| The Heihe River Basin (HRB) belongs to the arid inland basin, and contains many natural landscapes, such as glacier, the Gobi, desert and the oasis and so on.In HRB, water vapor sources for precipitation are complex and the spatial and temporal distribution varies greatly. Precipitation has an important impact on the ecological environment, and studying on atmospheric precipitation characteristics of HRB and water vapor sources has an important significance on a whole-depth study of water cycle characteristics of HRB, even the northwest arid zone, and guide reconstruction and rehabilitation of the ecological environment in arid areas. Using of environmental isotopes to analysis precipitation of large-scale basin and water vapor sources has its advantages. Water samples of atmospheric precipitation were collected from six sampling points in the upper reach and middle reach of HRB during October 2002 to September 2004. Based on laboratory analysis of samples and data analysis, the spatial and temporal distribution of hydrogen and oxygen isotopes in atmospheric precipitation and the relationship between it and several of impact factors were studied and discussed deeply. Combined both Backwards Trajectory and stable isotope value in precipitation, water vapor sources of Heihe river basin were studied, at the same time,hydrogen and oxygen isotope in precipitation of Xishui were simulated according to meteorological data of Xishui Station from 2002 to 2003. Based on the comprehensive study and analysis, results obtained were as follows:1.δ18O in precipitation of every precipitation event values and monthly weighted average values showed that:there is clearly the "temperature effect" inδ18O andδD of precipitation in HRB. The linear relationship betweenδ18O and temperature had a certain relationship with the altitude of sampling area, that was, the altitude higher, the slope greater, andδ18O/ΔT gradient greater from downstream to upstream. "Precipitation Effect" was not observed by monthly weighted average values ofδ18O in precipitation, but it was existent in successive precipitation events value ofδ18O. The monthly weighted average values ofδ18O in precipitation ranged from the lower reaches of the Gobi desert to the mountains is-2.2‰to -9.2‰, with the increase of altitude of sampling points,δ18O showed a downward trend. "Altitude Effect" is observed vividly between Yingluoxia and Xishui, the altitude of them is 1690m and 2569m, respectively. By analyzing weighted average values ofδ18O in precipitation of the Xishui, Yingluoxia, Ganzhou, Pingchuan and Zhengyixia, the height gradient ofδ18O is -0.47‰/100 m, which can be obtained. Through simulating changes ofδ18O value over time, seasonal changes ofδ18O values in precipitation of each sampling point showed apparent sine wave trend.2. The local meteoric water line (LMWL) equation of HRB was established byδD=7.82δ18O+7.63 and the correlation coefficient r= 0.99. The slope of precipitation line was less than the slope of the global meteoric water line, which is 8. This was consistent with dry conditions and strong evaporation conditions in HRB.3. The distribution and influencing factor of d-excess in precipitation of the basin were studied; water vapor sources which formed the atmospheric precipitation in HRB were analyzed combined with Backwards Trajectory. Results obtained as follows:water vapor of westerlies was dominant in cold seasons, followed by local circulation of water vapor; water vapor sources in warm seasons were very complex, and water vapor of westerlies, inland and southeast monsoon were the most pronounced, d-excess values of 162 samples were greater than +10‰, and the values of 27 samples were even greater than +20‰in 301 water samples of the precipitation, which showed water vapor of local evapotranspiration was in a large proportion in water vapor cycle of HRB, especially in the warm seasons.4. Hydrogen and oxygen isotope values in the atmospheric precipitation of Xishui were simulated by Rayleigh fractionation model and the simulation results in cold season were significantly better than in the warm season, and the efficiency of model for cool season was over 90%. This probably because that Heihe River Basin was mainly controlled and affected by water vapor of westerlies, and local evaporation was weak in cold season. Water vapor sources in warm seasons were complex, and there was no case that single water vapor was dominant. Hydrogen isotope values in the whole atmospheric precipitation were great influenced by water vapor of evapo transpiration owing to the higher temperature.
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