An investigation of moisture sources and secondary evaporation in Lanzhou, Northwest China

The purpose of this study is to investigate the atmospheric water cycle in Lanzhou and surrounding areas, a place sensitive to climatic conditions and located in the vertex of the “Monsoon Triangle” of China; this study obtained 243 event-based precipitation samples from four stations in Lanzhou, Yongdeng, Yuzhong and Gaolan for 1 year from April 2011 to March 2012. The seasonal variations of δ ¹⁸O and d excess indicate that westerly water vapor, local moisture and summer monsoon all have an influence in this region on a large scale. The westerlies play a dominant role. However, the impact of monsoon moisture has a seasonal limitation, mainly during the period from June to early August. On a local scale, the transportation of moisture appears via two routes. The contribution rate of recycling moisture, over the region, is only 3.6 % on average due to the deficiency of water resource in arid and semi-arid land. Additionally, the effect of secondary evaporation has also been discussed, and the results show that relative humidity, temperature and precipitation amount have different impacts on the effect. However, the influence of precipitation amount is not obvious when the rainfall amount is below 10 mm, while the meteorological parameters of relative humidity and temperature play a significant role in that scope.