Simulations of Rainfall Changes Impacts on Streamflow in the Bogowonto Watershed Using the Hydrologiska Byråns Vattenbalansavdelning (HBV) Model

Climate change due to global warming directly affects the spatial and temporal patterns of rainfall, which in turn influence streamflow characteristics within watershed areas. One of the most critical streamflow characteristics is peak flow, which can lead to flooding and cause significant losses. Bogowonto Watershed, one of the watersheds in Indonesia has frequently experienced flooding since the 19 th century. This study aims to assess the impacts of projected rainfall changes on streamflow characteristics in the Bogowonto Watershed based on future climate scenarios. Historical climate data (2010-2018) from the River Basin Organization of Serayu-Opak (BBWS SO) and projected climate data (2020-2045) from CORDEX were utilized. A conceptual HBV model was developed using historical data to simulate the hydrological response of the watershed under RCP 4.5 and RCP 8.5 scenarios. The results showed changes in rainfall characteristics, including an increase in annual rainfall of up to 32 %, an increase in wet-season rainfall by 63 %, and a decrease in dry-season rainfall by 54 %. These changes led to an increase in average annual streamflow by 5 % and 17 % under RCP 4.5 and RCP 8.5, respectively, with runoff coefficients of 0.40 and 0.41. These findings indicate a potential increase in hydrometeorological disaster in the region.