The Spatial-temporal Variation And Influence Factors Of Soil Evaporation In The Southern And Northern Mountains Of Lanzhou City Based On Hydrogen And Oxygen Stable Isotopic

The hydrogen and oxygen stable isotopes(²H and ¹⁸O)as the components of water molecules,which are the natural tracers to describe the water cycle process.Precipitation infiltration recharges the soil and translated into soil water,soil water is continuous changing due to the influence of precipitation recharge and soil evaporation,and soil evaporation is the main output of the regional water cycle process.The southern and northern two mountains of Lanzhou as the study area of this paper,the forest and grass coverage are low,vegetation sparse,ecological environment is fragile,and environmental conditions differentiation is significant.It is of great significance for improve ecological and environmental quality and promote urban construction to southern and northern two mountains greening projects.The primary contradiction is the problem of water during the greening process.In this study,six sampling points in the southern and northern two mountains of Lanzhou City were selected based on month scale.From April to October 2018?plant growth season?,precipitation,river water,soil water,and irrigation water samples were collected at each sampling point.And the Craig-Gordon model was used to calculate soil evaporation intensity at different sampling points.The following conclusions are obtained by research:?1?From April to October 2018,the local meteoric water line LMWL of Lanzhou City is that:?²H=7.04?¹⁸O+3.47?r²=0.93,p<0.01,n=182?.The slope is smaller than GMWL,indicating that the lower relative humidity,and raindrops are influenced by sub-cloud secondary evaporation;the annual variance of hydrogen and oxygen isotopic composition of river water are very small,indicating that the river water is relative stable than the precipitation and the soil water;as the increases of soil depth,soil water?²H and?¹⁸O shows regularly depleted,from shallow layer 0-10 cm to deep layer 60-120 cm,the slope of the soil water line SWL shows regularly increase.The smaller the water line slope,indicating that kinetics fractionation is stronger.that is,with the increase of soil depth,the evaporation fractionation gradually weaker.?2?On temporal,soil water exists weaker evaporation fractionation in April,May,September and October,while it was more stronger from June to August.Combined with the annual variance of the slope of the local evaporation lines(S_LEL),which is larger in April,indicated that the soil evaporation is smaller.from April to June,the S_LELEL began decreases,and the soil evaporation increases,from June to August,the S_LELEL tend to stabilizes,and the soil evaporation is most stronger in July.the S_LELEL increase since August,soil evaporation begun to decrease,and has been decreased to October;spatially,from April to October,the evaporation losses f of Baoli,Dashagou and the western of airport in Northern mountain is significantly higher than the evaporation losses f of Lanlian,Renjiazhuang,and Dawa mountains in Southern mountain,which indicating that the soil evaporation in northern mountain is more stronger than that in southern mountain.?3?The annual variance trend of evaporation losses f in Lanzhou is consistent with the air temperature,which variation with the temperature variation,the altitude higher,the smaller the evaporation losses f;from the perspective of the entire plant growing season,the soil evaporation variations with relative humidity.Moreover,soil evaporation is related with soil water content,the larger of soil water content,the greater of the evaporable amount of water in soil,and the stronger of soil evaporation.?4?There is a significant hysteresis that the time of the evaporation losses f reaches the peak value and the valley value compared with the precipitation?¹⁸O.That is,precipitation infiltration into the soil matrix,and during the migration from the surface layer to the deep layer,there will be residence in the soil matrix,which stays to the next month even longer.During the single precipitation event,on the first day after precipitation,the soil water tend to saturation,soil water content is maximum,the slope of soil water line is minimum,and soil evaporation is the strongest.As time goes on,soil water content decrease,the slope of soil water line remarkable increase from the first day to the third day after precipitation,soil evaporation rapidly weaken.The soil capillary water was completely broken to the seventh day after precipitation,and the soil water content was the minimum,the soil evaporation also became the minimum and tended to be stable at this time.The above phenomena reflected in the variations process of soil water isotope is that soil evaporation variations with the variations of soil water isotope,showing a positive correlation.