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Northwest A&F University (2021)

Study on Soil Evaporation and Groundwater Recharge Based on Stable Isotopes in the Loess Plateau at Regional Scale

向伟

Titre : Study on Soil Evaporation and Groundwater Recharge Based on Stable Isotopes in the Loess Plateau at Regional Scale

Auteur : 向伟

Grade : Doctoral Dissertation 2021

Université : Northwest A&F University

Résumé partiel
Characterized by a typical arid and semi-arid climate,the Chinese Loess Plateau(CLP)is extremely lacking of water resources,which is futher intensified by the im-plementation of a series of ecological projects,especially the“Grain for Green”pro-ject.Therefore,it is urgently to understand the hydrological processes of the CLP for relizing the sustainable use and management of water resources.The hydrological process of the CLP has an unique spatial pattern due to the comprehensive effects of precipitation,tography,soil,vegetation,and anthropokinetics.To the beset of our knowledge,previous studies on the hydrological processes mainly focused on precip-itation,surface runoff,and soil water content,or were carried out at the plot and wa-tershed scales ;however,soil evaporation,groundwater recharge,and transpiration were poorly understood,especially at the regional scale.Therefore,this study mainly focused on soil evaporation and groundwater recharge,based on the advantages of the stable isotope technogy.At the regional scale,I analysed the spatial characteristics of stable isotopes in precipitation,deep soil water(2~10 m),and shallow groundwater.I revealed the spatial distribution pattern of soil evaporation,and evaluated the fraction of soil evaporation to precipitation using the presented isotope-based method.I inden-tified the recharge sources of shallow groundwater,and quantitatively evaluated the relative contribution of piston flow and preferential flow.At the plot scale(in Changwu tableland of the CLP),I quantitatively evaluated the hydrolopgical process-es and the response of land use change using multiple tracer technology(δ18O,δ2H,3H,Cl-,and NO3-),combined with water blance,end-member analysis,and Bayesian mixing model,and paired experiment.The main conclusions are as follows:1.The precipitationδ18O varied from-9.8‰to-5.8‰,showing a spatial pat-tern that precipitation enriched heavy isotopes in the south-east and north-west region while isotopicaly depeletd in the middle region.I found that only the Online Isotopes in Precipitation Calculator(OIPC)can simulate the spatial pattern of precipitation isotope at the CLP ;however,the precision of isotope data obtained from OIPC should be improved.The meanδ18O of deep soil water varied from-10.1‰to-6.7‰,dis-playing an obviously spatial pattern that isotopically depeleted in the south but iso-topically enriched in the west and north regions.There is a strong relationship be-tween deep soil waterδ18O and mean annual precipitation.The shallow groundwaterδ18O ranged between-11.4‰and-5.9‰,which were poorly correlated the spatial and climate parameters.However,the meanδ18O of 14 catchments varied obviously,showing a strong relationship between mean groundwaterδ18O and mean annual pre-cipitation.2.The deep soil water line-conditioned excess(lc-excess)from 33 sites of CLP were consistently smaller than that of precipitation,indicating that deep soil water re-tained isotope fractionation signal due to soil evaporation at land surface.Deep soil water lc-excess decreased from the south to north and from south-east to north-west,and were correlated to the latitude,mean annual precipitation,potential evapotranspi-ration and dryness index.I also found that 64%of the variance of deep soil water lc-excess can be explained by the dryness index

Mots clés : water cycle ; stable isotope ; soil evaporation ; groundwater recharge ; pattern ;

Présentation (CNKI)

Page publiée le 12 novembre 2021