Lancang-Mekong dams key in mitigating aridity

Irregular weather, not retained water, led to last year's drought, report says

China shares a common fate with other countries in the Lancang-Mekong River Basin as the region increasingly suffers more frequent and severe droughts.

The country's dams play an important role in mitigating aridity in the river's downstream areas, a recent report said.

As data from the past 119 years show, China is among those that suffer the most from drought. Severe and exceptional droughts have occurred more frequently in the past 59 years compared with the previous 60 years in the basin, and drought hot spots were located principally in the middle and upper parts of the Lancang subregion, it said.

"The frequency of severe and exceptional droughts in the Lancang-Mekong River Basin is about 7 percent, and it has reached as high as about 12 percent in the upper and middle areas of the Lancang subregion over the past 59 years," said the report, jointly published in mid-July by the Center for International Transboundary Water and Eco-Security of Tsinghua University and the Department of Hydraulics of China Institute of Water Resources and Hydropower Research.

Originating from the country's Qinghai-Tibet Plateau, the Lancang River is known as the Mekong after it flows out of China and winds through Myanmar, Laos, Thailand, Cambodia and Vietnam.

The report said about half of the basin's downstream area has experienced an increase in severe and exceptional droughts, principally in Thailand, Cambodia and Vietnam.

The proportion of drought occurring in the dry season is significantly higher than that in the wet season in the basin, and this enables China's cascade reservoirs to play an important role in alleviating drought in downstream areas, it said.

"The Lancang cascade reservoirs store floodwater in the rainy season and discharge more water in the dry season, which effectively increases the dry season stream-flow of the Mekong River ... the supplementary role of the Lancang reservoir cascade can generally alleviate drought occurring in the Mekong River Basin," it said.

US consulting firm Eyes on Earth published a report in April claiming that the severe lack of water in the lower Mekong during the wet season of 2019 was largely caused by the restriction of water flowing from the upper Mekong.

The Chinese report said, however, the severe drought last year occurred due to insufficient rainfall with abnormal monsoons, high temperatures and evapotranspiration created by El Nino.

The conclusion was also endorsed by the Mekong River Commission－an intergovernmental river basin organization.

"During a normal year, monsoon rains usually start in late May and end in October. Nevertheless, in 2019, they began almost two weeks late and stopped about three weeks earlier than usual," the commission said in a commentary note in April.

"The basin lost about five weeks of rain and only received about 75 percent of rainfall compared to previous years."

The Chinese report also called to strengthen cooperation in the basin to cope with aridity, considering the limited role China's reservoirs could play for flood mitigation in areas far away.

The purpose of publishing the report is to "build knowledge and trust for further collective-action of basin-wide drought relief," it said.

Lancang contributes 64.4 percent of annual discharge at Chiang Saen, Thailand. However, its discharge contribution in Stung Treng, Cambodia, is just 14.3 percent. The help the country's reservoir could offer in drought alleviation decreases substantially as the river winds to the south, it said.

"Joint operation of all the reservoirs located in both mainstream and tributaries can be more supportive for the downstream drought relief," it said.