Impacts of moisture sources on the isotopic inverse altitude effect and amount of precipitation in the Hani Rice Terraces region of the Ailao Mountains

Abstract

Generally, the isotopic composition of precipitation on the windward side of gigantic mountains has a negative altitude gradient. However, an inverse isotopic altitude effect occurs when there are diverse vapor sources. This paper takes the Quanfuzhuang River Basin (QRB), which lies in the Hani Rice Terraces region of the southern Ailao Mountains in southwest China, as the study area. The study analyzes the isotopic data of 42 precipitation samples collected between an elevation range from 1500 m to 2024 m a.s.l. during the rainy season (from May to October) of 2015. The results indicate that there is an inverse isotopic altitude effect of precipitation with a positive isotope altitude gradient of 0.47‰/100 m and 1.10‰/100 m for δ18O and δ2H, respectively, at the mountaintop, while the precipitation amount increases at related elevations. A mixture of over peak airflow and recycled vapor is responsible for the inverse altitude effect as well as increasing rainfall amount. The positive precipitation isotopic altitude gradient is primarily caused by the local water cycle, and the increased precipitation amount is mainly caused by over peak airflow. The inverse isotopic altitude effect is also found in the Colorado Rocky Mountains, the Central Hindu Kush Mountains and the Tianshan Mountains, and findings from those mountains support these findings on the influence of mixed moisture sources on isotopic inverse altitude effects.