Observed Changes in Extreme and Persistent Areal Precipitation over Standardized Watersheds in the Korean Peninsula

Extreme and persistent rainfall plays a critical role in shaping hydroclimatic risks, yet its long-term behavior at the watershed scale remains poorly characterized. This study examines observed changes in areal precipitation across standardized watersheds over the Korean Peninsula, with an emphasis on rainfall persistence, spatial variability, and extreme events. Daily areal precipitation for the period 1973–2024 was calculated from surface observations of the Korea Meteorological Administration using the Thiessen polygon method to improve spatial representativeness.

The analysis identifies clear multi-decadal shifts in precipitation characteristics. Mean annual areal precipitation increased from the 1970s to the early 2000s, reaching approximately 1,370 mm, before showing a slight decrease in recent years. Despite this moderation in mean values, heavy rainfall events exceeding 50.0 mm day⁻¹ exert a dominant influence on annual precipitation totals, with a strong correlation (Pearson r ≈ 0.95). This indicates that year-to-year variability in water availability is largely controlled by a small number of intense rainfall events rather than by changes in average conditions.

Spatial variability of heavy rainfall has increased notably since the early 2000s, as reflected by a rising coefficient of variation among watersheds. Rainfall persistence analysis further shows that moderate rainfall events (≥10.0 mm day⁻¹) commonly persist over consecutive days, with a mean Rainfall Persistence Index of approximately 1.4, highlighting the importance of sustained wet periods for hydrological processes. Frequency analysis based on the Generalized Extreme Value distribution reveals that, in the post-2013 period, estimated 100-year return levels of daily areal precipitation exceed 800 mm in several watersheds, indicating an increased potential for extreme rainfall hazards.

Overall, the results demonstrate that hydroclimatic change over the Korean Peninsula is expressed more strongly through shifts in rainfall persistence, spatial heterogeneity, and extremes than through changes in mean precipitation. The findings support the use of watershed-scale areal precipitation analyses for improved assessment of climate-related hydrological risks.