Characterization Of Soil Water Infiltration In The Shallow Subsurface Based On GPR And ERT

Soil infiltration plays a pivotal role in the hydrological and energy cycle in Earth’s Critical Zone,influencing phenomena such as surface runoff,slope runoff,plant water availability,and groundwater recharge.However,challenges remain in monitoring and characterization of this highly dynamic process.The application of geophysical methods can achieve non-invasive(or minimally invasive)continuous monitoring of soil infiltration at multiple spatial and temporal scales.Yet most of the existing studies still use a single geophysical method for qualitative investigation,resulting in limited hydrodynamic information,and consequently lacking the ability to fully characterize the transient process of soil water infiltration.This study presents an integrated geophysical approach to characterize water infiltration process in the shallow vadose zone.The main findings and conclusions are summarized as follows.(1)A physical model of soil vadose zone was constructed to simulate water infiltration test at the lab scale,petrophysical models linking soil water content to geophysical parameters were established based on multiple regression analysis by combining time-lapse electric resistivity tomography(ERT),ground penetrating radar(GPR)and direct hydrogeological measurement.The results showed that: there was a good power law correlation between the resistivity and water content of the subsoils in the study area.The relationship between dielectric constant and water content of sandy clay is cubic spline,while for sandy clay,a polynomial relation considering the influence of dry bulk density and clay content of soil matrix was more suitable.(2)By integrating GPR,ERT and probe sensing method,the spatial-temporal evolution of soil water content caused by infiltration event were revealed,and the influence of multiple environmental factors was analyzed.The results showed that the moisture sensors could monitor the vertical migration of water with a relatively high temporal resolution whilst providing prior information for geophysical data inversion.While time lapse GPR method clearly captured the change of water content in different infiltration stage,time lapse ERT profiled the water infiltration path and pattern within soils with high spatial-temporal resolution.Through mutal complementation and verification,the proposed integrated geophysical method could achieve dynamic monitoring and accurate characterization of water infiltration within shallow vadose zone.(3)Based on high resolution geophysical time series data,the hydraulic conductivity of unsaturated soil was solved by an improved transient profile method,and the relation between the unsaturated hydraulic conductivity and geophysical parameters was estimated.The results showed that the hydraulic conductivity function could be quickly and accurately computed by the improved inversion algorithm,thus resolving the drawbacks of conventional testing methods which are time consuming,laborious and under-representative.The innovative constitutive model between unsaturated hydraulic conductivity and geophysical parameters was meaningful in hydrodynamic investigation of large-scale heterogeneous soil at field scale.(4)Based on ERT and 3D GPR method,high-resolution inversion of spatial structure and geophysical parameters of heterogeneous soil was achieved to map the infiltration related response in the shallow vadose zone in Sponge city.The results showed that the spatial-temporal distribution and variation of soil water content in shallow urban areas were closely related to the structure and permeability of urban surface and underlying soils.Based on the observed changes in critical hydrological parameters,a conceptual model describing the subsurface hydrodynamics of Sponge city was established,further confirming that the construction of sponge cities can enhance the "water capacity" of urban pedosphere and improve the quality of urban water environment.