Lancang River reservoirs regulate flood, drought

The cascade reservoirs in the Lancang River in China played a significant role in mitigating floods and droughts in Mekong River countries in 2021, a report has found.

The Mekong River, known as the Lancang River in China, is a vital waterway that also stretches across Laos, Myanmar, Thailand, Cambodia and Vietnam.

For the Lancang River Basin, 2021 was a relatively dry year. Compared with the long-term average, precipitation in the basin in the year was 11.6 percent less and the annual runoff was about 20 percent less, said Zhou Dongni from the Bureau of International Cooperation, Science and Technology at the Changjiang Water Resources Commission.

Introducing the "Report of Hydrological Situation of the Lancang-Mekong River Basin in 2021", she noted uneven distribution of rainfall in the basin throughout the year.

About 74.5 percent of the annual precipitation in the basin happened in the rainy season, which lasts from May to October, she said. The precipitation in August, the rainiest month, was 50 times more than the driest month of March.

Research by her team has found, however, the cascade reservoirs in China had greatly relieved Mekong River countries from the adverse impacts of potential floods and droughts, she said, addressing an event held virtually as part of the 2022 LMC Week Activities on Water Resources.

"Without the regulation of cascade reservoirs, during the dry season, the monthly average discharge at Chiang Saen station would be decreased by 60 to 83 percent," she said.

In March, the driest month, discharge at the station in Thailand would only be 290 cubic meters per second, "which may bring great adverse effects to people living near the river", she continued.

During the flood season, the water levels of Mekong's mainstream would rise significantly without the operation of cascade reservoirs in China, she said. At Chiang Saen station, the monthly average discharge would be increased by 23 to 51 percent.

"Therefore, the regulation effect of Lancang cascade reservoirs was significant in storing the floodwater during the flood season and supplementing extra water during the dry season downstream in 2021," she said.

In a prepared video played at the opening ceremony for the Tuesday event, Li Guoying, minister of water resources, stressed the role of China's water conservancy and hydropower projects on the Lancang in helping downstream countries in disaster mitigation.

China's development and utilization of the Lancang is mainly for hydropower. The hydropower projects do not consume water resources, but use water energy, and they can regulate natural runoff, he said.

"It relieves the flood control pressure of people in downstream countries," he said. "Also, it meets the water demand of people along the river downstream during dry seasons."

In a move to help Mekong countries cope with the grim challenges caused by climate change, China started to provide year-round hydrological information on the Lancang to these nations and the Mekong River Commission in 2020.

Before that, it had offered them flood-season hydrological information since 2003.