Assessing Water Use Efficiency and Stress in Thailand’s River Basins: Trends, Challenges, and Policy Strategies

Water use efficiency (WUE) and water stress (WS) are keys indicators of water sustainability, particularly in regions with rising demand and limited supply. In Thailand, increasing water use across sectors and climate variability have raised concerns about long-term availability. This study applied Sustainable Development Goal (SDG) indicators 6.4.1 (WUE) and 6.4.2 (WS) at the river basin level, covering 22 basins from 2015 to 2022, to provide a more localized perspective than national assessments. A modified version of the FAO’s monitoring framework was applied, using standardized formulas based on sectoral water withdrawals and economic productivity. Supplementary data were gathered through estimation techniques, field surveys, and stakeholder consultations. The results showed a 21.0% decline in WUE and a rise in WS from 9.68% to 13.8%, indicating increased pressure on water resources. A very strong negative correlation was found between WUE and WS (r = −0.97, p < 0.001), although causation could not be inferred. Regional differences were evident: basins such as Tha Chin and Chao Phraya showed worsening conditions, while the Peninsula–West Coast remained relatively stable. These findings suggest the need for targeted policies to improve water use efficiency, especially in agriculture, and to enhance monitoring systems. Increasing wastewater reuse and implementing efficiency measures could help to reduce stress in vulnerable basins and support Thailand’s progress to achieving SDG 6.4.