International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 08 Issue: 08 | Aug 2021 www.irjet.net p-ISSN: 2395-0072 © 2021, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 129 Climate Change Impact on Hydropower Projects in Marsyangdi Basin, Nepal: A Comparative Study using GCM-led Top-down and Bottom-Up Approaches Nirajan Chiluwal 1 , Divas B. Basnyat 2 , Mukesh Raj Kafle 1 , and Dibesh Shrestha 2 1 Institute of Engineering, Tribhuvan University 2 Nepal Development Research Institute ---------------------------------------------------------------------***---------------------------------------------------------------------- Abstract - Nepal’s power (electricity) system is hydropower- dominated. The high climate and hydrological variability are a major challenge for hydropower generation in Nepal. Climate change is expected to further exacerbate the hydrological variability and impact hydropower generation in the future. Major climate change impacts on the hydropower projects are related to increased risks due to changes in water availability, extreme events like flood, droughts and sediment transport and subsequently on energy generation. This study adopts the combined form of General Circulation Model (GCM)-led top- down and bottom-up climate change impact assessment approaches to assess the impacts of climate change on hydropower projects in the Marsyangdi River Basin of Nepal. The combination of the two approaches is adopted because of the high uncertainty of the future climate among the available models (GCMs). The projected changes in climate data with other hydrological parameters were input into the Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) to simulate the response of the hydrological system. Four representative s GCMs under the Representative Concentration Pathways (RCP 4.5 and RCP 8.5) climate scenario selected using an Envelope Method were used for the top-down method. Future climate scenarios simulated using a Stochastic Weather Generator for a range of future changes in climate conditions were used for the bottom-up approach. The impacts of future climate scenarios of the two methods were then compared to assess the climate risks to hydropower projects in term of water availability, extreme floods and energy generation. Key Words: Climate Change; Climate Risk Assessment, HEC-HMS, GCM, Bottom-up Approach, Hydropower, Marsyangdi River Basin 1. INTRODUCTION Climate change has become a major global issue at national and international levels. Nepal has not remained indifferent from climate change risk due to the result of complex characteristics like extreme topography, quick responding catchments with intense seasonal and climatic variability [1]. Nepal has an estimated economic hydropower potential of 43,000 MW and has plans to increase its current installed capacity of about 1600 MW by 10,000 – 15,000 MW of additional capacity in the next decade [2]. Assessment of the risks of climate change to hydropower projects is critical for countries like Nepal, as hydropower project are directly influenced by hydrological, meteorological, geotechnical, glacial and geological processes, which are affected by the climate change [3, 4]. The analysis of the past recorded rainfall time series data in many regions shows increasing trend of the intensity of extreme rainfall events [4, 5]. This directly impacts on alteration of the intensity, frequency, amount and type of precipitation. Considering the impact of climate change on water availability and extreme events in Nepal, a clear understanding of climatic variability and change is very important for the development and management of the hydropower sector in the long run [6]. There are basically two approaches used in climate change impact studies of water and hydropower projects, namely, the General Circulation Models (GCM)-led top-down approach and the stakeholder driven bottom-up approach [7]. Top–down (or ‘scenario-centered’) method involves downscaling climate projections from GCMs under a range of emissions scenarios, providing inputs for hydrologic and management models to estimate potential impacts and, finally, to analyze adaptation measures. In the top-down approach information is cascaded from one step to the next with uncertainty increasing at each step of this process. It provides results highly uncertain for decision making [8]. The bottom–up approach analyzes vulnerability and adaptive capacity to climate variations to make adaptation decisions (decision-centered approaches) [7]. The high uncertainty associated with the effect of global change on water resource systems calls for a better combination of conventional top–down and bottom–up approaches, in order to design robust adaptation plans at the local scale. The two approaches meet and feed each other through the development of an integrated water resources management model to support the definition of a climate adaptation strategy for global change. The results derived from the integration of the bottom–up and top–down approaches illustrate the sensitivity of the adaptation strategies to the