Water Cycle Research In Lanzhou City Based On Stable Isotope In Precipitation

Precipitation plays an important role in the process of water circulation.Stable hydrogen and oxygen isotopes in precipitation are very sensitive to changes in the environment,and can record the evolution of water cycle.Due to the transitional zone among the monsoon area of East China,the arid area of Northwest China and the cold area of Tibetan Plateau,the regional water cycle is very complex in Lanzhou City of Gansu Province.It is of great significance to study stable isotopes in precipitation,which is useful to understand the regional hydrological process.Based on the event-based precipitation samples collected at Anning,Yuzhong,Gaolan and Yongdeng from April 2011 to October 2014 and at Renshoushan,LREVC?Lanzhou Resources&Environment Voc-Tech College?,Wushengyi,Daheng,Gongjingxiang and Hezui from October 2013 to October 2014,the basic characteristics of stable hydrogen and oxygen in precipitation were investigated,and the water vapor source,moisture recycling and below-cloud secondary evaporation were discussed by using isotopic approach.The main results are as follows:?1?Spatial diversity and seasonal variation of precipitation isotopes can be detected in the study region.The values of?D and?¹⁸O in precipitation are usually higher in summer and lower in winter months.In the four sites with long observations,the correlations between?¹⁸O and air temperature are statistically significant at the0.01 level,but there are differences in seasonal pattern.Generally,precipitation amount negatively correlates with isotopes,and the values of?¹⁸O negatively correlate with relative humidity for the four sites.The correlations between?¹⁸O and water vapor pressure are negative at Gaolan and Yuzhong,but positive at Anning and Yongdeng.Positive correlation is also given priority between dew point temperature and?¹⁸O in precipitation.?2?Compared with global meteoric water line?GMWL?,the slope and intercept of meteoric water line in Lanzhou City are smaller.There are differences of the slopes and intercepts in four seasons.The meteoric water lines can be calculated by using six approaches of linear regressions,and the calculated slopes and intercepts are slightly different.Among these approaches,PWLSR?precipitation-weighted least squares regression?usually has the minimum rmSSE_av,indicating that the result of PWLSR is relatively stable.?3?The values of d?D-excess?in precipitation vary seasonally,and the values in dry season are larger than those in wet season.Regarding the correlation between d value and meteorological parameter,most stations have statistically negative correlations between d value and temperature or vapor pressure,except Yuzhong for the relationship between d and vapor pressure.Based on the global reanalysis data,the Lagrangian method was applied to investigate the air backward trajectory,and the water vapor flow was acquired for total air column.The results based on reanalysis data are generally consistent with the finding using isotopic approach,that is,the monsoon moisture plays an important in summer months.?4?Using the two-component and three-component mixing models,the proportion of recycled moisture was calculated,respectively.The recycled moisture is less than 10%by using a two-component mixing model.Compared with a three-component mixing model,transpiration is totally underestimated by a two-component mixing model.The three-component mixing model indicates that the contribution of surface evaporation to precipitation is larger than that of transpiration in urban area,and the relationship is different at rural area.In addition,the contribution of transpiration to local precipitation is up to 30%during some months.?5?The daily values of?¹⁸O and d in precipitation indicate that there is secondary evaporation in Lanzhou city.Secondary evaporation is obvious when the rain amount is light,and secondary evaporation is weak when there is heavy rain or snow.Secondary evaporation becomes stronger as air temperature rises.Compared with snow and heavy rain,water vapor pressure has an obviously negative influence on secondary evaporation of light rain.Relative humidity has significant influence on d value as well as slope and intercept of meteoric water line for light rainfall,but has a small influence when snow or heavy rain occurs.A linear correlation is seen between d variation and raindrop mass evaporation from cloud base to ground.Under conditions of low air temperature,high relative humidity,heavy precipitation amount and large raindrop size,a good linear correlation is found;In contrast,under opposite conditions,the correlation is weak.The above findings about vapor source tracing,moisture recycling assessment and below-cloud secondary evaporation in Lanzhou City is of great significance to investigate controlling factors of precipitation isotopes at a city scale,and establish regional water budget.The results are also useful to understand the internal regime of water cycle at a city scale under arid and semi-arid climate backgrounds,and provide a scientific reference for water resource management.