Research Reports

Publications - Understanding the Mekong Rivier's hydrological conditions (2020)

A brief commentary note on the "Monitoring the Quantity of Water Flowing Through the Upper Mekong Basin Under Natural Conditions" study by Alan Basist and Claude Williams (2020) Summary of the commentary • Dam development in the upper reaches of the Mekong has led to changes in seasonal flows, with increased dry season flows and decreased wet season flows. Both phenomena are observed in the LMB, with the impact on the flow regime becoming progressively less noticeable further downstream. • The findings of the Eyes on Earth study have not yet well taken into account the complexities of rainfall and runoff, thus not reflecting the actual hydrological conditions in the Basin. The conclusions drawn are based on calculated flows and not an analysis of the actual flows at Chaing Saen set against the long term trends.  • As the methods presented by the authors are highly empirical and calibrated on the period 1997 2001, various complex hydrological and physical processes that determine the runoff from the catchment and the river are not captured in the regression functions. • Our preliminary analysis of hydrological and rainfall data showed that the 2019 drought in the basin was due largely to insufficient rainfall during the wet season with a delayed arrival and earlier departure of monsoon rains and an El Nino event that created an abnormally higher temperature and higher evapotranspiration. • Inadequate information on water infrastructures in the Mekong River Basin and the way these infrastructures are operated have made it challenging to forecast short term impacts

The annual flow regimes at Chaing Saen, where the Mekong enters the Lower Mekong Basin (LMB), have been altered by hydropower projects in China. These projects reduce wet season flows and increase dry season flows. This situation brings both opportunities and challenges. The impact of these changes becomes less evident further downstream due to the inflows from tributaries in the LMB and the operations of the tributary hydropower dams. The Eyes on Earth Study draws its conclusions from calculated flows at Chaing Saen, the most upstream point in the LMB. These calculations do not reflect the actual dry season flows over the last two years. The actual flows recorded in the Mekong River Commission’s Procedures for the Maintenance of Flows on the Mainstream (PMFM) in the 2019 and 2020 dry seasons showed that inflows from China were higher than typical for the dry season. The record low flow at Vientiane in Lao PDR noted in the 2019 and 2020 dry seasons was most likely due to the change in rainfall volume and pattern. The MRC’s data also showed that these recorded low flows do not persist further downstream of Vientiane due to the operations of the tributary hydropower dams. The PMFM showed that dry season inflows into Cambodia were higher than typical during 2019 and 2020. 

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Mekong River Commission for Sustainable Development