Ecological drought monitoring through fish habitat-based flow assessment in the Gam river basin of Korea

•Global warming has reinforced irregular hydrologic cycle patterns, causing serious changes and threats to the ecosystem.•This study targeted the aquatic ecosystem and assessed ecological droughts.•The Ecological Drought Index for Fish Habitats (EcoDI_FH) was developed to evaluate the severity of the ecological drought. Global warming has reinforced irregular hydrologic cycle patterns causing serious changes and threats to the ecosystem. This study targeted the Gam River in South Korea and assessed ecological droughts that could occur in the river ecosystem due to long-term meteorological droughts. To assess ecological droughts, this study (1) focused on a representative fish species in the river, (2) analyzed the required ecological flow in the river to ensure a healthy fish habitat, (3) developed a fish habitat-based ecological drought index (EcoDI_FH) calculated by analyzing the river’s ecological flows and flow regimes with double thresholds to monitor how ecological droughts proceed in extreme low flow, and (4) evaluated the applicability of the EcoDI_FH. The following are the salient points of this study: (1) The dominant species, Zacco platypus, and endangered species, Squalidus gracilis majimae were chosen as the target fish species in the study area. To determine the ecological flow considering its habitat the optimal ecological flow was estimated using the River2D hydrodynamic model. (2) Due to the construction of Buhang Dam in the Gam river basin, the pattern of river flow has changed significantly. (3) By applying the developed ecological drought index, the ecological flow was used as the first threshold for determining drought occurrence. Droughts were divided into four stages, and the extreme low flow calculated using indicators of hydrologic alteration was set as a second threshold or a serious stage. (4) For the past drought periods that took place in the Gam river basin, the drought state was analyzed before and after the construction of the dam. In the drought period that occurred before Buhang Dam was built, an ecological drought occurred in the serious stage during the drought period. During the droughts that occurred after the dam was built, the serious drought stage was alleviated quite substantially. It is expected that the results of this study will be utilized as one of the main tools for establishing countermeasures and adaptation strategies against ecological droughts in the river ecosystem.