Abstract: Stable isotopes are considered as a diagnostic tool which has been utilized in different media and widely used in geosciences and environmental studies, including use of hydrogen and oxygen isotopes in rivers, lakes and groundwater to investigate the circulation mechanism as well as the surface runoff composition in drainage basins, and use of isotopic data from speleothems, tree rings and ice cores to reconstruct paleoclimate. Precipitation is a main input factor in atmospheric water cycle and contains two natural tracers (¹⁸O and ²H) with strong signals for tracking the trajectories of water vapor. Rayleigh model is a popular model used in the methods to investigations the changes in moisture sources. Many investigators have used the model to simulate the variations of δ values in different study areas and got better results. In this paper, the study area in Southeast China is mainly influenced by summer monsoon during the period from June to September. However, with depletion of moisture in clouds, the impact of monsoon moisture changes from coast to inland. Based on Rayleigh theory and an evaporative model used by many researchers to calculate the contribution rate in different areas, we investigated the atmospheric water cycle mechanism, the contribution rate of evaporative vapor and the effect of secondary evaporation in Southeast China during the summer monsoon. (1) The comparison between the modeled values and the observed values indicated that the movement of water vapor abided by Rayleigh theory. (2) It was found that the supply of evaporative vapor from surface increased from coast to inland. The contribution rate of evaporative vapor, varying from 1.4% to 4.1% in the area, was 2.2% on average. (3) By comparison of the observed d excess to the global average d excess (10‰), it was inferred that the supply of evaporative vapor from surface and the effect of secondary evaporation both existed in this area. However, the effect of secondary evaporation decreased from coast to inland, suggesting that the decrease of the secondary evaporation may have been compensated by the supply of evaporative vapor from land. Based on the results in this research, it was concluded that the supply of evaporative vapor from surface area and the effect of secondary evaporation both had influences on water circulation in the study area. However, the value of the supply of evaporative vapor and the impact of the secondary evaporation could only be roughly estimated. Related investigations on the supply of evaporative vapor and the effect of secondary evaporation are few and far between in the area. If the problems above can be comprehensively solved, it will be of great significance not only studying the regional water cycle, but also providing basic data for agriculture, meteorology and other purposes. Thus, more sampling sites should be built in this area for detailed studies.

Key words: contribution of moisture, effect of secondary evaporation, Southeast China, stable isotopes of hydrogen and oxygen