The application of water footprint for assessing the crop water resources stress in Nanjing City, China

Accurate evaluation of crop water use efficiency for planting structure optimization is essential for sustainable agricultural water management. Yet, how to comprehensively improve the efficiency based on the objectives of maximizing crop production and minimizing crop water consumption? It is still unclear. This study employed the CROPWAT model and Mann-Kendall trend analysis to assess the spatiotemporal changes in the crop water footprint (WF) of major crops in Nanjing City from 2002 to 2022. The crop WF both in the new industrial and ecological agricultural areas of Nanjing City accounted for greater than 90 % of the total crop WF. The crop WF decreased by 11.67 %, from 1.08 x 1012 m3 in 2002 to 0.959 x 1012 m3 in 2022. Over 90 % of the WF was concentrated outside the urban core areas. Additionally, the Agricultural Water Footprint Intensity (AWFI) remained stable, whereas the Agricultural Water Scarcity Index (AWSI) exhibited a downward trend. From 2002 to 2022, the average Blue Water Scarcity Index (BWSI) was 0.108, and the Water Use Efficiency Index (WUEI) was 0.365. Ultimately, the Comprehensive agricultural water use indicator (CWI) was 0.311, indicating a moderate level of overall crop water use efficiency in Nanjing City. The results of the optimization scenarios showed that crop water consumption could be reduced by 0.91 billion m3. And Nanjing City could save arable land at 7,148.58 hm2. The findings provide a scientific basis for informed agricultural water governance in rapidly urbanizing regions like Nanjing City.