Influence of time-space variability of rainfall infiltration recharge on fresh groundwater lens

The freshwater lens (FGL) represents a critical subsurface freshwater resource on islands, primarily recharged through rainfall infiltration and artificial recharge mechanisms. While existing research has predominantly examined the influence of rainfall infiltration rates on FGL dynamics, less attention has been given to the substantial spatiotemporal variability inherent in tropical rainfall patterns. Disregarding these spatiotemporal distribution characteristics may lead to an incomplete or inaccurate understanding of the freshwater lens's dynamic behavior. In this study, utilizing field survey data from Yongxing Island, a two-dimensional numerical model of the island's FGL was developed to investigate the effects of spatiotemporal variability in rainfall infiltration recharge on the lens's volume and thickness. The findings reveal that neglecting temporal variability in rainfall infiltration results in an overestimation of freshwater storage. Under a fixed infiltration scenario (based on averaged annual rainfall), the proportion of freshwater exhibited a consistent increase or decrease throughout the year. In contrast, under a monthly variable infiltration scenario, the freshwater proportion fluctuated in alignment with precipitation variations. During the rainy season, the freshwater proportion surpassed that observed under constant rainfall infiltration conditions. Furthermore, artificial recharge at the island's center significantly enhanced both groundwater levels and freshwater storage, with the peak hydraulic head increasing by 24.39% compared to the average infiltration scenario. This study highlights the effects of precipitation variability and artificial recharge on the FGL, offering valuable theoretical insights for the sustainable development and management of freshwater resources on islands.