Quantitative attribution of seasonal discharge alterations in the Vietnamese Mekong Delta: regulation and ENSO

Abstract

ABSTRACT The Vietnamese Mekong Delta (VMD) is a region of immense socio-economic and ecological importance, that has experienced significant alterations to its hydrological regime. The objective of this study is to quantitatively disentangle and attribute these changes to their primary drivers: anthropogenic interventions from upstream civil works (UCWs) and climatic variability, particularly the El Niño-Southern Oscillation (ENSO). A 25-year (2000–2024) dataset of daily water discharge and local precipitation from key stations across the VMD was analyzed. The Oceanic Niño Index (ONI) was used as a proxy for ENSO events. A multi-method framework was employed, combining non-parametric trend analysis (Mann-Kendall test), Pearson’s correlation, and a multiple linear regression model. The VMD’s hydrology has been fundamentally reshaped and characterized by a significant increase in dry-season discharge (+45-48%) along with a decrease in rainy-season peak flows (-18-21%). The attribution analysis finds that UCWs are the dominant driver of these seasonal shifts, accounting for approximately 60–70% of the observed changes based on standardized regression coefficients. Climatic factors, primarily strong ENSO events, underlie the remaining 30–40% by modulating inter-annual variability. Local rainfall showed a minor but statistically significant contribution in the regression model, despite a weak direct correlation with discharge. By characterizing the VMD as a hybrid hydro-climatic system, the results underscore the need for adaptive transboundary water management in heavily modified and climate-sensitive deltas.