Browsing by Subject "Vietnam Mekong Delta"

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Evaluation of geo-physics methods to study the effects of land use on salinity in rice production systems in the Vietnam Mekong Delta
(2023) Nguyen, Van Hong; Asch, Folkard
In the Vietnam Mekong Delta (VMD), salinity is a major concern for rice production, which is highly susceptible to saltwater intrusion due to its proximity to the sea and tidal influences. Climate change induced sea level rise, reduced upstream freshwater flows and land subsidence are exacerbating the problem. As a result, saltwater intrudes into the rivers, canals and aquifers of the VMD, reducing the availability of freshwater for irrigation and agricultural use. As the worlds largest rice exporter, the impacts of salinity on rice production in the VMD is significant and poses a serious threat to food security. Addressing the impact of salinity on rice production in the VMD requires a comprehensive approach to assess salinity from the topsoil to the subsoil layers. Therefore, this study was conducted to evaluate salinity issues in rice production systems and figure out the link between rice production systems in the VMD and salinity by applying geophysical methods. Geophysical methods were used in this study including Electromagnetic Induction (EMI) and Electrical Resistivity Tomography (ERT). EMI measures electrical conductivity, while ERT measures electrical resistivity, which is the inverse of conductivity and is closely related to soil salinity. ERT was employed to assess salinity of the subsurface to a depth of 40 m, while EMI was used to detect topsoil salinity up to 1.5 m depth. The case study, Tra Vinh province, in the VMD was chosen for the soil salinity investigation. Soil salinity measurement was conducted during dry season in different land-use types related to rice production systems in the VMD. The field measurements were carried out in two consecutive dry seasons, the dry season of 2019-2020 and dry season of 2020-2021. The first measurement was carried out at five case study sites with different cropping patterns to validate the ERT data and to compare the two methods to determine the best method for investigating soil salinity. Five boreholes were drilled to a depth of 40 m to validate the ERT measurement for subsurface salinity. In the following dry season, ERT and EMI were measured in the extensive survey, with the measurement sites selected along four typical geological transects in the Tra Vinh province. With a desire to use ERT alone to assess soil salinity, ERT data was then used to predict topsoil salinity along with EMI measurement. However, the results from ERT measurement seems to underestimated topsoil salinity due to the lack of measurement on the fields compared to the EMI. The conductivity data collected from different land use types, showed that double rice crop fields are the most prone to salinity than other cropping patterns such as, triple rice, triple/double rice, and single rice. In general, however, topsoil salinity is not a critical issue in the study area compared to potential salinity from the near surface water table, which varies from a relatively shallow depths, from 2 m to 5 m depth, identified by ERT using a resistivity of less than 3 Ωm as the threshold for saline water. The saline water table is the Tra Vinh province increase with the proximity to the sea. As analyzing from resistivity maps, saltwater intrudes the subsurface groundwater from two directions: rivers along the province, and from the sea. Therefore, the double rice, single rice fields and small area of triple rice fields distributed along the two main rivers in the Tra Vinh province are highly affected by saline subsurface water. From the results we see that salinity affect land use in rice production system, and not another way round. Furthermore, we would like to prove for the first time the capability of ERT and EMI in evaluating soil salinity in the rice cultivation fields in the VMD. In addition, we suggested the powerful methods to capture and monitor saltwater intrusion into the rice fields from top to subsurface, which is necessary to improve and protect rice production.