Identifying how human activities affect the water cycle is essential for improving water resource management. This study analyzes the differences in water Mean Residence Time (MRT) across different reservoir types in the Shiyang River Basin (SRB) using precipitation, surface water, and groundwater isotopes, aiming to identify the influence of specific human factors on water resource allocation. The study concluded: (1) The barrier effect of the dams led to a significant isotopic enrichment in the basin reservoirs, while the hydrological connection between reservoir water and groundwater was strengthened in the reservoir distribution area. (2) The MRT for reservoir water is generally longer than those for other surface water, with surface water having MRT in that order: Hydroelectric Reservoirs > Irrigation Reservoirs > Urban Landscape Dams > River Water (Runoff); (3) The Young Water Fraction (F-yw) is negatively correlated with MRT, exhibiting a sharp decline from natural headwaters to the reservoir systems, with the lowest recorded contribution ratio of 11.2% in The Xiying Reservoir region. Our research demonstrates that water infrastructure, particularly large-scale hydroelectric and irrigation reservoirs, fundamentally determines the storage and transport of water in arid inland basins and exerts a significant influence on hydrological patterns.