Efficient allocation of agricultural land and water resources for soil environment protection using a mixed optimization-simulation approach under uncertainty

This study proposes an optimization-simulation approach for simultaneously determining crop patterns and allocating irrigation water to improve soil environment. The approach incorporates an optimization model (a Chance-Constrained Programming (CCP) based multi-objective non-linear programming) integrated with a carbon footprint (CF) model and irrigation water use efficiency (IWUE) index, a soil water balance model and a groundwater dynamics model. The output of the optimization model constitutes the input to the soil water balance model which is calculated based on soil water content from soil samples representing the basin under study, and the resulting output is the input to the groundwater dynamics model. The output of the groundwater dynamics model is examined by whether the optimal results give rise to soil salinization. The proposed approach has advantages in addressing the tradeoffs of land and water resources for different crops in irrigation districts to reduce soil carbon emissions and improve land and water resources allocation efficiency, dynamically reflecting water transformation among precipitation, surface water, soil water and groundwater, and dealing with non-linearity and uncertainties. The approach was applied to identify optimal land and water resources allocation schemes in the oasis of Heihe River basin, northwest China. Results demonstrated that in the studied area, soil carbon emissions decreased and water use efficiency increased by the simultaneous allocation of the interactive agricultural land and water resources in space and time. Choosing the violation probabilities in the range of 0.05 and 0.1 might be more beneficial to the comprehensive benefits of the contradictory objectives (ie. reducing soil carbon emissions and increasing irrigation water use efficiency), because during this range the value of IWUE tended to be the largest and the value of CF was beginning to stabilize. From the perspective of spatial distribution, the value of CF showed an increasing trend from northwest to southeast, while the changes of groundwater table in the southern part were higher than in the northern part of the oasis. As the main limiting factor, increasing water availability led to an increase of cultivated land within a certain range, which significantly affected soil environment, leading to the necessity to contribute towards the efficient utilization of irrigation water resources and field activities to reduce soil pollution. Therefore, the proposed modelling framework can help to comprehensively manage agricultural land and water resources under complexity in an efficient and environmental-friendly way, and thus promote soil environment protection.