The annual flood pulse of the Mekong River is crucial to sustain agriculture production, nutrition, and the livelihood of millions of people living in the Vietnamese part of the Mekong Delta (VMD). However, climate change impacts on precipitation, temperature and sea-level combined with land subsidence, upstream hydropower development, and water infrastructures (i.e. high-dykes construction) are altering the hydrological regime of the VMD.This study investigates future changes in flood hazard and agricultural production caused by these different scales of human-induced stresses. A quasi-two-dimensional (quasi-2D) hydrodynamic model was used to simulate eight scenarios representing the individual and compound impacts of these drivers for a baseline and future (2036-2065) period. The scenarios map the most likely future pathway of climate change (RCP 4.5) combined with the best available Mekong upstream hydropower development, and land subsidence scenarios as well as the current delta development plan. We found that sea-level rise and land subsidence would cause the highest changes in flood hazard and damage to rice crop, followed by hydropower and climate change impacts. Expansion of high-dyke areas in two northernmost delta provinces (An Giang and Dong Thap) would have the smallest impact. The combination of all modelled drivers is projected to increase delta inundation extent by 20%, accompanied with prolonging submergence of 1-2 months, and 2-3 times increase in annual flood damage to rice crops in the flood-prone areas of the VMD. These findings of likely increasing risk of tidal induced flood hazard and damage call for well-planned adaptation and mitigation measures, both structural and non-structural. Highlights Climate change, hydropower, dyke development, sea-level rise and delta land subsidence strongly alter the delta future flood hazard and damage.  Higher flood extent (22-26%) and prolong inundation (1-2 months) are mainly driven by effective sea-level 5 rise and climate change.  Hydropower development might counteract the increase flood hazard and damage driven by climate change.  A reliable seasonal flood forecast (by August) would effective help to minimize agriculture flood damage.  Emerging call to develop flood management plan with focus on tidal induce inundation hazard.

Abstract. This study analyzes the influence of local and regional climatic factors on the stable isotopic composition of rainfall in the Vietnamese Mekong Delta (VMD) as part of the Asian monsoon region. It is based on 1.5 years of weekly rainfall samples. In the first step, the isotopic composition of the samples is analyzed by local meteoric water lines (LMWLs) and single-factor linear correlations. Additionally, the contribution of several regional and local factors is quantified by multiple linear regression (MLR) of all possible factor combinations and by relative importance analysis. This approach is novel for the interpretation of isotopic records and enables an objective quantification of the explained variance in isotopic records for individual factors. In this study, the local factors are extracted from local climate records, while the regional factors are derived from atmospheric backward trajectories of water particles. The regional factors, i.e., precipitation, temperature, relative humidity and the length of backward trajectories, are combined with equivalent local climatic parameters to explain the response variables δ 18 O, δ 2 H, and d-excess of precipitation at the station of measurement.The results indicate that (i) MLR can better explain the isotopic variation in precipitation (R 2 = 0.8) compared to single-factor linear regression (R 2 = 0.3); (ii) the isotopic variation in precipitation is controlled dominantly by regional moisture regimes (∼ 70 %) compared to local climatic conditions (∼ 30 %); (iii) the most important climatic parameter during the rainy season is the precipitation amount along the trajectories of air mass movement; (iv) the influence of local precipitation amount and temperature is not significant during the rainy season, unlike the regional precipitation amount effect; (v) secondary fractionation processes (e.g., sub-cloud evaporation) can be identified through the d-excess and take place mainly in the dry season, either locally for δ 18 O and δ 2 H, or along the air mass trajectories for d-excess. The analysis shows that regional and local factors vary in importance over the seasons and that the source regions and transport pathways, and particularly the climatic conditions along the pathways, have a large influence on the isotopic composition of rainfall. Although the general results have been reported qualitatively in previous studies (proving the validity of the approach), the proposed method provides quantitative estimates of the controlling factors, both for the whole data set and for distinct seasons. Therefore, it is argued that the approach constitutes an advancement in the statistical analysis of isotopic records in rainfall that can supplement or precede more complex studies utilizing atmospheric models. Due to its relative simplicity, the method can be easily transferred to other regions, or extended with other factors.The results illustrate that the interpretation of the isotopic composition of precipitation as a recorder of local climatic conditions, as for...

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