Modelling hydrological dynamics under dramatic shift from drought to flood in the small mountainous catchment

In summer of 2025, an extreme drought occurred in the east cost region in South Korea. The Gangneung city encountered a critical shortage of water supply from July to mid-September and a ‘state of disaster’ was declared by the government. The Obong reservoir of 100 km2 catchment and 1.4 million m3 storage is operated for the water supply for 200,000 residents and 480 ha of irrigated paddy fields. This severe drought was ended with abnormal rainfall after mid-September and reservoir storage was returned to normal levels.To better understanding the hydrologic response in small mountainous catchment, the 25 years of climate and hydrologic data (2000~2025) analysed and daily record of water supply (2023~2025) for municipal and irrigation from the reservoir were collected. The catchment characteristics including topography, soil type and shallow aquifer properties were also analysed using DEM and digital soil and land-use map. The hydrological dynamics including soil moisture and groundwater level changes are simulated using physically-based hydrologic model, DWAT (Dynamic Water Resources Assessment Tool). The DWAT is run on a daily time step to generate hydrologic processes in the catchment from the rainfall and climate data (temperature, humidity and wind velocity, sunshine hours).The hydrologic behaviour was simulated from 2000 to 2025 and the simulations result showed that the dramatic changes of soil moisture after mid-September rainfall and which subsequently increase of streamflow from the catchment. The simulation results highlighted the strong influence of antecedent soil moisture conditions on catchment response. During the drought period, soil moisture was critically low, limiting runoff generation even during heavy rainfall events. For example, on September 13, even the daily rainfall was exceeded 100 mm, the streamflow was not increased significantly because of the low soil moisture in the catchment. Once the soil was saturated, interflow increased streamflow, followed later by baseflow from the shallow aquifer. The water storage of Obong reservoir was lowest to 11.5% in mid-September and dramatically recovered over 90% by 300 mm of rainfall until the early of October.This study demonstrates the value of long-term hydrologic modelling for understanding drought resilience in small mountainous catchment. The DWAT simulations provided insights into the interactions between rainfall, soil moisture, streamflow, and groundwater, highlighting the critical role of antecedent conditions in shaping hydrologic response. Moreover, the case of Gangneung in 2025 illustrates how extreme droughts may be rapidly reversed by anomalous rainfall, yet such reliance on unpredictable events poses significant risks for water resource management.This work was supported by Korea Environment Industry & Technology Institute(KEITI) through Water Management Program for Drought Project, funded by Korea Ministry of Climate, Energy and Environment(MCEE).(2022003610002)