The Japanese environmental flow (e-flow) concept and framework are outlined in the current study. We first review the natural and historical background related to the establishment and implementation of the e-flow framework in Japan. We then summarize the national guidelines used to assess e-flows. Finally, we discuss a river for which the guidelines have been applied to set e-flow requirements. It is legally mandated that flows be established to maintain the riverine environments of class A rivers managed by the national government. These correspond to e-flows, which are referred to as maintenance flows in Japan. A Guideline established in 1997 provides detailed instructions on how to assess e-flows, and the e-flow framework has been implemented for 100 class A rivers according to the guidelines. E-flow objectives in Japan are intended to benefit ecosystems through the protection of river habitats and fisheries and the protection and maintenance of water quality, groundwater levels, riverine scenery, and navigable waterways. E-flow objectives also address cultural and social needs, such as the prevention of salt damage and estuary clogging, and the protection of wooden infrastructure management facilities. Although Japanese rivers are characterized by large fluctuations and sudden peaks in flow, there has been little discussion about e-flow evaluation methods that take flow variability into consideration. This is because dams on Japanese river basins control only a small percentage of total runoff volume. This is one reason why the guidelines for setting e-flows focus only on the minimum flows required to prevent extreme low-flow conditions.
Environmental flow (e-flow) practices were investigated for 100 class A rivers in Japan, which are managed by the central government. We examined how much e-flow requirements (EFRs) are allocated for each river, and investigated the seasonally variable flow components, objectives, and target organisms. The EFRs of the 100 rivers ranged from 5 to 51% as a percentage of the mean annual discharge (MAD). Protecting physical habitat was an e-flow objective for all target rivers.Protecting water quality and cultural scenery, preventing salt damage, and maintaining groundwater levels were also common objectives. However, the minimum flow required for the migration and spawning of 12 representative fish species was the most influential factor. The Japanese e-flow framework is a holistic approach that takes not only individual species but also multiple aspects of ecosystem services, including its social and cultural value, into account. E-flows for all target rivers were given as a form of minimum flow. Eighty-six percent of the rivers considered seasonally variable flow targets, while 14% designed constant minimum flow throughout a year. The reason why the EFRs do not consider natural flow variability is that the storage and control capacity of Japanese dams is minimal, and that problems associated with the modified flow regime is not critical at present. The Japanese e-flow framework has been successfully implemented, and it has been institutionally integrated into river management for 30 years. However, it will require adaptive management and adjustment in the future as social structure, climate, and hydroecological characteristics change.
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