Dynamic Change Characteristics Of Stable Hydrogen And Oxygen Isotopes In Soil Water At The North And South Mountains Of Lanzhou City And Its Response To Precipitation

Soil water is the most important and complex part of water resources and plays a vital role in the ecosystem.As an important external supply source of soil water,precipitation not only affects the migration process of soil water,but also determines the dynamic change characteristics of stable hydrogen and oxygen isotopes in soil water.Precipitation infiltration is the main controlling factor affecting the change of isotopic composition of soil water during precipitation.Based on the measured data of stable isotopes of soil water,oxygen,hydrogen.And oxygen and hydrogen stable isotopes of precipitation at six sampling sites in the north and south mountains of Lanzhou city from April to October,2018.This paper studied the dynamic variation characteristics of stable isotopes of soil water,hydrogen and oxygen in the north and south mountains of Lanzhou.Characteristics of stable isotopic changes of soil water,hydrogen and oxygen and precipitation contribution rate in the process of primary precipitation infiltration.And the influence on the dynamic change characteristics of stable isotopes in soil water,oxygen and hydrogen during the period from April to October.The main conclusions were as follows:?1?The local atmospheric waterline of Lanzhou city was?²H=7.04?¹⁸O+3.47?R²=0.93,P<0.01?,the mean value was-40.80‰and-6.29‰,and the standard deviation was 24.99‰and 3.43‰,respectively.The waterline in the soil water was?²H=3.70?¹⁸O-29.46?R²=0.85,P<0.01?,and its mean value and standard deviation were-43.40‰and 17.96‰,-3.75‰and 4.40‰,respectively.Most of the?²H and?¹⁸O values of soil water were below the hydrogen and oxygen isotope values of precipitation,indicating that soil water is greatly affected by atmospheric precipitation.From April to June,stable hydrogen and oxygen isotopes of meteoric precipitation in Lanzhou city were relatively enriched,while from July to September,they were obviously depleted.d-excess fluctuates little with time and shows a trend opposite to the stable isotope value of precipitation hydrogen and oxygen.The stable isotope value of soil water,hydrogen and oxygen fluctuates less with the depth,while the d-excess value increases with the depth.?2?The stable isotopes of soil water,hydrogen and oxygen in each sampling point have the same time-varying characteristics as the precipitation isotopes.The stable isotopes of soil water,hydrogen and oxygen vary with the depth of each sampling site,but they all decrease with the depth.Each sampling point of soil water hydrogen and oxygen stable isotopic differences vary with depth,the?¹⁸O value and the?²H of soil water accumulation degree was sample N3>S3>S2>S1>N1>N2,The d-excess value was sample N2>S1>S2>N1>S3>N3.The degree of soil water enrichment was different in different seasons.?3?A precipitation process in the different time due to rainfall infiltration hydrogen and oxygen stable isotopic values in different soil water,the first day after the precipitation infiltration velocity of 16.67 mm/h,then gradually decreases.And soil moisture content has obvious change in different times and different depth,there may be preferential flow,through preferential flow and rainfall infiltration model of piston flow supply of soil water in different ways.Within different number of days after precipitation precipitation a contribution rate changes significantly,about 10 to 20 cm in soil water precipitation on the first day and on the third day after precipitation to its contribution rate was highest.30-40 cm place on the third day after precipitation precipitation to its contribution rate was highest.50-90 cm is the single rainfall different times contribution rate is higher in the process of soil layer,and 110-120 cm rainfall contribution rate higher time was on the third day after the occurrence of precipitation.The effective contribution time of precipitation contribution rate was different with different depths.?4?The contribution of precipitation in different periods to the stable isotope changes of soil water,hydrogen and oxygen was different.The farther away from the sampling time,the higher the contribution rate was to the deep soil water,whereas the higher the contribution rate was to the shallow soil water,especially 10-30 cm.The longer the distance sampling time interval,the shorter the retention time of the effective influence precipitation.On the contrary,the longer the retention time of the effective contribution precipitation.The variation of precipitation contribution rate in different periods also affects the dynamic change characteristics of hydrogen and oxygen stable isotopes in soil water.The factors that directly affect the contribution rate of precipitation were precipitation and evaporation.The difference of precipitation infiltration and the degree of evaporation determine the contribution rate of precipitation,while vegetation coverage,soil texture,root distribution,soil porosity,ground temperature,air temperature,relative humidity and wind speed affect the change of determining factors.