Study On Typical Hydrological Characteristics Of The Vadose Zone And Spatiotemporal Evolution Of Potential Groundwater Recharge In The Chinese Loess Plateau

Water resources are the basis of human survival and development.Understanding the current situation and evolution mechanism of regional water resources is the premise of effective regulation and sustainable development.The Chinese Loess Plateau has effective water resources,fragile ecological environment,and is extremely sensitive to climate and environmental change.In recent years,with the deep implementation of the project and the rapid development of regional economy and society,the land use and vegetation cover of the Chinese Loess Plateau have changed significantly,which has curbed soil erosion and reduced soil erosion.However,it is also faced the“new problem"with excessive consumption of water resources in the vadose zone and of whether the regional water resources carrying capacity and vegetation restoration intensity are balanced and sustainable.Therefore,it is very important to fully understand the potential groundwater recharge evolution under the natural state,analyze the temporal and spatial response mechanisms of the potential groundwater recharge,and better guide the regional water resource system planning and the sustainable development of the ecological environment,and realize the high-quality development of the Loess Plateau in the new era.Based on the obvious zonal characteristics of the climate and soil in the Chinese Loess Plateau,a lot of investigations and samplings are carried out on the soil properties and hydrological information in the vadose zone,and atmospheric chloride input.The soil bulk density,hydrological process and atmospheric chloride input in the typical vadose zone of the Chinese Loess Plateau are studied in depth by using a variety of spatiotemporal data analysis methods and multiple hydrological tracing techniques.The chloride tracer technology is developed to reveal and reconstruct the abundant information of hydrology and climate in the vadose zone of different climatic regions of the Chinese Loess Plateau.In order to make up for the lack of high-resolution hydroclimatic records in the Chinese Loess Plateau for nearly one thousand years,this paper discusses the evolution process of potential groundwater recharge and its response to climate change in the time dimension,and analyzes the distribution characteristics and main control factors of potential groundwater recharge in the spatial dimension.The main conclusions are as follows:1.Characteristics of soil bulk density in typical vadose zone of the Chinese Loess Plateau:The distribution of soil bulk density in the loess vadose zone is similar in the horizontal direction of the region,which is basically not affected by land-use type and landscape location;but in the vertical direction,the variation of soil bulk density in the vadose zone is large,which mainly shows the logarithmic increase of large-scale?>30 m?and the fluctuation change of mesoscale?3–13 m?with the change of soil depth.Among them,the depth of soil layer is the dominant factor for the large-scale change of soil pressure in the vadose zone,and the magnetic susceptibility can better explain the relevant variation of medium-scale bulk density than soil texture and organic carbon parameters.It is found that the distribution characteristics and main influencing factors of soil bulk density in shallow and deep soil of the vadose zone are quite different.When estimating the reserves and fluxes of water,solute and other substances in the vadose zone or simulating the relevant circulation process,it should pay special attention to the value of soil bulk density parameters.Replacing the empirical value or predicted value of shallow soil bulk density may cause large errors in the research results.According to the vertical distribution of soil bulk density in vadose zone,it is mainly composed of"large-scale logarithmic growth"contributed by compaction and"mesoscale oscillation"controlled by soil texture pores.The optimal explanatory variables of compaction and texture pores are soil depth,and magnetic susceptibility and clay content,respectively.A multi-scale bulk density concept model is proposed.The model can explain the variation of soil bulk density about82%.Compared with the traditional pedo-transfer functions of soil bulk density,it can greatly improve the accuracy and accuracy of soil bulk density prediction,and it can support the estimation of soil water,solute,carbon and other material reserves and fluxes,and promote the soil The development of the research and application of hydrology and earth key zone is of great significance.2.Hydrological process of typical vadose zone in the Chinese Loess Plateau:The dynamic fractionation and non-uniform flow?preferential flow or macropore fast flow?infiltration of soil water in the loess vadose zone can only affect the shallow soil?0–2m?,while the deep soil water?>2m?is dominated by uniform inflow and is not affected by dynamic fractionation,which has a certain space-time stability.The rainfall in both wet and dry seasons can recharge the shallow soil water,while the deep soil water recharge mainly comes from the heavy rainfall in wet season.The mechanism of water movement in the deep loess vadose zone is dominated by piston flow,and the loess vadose zone is stratified and horizontally homogeneous,which makes the soil hydrological physical and chemical information have spatial timing and spatial consistency in the vertical and horizontal directions,respectively.It has great potential as an agent index of hydroclimate,and to reconstruct the local long-term potential groundwater recharge history?3.Assessment of atmospheric chloride input flux in the Chinese Loess Plateau:In order to enrich the assessment method of atmospheric chloride input,alleviate the shortage of the data of atmospheric chloride input in the Loess Plateau,and provide a new means for the assessment and research of the global atmospheric chloride input flux in the future.Based on the coupling of tritium and chloride profiles in vadose zone,the indirect estimation methods of ICMB and ICCA are proposed.Compared with the actual monitoring results,it is found that the reverse chloride accumulation age method has higher reliability and accuracy,while the reverse chloride mass balance method may be more suitable for the reverse estimation of atmospheric chloride input in farmland or polluted areas,but the selection of the average concentration of chloride ions in soil water should be paid attention to.The spatial distribution of atmospheric chloride input in the Chinese Loess Plateau shows a trend of"high in the southeast and northwest,low in the middle".Among them,the southeast is dominated by the East Asian monsoon,and the air chloride input flux is mainly controlled by the sea land distance,while the northwest inland chloride input source is complex,the sea land distance effect is not obvious,and the chloride concentration is negatively related to the precipitation,showing a strong leaching effect.4.Reconstruction of potential groundwater recharge history based on chloride tracer technique:The potential groundwater recharge series of Weinan,Changwu,Haiyuan and Jingtai vadose zones,which represent different climates of the Loess Plateau,have similar change trend with time,and both of them have significant positive correlation,indicating that the potential groundwater recharge in different regions of the Chinese Loess Plateau has the same change in time.Among them,in AD 1350–1400,1500–1600,1750–1800 and 1950–1995,the Chinese Loess Plateau showed a relatively wet period with high potential groundwater recharge,while in AD 1400–1500,1620,1740 and 1800–1950,the Chinese Loess Plateau was relatively dry with low potential groundwater recharge.The potential groundwater recharge history of the Chinese Loess Plateau is basically consistent with the change trend of tree ring and karst cave climate records nearby,which verifies the reliability of using chloride tracer to reconstruct regional hydrological and climate history.The feasibility of reconstructing the potential groundwater recharge history of the Chinese Loess Plateau by chloride tracer technique in the vadose zone.On the one hand,the variation trend of potential groundwater recharge series in different climatic regions?Weinan,Changwu,Haiyuan and Jingtai?is similar with time,which indicates that the potential groundwater recharge in different regions of the Chinese Loess Plateau is consistent with time.Specifically,the Chinese Loess Plateau showed relatively high potential groundwater recharge rate during ad AD1350–1400,1500–1600,1750–1800 and 1950–1995.In AD 1400–1500,1620,1740,1800–1950,the Chinese Loess Plateau was relatively dry,and the potential groundwater recharge was relatively low.On the other hand,compared with the climate records of tree rings and caves nearby,the trend of climate change was basically the same,which verified the reliability of reconstructing regional hydrological and climate history by using chloride tracer in the loess vadose zone.The performance of chloride tracer technology in vadose zone to reconstruct the potential groundwater recharge history.Compared with the arid desert area,higher water flux and atmospheric chloride input make the vadose zone more sensitive to the alternation of dry and wet climate in short time scale and stronger in recording ability.On the one hand,in arid areas?Badain Jilin desert and Jingtai?,the chloride signals of 50 years and 43 years have been accumulated in the shallow vadose zone?0-2 m?,respectively,while the chlorde signals of 12years and 14 years have been accumulated in the semi humid areas?Weinan and Changwu?.This shows that the vadose zone with large water flux in the semi humid areas is helpful for the short-term humid climate signals to enter the deep drainage area quickly,and is more conducive to the recording and reconstruction of the high-resolution hydroclimatic evolution information.On the other hand,it can be seen from Cook model simulation that the storage capacity of vadose zone for hydroclimatic events depends on two factors:soil water content and water flux.The higher water flux is helpful to the better separation of"climate events",which makes it difficult to mix and smooth the signals of adjacent climate events due to dispersion and diffusion,so that they can be kept for a long time.5.Spatiotemporal evolution of potential groundwater recharge in the Chinese Loess Plateau:The temporal and spatial evolution analysis of potential groundwater recharge in the Chinese Loess Plateau.In the time dimension,the potential groundwater recharge history of the Chinese Loess Plateau in the past 1000 years is significantly positively correlated with the solar radiation?activity?and the Northern Hemisphere temperature anomaly indicators,which represent the monsoon climate change.With the global change?Asian monsoon?,it shows that:the corresponding relationship between warm and humid climate,high potential groundwater recharge,dry and cold climate,low potential groundwater recharge rate and their alternating fluctuations.This indicates that the change of East Asian monsoon intensity may be the main driving force for the change of potential groundwater recharge time in the Chinese Loess Plateau.In spatial dimension,from southeast to northwest of the Loess Plateau,the distributionof potential groundwater recharge fluxes in different climatic regions in recent 200 years is3.9±2.6 mm yr^-1,15.1±9.5 mm yr^-1,25.5±15.6 mm yr^-1 and 72.0±29.1 mm yr^-1 has obvious zonal characteristics.Among them,heavy rainfall?>20 mm?and high rainfall?>30 mm?can explain the spatial variation of 82%and 91%of the potential groundwater recharge in the Loess Plateau,which is the most important control factor of the potential groundwater recharge in the Loess Plateau,while the shallow soil texture?clay content?controls the contribution of the precipitation event of<20 mm to the potential groundwater recharge in the Loess Plateau.This study not only deepens our understanding of the basic characteristics of soil and hydrology in the deep vadose zone,clarifies the information of chlorine cycle driven by the hydrological process,develops the application potential of chlorine tracer technology in the long-term and high-resolution potential groundwater recharge process in the Chinese Loess Plateau,but also promotes the analysis of the temporal and spatial evolution of water resources recharge in the Chinese Loess Plateau.Enhanced the systematic understanding of the evolution law of terrestrial water resources recharge in the Chinese Loess Plateau under the background of global change,which provides important technical support,data reference and scientific guidance for the prediction and sustainable development of terrestrial water resources in the future.This is of great practical significance to the construction of ecological civilization on the Chinese Loess Plateau,the ecological protection and high-quality development of the Yellow River Basin.