Applicability of Environmental Flow Techniques in Himalayan Hydropower Considering Hydrologic and Energy Perspective: A CASE study of Kali Gandaki River Basin

Abstract

Environmental flow (e-flow) management is a critical issue in Himalayan river basins, particularly in the context of rapid hydropower development. The prevailing practice of maintaining a uniform, year-round static e-flow release (typically 10% of the minimum monthly flow) is often inadequate to sustain riverine ecological health while balancing hydropower generation objectives, especially during the dry season. This study evaluates the applicability of seven e-flow assessment methodologies by integrating hydrological reliability and hydropower generation impacts in Kali Gandaki river basin. Different e-flow methods are assessed in terms of hydrologic failure and the resulting alterations in energy production. Results showed that method-wise variability in e-flow demand does not differentiate between rain-fed and snow-fed hydrological characteristics. Annual Distribution Method (ADM), Department of Electricity Development (DoED) guideline, and Modified Minimum Flow Method (MMFM) exhibit negligible hydrological failure rates. Methods such as ADM and MMFM demonstrate relatively lower impacts on annual and dry-season energy generation. In contrast, more stricter approaches such as the Variable Monthly Flow (VMF), Tessmann, and Tennant methods show lower hydrological reliability and significantly higher impacts on energy generation. The findings highlight the need for a comprehensive evaluation of dynamic e-flow strategies to support sustainable planning and operation of hydropower projects in the Himalayan region.