Hydropower is expected to remain the world’s largest source of renewable electricity generation and play a critical role in decarbonising the power system and improving system flexibility and storage. In 2020, hydropower supplied one sixth of global electricity generation, making it the single largest source of low-carbon power – and more than all other renewables combined. Its output has increased 70% over the past two decades. Many hydropower plants can ramp their electricity generation up and down very rapidly compared with other power plants such as nuclear, coal and natural gas. This makes sustainable hydropower an attractive foundation for integrating greater amounts of wind and solar power, whose output can vary, depending on factors like the weather and the time of day or year. Global hydropower capacity is expected to increase by 17% between 2021 and 2030 – led by China, India, Turkey and Ethiopia – according to the Hydropower Special Market Report, part of the IEA’s Renewables market report series. However, the projected growth for the 2020s is nearly 25% slower than hydropower’s expansion in the previous decade. Reversing the expected slowdown will require a range of strong research foundations. These research topics include the climate fluctuation impacts on hydropower and other renewable energy, the regulatory role of hydropower for a renewable energy system, and the integrated energy production among hydropower, wind, and solar power in the international energy markets. This study is focused on investigating the regulatory role of hydropower in a multi-energy renewable system, considering the climate fluctuations and the demand of international energy markets.