Report on LM Drought and Reservoir Regulation.pdf
Author：Tsinghua University,IWHR
The Lancang-Mekong River Basin (hereinafter referred to as LMRB) has been experiencing increasing frequency and intensity of drought during recent decades. Due to insufficient rainfall with abnormal monsoon and high temperature and evapotranspiration created by El Nino, the Basin witnessed a severe drought in 2019, which has attracted attention and various views from within and beyond the basin. To build knowledge and trust for further collective-action of basin-wide drought relief, this study collected hydrometeorological data covering both upstream and downstream areas and utilized state-of-the-art methods to investigate the drought characteristics and the impacts of reservoir regulation on the mainstream discharge of the Mekong River.
The objectives of this study include: (1) to investigate drought frequency and its spatiotemporal features, (2) to quantify the natural runoff composition along the mainstream of the Mekong River, (3) to investigate impacts of the regulation of Lancang cascade reservoirs on the mainstream discharge of the Mekong River.
In this study, two meteorological drought indices (SPEI and SPI) were adopted to investigate the drought characteristics in the LMRB. Two sets of long-term reanalysis dataset, i.e., CRU TS dataset from 1901 to 2019 and CHIRPS dataset from 1981 to 2019, were adopted for analysis. A distributed hydrological model (THREW) was established for the whole LMRB (above Phenom Penh). The model was run for 1991-2019 driven by gauge and remote sensing data. The gauged daily runoff data in 1991-2019 were collected from the MRC and the MWR of China from 8 main hydrological stations along the Lancang-Mekong River (i.e., Jinghong, Chiang Saen, Luang Prabang, Nong Khai, Nakhon Phanom, Mukdahan, Pakse, and Stung Treng).
It is found that the LMRB is experiencing high frequency of drought, and the proportion of drought occurring in the dry season is significantly higher than that in the wet season. The 2019 drought is among the most severe droughts in the past century. The regulation of reservoirs in the LMRB could play an active role in dealing with droughts in the Basin. The main findings are as follows.
(1) Drought characteristics: The frequency of drought in the LMRB is high, and the average frequency of severe meteorological drought (SPEI<-1.5) is 7%. The highest frequency of severe meteorological drought occurred in the middle and upper areas of Lancang sub-region, reaching more than 12%. The severe and exceptional droughts occurred more frequently during the recent 59 years compared to previous 60 years. Also, the proportion of drought occurring in the dry season is significantly higher than that in the wet season, which implies that the normal operation mode of reservoirs, i.e., store water in the flood season and release water in the dry season, is conducive to drought relief in the LMRB as a whole. For the year of 2019, the LMRB experienced one of the most severe droughts in the past century, the worst hit area was located in the region from the lower Lancang to the upper Mekong (Nong Khai). The 2019 drought is characterized by a long duration and severely less precipitation since the early wet season.
(2) Natural runoff composition: the Lancang River contributes significantly to the annual discharge at Chiang Saen, accounting for 64.4%. When it comes to the downstream of the Mekong River, the contribution rate decreases substantially, with 39.5% at Nong Khai, 24.9% at Nakhon Phanom, and 14.3% at Stung Treng. This means that the reservoirs located on the Lancang River may not supplement enough water when drought occurs in the downstream area of the Mekong River. Joint operation of all the reservoirs located in both mainstream and tributaries can be more supportive for the downstream drought relief.
(3) Impacts of reservoir regulation on the mainstream discharge: The Lancang cascade reservoirs store flood water in the rainy season and discharge more water in the dry season, which effectively increases the dry season streamflow of the Mekong River. Considering that the demand for agricultural water in the Mekong sub-region peaks in the dry season, and the drought statistically occurs more in the dry season than in the wet season, the water supplementary role of the Lancang cascade reservoirs can generally alleviate drought occurring in the Mekong sub-region.
Following recommendations were proposed to enhance drought relief capacity in the Basin.
(1) Integrated structural and non-structural measures to alleviate drought. Droughts occur more frequently in the dry season than in the wet season in the LMRB. As the peak of agricultural water demand of the Mekong sub-region occurs in the dry season and water shortage is more likely to affect agricultural production during this period, it is recommended that holistic measures be taken to deal with the drought in the dry season.
Both structural and non-structural measures should be included, such as strengthening the construction of water storage project and supporting canal system to improve the water supply capacity, adjusting the agricultural planting structure and selecting drought-resistant crop types, promoting the drought monitoring and early warning system, developing water-saving and drought-resistant irrigation and cultivation technology.
(2) Joint operation of mainstream and tributary reservoirs for flood prevention and drought relief. According to the runoff composition analysis, the contribution rate of the Lancang River to the mainstream discharge decreases to 39.5% at Nong Khai, and continues to decrease to 14.3% at Stung Treng. It should be noted that the overall storage capacity of reservoirs in the tributaries of the Mekong River reaches more than 37.2 billion m3 according to the dataset from the CGIAR research program on Water, Land and Ecosystems, and the number will exceed 100 billion m3 by 2030, which could play an important role of runoff regulation on the mainstream discharge of the Mekong River. The drought in the LMRB is characterized by significant spatiotemporal asynchrony. It’s recommended that the riparian countries strengthen relevant research on joint operation of reservoirs in the upstream and downstream as well as in the mainstream and tributaries, so as to make good use of these reservoirs and provide technical support for the benefits of the whole basin.
(3) Joint research on the whole-basin flood and drought forecasting system. Strengthened cooperation is needed to cope with challenges as well as share benefits from and beyond the river. It is suggested that experts from the riparian countries carry out joint research to lay a solid foundation for reciprocal cooperation mechanisms. A whole-basin flood and drought forecasting system will aid in flood prevention and drought relief, and joint research efforts should be implemented.
(Source: LMC Water Cooperation, http://www.lmcwater.org.cn/)