# Recent Spatiotemporal Changes of Subhourly Extreme Rainfall Events in Seoul
## 5 Discussion
Additionally, Figure 3 reveals that while the median value of the box plot for 2018 was low, there were outlier points significantly above the median. These outliers likely represented extreme rainfall events much higher than typical values, contributing to urban flooding and associated damage. These findings confirm that the urban flooding in Seoul in 2018 resulted from localized heavy rainfall events. Furthermore, the high CV in 2022 suggests that the flooding events in that year were also caused by localized heavy rainfall. The spatial heterogeneity of extreme rainfall events caused by localized heavy rainfall cannot be adequately addressed by traditional at-site frequency analysis. Each site is analyzed separately to estimate the rainfall quantiles. In cases where localized heavy rainfall results in extremely high AMP values at certain points, the rainfall quantile values will increase, while at other points, the AMP may show a decreasing trend, leading to a reduction in the rainfall quantile values. Since localized heavy rainfall has occurred within the city, there remains the possibility of such events happening at any other point. Therefore, if the rainfall quantile is not calculated considering this phenomenon across the entire region of the city, this could lead to an underestimation of the rainfall quantile and consequently an increase in the vulnerability of hydraulic structures to climate change. As an alternative to address these issues, it is necessary to use regional frequency analysis (RFA) for estimating rainfall quantiles in urban areas. RFA involves performing frequency analysis by grouping sites that possess homogeneous hydrological characteristics [40]. By conducting frequency analysis on rainfall sites located in proximity and determined to be homogeneous regions, it is believed that the issues arising from spatial heterogeneity can be mitigated. RFA has the advantage of being able to be adopted swiftly, as considerable research has already been done on nonstationary frequency analysis for hydrometeorological variables [41; 42; 43]. It has been confirmed that the spatiotemporal characteristics of extreme rainfall differ significantly according to duration and exhibit different patterns of change in Seoul. For the design of hydraulic structures within cities, rainfall frequency analysis is conducted, and the results are used to derive the intensity-duration-frequency (IDF) curve to calculate the value of design criteria for hydraulic structures [44; 45]. Existing IDF formulas have the drawback of not adequately considering these spatiotemporal variations. Although studies on IDF curves considering nonstationarity were conducted [46; 47; 48], there was a limitation in not addressing the nonstationarity in the spatial aspect. A newly proposed IDF curve that considers spatiotemporal characteristics is necessary for addressing spatiotemporal changes. In this study, while the analysis based on annual maxima allowed us to examine overall phenomena, there are some limitations in making strict judgments about the intensification of localized heavy rainfall. These constraints arise because the analysis focused on AMP values rather than specific rainfall events representing extreme rainfall in Seoul. The data used in our analysis spanned 28 years, with one datapoint added each year, making the trend detection test results somewhat reliable but accompanied by significant uncertainty. Furthermore, variations in the starting years of the rainfall gauge stations introduced additional uncertainty to the findings. Urban inundation and flood damage result not solely from increased rainfall intensity but from a combination of factors, such as pipeline maintenance, operation and installation of pumping stations, increase in impervious surfaces, and neglect in managing trenches. Therefore, factors beyond those identified in this study may also contribute to the rise in urban inundation and flood damage.