Assessing the Impact of Hydropower to Multipurpose Dam Reoperation on Downstream Hydrology and Water Quality

Dams play a vital role in hydropower generation and water management, yet growing environmental and societal demands increasingly require reoperation to balance multiple purposes. Despite this shift, few studies have quantitatively evaluated how converting an existing hydropower dam to multipurpose operation affects downstream hydrology and water quality. This study examines the Hwacheon Dam in South Korea, which was converted to multipurpose use, to assess the effects of this operational change. The analysis focused on changes in the number and frequency of non-release days, downstream flow regime variation, and water quality responses evaluated using the national water quality standards and the real-time water quality index (RTWQI). After the transition, non-release days decreased sharply and the dam maintained continuous releases of 22.2 cubic meters per second (CMS), ensuring stable water supply and consistent downstream flow. Results show that flood, high, and normal flows decreased by 51.4% (from 1037.0 CMS to 503.5 CMS), 21.7% (from 54.4 CMS to 42.6 CMS), and 13.0% (from 23.9 CMS to 20.8 CMS), respectively. In contrast, the low flow increased by 7.4 times (from 2.4 CMS to 20.2 CMS) after the transition to multipurpose operation, while the drought flow increased from 0 CMS to 17.8 CMS. Water quality also improved across all downstream stations, with both national water quality standards and RTWQI scores indicating excellent and stable conditions. The multipurpose operation of the Hwacheon Dam stabilized hydrologic and water quality conditions, demonstrating the potential of dam reoperation for sustainable water and ecosystem management.