Evaluating topsoil salinity via geophysical methods in rice production systems in the Vietnam Mekong Delta

The Vietnam Mekong Delta (VMD) is threatened by increasing saltwater intrusion due to diminishing freshwater availability, land subsidence, and climate change induced sea level rise. Through irrigation, saltwater can accumulate in the rice fields and decrease rice production. The study aims at evaluating topsoil salinity and examining a potential link between topsoil salinity and rice production systems in a case study in the Tra Vinh province of the VMD. For this, we applied two geophysical methods, namely, 3D electrical resistivity tomography (ARES II) and electromagnetic induction (EM38-MK2). 3D ARES II measurements with different electrode spacings were compared with EM38-MK2 topsoil measurements to evaluate their respective potential for monitoring topsoil salinity on an agricultural scale and the relationship between land-use types and topsoil salinity. Results show that EM38-MK2 is a rapid and powerful tool for obtaining high-resolution topsoil salinity maps for rice fields. With ARES II data, 3D maps up to 40 m depth can be created, but compared with EM38-MK2 topsoil maps, topsoil salinity was underestimated due to limitations in resolution. Salt contamination of above 300 mS m-1 was found in some double-cropped rice fields, whereas in triple-cropped rice fields salinity was below 200 mS m-1. Results clearly show a relation between topsoil salinity and proximity to the saline water sources; however, a clear link between rice production and topsoil salinity could not be established. The study proved that geophysical methods are useful tools for assessing and monitoring topsoil salinity at agricultural fields scale in the VMD.