Science 12 March 2010
XISHUANGBANNA, CHINA—Smoky haze hangs over the hills in this subtropical corner of China bordering Laos and Myanmar. The smoke is familiar: During the dry season, farmers across Yunnan Province burn fallen leaves, banana fronds, and more to make ash-based fertilizer. More unusual here, and more troubling, are the sickly yellow bamboo stands and the exposed bed of the Lancang River. "It's the worst drought in that region since 1949," the founding of the People's Republic of China, says Lu Juan, vice director of the Institute of Water Resources and Hydropower Research in Beijing.
Southwest China's monsoon-driven climate doesn't bring much precipitation in autumn and winter. But this year's dry season—coupled with a late start and early end to last year's rainy season—has left the region parched. Yunnan officials estimate that some 6 million people are short of drinking water and that the dry spell has ravaged winter wheat and other crops, inflicting $1.5 billion in losses.
The drought's effects have spilled across China's borders, stoking tensions with neighbors and prompting scientific debate. Rice yields in Thailand are expected to take a big hit, and the Mekong River—the name for the Lancang south of China—is in many stretches less than a meter deep, its lowest level in decades, making it impassable to tour boats and cargo ships. Researchers worry about how the low water level may affect fisheries and critically endangered species such as the Mekong giant catfish, which in the coming weeks would normally spawn in the upper Mekong.
Environmental groups in Thailand and elsewhere lay at least part of the blame on China's doorstep. They claim that China's management of a series of dams on the Lancang has aggravated the unfolding crisis. The Thai media has helped stir up emotions; one editorial in the Bangkok Post last month was headlined "China's dams killing Mekong." Yet Chinese engineers and some other scientists say the criticism is unfounded.
Rising tensions in Asia could usher in a protracted regional conflict over resources, especially as many key rivers cross several borders. In Asia, "competition for transboundary water utilization will be fierce," says He Daming, director of the Asian International River Centre of Yunnan University in Kunming. China will be at the center of many squabbles. With some 110 rivers and lakes straddling its borders with 19 countries, says He, "China is the most important upstream riparian country in Asia, even in the world."
A major feature in this vast waterworks is the 800,000-square-kilometer Lancang-Mekong basin, home to some 60 million people. From glacier-fed headwaters on the Qinghai-Tibetan plateau, the Lancang wends 2160 kilometers through southwestern China before entering the Golden Triangle region of Burma, Laos, and Thailand. The river finally spills into the South China Sea off Cambodia. In the late 1980s, China began work on eight cascades, or hydroelectric dams, on the Lancang's lower reaches, aiming to supply 15.6 gigawatts a year. Four have been completed, including Xiaowan, the tallest at 292 meters.
Some environmental groups contend that the Mekong flow regime has been altered by dredging and dam construction, suppressing fish catches. Living River Siam, a nonprofit based in Chiang Mai, Thailand, has called on governments to "immediately stop all works on hydropower and river development on the Lancang-Mekong."
Yet the dams on the lower Lancang reduce runoff only during the rainy season, when reservoirs are filling, according to Chen Guanfu of Hydrochina Corp. Dry season water releases should increase river volume by 35%. "There are a lot of accusations that the dams in China are exacerbating the current low water levels, but the Chinese have informed [downstream nations] that they will not fill any reservoir during the dry season," says Roger Mollot, a fisheries expert with the World Wide Fund for Nature in Vientiane, Laos. The dams would also help rein in flooding, says Zhou Shichun of the General Institute of Hydropower and Water Resource Planning and Design in Beijing.
The biggest ecological impact could be less sediment swept downstream as silt accumulates in the reservoirs. But that would be a good thing, Zhou insists: It would "facilitate irrigation and navigation" on the Mekong. Others, however, point out that decreased sediment loads will likely lead to erosion of downstream riverbanks and the Mekong Delta.
Hydropower authorities have taken ecological effects into consideration, Zhou says. Work on one dam—the Mengsong Cascade, which would be sited nearest the border—has been postponed indefinitely, he says, to protect four species of migratory fish, including the giant pangasius (Pangasius sanitwongsei), whose conservation status is uncertain (Science, 22 June 2007, p. 1684). The freshwater goliath has not been reported above the Mengsong dam site, so the other dams would not affect it, Zhou says.
The first victim of an ecological crisis could be the Mekong giant catfish, which has been on the ropes for years. "It is not clear if the current drought conditions will impact successful spawning of the wild population of giant catfish, but low water levels may make them more vulnerable to fishing pressure," says Mollot.
Things may get worse due to climate change. After examining weather and tree ring data, Fan Ze-xin, a tree physiologist at Xishuangbanna Tropical Botanical Garden, has found that in the past 40 years Yunnan has grown warmer and drier—a trend that started long before the dams were built. In a nature reserve near the botanical garden, he grabs leaves from a seedling; dry as parchment, they disintegrate. "Some of these leaves are fresh," Fan says. "I haven't seen it as bad as this."