Satellite data showed that water sequestered in 7,245 reservoirs across the world fell from 1999 to 2018, despite a 28 cubic kilometre annual increase in capacity, a study published by Nature Communications said.
Climate change was a “critical factor” in reducing reservoir efficiency, said lead author Huilin Gao of Texas A&M University, but rising water demand also played a role.
“Even if temperatures stop rising, increasing demand and new construction are likely to continue,” she added.
The decline in storage volumes was concentrated in the south, especially Africa and South America, where water demand increased rapidly and new reservoirs didn’t fill up as quickly as expected.
The study did not account for the impact of sedimentation, a persistent problem that is predicted to cut storage capacity by a quarter by 2050, according to a January paper by the United Nations University.
Lengthy droughts have raised questions about the feasibility of large reservoirs. China saw hydropower output plummet last summer as a result of record-high temperatures across the Yangtze basin.
The International Hydropower Association said last week that new dams and reservoirs played a “crucial mitigating role in an era of increasing climate extremes”, making it easier to regulate water flows.
“As the climate gets more volatile, we will need more, not less, water infrastructure, with the bonus of much-needed low-carbon electricity,” it said.
China has also repeatedly said its enhanced ability to store and release water on the upper reaches of the Yangtze has alleviated downstream floods and droughts.
Unlike many regions, China’s storage levels increased slightly over 1999-2018 as a result of higher run-off in major river basins, suggesting it will benefit from new reservoirs, Gao said.
“But this highly depends on future climate, especially since most regions have experienced decreasing run-off,” she said.