Low-carbon hydropower capacity is vital for a faster integration of wind and solar power, but its growth is set to slow by 23% this decade without a sweeping policy and investment push, the International Energy Agency (IEA) said on Wednesday.

"Hydropower is the forgotten giant of clean electricity, and it needs to be put squarely back on the energy and climate agenda if countries are serious about meeting their net zero goals," said IEA Executive Director Fatih Birol in a statement.

In addition to producing massive quantities of low-carbon electricity, many hydropower plants can ramp generation up and down very rapidly, allowing for the integration of more intermittent wind and solar power output, the Paris-based agency said when presenting its Hydropower Special Market Report.

But new projects often face long lead times, lengthy permitting processes, high costs and risks from environmental assessments, as well as opposition from local communities, it said.

In 2020, hydropower supplied one sixth of global electricity generation and more than all other renewables combined, meeting the majority of electricity demand in 28 emerging and developing economies with a total population of 800 million, the IEA added.

Global hydropower capacity is expected to increase by 17% or 230 gigawatt (GW) between 2021 and 2030, led by China, India, Turkey and Ethiopia, but this was nearly 25% slower than expansion in the previous decade, according to the report.

Around half of hydropower's economically viable potential worldwide remains untapped, especially in emerging and developing economies, the IEA said.

Policymakers needed to address the hurdles and set robust sustainability standards to ensure projects were economically viable and garnered investor interest. Doing so could unblock existing project pipelines and potentially lift capacity additions by 40% through 2030, the agency said.

Still, hydropower would need to grow twice as fast through 2030 to meet renewables targets outlined by the IEA's recent report on reaching net zero emissions by 2050.