| Loess hilly region is located in the semi-arid and semi-humid region of northwest China,which is one of the most serious area of soil and water loss in China.In loess hilly regions,Precipitation and soil water are vital for agriculture and forestry,runoff and groundwater are the main water sources for local residents.In recent 20 years,vegetation types in the loess hilly region have changed significantly through ecological construction projects such as returning farmland to forest and grassland,which affects the dynamic transformation process of soil water.Therefore,it is of great significance for the efficient utilization of water resources in this area to understand the mechanism of soil water movement and clarify the transformation relationship of"four waters".This paper selects the surrounding areas of Dunshan Mountain in Ansai as the research object.Samples of precipitation,river water,groundwater and soil under four vegetation types（Setaria italica terrace,Cirsium setosum grassland,Hippophae rhamnoides shrubland,Robinia pseudoacacia woodland）were collected from May to October 2019.The soil moisture content and hydrogen and oxygen isotope content in each soil layer were determined,and the dynamic change law of soil moisture under different vegetation types was clarified,and the recharge and transformation relationship between soil water and other water bodies was revealed.It is expected to provide reference for ecological restoration and efficient utilization of water resources in the loess hilly region.The main conclusions are as follows:（1）The soil water content of the four planting cover types showed obvious time characteristics,with lag effect.The average soil water content at 0～300 cm depth showed as follows:Setaria italica terrace>Cirsium setosum grassland>Hippophae rhamnoides shrubland>Robinia pseudoacacia woodland.With the increase of soil depth,the seasonal variation coefficient of soil water content decreased gradually,and the seasonal variation coefficient tended to be stable when the soil depth was below 100 cm.The soil water content from the topsoil layer to the deep soil layer is in the shape of"S",and the water content first increases and then decreases,with obvious vertical change characteristics.The similarity of soil water changes showed as follows:Setaria italica terrace>Hippophae rhamnoides shrubland>Cirsium setosum grassland>Robinia pseudoacacia woodland.Robinia pseudoacacia woodland and Hippophae rhamnoides shrubland need to consume more deep soil water,which is prone to soil desiccation.The soil water condition of Setaria italica terrace and Cirsium setosum grassland was better,and the Setaria italica terrace had better regulation effect on soil water,and the soil water changed gently in the vertical direction.（2）The isotopic composition of soil water in the study area ranged from-39.34‰to-77.31‰（δD）and-3.19‰to-10.52‰(δ18O).TheδD andδ18O of soil water showed similar time variation characteristics,are presented for July>August>October.The maximum variation of hydrogen and oxygen stable isotopes of soil water is in the shallow layer of 0～100 cm soil depth,and the fluctuation of soil water content is also great in the shallow layer.In the process of water infiltration,piston flow and preferred flow coexist,and the occurrence of preferred flow is related to vegetation type.In addition,the difference of soil evaporation under the influence of vegetation was the main reason for the variation of hydrogen and oxygen isotopic composition of soil water in the four planting cover types.The evaporation fractionation degree of soil water in Setaria italica terrace,Hippophae rhamnoides shrubland and Cirsium setosum grassland was low,and the amount and speed of precipitation infiltration were larger and faster,and the soil water supply was better.The degree of evaporation fractionation in Robinia pseudoacacia woodland is high,and the precipitation in small precipitation events is easy to evaporate and not easy to infiltrate,and only when the precipitation is large can the soil water be replenished.The change of vegetation configuration（Setaria italica,Hippophae rhamnoides and Cirsium setosum are transformed into Robinia pseudoacacia）will make it more difficult for precipitation to replenish deep soil water and groundwater,which is not conducive to the sustainable utilization of water resources in the hilly region of the Loess Plateau.（3）The local meteoric water line wasδD=7.90δ18O+22.08（R2=0.98,n=9,p<0.01）,compared with global atmospheric precipitation line,small local meteoric water line slope,intercept slants big,reflect the local dry climate and local recirculation water vapor.River water,groundwater and soil water evaporation line respectivelyδD=4.63δ18O-12.19（R2=0.43,n=8,p<0.01）、δD=4.42δ18O-16.75（R2=0.77,n=31,p<0.01）、δD=4.68δ18O-24.23（R2=0.85,n=96,p<0.01）,the slope and intercept of river water,groundwater and soil water evaporation line was low,its point of isotope both in the local meteoric water line right description information of river water,groundwater,soil water supplied by the precipitation,And is subjected to strong non-equilibrium evaporation.（4）The hydrogen and oxygen isotopic characteristics of precipitation,soil water,river water and groundwater around Dunshan Mountain were analyzed by two-terminal element mixed model,and the transformation relationship of different water bodies was obtained.The proportion of precipitation and groundwater replenishment of river water was 43.90%and 56.10%,respectively.The ratio of soil water and river water to groundwater was 68.86%and 31.14%,respectively.Based on the analysis of all groundwater recharge sources by using the Iso Sources multi-mixed model,it was found that the groundwater recharge in the study area was closely related to 0～30 cm surface water and 200～300 cm deep soil water during July to October,and the water contribution rate was 19%and 21%,respectively. |
