Defining a new hydrological normal for the Vietnamese Mekong Delta: distinguishing upstream regulation from climate variability
Nguyen Cong Thanh, Tran Thi Mai Huong, Dang Truong AnPurpose
The Vietnamese Mekong Delta (VMD) is confronting a fundamental hydrological shift. This study aims to quantify this emerging hydrological normal by distinguishing the effects of upstream regulation from basin-wide climate variability.
Design/methodology/approach
We analyzed daily discharge and rainfall data for 2000–2024 at Tan Chau and at upstream stations (Luang Prabang, Pakse, Mukdahan and Kratie). The analysis combines Mann–Kendall tests, Sen’s slope estimator, the range of variability approach (RVA), the hydrologic alteration factor (HAF) and Mann–Whitney U tests for the 2000–2009 baseline and 2010–2024 regulated period.
Findings
The analysis reveals a profound hydrological homogenization of the annual hydrograph. Key findings include: While upstream rainfall trends were non-significant or decreasing (Z < 0), dry-season discharge increased significantly across upstream and mid-basin stations (e.g. + 110.0 m³/s/year at Luang Prabang), whereas Tan Chau showed no statistically significant trend, indicating downstream attenuation of the regulated signal; RVA analysis shows high alteration levels for discharge (HAF∼−0.87 to −1.00), whereas rainfall RVA/HAF includes some high-magnitude monthly values but is mostly not statistically significant in Mann–Whitney tests, supporting discharge-rainfall decoupling; The flood pulse has been severely dampened. Notably, the strong La Niña year of 2021 produced flood peaks 41.3% below the 2000–2008 baseline at Pakse and 25.5% below the baseline at Kratie, confirming that reservoirs are systematically overriding natural climate signals.
Originality/value
The revised analysis shows that upstream regulation has reshaped the basin hydrograph most clearly in the upper and middle Mekong, while the delta gateway response is buffered by tributary inflows, Tonle Sap storage and channel-scale attenuation. This distinction provides a more defensible basis for adaptation planning in the VMD.