Recent climate change has brought about irregular rainfall patterns along with an increased frequency of heavy rainfall, and flood damage in Korea is increasing accordingly. The increased rainfall amount and intensity during the rainy season lead to flood damage on a massive scale every year in Korea. In order to reduce such flood damage and secure the stability of hydraulic structures, evaluation of hydrologic risk corresponding to design floods is necessary. As Korea’s current climate change scenarios are generally applied to mid-sized watersheds, there is no practical application method to calculate the hydrologic risk of local floods corresponding to various future climate change scenarios. Using the design flood prediction model, this study evaluated the hydrologic risks of n-year floods according to 13 climate change scenarios. The representative concentration pathway (RCP) 8.5 scenario resulted in the 100-year floods increasing 134.56% on average, and 132.30% in the Han River, 132.81% in the Nakdong River, 142.42% in the Gum River, and 135.47% in the Seomjin-Youngsan River basin, compared with the RCP 4.5. The 100-year floods at the end of the 21st century increased by +3% and +13% according to the RCP 4.5 and 8.5, respectively. The corresponding hydrologic flood risk increased by 0.53% and 8.68% on average according to the RCP 4.5 and RCP 8.5, respectively, compared with the current level of hydrologic risk of a 100-year flood.

<p>Recently, due to the climate change, the frequency of extreme hydrological disasters such as drought and flood is increasing worldwide. Especially, sudden change in precipitation cause drought and flood often occur alternately in a short period of time, which is defined as a drought-flood abrupt alternation event. In this study, daily Standardized Precipitation Index (SPI) for the entire basin of South Korea is used to analysis the characteristics of the short-term abrupt alternation events of drought and flood, focusing on the short-term perspective rather than the monthly SPI. When the SPI value is less than –1.5 for 10 consecutive days, a drought event begins, whereas when the SPI value is more than 0.5 for 7 consecutive days, a drought event ends. On the contrary, when the SPI value is more than 1.5 for 10 consecutive days, a flood event begins, whereas when the SPI value is less than –0.5 for 7 consecutive days, a flood event ends. When the time interval between the end of drought event and the start of flood event is less than five days, a drought-flood abrupt alternation event is identified. The severity of drought-flood abrupt alternation event is defined similarly to the severity of drought using the SPI. We classified the severity into two types: SW(severity of whole period) and ST(severity of transition period). We used the additional statistical risk grade analysis. Nackdong River basin (southeastern region of Korea) has most severe grade of the SW rather than the other basins and the ST is lower than other basins. On the contrary, Yeongsan River basin (southwestern region of Korea) has most severe grade of ST rather than the other basins and the SW is lower than other basins. In conclusion, using daily SPI can determine the risk-prone areas through evaluating the frequency and severity of drought-flood abrupt alternation events. Due to climate change, increasing variability of precipitation, and frequent flood abrupt alternation events in the future, our results will cornerstone to predict the vulnerable and risk-prone region or preventing disasters.</p> <p><strong>Acknowledgement:</strong> This research was supported by a grant(2022-MOIS63-001) of Cooperative Research Method and Safety Management Technology in National Disaster funded by Ministry of Interior and Safety(MOIS, Korea).</p>