Recently, the Korean peninsula faced severe drought for more than 3 years (2013–2015). Drought in this region is characterized by multidecadal variability, as seen from one of the longest systematic records available in Asia from 1770 to 2015. This paper explores how the return period of the 2013–2015 drought varies over this historical period to provide a context for the changing climate and drought severity in the region. A nonstationary, multivariate, Bayesian copula model for drought severity and duration is developed and applied. Given the wetting trend over the last 50 years, the recent drought appears quite extreme, while such droughts were common in the eighteenth and nineteenth centuries.

Watershed health, including the natural environment, hydrology, water quality, and aquatic ecology, is assessed for the Han River basin (34 148 km²) in South Korea by using the Soil and Water Assessment Tool (SWAT). The evaluation procedures follow those of the Healthy Watersheds Assessment by the U.S. Environmental Protection Agency (EPA). Six components of the watershed landscape are examined to evaluate the watershed health (basin natural capacity): stream geomorphology, hydrology, water quality, aquatic habitat condition, and biological condition. In particular, the SWAT is applied to the study basin for the hydrology and water-quality components, including 237 sub-watersheds (within a standard watershed on the Korea Hydrologic Unit Map) along with three multipurpose dams, one hydroelectric dam, and three multifunction weirs. The SWAT is calibrated (2005–2009) and validated (2010–2014) by using each dam and weir operation, the flux-tower evapotranspiration, the time-domain reflectometry (TDR) soil moisture, and groundwater-level data for the hydrology assessment, and by using sediment, total phosphorus, and total nitrogen data for the water-quality assessment. The water balance, which considers the surface–groundwater interactions and variations in the stream-water quality, is quantified according to the sub-watershed-scale relationship between the watershed hydrologic cycle and stream-water quality. We assess the integrated watershed health according to the U.S. EPA evaluation process based on the vulnerability levels of the natural environment, water resources, water quality, and ecosystem components. The results indicate that the watershed's health declined during the most recent 10-year period of 2005–2014, as indicated by the worse results for the surface process metric and soil water dynamics compared to those of the 1995–2004 period. The integrated watershed health tended to decrease farther downstream within the watershed.

The impacts of future climate change on the agricultural water supply capacities of irrigation facilities in the Geum River basin (9645.5 km²) of South Korea were investigated using an integrated modeling framework that included a water balance network model (MODSIM) and a watershed-scale hydrologic model (Soil and Water Assessment Tool, SWAT). The discharges and baseflows from upland drainage areas were estimated using SWAT, and the predicted flow was used to feed agricultural reservoirs and multipurpose dams in subwatersheds. Using a split sampling method, we calibrated the daily streamflows and dam inflows at three locations using data from 6 years, including 3 years of calibration data (2005–2007) followed by 3 years of validation data (2008–2010). In the MODSIM model, the entire basin was divided into 14 subwatersheds in which various agricultural irrigation facilities such as agricultural reservoirs, pumping stations, diversions, culverts and groundwater wells were defined as a network of hydraulic structures within each subwatershed. These hydraulic networks between subwatersheds were inter-connected to allow watershed-scale analysis and were further connected to municipal and industrial water supplies under various hydrologic conditions. Projected climate data from the HadGEM3-RA RCP 4.5 and 8.5 scenarios for the period of 2006–2099 were imported to SWAT to calculate the water yield, and the output was transferred to MODSIM in the form of time-series boundary conditions. The maximum shortage rate of agricultural water was estimated as 38.2% for the 2040s and 2080s under the RCP 4.5 scenario but was lower under the RCP 8.5 scenario (21.3% in the 2040s and 22.1% in the 2080s). Under the RCP 4.5 scenario, the projected shortage rate was higher than that during the measured baseline period (1982–2011) of 25.6% and the RCP historical period (1982–2005) of 30.1%. The future elevated drought levels are primarily attributed to the increasingly concentrated rainfall distribution throughout the year under a monsoonal climate, as projected by the IPCC climate scenarios.