The Complexity of Moisture Sources Affects the Altitude Effect of Stable Isotopes of Precipitation in Inland Mountainous Regions

From a global perspective, the stable isotope altitude effect is crucial for understanding climate information. However, the intensity of this effect can be influenced by the source of moisture, particularly in inland mountainous regions where the moisture sources are complex. Different combinations of moisture sources might affect the altitude effect. Focusing on the upper Shiyang River in the northern part of the Qilian Mountains in China, this study calculated the proportion of recycled moisture in precipitation and utilized the HYSPLIT model to determine the source of advective moisture. It explored the characteristics and mechanisms by which moisture sources affect the spatiotemporal variations in precipitation isotope effects within the study area. The findings indicated that: (a) The altitude effect follows a seasonal pattern: winter < autumn < spring < summer, with a reverse effect in winter. (b) As the contribution of recycled moisture to precipitation increases, the altitude effect of stable isotopes weakens, primarily due to the disruptive influence of recycled moisture on this effect. (c) The altitude effect of stable isotopes in precipitation is determined by the direction of the moisture source and its attributes. When the primary source of advective moisture runs perpendicular to the mountain range and the moisture migration speed is slow, the altitude effect is pronounced. Thus, although temperature directly causes the altitude effect, water vapor sources significantly influence it in inland mountainous regions.