The stable isotope composition of precipitation has been widely applied to trace moisture sources. In the westerly dominated areas of central Asia, the isotopic composition of precipitation significantly correlates with surface air temperature, and the temperature effect is widely accepted. However, the significance of isotopes in tracing water vapor source is not clear for this region, and a further investigation using event‐based observations is still needed. Based on a network around the Tianshan Mountains, the westerly regime of precipitation isotopes is investigated. Although the seasonality of precipitation isotopes for the northern and southern basins of the Tianshan Mountains is similar, the more arid climate and leeward landform of the southern basin lead to weak moisture transport and relatively enriched oxygen isotopes. Of the synoptic systems resulting in heavy precipitation, the central Asian vortex more commonly leads to depleted isotopic ratios in precipitation, which is related to the contribution of northerly moisture and/or surface moisture. The specific humidity‐adjusted Lagrangian diagnostic indicates that the heavy precipitation on the southern basin stations usually has relatively local moisture sources in this arid climate and that terrestrial moisture evaporated from Europe and central Asia, rather than marine moisture from the Atlantic Ocean, is more likely to be the direct moisture source. A sensitivity analysis for target height and adjusting threshold of specific humidity is also applied to the improved Lagrangian diagnostic. Overall, the findings were unaffected by different conditions, and the moisture was more likely to be locally sourced, as indicated by higher specific humidity thresholds.

Different water sources exploited for public use have different exposure to risks associated with climatic and environmental change. Isotope ratios of tap water have previously been studied as a potential tool to link public supply waters with water source characteristics at local to continental scales, providing information on the footprint of and potential risks associated with the water sources used. Work that combines intensive spatial and temporal sampling with independent water management data has been limited, however. In this study, an extensive observation network was established during 2014–2016 to provide monthly tap water sampling across China. We show that the spatial distribution of annual mean tap water isotope ratios is generally consistent with that of local precipitation across China. We identify seasonal correlation between tap water and precipitation isotope ratios in south China, where use of surface water is prevalent. In contrast, relatively invariant tap water isotope ratios elsewhere in China, which are not correlated with seasonal variation of precipitation isotope ratios, can be attributed to use of groundwater or water from river basins with longer storage times. The tap water isotope signatures identified here could be widely applied to characterize water supplies and associated sustainability challenges in different regions worldwide.

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