The Investigation Of Moisture Sources In Lanzhou Based On Precipitation Isotopes

To investigate the isotopic effects （Temperature effect, amount effect, humidity effectand so on） and the atmospheric water cycle on different scales in Lanzhou, a placesensitive to climatic conditions and located in the vertex of the “Monsoon Triangle”of China, and the effect of secondary evaporation, combined with the data in theGlobal Network of Isotopes in Precipitation （GNIP）, the research obtained243dailyprecipitation samples at4stations in the region （Lanzhou, Yongdeng, Yuzhong andGaolan） for one year from April2011to March2012. The paper got the results asfollows:（1） After a comprehensive analysis of the variation of δ¹⁸O and local meteoric waterlines, it is not difficult to find that the range of δ¹⁸O values have a similarity to somedegree. It indicates that the4stations may have been affected by the same moisturesources on a large scale. Compared with the global meteoric water lines （GMWL）, theslope and intercept of local meteoric water lines （LMWL） were relatively lower. Onthe other hand, the slope and intercept of LMWL of Lanzhou were more close to themeteoric water lines in northwest China. In addition, the temperature effect is notobvious in the annual scale, but the effect is stronger in summer time than in wintertime, which may be caused by the complex moisture sources in summer time.Moreover, there exists amount effect in summer time, which may be also caused bywater vapor sources in monsoon period. In the4sampling sites of the study area, theδ¹⁸O values and the relative humidity are negatively correlated. It indicated that thesecondary evaporation under the cloud base may be the primary reason to cause thisfact. Finally, the variation of δ¹⁸O values in annual scale shows a relatively highervalues of δ¹⁸O in summer, and lower values in winter.（2） The variations of observed δ¹⁸O and d excess show that westerly water vapor,local moisture and summer monsoon all have an influence on this region on a largescale. The westerlies play a dominant role. However, the impact of monsoon moisturehas some seasonal limitations, mainly during the period from June to early August.On a local scale, the transportation of water vapor appears via two routes. The firstone is from Lanzhou and Yongdeng to Gaolan and Yuzhong, and the other trajectoryis transported through Gaolan firstly, and then to Yongdeng, Lanzhou and Yuzhong, respectively. Moreover, the quantitative estimations for the contribution rate ofrecycling moisture from land surface indicate that the rate is3.6%on average, rangingfrom2.0%to6.3%. Additionally, secondary evaporation exists in the area. Relativehumidity, temperature and precipitation amount have different impacts on evaporation.However, the effect of precipitation amount is not apparent when the rainfall isbetween0mm and10mm, while the indicators of relative humidity and temperatureare playing a significant role in that scope. By using a water droplets modelestablished under well-defined library conditions the evaporation ratio in summer hasbeen calculated. The ratio is5.8%on average. The low ratio can be attributed to shortdurations of the falling of raindrop in higher elevations relative to some regions withlow elevations.