Influence of Evaporating Under the Clouds on the Precipitation Stable Isotope in the Transition Zone Between Tibetan Plateau and Arid Region of China

Consideration of stable isotopes in precipitation is valuable for investigating hydrological processes. Therefore, correcting the measured isotopic composition of precipitation under below-cloud evaporation is necessary. An accurate description of the underlying processes affecting stable isotopic composition of precipitation could help improve our understanding of the water cycle. The transitivity between monsoonal and arid climates was reflected by the evaporation rate of falling raindrops in precipitation in the Qilian Mountains, a typical transition zone between Tibetan Plateau and arid region of China. Considering 1310 precipitation event-scale samples, based on stable isotope analysis method, the mean below-cloud evaporation rate ( f ) in the study area was measured as 12.00% during the summer half-year (May–October). The evaporation rate on the northern slopes (12.70%) of the Qilian Mountains in China was significantly higher than that on the southern slopes (9.98%). The transition between monsoonal and arid climates was reflected in the evaporation rate of falling raindrops during precipitation in the Qilian Mountains of China. Below-cloud evaporation contributed to a noticeable enrichment of stable isotopes in the precipitation in the study area. The monthly precipitation δ 18 O enrichment rate in the Qilian Mountains of China from May to October was 29.18%, 23.35%, 25.60%, 22.99%, 31.64%, and 14.72%, respectively. For every 1.00% increase in the evaporation rate of raindrops in Qilian Mountains of China, the changes in the concentration of oxygen isotopes from the bottom of the clouds to the ground increased by 0.92‰; however, with an evaporation rate of < 5.00%, for every 1.00% increase in the evaporation rate of raindrops the changes in the concentration of oxygen isotopes from the bottom of the clouds to the ground increased by 1.00 ‰ could also be observed. Furthermore, altitude was an important factor affecting below-cloud evaporation in the study area.