Microclimatic Environmental Significance Of Stable Isotopes Of Precipitation In Tianshui Area And Analysis Of Sub-cloud Secondary Evaporation

As an ideal natural tracer of water vapor,the hydrogen and oxygen isotopes in precipitation provide an important reference for studying paleoclimate and paleogeographical environment as well as the ecological hydrological cycle process.The environmental significance of stable isotopes of falling water in different geographic environments is also different.It is especially obvious in the transition zone of climate change.The Tianshui area in the middle of Gansu Province is located in the transition zone from semi-humid to semi-arid.The regional microclimate changes obviously,and the precipitation process is complicated and changeable.In view of this,this study established a precipitation stable isotope monitoring network in Tianshui area from April 2019 to May 2020.Combined with atmospheric convection activities,the time series of stable isotopes of precipitation are systematically analyzed.At the same time,the raindrop evaporation model is used to analyze and study the secondary evaporation under the cloud at 7 sampling sites in Tianshui area.The specific research results are as follows:(1)The 7 sampling points in Tianshui area showed two obvious low-value areas during the sampling period of?¹⁸O,which were around September 14,2019 and the end of November 2019.The reason for the low value area of the former is that strong convective activity leads to the continuous depletion of water vapor in the heavy isotopes during transportation,while the latter is because there is basically no evaporation process due to the equilibrium fractionation of isotopes in solid precipitation.At the same time,stable isotopes of precipitation before and after monsoon and the change characteristics of large-scale convective activities can clearly record the advancing and retreating time of the monsoon.Around May 7 and September21,this article captured this important time node.(2)The microclimate environment of Tianshui area indicated by stable isotopes in precipitation is obviously significant.During non-monsoon periods(Pre-monsoon and Post-monsoon),the change of?¹⁸O in precipitation is mainly controlled by temperature.During monsoon periods,it also shows a weak precipitation effect.There is a clear negative correlation between?¹⁸O in precipitation and water pressure during the period.During the entire study period,the surface temperature of Weibei,River Valley and Guanshan District has a significant relationship with the?¹⁸O value in precipitation,and the change gradients are 0.37‰/?,0.20‰/?and 0.32‰/?respectively.As far as the slopes of the local meteoroic water line is concerned,the maximum value is 7.76(Maiji)and the minimum value is 7.14(Gangu).Compared with the slope of the meteoroic water line,the intercept changes more obviously,and the maximum relative difference is The value reached 8.46,but the relative maximum and minimum values also appeared at the Maiji and Gangu sampling sits.(3)The seasonal variation of the evaporation rate(r_ev)and the remaining evaporation ratio(f)in Weibei District is the most obvious.The main manifestation is that the evaporation rate gradually increases from January to July and reaches the historical maximum in July.After that,the raindrop evaporation rate began to decrease,reaching the historical minimum in November.The maximum raindrop evaporation rate and evaporation residual ratio in Guanshan District appeared in May.The precipitation and evaporation rates of Maiji in the river valley as well as Gangu and Qin'an stations in Weibei district are higher than those of other stations.?d and f are obviously affected by relative humidity,temperature,precipitation and raindrop diameter,and their correlations also fluctuate with the meteorological conditions.