Water-centric nexus assessment and critical land-use thresholds for economic crops in central Thailand

This research develops a water-centric nexus assessment framework for Thailand's major crops: rice, sugarcane, and cassava, within shared cultivation areas. The framework examines interaction among water use, agricultural productivity, and economic performance, positioning water as the central analytical dimension. A 10-year monthly dataset (2013–2022) from official sources was examined, and farmer interviews confirmed the relevance of variables, cost structures, and adaptive capacity. Applying a system-thinking approach using VENSIM software to construct causal loop diagrams and perform scenario-based analyses rather than full dynamic simulations. The framework integrates three normalized components: water mass productivity, economic water productivity, and water security, into a composite nexus index. Results indicate that sugarcane has the highest water productivity (0.010 tons/m3), followed by cassava (0.004 tons/m3) and rice (0.00031 tons/m3). Sugarcane also demonstrates superior economic water productivity, yielding higher profits per unit of water consumed. Sensitivity analysis reveals that expanding cultivation areas generally reduces water security; however, productivity and water-use efficiency improvements can offset these impacts up to crop-specific land-use thresholds. The sustainable annual expansion rates are estimated at 0.68% for sugarcane and 0.75% for cassava, while rice requires a 0.26% reduction in cultivated area to sustain a positive Water–Economy–Food nexus. Beyond these thresholds, the nexus turns unfavorable. Overall, the findings highlight the importance of demand-side water management, strategic crop allocation, and farmer adaptability in sustaining a stable water-centric nexus. The proposed framework provides a practical decision-support tool for integrated agricultural water management and policy development in Thailand.