This study is to evaluate the future potential impact of climate change on the water quality of Chungju Lake using the Water Quality Analysis Simulation Program (WASP). The lake has a storage capacity of 2.75 Gm3, maximum water surface of 65.7 km2, and forest‐dominant watershed of 6,642 km2. The impact on the lake from the watershed was evaluated by the Soil and Water Assessment Tool (SWAT). The WASP and SWAT were calibrated and validated using the monthly water temperatures from 1998 to 2003, lake water quality data (dissolved oxygen, total nitrogen [T‐N], total phosphorus [T‐P], and chlorophyll‐a [chl‐a]) and daily dam inflow, and monthly stream water quality (sediment, T‐N, and T‐P) data. For the future climate change scenario, the MIROC3.2 HiRes A1B was downscaled for 2020s, 2050s, and 2080s using the Change Factor statistical method. The 2080s temperature and precipitation showed an increase of +4.8°C and +34.4%, respectively, based on a 2000 baseline. For the 2080s watershed T‐N and T‐P loads of up to +87.3 and +19.6%, the 2080s lake T‐N and T‐P concentrations were projected to be 4.00 and 0.030 mg/l from 2.60 and 0.016 mg/l in 2000, respectively. The 2080s chl‐a concentration in the epilimnion and the maximum were 13.97 and 52.45 μg/l compared to 8.64 and 33.48 μg/l in 2000, respectively. The results show that the Chungju Lake will change from its mesotrophic state of 2000 to a eutrophic state by T‐P in the 2020s and by chl‐a in the 2080s. Editor's note: This paper is part of a featured series on Korean Hydrology. The series addresses the need for a new paradigm of river and watershed management for Korea due to climate and land use changes.

This study is to assess the future impact of climate change on hydrological behavior considering future vegetation canopy prediction and its propagation to nonpoint source pollution (NPS) loads. The SWAT (Soil and Water Assessment Tool) model was used for the assessment. For a forest dominant ChungjuDam watershed of South Korea, the MIROC3.2hires climate data of SRES A1B and B1 scenarios were adopted and downscaled for the watershed. The future vegetation canopy information was projected by the monthly relationship between Terra MODIS (MODerate resolution Imaging Spectroradiometer) LAI (Leaf Area Index) and temperature. The future predicted LAI increased up to 1.9 in 2080s April and October because of the temperature increase 3.6 degrees C and 5.3 degrees C respectively. By reflecting the future LAI changes, the future estimated percent changes of maximum annual dam inflow, SS, T-N, and T-P were + 42.5% in 2080s A1B,-35.6% in 2020s A1B,+73.7% in 2080s A1B and-21.0% in 2080s B1 scenario respectively. The increase of T-N load was from the increase of subsurface lateral flows and the groundwater recharges by the future rainfall increase. The decrease of T-P load was by decrease of sediment load during wet days because the effect of LAI increase is greater than the increase of rainfall.