Research On Water Cycle And Plant Water Sources In Shan Dian River Basin Based On Stable Isotopes

Shan Dian River Basin is a key area of desertification monitoring and control,and an important ecological function area in the Beijing-Tianjin-Hebei region.Taking Shan Dian River Basin（Inner Mongolia Section）as the research object,this thesis measuredδD andδ¹⁸O by collecting precipitation,soil water and plant xylem water during May-October of2020-2021 and surface water and groundwater during abundant（August）and dry（October）water periods of 2021.Based on the analysis of isotopic variation characteristics,The relationship between water isotope and environmental factors was discussed.The water vapor source was tracked by HYSPLIT model,and the water transformation relationship was revealed by hydrodynamics and isotope tracer principle.Direct contrast method and multi-source linear mixing model were used to identify water use strategies of different plants in response to seasonal changes,and the effects of roots on water use of vegetation were analyzed.The results are as follows:（1）Precipitation isotopes in the study area are enriched--depleted--enriched from May to October,enriched in summer（July-September）,and depleted in spring and autumn（May,June,October）,and varied widely in summer,with the overall variation characteristics of"high in summer and low in spring".The slope and intercept of the local precipitation line were both smaller than that of the global atmospheric precipitation line,and the water vapor mainly came from the Arctic Ocean and the Atlantic Ocean transported by the westerly belt,the Southeast Asian summer monsoon and the local evaporation water vapor in China.d-excess was high in spring and autumn,but low in summer,and there was obvious"dilution effect"in summer,and the precipitation isotope had significant temperature effect.（2）In terms of time,both surface water and groundwater isotopes are more enriched in dry season than in wet season,and d-excess values are lower than the global average,showing strong local evaporation.Spatially,theδ¹⁸O values of surface water have the same variation characteristics during the seasons of abundance and depletion.The evaporation intensity increases gradually with the widening of downstream water surface and the decreasing of discharge,and the hydrogen and oxygen isotopes are gradually enriched from upstream to downstream.Theδ¹⁸O high value area of groundwater is unevenly distributed,and theδ¹⁸O value becomes more depleted with the increase of buried depth.The slope of the underground waterline is very close to the local atmospheric precipitation line and surface water line,indicating that there is a close hydraulic relationship between the"three rivers"in the wet period.The trend of groundwater recharge is obtained by analyzing the groundwater flow pattern based on the contour map of groundwater level.Combined with the endmember mixing model,it is concluded that the surface water receives more than 70%of the precipitation recharge and less than 30%of the groundwater runoff recharge,and there is 25%evaporation loss in the recharge process.（3）The isotope enrichment degree of each water body is groundwater<xylem water<soil water<precipitation,and precipitation has the worst stability and greater variability.Theδ¹⁸O values and soil water content in the shallow layer of poplar and yellow willow forests changed significantly,but the changes in the middle and deep layers were small.The soil moisture content of 80～120 cm layer of yellow willow and 60～140 cm layer of poplar varied greatly under the influence of plant canopy width and different replenishment sources.Theδ¹⁸O values in shallow and middle soil water showed a more significant response to precipitation,in the order of before rain>the first day after rain>the second day after rain>the third day after rain,and the recharge to yellow willow was more significant in the middle degree of precipitation,and the water infiltration recharge to the middle level was more.Poplar mainly uses shallow soil water,followed by middle soil water,and also uses a small amount of deep soil water and groundwater.The water source of yellow willow is mainly in the shallow soil water,and it has a certain proportion of the middle soil water.The strong root system enables poplar to draw water from the deeper soil,while the shallower root system of yellow willow makes yellow willow concentrate water in the shallower layer.