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干旱区盆地排泄区地下水与土壤水动态的控制机理

Control Mechanism of Groundwater and Soil Water Dynamics in Discharge Areas in Arid Region

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【Author in Chinese】 赵可玉

【Supervisor】 万力蒋小伟

【Author's Information】 中国地质大学(北京), 水文地质学, 2020, 博士

【Abstract in Chinese】 潜水面作为包气带和饱水带的分界面,其动态变化受控于陆面过程,同时也对包气带水分运移及深部地下水多尺度循环具有控制作用。在地下水排泄区,由于埋深浅,浅层地下水与大气条件通过包气带耦合密切,而深层地下水受到补给区潜水面动态控制。本研究选取鄂尔多斯盆地乌杜淖小流域,在补给区和排泄区建立多组地下水、土壤水监测点,在多年野外监测数据的基础上,讨论干旱-半干旱气候下排泄区内非饱和带和饱和带不同深度的水动力要素变化的形成机理。传统盆地尺度区域地下水模型以多年平均水位作为上部边界,基于补给区波动较排泄区波动强烈的野外现象,本研究将上边界改进为潜水面季节性波动,推导Toth盆地在季节性水位下的非稳定流解析解,刻画排泄区深部地下水的动态特征。结果表明其流场在非稳定流条件下发生变形,内部驻点呈现环形运动轨迹。排泄区地下水波动受补给区潜水位动态控制,水头振幅随着深度增大而增加,排泄区发育的自流区也随潜水面下降而自流能力减弱。同时,补给区与排泄区分布范围也呈现季节性变化。在研究潜水面动态对于深部地下水流系统的影响之后,本论文进一步结合野外土壤水动态数据,对比分析排泄区不同埋深处土壤剖面垂向含水量对于降水的响应时间特征,出现土壤水未入渗到达潜水面,但潜水面已抬升的季节性现象。利用变饱和度模型FEFLOW建立二维土壤水-地下水模型,模拟得到在单次降水理论上边界下,降雨入渗在浅埋深区优先形成水丘,水丘横向运移过程则成为本区深部土壤水波动的来源之一,并且造成不同埋深处含水量波动幅度垂向变化模式的差异性。基于本研究区排泄区内深层土壤水动态受水丘横向运动控制的认识,利用HYDRUS-1D建立一维大气边界-波动水位实例模型,分析了潜水面波动对于蒸发与补给通量的抑制作用。针对野外出现的土壤水双向衰减现象,利用理论模型分别对水位埋深、气候条件、土壤质地和植被分布等因素进行敏感性分析,验证了该现象的普遍性。本研究加深了对干旱条件下排泄区地下水和土壤水动态特征、形成原因的认识。

【Abstract】 As the interface between the vadose zone and the saturated zone,the dynamics of water table is controlled by the land surface process,and it also controls the water movement in the vodase zone and the multi-scale groundwater flow system in deep.In groundwater discharge areas,groundwater in the shallow part is closely coupled with atmospheric conditions through the vodase zone,while groundwater in deep part is controlled by the fluctuating waterlevel in recharge area.In this study,the Wudu lake watershed in Ordos basin was selected,and several groundwater and soil water monitoring points were established in the recharge and discharge areas.Based on the mutli-year field monitoring data,under the arid and semiarid climate the formation mechanism of unsaturated and saturated zones including multi-depth hydrodynamic features in the discharge area was discussed.The traditional Tothian basin uses the multi-year average water level as its upper boundary.Based on the field phenomenon that the fluctuation of recharge area is more intense than that of discharge area,this study improves the upper boundary to seasonally fluctuating groundwater table in recharge area,deduces the analytical solution of seasonal variation of unsteady flow,and depicts the dynamic characteristics of deep groundwater in discharge area.The results show that the unsteady flow field will be distorted,and the stagnation points moves in a circular motion.The groundwater fluctuation in the discharge area is controlled by the dynamic groundwater level in the recharge area.The amplitude of hydraulic head increases with depth,and the artesian phenomenon in the discharge area decreases or even disappears when the groundwater level in the recharge area decreases.Meanwhile,the distribution of recharge and discharge area changes seasonally.After investigating the effect of the watertable dymanics on the groundwater flow system,this study further combines the soil water dynamic data to compare and analyze the vertical water content response time characteristics of soil profiles at different depths in the discharge area.There is a seasonal phenomenon that soil water does not infiltrate into the watertable,however,the watertable has been raised in the field.By using FEFLOW,the two-dimensional soil water-groundwater model simulates that,under the influence of rainfall events,infiltration forms a groundwater mound in shallow depth preferentially.The groundwater mound transverse transfer process becomes one of the sources of deep soil water fluctuations in this area,and the vertical variation pattern of water content fluctuation amplitude is different at different soil profiles.Based on the lateral movement of water mounds controlling the deep soil water dynamics in the discharge area of this study area,a one-dimensional atmospheric boundary-fluctuating water level model was established by using HYDRUS-1D to analyze the influence of flucatuating water table on evaporation and recharge fluxes.Based on the sensitivity analysis in theoretical models of water table depth,climatic conditions and soil texture and vegetation distribution,the universality of the two-directional damping of soil water content in the field is verified.This research enhances the understanding of formation and characteristics of groundwater and soil water dynamics in the arid region.

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