Characteristics of Hydrogen and Oxygen Isotopes in Different Waters of the Loess Hilly and Gully Region
In order to study the characteristics of hydrogen and oxygen isotopes in precipitation,surface water and soil water of the loess hilly and gully region systematically,the Zhifanggou watershed,which is located in Ansai County,Yan'an City of Shaanxi Province,was selected to be the research object.Precipitation samples,surface water samples and soil samples in locust forest and waste grassland were collected from March to September of 2015 to test the compositions of the hydrogen and oxygen isotopes.By analyzing the seasonal variations of hydrogen and oxygen isotopes and the relationships between δD and δ18O in the precipitation,surface water and soil water of Zhifanggou watershed and their seasonal changes,the effects of soil depth,vegetation type,and slope direction and position on characteristics of hydrogen and oxygen isotopes in soil water were explored.The results showed that the local meteoric water line equation was δD=6.71δ18 O-3.22 (n=18,R2 =0.96),the evaporation line equation of the surface water was δD=6.77δ18O-5.32 (n=13,R2 =0.72),and the evaporation line equation of the soil water was δD=3.50δ18 O-34.00 (n=756,R2=0.76).δ18O in different water listed in the order of soil water (locust forest)≈soil water (waste grassland)＞precipitation＞surface water.The seasonal variations of δ18O in precipitation and soil water were obvious,while that in the surface water was not obvious with small variable amplitude and stable composition.It can be concluded that the average transmission time of soil water in the 0-50 cm soil layer was about a month,and the surface water came from a blend of multiple historical precipitations.Finally,the influence degrees of underlying surface factors (soil depth,vegetation type,slope direction and position) on the hydrogen and oxygen isotopes in soil water were calculated,which listed in the order of soil depth＞ slope direction＞vegetation type＞slope position.The influence of soil depth on δ18O was extremely significant in the surface layer,and the difference of δ18O between different layers increased with the difference of soil depth.The difference of δ18O tended to be very significant when the difference of soil depth was greater than 15 cm.Therefore,the 0-15 cm soil layer was the active zone of soil moisture,which was mainly caused by precipitation and soil moisture evaporation.
loess plateauZhifanggou watershedhydrogen and oxygen stable isotopesprecipitationsurface watersoil water
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