Water was the primary limiting factor for plant survival, and understanding water utilization in urban plantations was critical for ecological conservation and sustainable vegetation management. This study focused on the water use characteristics of species in four distinct plantation configurations on the campus of Qinghai University in Xining, Qinghai Province: pure Populus cathayana plantations, pure Picea crassifolia plantations, mixed P. cathayana and P. crassifolia plantations, and mixed P. cathayana, Syringa reticulata, and Amygdalus triloba plantations. Using hydrogen and oxygen stable isotope techniques combined with the MixSIAR model, we analyzed the water sources of species in these plantations during the 2023 growing season (May-October). The results revealed that the total precipitation from May to October was 406.88 mm, accounting for 89% of the annual precipitation (454.7 mm). In the early growing season, P. cathayana and P. crassifolia in pure plantations primarily utilized soil water from above 60 cm depth, with utilization ratios of 70.58±58.15% and 72.21±61.33%, respectively. In contrast, species in mixed plantations exhibited a more diversified water uptake strategy, utilizing soil water from all depth layers. During the mid-growing season, increased precipitation led to higher soil water content, and most species, except A. triloba in the MPSA community, predominantly absorbed surface soil water (0-20cm depth). At the end of the growing season, as precipitation decreased, soil water content stabilized at 13.61±0.94%. In pure plantations, P. cathayana and P. crassifolia shifted to using soil water from the 20-60 cm depth as their main source, with utilization ratios of 35.90±29.63% and 43.17±33.73%, respectively. In mixed plantations, these species, along with S. reticulata, began to utilize deeper soil water, while A. triloba continued to use water from all soil layers throughout the growing season. These findings indicated that the water sources for plants exhibited significant seasonal variation, and the water utilization patterns of the same species differed between pure and mixed plantations. The configuration of plantations altered the competitive dynamics of water sources both between and within species. In mixed plantations, interspecific competition for water resources led to niche partitioning, enabling different species to utilize water from varying soil depths, thereby reducing direct competition. In contrast, pure plantations exhibited intraspecific competition, where individuals of the same species competed for similar water sources, potentially leading to resource depletion in specific soil layers. The water utilization patterns of plants were closely related to their plantation configuration, root distribution, and the spatial-temporal variability of soil water content.