Hydrological Impacts of the Lancang Hydropower Cascade on Downstream Extreme Events.pdf
Author：MRC, LMC Water Center, IWHR, IWMI
The Lancang River originates from the northern slope of the Tanggula Mountains in the Yushu Tibetan Autonomous Prefecture of the Qinghai Province, China. The river flows south through the Dai Autonomous Prefecture of Xishuangbanna in Yunnan Province before leaving China. Also, known as the Mekong River in downstream countries, it passes Myanmar, the Lao PDR, Thailand, Cambodia, and Viet Nam, before emptying into the Sea. The Lancang-Mekong Basin is exposed to natural disasters such as floods and droughts. The Mekong region experienced more than 300 floods and storm surges between 1970 and 2012. In the Yunnan Province of China, the frequency of droughts has increased recently, with the years 2009-2011 experiencing continuous drought at an unprecedented level. Frequent floods and droughts pose major threats to the livelihoods, properties and lives of the people in the riparian countries.
The Lancang-Mekong Cooperation Mechanism was officially inaugurated during the first LancangMekong Cooperation (LMC) Leaders' Meeting held in Sanya, China on 23 March 2016. It marks a closer collaboration for ‘shared river, shared future’. Among the five priority areas agreed, water cooperation is identified as a flagship area in the LMC.
The Lancang-Mekong River is a transboundary river flowing through six countries, therefore water resources development and conservation of this important river is a concern shared by the riparian countries. Six hydropower reservoirs have been commissioned on the mainstream of the middle and lower reach of the Lancang River, namely Gongguoqiao, Xiaowan, Manwan, Dachaoshan, Nuozhadu and Jinghong. Of these dams, the Xiaowan and Nuozhadu reservoirs have multi-year regulation capacity, totalling 21.2 km3. In the Mekong River Basin, existing hydropower reservoirs have been mainly built on the tributaries. While 11 dams have been proposed, and are in various stages of planning on the mainstream, Xayaburi is the first one on the Mekong River where construction commenced in the Lao PDR in 2012.
Hydropower development can provide important contributions to poverty reduction and sustainable development in the region. It provides much needed energy for rapid economic growth, and reduces reliance on non-renewable energy. Large dams such as Xiaowan and Nuozhadu have potential role to store flood water and increase low flows. The impacts of hydropower are also a subject of debate in the international development agenda. One controversial question is about the role and contribution of the hydropower dams built on the Lancang River in China to recent flood and drought events experienced by downstream countries.
To assess the role of the Lancang cascade reservoirs on downstream floods and droughts and to explore the potential for win-win upstream-downstream collaborations, the six riparian countries agreed, during the 20th MRC Dialogue Meeting (with China and Myanmar) in July 2016, to conduct a joint assessment to examine the links between floods and droughts with the Lancang cascade reservoirs. Also, it fits well in the framework of the proposed Lancang-Mekong Water Resources Center.
The research activity is a joint effort between the China Institute of Water Resources and Hydropower Research (IWHR), International Water Management Institute (IWMI), Lancang-Mekong Water Resources Cooperation Center (LMWRCC), Mekong River Commission Secretariat (MRCS), and National Mekong Committees.
The project is expected to: (1) build trust, foster technical cooperation, and help clarify the impact of Lancang-Mekong hydropower development for the riparian countries and international development community; (2) contribute to improved cooperation between the riparian countries for flood protection and drought mitigation; and (3) help establish models of successful cooperation, and facilitate greater collaboration for peace and development in the Lancang-Mekong region.
1. Comparative analysis of the droughts of 2009-2010 and 2012-2013
The Lancang-Mekong Basin experienced a severe drought from October 2012 to April 2013, with an estimated return period of 50-year to 100-year. This drought was similar in terms of spatial distribution and magnitude to the drought that occurred between October 2009 and April 2010. The main hydrological difference between these two droughts is likely caused by the Xiaowan Dam that was operational by September 2012 and already achieved its operational water storage target, releasing water from November 2012 to April 2013. The comparative analysis of the droughts of 2009-2010 and 2012-2013 aims to better understand the respective influences of the climate and the operation of the Lancang cascade reservoirs on downstream low flows. Experience on successful dam operations at times of drought was also be synthesized for improved collaborations between China and downstream riparian countries in the future.
2. Analysis of extreme drought of 2015-2016
The drought of 2015-2016 in the Lancang-Mekong River basin is the most serious of the past decades. As the drought situation worsened, a ‘three phases’ emergency water supplement was carried out by the Ministry of Water Resources of China to supply water from the Lancang River to the Mekong River to help mitigate impact of the drought on downstream countries. The findings reveal evidence that explain the positive hydrological impacts of the Lancang cascade reservoirs on the downstream droughts.
3. Analysis of the flash flood of December 2013
Between the 13 and 15 December 2013, a significant intense rainfall event has been recorded downstream of the Jinghong reservoir (248.5 mm at Guanlei station). This series of major rainfall events resulted in a series of unusually high flows. There were suspicions (Financial Times, July 18, 2014, by Pilita Clark) that the flash flood was caused by releases from the Lancang cascade reservoirs. A hydrological assessment of such extreme events would help improve the understanding of the regional floods and help better prepare in downstream countries to reduce damages.
The main findings are as followed.
(1) The dry season drought in 2009-2010 and 2012-2013 is comparable in the upper reaches of the Lancang-Mekong River Basin. The drought in the lower reaches of the Lancang-Mekong River Basin in 2012-2013 is more severe than that of 2009-2010. The rainfall was less than average in 3 months during the two drought events in the Jinghong subbasin. The drought in 2009-2010 mainly occurred from December to February, and that of 2012-2013 mainly occurred from November to January. The two droughts reached moderate or severe level. The SPI6 results in the Stung Treng subbasin show that the dry season of 2012-2013 mostly belongs to moderate drought, and that of 2009-2010 mostly belongs to light drought.
(2) In the dry season of 2012-2013, no hydrological drought occurred along the Mekong mainstream. The results of dry season SRI6 show that the SRI values of hydrological stations along the Mekong mainstream in 2009-2010 ranged from -0.59 to -1.16. That of 2012-2013 ranged from 0.3 to 1.13, indicating that the discharge along the Mekong mainstream was slightly or significantly greater than the multi-year average, and there was no hydrological drought occurred. The analysis of hydrological frequency in dry season shows that the drought recurrence period of the minimum daily and monthly discharge of Chiang Saen Station in 2009-2010 is more than 12 years, while the discharge of 2012-2013 dry season has reached the multi-year average.
(3) The Lancang hydropower cascade has a positive impact on the discharge and water level of the Mekong mainstream in dry season. The water level of Jinghong station from January to May 2013, is 0.3~0.71m higher than the multi-year average; the monthly discharge of Chiang Saen station in dry season of 2012-2013 is higher than the multi-year average, and the water level is 0.46-1.11 meter higher than the historical average; the monthly discharge and water level of other hydrological stations along the Mekong mainstream after January 2013 is higher than the multi-year average.
(4) The water supplement of Lancang hydropower cascade has increased the water volume of the Mekong mainstream in the dry season. In the dry season of 2012-2013, the water volume at Jinghong station was 5.08 billion m3 more than the multi-year average, and 6.70 billion m3 more than that of 2009-2010. For the dry season water volume at Chiang Saen station in 2012-2013, it was increased from multi-year average 17.79 billion m3 to 23.15 billion m3, with an increase of 5.36 billion m3, and it was also 5.89 billion m3 more than that of 2009-2010.
(5) It is found from the Joint Observation and Evaluation of the Emergency Water Supplement from China to the Mekong River that the emergency water supplement from China in the dry season of 2015-2016 played a key positive role on the alleviation of the drought of Mekong River, through the regulation of Lancang cascade reservoirs.
(6) The flash flood of December 2013 was attributed to rainfall happened in the downstream sections of the Lancang River, not the regulation of the Lancang hydropower cascade.
(Source: LMC Water Cooperation, http://www.lmcwater.org.cn/)