a decreasing trend--suggesting that design criteria for hydraulic structures should be reduced and urban flood damage should decrease--the frequency of urban flood damage has increased. This paradoxical phenomenon can be partially explained by the findings of this study. Although the overall AMP decreased, the increased frequency of localized heavy rainfall events can lead to high-intensity rainfall in small areas that are part of the watershed in the city, causing urban flooding and damage. In large, sparsely populated river basins, decreases in AMP can lead to reduced flood damage because they may not result in increased water levels within river channels. However, in densely populated urban areas, localized heavy rainfall can occur more frequently and cause damage, as high-intensity rainfall events can lead to temporary inundation in small, concentrated regions. This suggests that recent urban flooding and inundation in Seoul may be largely driven by the intensification of localized extreme rainfall events. Consequently, there is a pressing need to reevaluate urban infrastructure and drainage systems, emphasizing the importance of implementing more meticulous countermeasures to address the impact of climate change. This point must be considered in research and policy decisions such as stormwater management, sewer management, urban planning, and emergency action plans for flash urban inundation [33; 34; 35; 36; 37; 38]. In 2018, Seoul experienced significant urban flooding damage [39]. Upon examining Figure 4, it becomes evident that in 2018, AMP values were notably high during subhourly durations of 15, 30, 45, and 60 min. However, for durations exceeding 1 h, the AMP values in 2018 were not significant. Despite the lack of substantial AMP values for longer durations, urban flooding damage occurred that year. Figure 5 shows a high CV in 2018, indicating significant variability in rainfall intensities. Except for the 15 min duration, CV values were elevated for subhourly durations, and for hourly durations, the CV was higher than in other years.