NON-DESTRUCTIVE ESTIMATION OF UNSATURATED HYDRAULIC CONDUCTIVITY USING TIME-LAPSE GEOPHYSICAL METHODS

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Accurate estimation of soil unsaturated hydraulic conductivity is essential for understanding vadose zone flow, groundwater recharge, contaminant transport, and broader subsurface hydrological processes. Conventional point-scale measurements are typically invasive, labor-intensive, and limited in spatial and temporal coverage, making them inadequate for capturing natural field heterogeneity. To address these limitations, non-destructive geophysical techniques offer a promising alternative for continuous, large-scale monitoring of soil moisture dynamics and hydraulic behavior.

Integrated Geophysical Framework

This study introduces an innovative framework that integrates time-lapse ground penetrating radar (GPR) and electrical resistivity tomography (ERT) with an improved instantaneous profile (IIP) inversion approach. By combining electromagnetic and electrical measurements, the method captures changes in soil moisture and pore-water distribution over time. This integrated approach enables the indirect estimation of hydraulic conductivity without disturbing the soil structure, providing high-resolution spatial and temporal insights into subsurface flow processes.

Validation with Laboratory-Based Models

To ensure reliability, the conductivity estimates obtained from geophysical data were validated against laboratory-derived soil water characteristic curve (SWCC) models, specifically the van Genuchten–Mualem (VGM) and Childs–Collis-George (CCG) formulations. These widely accepted models relate soil water retention properties to hydraulic conductivity. The comparison demonstrates that the proposed method can produce results consistent with established theoretical predictions, reinforcing its suitability for practical hydrological applications.

Constitutive Relationships with Electrical Properties

The research further developed parsimonious logarithmic constitutive models linking hydraulic conductivity to relative permittivity and bulk electrical conductivity for two soil types. These relationships leverage measurable geophysical properties to infer hydraulic behavior, enabling rapid estimation across large areas. By translating electrical and dielectric measurements into hydraulic parameters, the approach bridges geophysics and soil physics, facilitating non-invasive subsurface characterization.

Model Performance and Uncertainty Analysis

Predictive performance was evaluated using root mean square error (RMSE) and coefficient of variation (CV) metrics. The models achieved an overall RMSE of 0.32 mm/min, indicating strong agreement with laboratory references. Uncertainty analysis revealed low variability in intermediate moisture conditions, with CV values around 2.5–5.4%, while higher variability (7.1–8.6%) occurred near saturation. This pattern reflects increased complexity in fully saturated soils due to thin-film flow and surface conduction effects.

Implications and Future Research Directions

The findings demonstrate that time-lapse geophysical monitoring can provide spatially explicit, non-destructive estimates of unsaturated hydraulic conductivity, particularly in drained to partially saturated conditions. However, model confidence decreases near saturation, highlighting the need for further refinement. Future work should focus on field-scale validation, joint inversion of multiple geophysical datasets, and improved modeling of saturation effects to enhance transferability across diverse soil environments. This approach holds significant potential for advancing hydrological modeling, agricultural management, and environmental monitoring.



#VanGenuchtenModel
#EnvironmentalEngineering
#SoilPhysics
#GroundwaterResearch
#GeotechnicalEngineering
#FieldHydrology
#EarthScienceResearch
#WaterResources
#CivilEngineering


 

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