River Discharge and Water Level Changes in the Mekong River: Droughts in an Era of Mega‐Dams

Abstract

While 1992 marked the first major dam -Manwanon the main stem of the Mekong River, the post-2010 era has seen the construction and operationalisation of mega dams such as Xiaowan (started operations in 2010) and Nuozhadu (started operations in 2014) that were much larger than any dams built before. The scale of these projects implies that their operations will likely have significant ecological and hydrological impacts from the Upper Mekong Basin to the Vietnamese Delta and beyond. Historical water level and water discharge data from 1960 to 2020 were analysed to examine the changes to streamflow conditions across three time periods: 1960-1991 (pre-dam), 1992-2009 (growth) and 2010-2020 (mega-dam). At Chiang Saen, the nearest station to the China border, monthly water discharge in the megadam period has increased by up to 98% during the dry season and decreased up as much as À35% during the wet season when compared to pre-dam records. Similarly, monthly water levels also rose by up to +1.16 m during the dry season and dropped by up to À1.55 m during the wet season. This pattern of hydrological alterations is observed further downstream to at least Stung Treng (Cambodia) in our study, showing that Mekong streamflow characteristics have shifted substantially in the post-2010 era. In light of such changes, the 2019-2020 droughtthe most severe one in the recent history in the Lower Mekong Basinwas a consequent of constructed dams reducing the amount of water during the wet season. This reduction of water was exacerbated by the decreased monsoon precipitation in 2019. Concurrently, the untimely operationalisation of the newly opened Xayaburi dam in Laos coincided with the peak of the 2019-2020 drought and could have aggravated the dry conditions downstream. Thus, the mega-dam era (post-2010) may signal the start of a new normal of wet-season droughts.

Quantifying hydrological responses to monsoon‐controlled precipitation across the soil‐groundwater‐stream continuum with long‐term high‐frequency hydrometric monitoring

“…Water levels responded to precipitation inputs in most rainfall events, though rise magnitude and intensity of groundwater level were far larger than those of stream stage (Figures 3b, 6, S1 and Table 2), hampered by the nearby dam (Dewey et al, 2022; Dott et al, 2022; Lu & Chua, 2021). Initial response activation threshold (Rt and Ti) for stream water and groundwater were small compared with soil moisture responses, especially in the groundwater during heavy rainstorms with large TP and rainfall intensity (Figures 5, 7a).…”

Drastic decline of floodpulse in the Cambodian floodplains (the Mekong River and the Tonle Sap system)

“…In addition, we identify an unreported change in rating curve at Stung Treng that depressed water discharge readings post-2005 (Lu and Chua, 2021 1960-20191962-19721995Bassac River Chaktomuk 1960-20191960-2019 Used the rating curve from MRC et al (2004) to estimate discharge for missing years Tonle Sap Lake…”

Can restoring water and sediment fluxes across a mega-dam cascade alleviate a sinking river delta?

“…1). The Lancang cascade has majorly altered the hydro-geomorphology of the entire Mekong Basin (16,17) and has been linked to recent hydrological droughts experienced downstream in the Lower Mekong Basin (LMB) (18,19). At the Vietnamese Mekong Delta (VMD), insufficient sediment supply from upstream sources is a key stressor threatening the continued existence of the delta landform; rendering the delta highly vulnerable to erosion, subsidence, and salt intrusion; and threatening the livelihoods of the 17 million people there (13,(20)(21)(22)(23)(24).…”