Understanding precipitation moisture sources of the Vietnamese Mekong Delta and their dominant factors during recent drought events

The Vietnamese Mekong Delta (VMD) is the most productive region in Vietnam in terms of agriculture and aquaculture. Unsurprisingly, droughts have been a prevalent concern for stakeholders across the VMD over the past decades. However, the VMD precipitation moisture sources and their dominant factors during drought conditions were not well understood. By using the ERA5 reanalysis data as inputs, the Water Accounting Model-2layers (WAM-2layers), a moisture tracking tool that traces moisture sources using collective information of evaporation, atmospheric moisture, and circulation, was applied to identify the VMD precipitation moisture sources from 1980 to 2020. The modelling simulation indicates that the moisture sources transported from the upwind regions dominate the VMD precipitation by 60.4% to 93.3%, and the moisture source areas vary seasonally with different monsoon types. The VMD precipitation moisture sources mainly come from the northeast area (e.g. the South China Sea) in dry seasons due to the northeast monsoon, while the southwest region (e.g. the Bay of Bengal) provides the primary precipitation moisture in wet seasons. Based on the causal inference algorithm, the driving factors in the process of moisture transport were also investigated. The results show that the specific humidity and wind speed are the dominant factors for driving moisture transport and determining the amount of VMD precipitation in dry and wet seasons, respectively. During the drought events in 2015-2016 and 2019-2020, the reduced moisture transport in the 2015 and 2016 dry seasons was mainly caused by the anomaly of both specific humidity and wind speed, while the negative anomaly of moisture sources in the 2020 dry season was dominant by the specific humidity. In the 2019 wet season, the wind speed anomaly led to the reduction of tracked moisture. These findings are important to understand the VMD precipitation moisture sources and their dominant factors during recent drought events.