A case study in the Naeseong-cheon Stream in Korea
Many countries have recognized the need for maintaining the quantity, timing, quality and certain amounts of flows to sustain freshwater, aquatic communities as well as the human livelihood depending on the ecosystem. Environmental flows entail both optimal flows that describe the amount of flow required to realize a complete function of stream ecosystems and minimum flow that represents the threshold below which the stability and health of an ecosystem cannot be maintained. To date, numerous methods: hydrological, hydraulic rating, habitat simulation, and holistic methods, for estimating environmental flows have been proposed. However, the notable significant changes in the environmental flows due the difference in the fundamental principles used in their estimation has raised a great concern amongst scientists. This has necessitated the need to quantify and compare environmental flows estimated using different methodologies taking into consideration the potential impact of environmental flows on habitat suitability of a river ecosystem.
To address these issues, a collaborative effort by Dr. Seung Ki Kim (postdoctoral fellow) and led by Professor Sung-Uk Choi from Yonsei University quantitatively compared the environmental flows estimated using various flow methodologies from a habitat suitability perspective. They focused on three methodologies: hydrological, hydraulic rating and habitat simulation methods. A 20km reach of Naeseong-Cheon stream from the Youngju Dam in Korea was chosen as a favorable study site due to its moderate size and normal flow. The work is published in the journal, Ecohydrology.
The findings showed that the environmental flows varied greatly depending on the methodology selection. Specifically, hydrological methods registered the least minimal flows while the optimal flow was largest by the hydraulic rating method. The habitat suitability for the environmental flows and particularly for the target fish was evaluated based on physical habitat simulation and normalized weighted usable areas. The authors observed that the habitat suitability could be degraded by lack of discharge for minimum flows (estimated by hydrological methods) or excessive discharge for optimal flows (estimated by hydraulic rating method).
Consequently, a habitat suitability-discharge relationship for the three environmental flow methodologies was established for the study reach presented. Despite giving a clear representation of the inherent features of the three approaches, the characteristics of the habitat suitability-discharge relationship for hydrological methods were observed to be different from the other two which are based on hydraulic simulations. Interestingly, the habitat simulations determined the optimal flow required to maintain the quality of a habitat.
Factors such as data availability, time constraint, and river management plan can be used to decide on the environmental flow methodology. Based on their results, the study showed that hydrological or hydraulic methods may give inappropriate environmental flows for aquatic life in a river. For instance, the hydrological methods will tend to underestimate the minimum flows while the hydraulic rating methods will overestimate the minimum flows as seen with the case of the target fish selected for the physical habitat simulations. In a statement to Advances in Engineering, Professor Sung-Uk Choi noted that their study provided important quantitative comparison of environmental flows under specific conditions with findings conforming to those in the existing literature.
Kim, S., & Choi, S. (2019). Comparison of environmental flows from a habitat suitability perspective: A case study in the Naeseong‐cheon Stream in Korea. Ecohydrology, 12(6).