Quang M. Nguyen
July 20, 2020
During the 2019 wet season, water levels in the Mekong river in northeast Thailand bordering with Laos dropped to record low [1-3]. Several explanations were suggested. According to the Mekong River Commission (MRC), it was caused by a deficiency of rainfall on the upper basin since the beginning of the year . Some scientists claimed that the Chinese cascade dams on the Lancang river controlled the river flow  or held water behind them . Fishermen in Thailand pointed to the newly completed Xayaburi dam in Laos as the primary culprit and organized protests [6-8]. China, of course, has long been rejecting such claims [9-10] and Laos and the builder of the Xayaburi dam also denied their responsibility for the record low water levels . This article is an attempt to find out the cause of this dramatic event by analyzing the water levels observed at the hydrological stations along the Mekong river from June to December 2019.
Hydrological stations in the Lower Mekong Basin
Figure 1: Hydrological stations in the Lower Mekong Basin. [Source: MRC]
In order to monitor the water levels in the Lower Mekong Basin (LMB), Mekong Committee, the predecessor of the Mekong River Commission, established a network of hydrological stations along the river in Thailand, Laos, Cambodia and Việt Nam. This network has 20 main stations, including, in order from upper to lower; Chiang Saen (Thailand); Luang Prabang (Laos); Chiang Khan (Thailand); Vientiane and Paksane (Laos); Nakhon Phanom (Thailand); Thakhek (Laos); Mukdahan (Thailand); Savannakhet (Laos); Khong Chiam (Thailand); Pakse (Laos); Stung Treng, Kratie, Phnom Penh Port, Phnom Penh Bassac, Neak Luong và Koh Khel (Cambodia); and Tan Chau and Chau Doc (Vietnam).
Chiang Saen is the gate to the LMB. Tan Chau and Chau Doc are the gate to the Mekong Delta in Vietnam. The Xayaburi dam is located between the Luang Prabang and Chiang Khan stations, as shown in Figure 1.
Water levels in the LMB during the 2019 flood season
The water levels in the Mekong river were measured daily and posted on the MRC website. The water levels from June to December are used in the analysis. First, the water levels at the first three hydrological stations in the LMB: Chiang Saen, Luang Prabang and Chiang Khan (Figure 2).
The daily water levels at Chiang Saen did not rise as usual. On the contrary, they were decreasing then fluctuated around 2.5 m (equivalent to a flow of 1,400 m3/sec) because of a lack of high flows from the upper basin. This may be caused by (1) low rainfall on the upper basin or (2) water was held back by the Chinese dams, as suggested by Eyes on Earth .
Ironically, the low amount of rainfall on the upper basin was verified by the same scientists at Stimson Center. According to these scientists, the amount of rainfall in wet season in the upper Mekong basin in 2019 varied from “just above average” to “very dry” compared to the average rainfall from 2000 to 2018, as shown in Figure 3.  Holding back water behind the dams remains an unproven hypothesis.
The water levels at Luang Prabang were also decreasing like Chiang Saen, but jumped up on July 27 and the fluctuated around 9.5 m (equivalent to a flow of 6,400 m3/sec) until September 19, when they dropped again and stabilized by the end of October at 8.5 m (equivalent to a flow of 5,250 m3/sec). The sudden changes of water levels at Luang Prabang were likely caused by the construction and operation of the 2nd phase of the hydropower project on Nam Ou, a major tributary of the Mekong river just upstream of Luang Prabang .
The water levels at Chiang Khan followed the pattern of those at Chiang Saen until August 30 and reached 8 m (equivalent to a flow of 4,900 m3/sec), then decreased steadily from the beginning of September to the middle of October. From there, they fluctuated around 4 m (equivalent to a flow of 1,400 m3/sec), which is 4 m lower than the level at the beginning of September.
The dropping of water levels at Chiang Khan was certainly caused by the construction and operation of the Xayaburi dam (Figure 3) , located upstream of Chiang Khan and downstream of Luang Prabang. Although it is a run-of-the-river dam, the water level behind the dam needs to rise to its operational level at the start, resulting in a reservoir with a volume of 1.3 billion cubic meters . The Xayaburi dam completely masked the impacts of the dam cascade on Nam Ou. The hydrologic impacts of the Xayaburi dam, however, are expected to disappear during the normal operations.
From Chiang Khan, flood peaks began to appear. The water levels at Vientiane, Nong Khai and Paksane, as shown in Figure 4, followed the pattern of water levels at Chiang Khan, decreasing steadily from June to July 20 then increasing and reaching the peak by the end of August. From there, the water levels decreased steadily then fluctuated at a lower level by the mid October. The water levels at Nong Khai increased to 6 m (equivalent to a flow of 5,300 m3/sec) by the end of September then decreased to 1.0 m (equivalent to a flow of 1.000 m3/sec) bay the end of October.
From Paksane, the flood peaks are more pronounced (Figure 5). They were caused by heavy rain in central Laos in August and September. The peaks of water levels at Nakhon Phanom and Mukdahan are relatively flat, likely caused by impacts of dams on Nam Theun. At Khong Chiam, the water level reached 15.72 m on September 5, higher than the flood level of 14.50 m but lower than the record level of 16.25 m. At Pakse, the water level reached 13.75 m on September 5, higher than the flood level of 12.00 m and the record level of 13.32 m. 
Summary and conclusion
The water levels in the Mekong river in northeast Thailand bordering with Laos dropped to the record low during the wet season in 2019. This dramatic event attracted international attentions on hydropower dams, especially the cascade of the Lancang river in Yunnan, China and the first dam of the Mekong mainstream in Laos, the Xayaburi. Some US scientists claimed that China controlled the river flow and limited the amount of water flowing downstream. Fishermen and activists in Thailand blamed Xayaburi as the culprit and voiced their protests. China always dismisses the claims. Laos and the dam builder also denied their responsibility.
Analysis of the water levels at the hydrological stations along the Mekong River, which were posted on the MRC website, shows that the water levels at Chiang Saen and Luang Prabang did not rise as usual and decreased due to the deficiency of rainfall in the upper Mekong basin. The water levels at Chiang Khan, Vientiane, Nong Khai and Paksane were certainly impacted by the newly completed Xayaburi dam, but these impacts are expected to disappear under normal operations. The water levels at Nakhon Phanom and Mukdahan were likely impacted by the operation of dams on Nam Theun. From Khong Chiam, the impacts caused by the Xayaburi dam and the dams on Nam Theun appeared to be masked by inflow from the tributaries in central Laos.
About the author
Quang M. Nguyen was a professional engineer of the States of Florida and California. He worked for the National Water Resources Commission in Saigon, Vietnam; the Broward County’s Water Resources Management Division in Florida; and the Stetson Engineers Inc. in Los Angeles County, California, specializing in water resources and groundwater contamination. He retired in 2016.
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