Quantifying diffusive groundwater recharge in chalk under arid conditions

Quantifying groundwater recharge (GR) is essential for conserving renewable groundwater resources. GR involves two primary processes: diffusive recharge and focused recharge. As aridity increases, focused recharge tends to become more prominent. However, diffusive recharge occurs over much larger areas, and its contribution to groundwater quantity and quality in many studies conducted in arid regions is frequently overlooked. This study aimed to quantify the GR diffusive component in chalk and in loess, which often overlays the chalk matrix, within an arid environment by testing and validating three models of varying complexity: the Richards equation, the dual porosity model, and the dual permeability model. Simulation results indicate that the chalk and loess materials exhibit dual porosity mechanisms, allowing for substantial diffusive recharge, which accounts for 2 to 19 % of annual precipitation. These findings were further validated against geochemical data collected from groundwater systems and the unsaturated zone. Additionally, the statistical factors in the rain that are related to GR were identified. The study reveals that GR is associated with high-intensity rainfall events in the loess material, while it is associated with both high-intensity and low-intensity rainfall events in the chalk. Overall, the results demonstrate that the preferential flow in both chalk and loess plays a similar role in replenishing groundwater under arid conditions. • Unsaturated loess and chalk in arid conditions exhibit dual porosity flow behavior. • Loess and chalk exhibit similar diffuse groundwater recharge rates, with 2–16% of the total rainfall in chalk and 3–19% in loess. • Diffuse groundwater recharge in loess and chalk in arid climates is associated with high-intensity rainfall events.