Study On Soil Evaporation And Groundwater Recharge Based On Stable Isotopes In The Loess Plateau At Regional Scale

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(?¹⁸O,?²H,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 precipitation?¹⁸O 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 mean?¹⁸O 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 water?¹⁸O and mean annual precipitation.The shallow groundwater?¹⁸O ranged between-11.4‰and-5.9‰,which were poorly correlated the spatial and climate parameters.However,the mean?¹⁸O of 14 catchments varied obviously,showing a strong relationship between mean groundwater?¹⁸O 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.Based on the Rayleigh fractionation model,I derived a novel lc-excess method to calculate the fraction of soil evaporation loss.The calculated soil evaporation loss fraction(11.3?23.9%)lined between the range of global observations under the similar climate region,suggesting that our re-sults were reliable.Stable isotopes of deep soil water in these sites corresponded to a timescale of decades,implying that deep soil is a novel archive for long-term soil evaporation loss.3.The shallow groundwater?¹⁸O values in most region of the CLP were iso-topically depeleted than that of recipitation,and the depeletion degree whithin differ-ent catchments were correlated to the mean annual precipitation.These indicated that the seasonal effects of groundwater recharge were influenced by the mean annual pre-cipitation.Resultes showed that shallow groundwater originated from the local pre-cipitation but had experienced evaporation during the recharge processes.Shallow groundwater lc-excess were consistent different from that of deep soil water,indicat-ing that piston flow and preferential flow coexist.Based on the isotope mass balance,I derived a new lc-excess balance equation to estimate the relative proportion of pis-ton flow and preferential flow.The pistion flow is the predominant groundwater re-charge mechanism in most region of the CLP,whereas preferential flow account for a large proportion in some regions of the Wei and Fen rivers.The spatial distribution factors affecting the relative contribution ratio of piston flow and preferential flow need to be further analyzed.4.The stable isotopes of daily precipitation at Changwu tableland were highly variable and the isotopically depeleted precipitation mainly occurred in the rainy sea-son(July to September),which were often the heay rainfall with the intensity of?30mm day^-1.The tracers within the 2?6 m soil layers can be used to study the response of soil water balance to the land use change,whereas the tracers in deep soils(?6 m)reflect the balance of precipitation and evapotranspiration under the historical farm-land conditions,which is useful for studing the groundwater recharge mechanism.The multiple lines of evidence showed that shallow groundwater was recharged by pistion flow and preferential flow.The end-member analysis and Bayesian mixing model showed that piston flow accounts for 45?62%of the total recharge,while preferential flow accounts for 38?55%.In further,I found that after conversion of farmland to ap-ple orchards,soil evaporation did not significantly changed,whereas deep soil water storage and piston flow recharge rates were significantly decreased,and piston flow were zero when the stand ages of apple trees over than 15 years.This work provides basic isotope data for studying water cycle in this region,is helpful closing the soil water blance on the regional scale,provides basic to sustaina-ble use and management of water resources,and provides references for understand-ing the evolution of hydrological processes against the background of the vegetation cover change.