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A positive altitude gradient of isotopes in the precipitation over the Tianshan Mountains: Effects of moisture recycling and sub-cloud evaporation

Kong, Yanlong, Pang, Zhonghe
Journal of hydrology 2016 v.542 pp. 222-230
altitude, basins, evaporation, groundwater recharge, isotopes, mixing, models, mountains, rain, rivers, water analysis, water distribution, watersheds, China
A negative stable isotope-altitude gradient is commonly observed on the windward side of a mountain. However, after the precipitation passes over a mountain range to the leeward side, the altitude effect becomes ambiguous as a result of an orographic rain shadow in addition to other complex processes such as sub-cloud evaporation and additional moisture mixing. In this study, we found a positive precipitation δ18O-altitude gradient with a value of 0.12‰/100m in the Urumqi River catchments on the leeward side of the Tianshan Mountains through an analysis of water isotopes sampled in this region. Processes including both sub-cloud evaporation and moisture recycling were found to be responsible for the positive gradient. A simple model was built to analyze the observations quantitatively. We defined the difference of the recycled (evaporated) fraction as the recycled (evaporated) fraction at the lower station minus the fraction at the higher station. The model showed that the δ18O-altitude gradient rises by 0.28‰/100m with the difference of the recycled fraction increasing by 1%/100m, and declining by 0.15‰/100m with the difference of the evaporated fraction increasing by 1%/100m. The effect of moisture recycling is more significant than that of sub-cloud evaporation on the leeward side of the Tianshan Mountains; therefore, the precipitation in the Tianshan Mountains has a positive δ18O-altitude gradient. The model also explains the distribution of water isotope data points in the δ2H-δ18O figure of Northwest China: while the data points of the mountainous water isotopes are located above the local meteoric water line (LMWL) because of moisture recycling, most data points of basin water isotopes are located below the LMWL because of evaporation. Accordingly, we concluded that the stable isotope-altitude gradient on the leeward side of a mountain is very sensitive to local atmospheric processes; an inference that should be taken into consideration while quantifying the paleo-altitude of orogeny and groundwater recharge area using stable water isotopes.