Is it worth it? Land-fallowing and saltwater intrusion control under uncertainty

Sea level rise and anthropogenic activities intensify saltwater intrusion, threatening coastal and river delta agriculture. Numerous previous studies relied on hydrologic models to understand how and to what extent sea-level rises and the alterations of river flow regimes affect saltwater intrusion. Some previous studies examined how changes in crop mix and land management can alleviate the impact of saltwater intrusion. However, little is known about the optimal level of land-fallowing needed to achieve a sustainable level of saltwater intrusion. In addition, how and to what extent a land-fallowing policy can affect the stochastic nature of saltwater intrusion remains poorly understood. Herein, we develop a probabilistic hydro-economic model by integrating a hydro-statistical model with the Inventory Theory to evaluate economic trade-offs between reducing saltwater intrusion and agricultural productivity losses resulting from a land-fallowing policy in the Mekong Delta, a major rice-producing region in Asia, by 2050. The results show that the non-exceedance probability of saltwater intrusion level is approximately 0.78 at the optimal point. Under a sea-level rise of 22 cm scenario, the optimal annual land-fallowing areas are estimated to be 124,000 ha to maintain saltwater intrusion levels that are less than or equal to the historical average level. This level of land-fallowing would lead to an annual crop profit loss of approximately $130.01 million. The model shows that saltwater intrusion control infrastructure can substantially reduce the areas affected by saltwater intrusion but still requires to fallow 45,000 ha annually to bring the saltwater intrusion to the historical average level. The estimated annual crop profit loss associated with this scenario is $47.18 million, plus an estimated total infrastructure cost of $8.2 billion. The results also show that the potential economic benefits of improved saltwater intrusion forecasts are substantial, ranging from $47.18-$51.37 million annually, implying that increased saltwater intrusion forecast accuracy can have a substantial economic value. The findings highlight the need for all countries in the Mekong River Basin to develop more sustainable and cooperative development plans and quickly establish a data-sharing mechanism to reduce adaptation costs. The finding also informs policy discussions on investments in enhanced regional and global monitoring and forecasting systems.