Spatial extrapolation in soil salinity and future land cover using hybrid machine learning and land change modeler: case study in the Mekong Delta and the Red River Delta

Soil salinity is a major ecological challenge that affects agricultural productivity, posed significant challenges on the ecological system, especially in the deltaic region vulnerable to human alterations and sea level rise. Assessing agricultural areas impacted by soil salinity change is very important to support decision-makers or planners in sustainable land use planning. To overcome limitations in current spatial extrapolation methods for a reliable prediction of salinity trends across extensive river deltas, an advanced synthesis approach was developed with the use of machine learning (ML) particularly appropriate to account for a multitude of factors representing land cover conditions, processes, and interactions. This study aims to: (i) address the extrapolation challenge in ML-based soil salinity mapping, and (ii) predict land cover changes due to soil salinity. The Mekong River Delta (MRD) and Red River Delta (RRD) were selected as case studies. A hybrid ML approach and land change modeler were used to analyze 39 contributing factors. To resolve the spatial extrapolation issue in soil salinity monitoring, we used 109 salinity-affected locations in the MRD and 72 in the RRD. Land cover data from 2000 and 2023, along with salinity maps, were used to project the 2050 land cover. Multiple ML models were used to cross-verify and obtain robust results. All models achieved R² scores above 0.85. Results show that crop areas will decline from 67% (2000) to 64% (2023) and 60% (2050), with 41 km² of cropland converted to aquaculture - mostly in high-salinity coastal zones. This study develops a powerful synthesis framework to address the problem of spatial extrapolation challenges related to natural hazard mapping in general and soil salinity mapping in particular, based on machine learning and on accurate prediction of land cover/land use (LULC) change under effects of soil salinity in the context of climate change.