Main findings 

(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 in the basin occurs 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 its previous 60 years  

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 flood season and release water in 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 hot spot of the 2019 drought was located in the region from lower Lancang to 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 

Lancang River contributes significantly to annual discharge at Chiang Saen at the ratio of 64.4%. When it goes to the downstream of the Mekong River, the contribution ratio decreases substantially, with the ratio of 39.5% at Nong Khai, 24.9% at Nakhon Phanom, and 14.3% at Stung Treng. This means that the reservoirs located in the Lancang River may not supplement enough water when drought occurs in the downstream part of the Mekong River. Joint operation of all the reservoirs located in both mainstream and tributaries can be more supportive for the downstream drought.  

(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 agricultural water demand peak of the Mekong sub-region happens in the dry season (Do et al., 2020), and the share of drought in the dry season is quite higher than that in the wet season, the water supplementary role of Lancang reservoir cascade can generally alleviate drought occurring in the Mekong River Basin.  

Recommendations  

(1) Integrated structural and non-structural measures to alleviate drought 

The drought occurs more frequently in the dry season than the wet season in the LMRB. As the peak of agricultural water demand of Mekong Basin occurs in the dry season and water shortage is more likely to reduce agricultural production during this period, it is recommended that holistic measures are to be taken in dealing with the drought in the dry season, which include structural measures as well as non-structural measures 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, etc. 

(2) Joint operation of mainstream and tributary reservoirs for flood prevention and drought relief 

According to the runoff composition analysis, the contribution ratio of Lancang River to mainstream discharge decreases to 39.5% at Nong Khai, and further decreases to 14.3% at Stung Treng. It should be noted that the overall storage capacity of reservoirs in the tributaries of Mekong River reaches more than 37.2 billion m³ according to the data set from CGIAR research program on Water, Land and Ecosystems, and the value will over 100 billion m3 until 2030 (Wang et al., 2017), which could play an important role of runoff regulation on the mainstream discharge of Mekong River. The drought in the LMRB is characterized by significant spatiotemporal asynchrony. It’s recommended that 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 a 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 the experts from 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 the endeavours of flood prevention and drought relief, and joint research efforts are deserved to be implemented.