Analysis of environmental flow for multiple fish species living in the Hwang River, South Korea

To preserve the health of aquatic ecosystems, it is important to analyze the weighted usable area (WUA) for fish species in response to flow variations and determine the suitable environmental flow ranges for their habitats. In this study, we focused on the Hwang River watershed, a part of the Nakdong River Basin known to have bad aquatic ecosystem health in Korea. The optimal and minimum environmental flows of the river were determined based on the WUA. The physical habitat simulation system (PHABSIM) model was used to estimate the environmental flows, and the Hydrological Engineering Centre-River Analysis System (HEC-RAS) program was used to obtain the input data. For the simulation, we considered the fish species that were common in the Hwang River watershed. We established flow boundary conditions for PHABSIM by analyzing flow duration by year from 2010 to 2019. Afterward, we calculated WUA that changes depending on flow using the habitat suitability indices of Pseudogobio esocinus (P. esocinus), Microphysogobio yaluensis (M. yaluensis), and Zacco platypus (Z. platypus) living in the Hwang River. The flow with the maximum WUA was calculated as the optimal flow, and the area value corresponding to the 4th quartile from the maximum WUA was calculated as the minimum flow. The optimal environmental flow ranges were 3.9–13.9 m³/s for P. esocinus, 3.2–10.4 m³/s for M. yaluensis, and 5.5–18.8 m³/s for Z. platypus. An analysis of 10 years of flow data revealed that the WUA was highest and lowest during winter and summer, respectively. If a separate nature-friendly habitat environment can be adjusted for periods of low WUA, efficient water resource management and habitat conservation will be possible. Our study provides the data essential for the water resource management of the Hwang River watershed, and the proposed method can be used to maintain the health of aquatic ecosystems.