Mapping saline groundwater under rice‐paddy fields in Vietnam's Mekong Delta

Abstract Climate change, decreased river flow and land subsidence lead to saltwater intrusion posing a significant threat to rice production in Vietnam's Mekong Delta (VMD), one of the world's largest rice exporting regions. Soil salinity in the VMD can be caused by saltwater intrusion into lowland areas through the canal system, or by capillary rise of water from the near surface saline water table, both resulting in salt accumulation in the top soil. Developing appropriate management strategies for adapting rice production systems of the VMD to climate change, both in terms of water and salinity management, requires characterizing and subsequently monitoring of the spatial distribution and temporal dynamics of salinity in the near‐surface aquifers underneath the rice producing area. The distribution of subsurface salinity was investigated using electrical resistivity tomography in the VMD's province, Tra Vinh, as a case study area. Soil salinity was measured for profiles of approximately 300 m length at 44 locations along geological transects in a case study area. Results show that saline water appears at a shallow depth, particularly along the coast and the lower reaches of rivers. Double‐cropped rice fields seem to be more susceptible to salinization via the near‐surface aquifer than other rice cropping systems. The study suggests that temporal fluctuations of the near‐surface aquifer and the dynamics of the exchange between the river and the shallow aquifer need to be investigated in future research.