Recharge zone of influence–based evaluation of groundwater recharge efficiency in a semi-arid hard-rock region

Groundwater recharge in semi-arid hard-rock areas mainly relies on surface water bodies like tanks and small reservoirs. These are key sources of recharge in regions that typically have low recharge rates. In many basins, only a few water bodies provide a significant portion of the total groundwater recharge. However, their actual effectiveness is often measured using surface indicators, such as storage capacity or area, without thoroughly examining how recharge moves through the subsurface. This study aims to fill this gap by assessing recharge efficiency through the identification of the recharge zone of influence (RZOI). The study took place in a 60 km² macro-watershed in a semi-arid area, which includes 14 significant water bodies that together account for nearly 60% of the total groundwater recharge. Understanding how recharge from these water bodies spreads within the aquifer is crucial because of their important role. A three-dimensional groundwater flow model was created using FEFLOW to simulate how the aquifer responds to recharge from the water bodies. The RZOI was defined as the subsurface volume calculated from the difference between groundwater head contours simulated with and without the influence of the water body recharge. Recharge efficiency was calculated as the ratio of the water volume in a water body to the corresponding RZOI volume. The findings reveal considerable differences in both RZOI extent and recharge efficiency throughout the study area. The largest water body, covering about 0.9 km², had a relatively large zone of influence extending over more than 2.5 km². Yet, its recharge efficiency was low, around 17–18%, mainly due to poor infiltration conditions. In contrast, a smaller water body showed the highest recharge efficiency, approximately 38–39%, due to a high infiltration capacity in the vadose zone, around 5–6 mm h⁻¹. Overall, the results suggest that recharge efficiency is mainly influenced by the infiltration capacity and hydraulic conductivity of the vadose zone, with the effect of surface area being relatively minor. The study emphasizes that larger water bodies do not always result in higher recharge efficiency and stresses the importance of assessing aquifers when planning and prioritizing recharge structures in semi-arid hard-rock regions.