Assessment of Groundwater Recharge Using Geophysical Methods in Semi-Arid Regions

Thesis Abstract

Semi-arid regions face acute water scarcity largely driven by complex interactions between climatic variability, land-use changes, and inadequate groundwater management. Groundwater recharge processes in these areas remain poorly characterized due to the typically scarce and heterogeneous subsurface data, which impairs sustainable water resource management. This study aims to quantify and spatially delineate groundwater recharge in semi-arid settings through integrated geophysical methods, thereby providing essential information for effective aquifer management and sustainable development. The specific objectives include (1) to delineate subsurface lithology and stratigraphy using electrical resistivity tomography (ERT) and seismic refraction surveys; (2) to estimate the true aquifer recharge rates employing geophysical-derived hydrogeological parameters and comparing them with empirical recharge models; and (3) to evaluate the influence of climatic variables and land-use changes on recharge variability within the study area. The research adopts an exploratory mixed-methods design focusing on field-based geophysical investigations complemented by meteorological and land-use data analysis. The study area encompasses approximately 150 square kilometers within a semi-arid basin characterized by low and sporadic precipitation and extensive ephemeral rivers. A stratified random sampling approach was employed to select twelve representative sites for geophysical surveys, ensuring coverage across different land-use zones and topographical features. Data collection involved high-resolution vertical electrical sounding (VES), 2D ERT profiles, and seismic refraction surveys, administered with portable resistivity meters and seismic equipment adhering to standard geophysical protocols. Climatic data were obtained from local meteorological stations spanning the past decade, while land-use information derived from satellite imagery analyzed via GIS tools. Instrument validity was ensured through calibration in controlled environments, and measurement reliability was tested via repeat surveys at selected sites. Analytical procedures included interpreting geophysical data through petrophysical inversion models, integrating the results into hydraulic conductivity and porosity estimations, and applying multiple regression analysis to relate recharge estimates to climatic and land-use variables. A conceptual model of recharge processes was developed based on the data, grounded in the Water Balance Theory and the Soil Water-Plant-Atmosphere Continuum framework. Expected findings indicate significant heterogeneity in subsurface lithology influencing recharge pathways, with zones of high permeability correlating with increased recharge rates, particularly in areas undergoing land-use change such as deforestation and urbanization. The geophysical-derived recharge estimates are anticipated to align closely with or refine existing empirical models like the Chloride Mass Balance method, providing a more precise spatial distribution of recharge rates. Climatic variability, notably annual precipitation and evapotranspiration rates, alongside land-use alterations, are projected to emerge as dominant factors modulating recharge dynamics. This research contributes novel insights into the application of integrated geophysical techniques for groundwater assessment in semi-arid ecosystems, bridging the knowledge gap between geophysical parametric data and hydrological models. The findings are expected to enhance the understanding of subsurface heterogeneity’s impact on recharge processes, informing more accurate groundwater management strategies tailored to semi-arid regions. The study also offers a methodological framework that can be adapted for similar settings elsewhere, advancing interdisciplinary approaches in hydrogeophysics. The main conclusion underscores the critical role of spatially detailed geophysical surveys in evaluating recharge, especially in data-scarce environments. Recommendations include adopting regular geophysical monitoring to capture temporal changes, integrating recharge assessments into water resource planning, and implementing land-use policies that mitigate adverse impacts on recharge. Future research should explore temporal variations using time-lapse geophysical methods and expand the scope to include additional hydrogeological factors such as aquifer trapping and contamination pathways, with the ultimate goal of fostering sustainable groundwater management in semi-arid zones.