Increased crop water requirements have exacerbated water stress in the arid transboundary rivers of Central Asia.

Water scarcity and ecological degradation as a result of the expansion of irrigated agriculture in arid regions have become global issues. A better understanding of the changes in crop water requirements (CWRs) is important for promoting sustainable development, particularly the water resource management of transboundary rivers. In this study, the latest and complete meteorological station and crop area data, the CropWat model, and the slope method were used to estimate the CWR in the Syr Darya Basin (SDB) of Central Asia from 2000 to 2018. The spatiotemporal variation of the water requirements for primary crops at the city scale was first assessed. The impacts of climate and cultivated land change on the CWR were quantified, and the associated impacts of the CWR on the water resources and environment were discussed. The results revealed that the mean unit area CWR of the SDB was 944.1 mm and the rate of increase was 7.6 mm/a from 2000 to 2018. The area of the primary crops expanded by 5851.6 km, and the total CWR increased at a mean rate of 2.0 × 10 m/a, with the majority of this change being concentrated between 2010 and 2018. By 2018, the total CWR reached 194.8 × 10 m. The lower reaches of the SDB were associated with a high CWR and a high rate of increase. Along with the reduction in basin water resources, the increased CWR has exacerbated the water stress in the SDB. Sensitivity analysis indicated that the dominant factors influencing the change in the CWR are cultivated land change (65.0%) and climate change (35.0%). Owing to a reasonable crop planting structure, the middle reaches maintained a relatively low CWR and rate of increase. Given the predicted changes in climate, optimizing crop planting structure and controlling the expansion of cultivated land in order to reduce the CWR can help to mitigate water scarcity.