Adaptive Rice Transplanting Schedules and Reservoir Use Reveal Emerging Climate Vulnerability in South Korea

Rice transplanting offers advantages over direct seeding, but it requires sufficient water resources in early spring before the monsoon season in East Asia. To provide water for rice cultivation, including transplanting, thousands of agricultural reservoirs have been built in South Korea, yet their water use patterns remain unknown. This study employs a rotated principal component analysis (rPCA) on long-term (1992–2022) daily water storage volume and ASOS-based precipitation data from 651 agricultural reservoirs across South Korea. The rPCA identifies three main patterns over the central, southern, and southcentral regions. During the rice transplanting period, the central region withdraws water from reservoirs for about two months, starting mid-April. The southern region starts water withdrawal about 20 days later than the central region. The southcentral region shows a DWSV pattern similar to the central region and a precipitation pattern similar to the southern region. Under county-specific 10th-percentile precipitation conditions, the mean estimated water-use rates are 50.9%, 35.2%, and 36.4% for the central, southern, and southcentral regions, respectively. We find contrasting temporal changes in regional water-use timing across South Korea. The central region shows a later start that may reflect adjustment to warmer conditions, whereas the southern region shows an earlier start possibly associated with cropping and cultivar changes. The southcentral region shows no statistically significant timing shift despite the largest positive fitted trend in post-heading temperature, suggesting potential climate vulnerability.