Estimating the Seasonal Variations of Multiple Recharge Sources’ Contribution in Mountainous Mediterranean Basins

ABSTRACT Groundwater in mountain–valley systems plays a crucial role in sustaining river baseflows, irrigation, and water security, yet the mechanisms and seasonal variability of aquifer recharge remain poorly quantified. This study investigates the seasonal variability of groundwater recharge sources in the Ñuble–Perquilauquén basins of Central Chile, a representative Mediterranean mountain system. A process‐based hydrological model (SWAT) was complemented with stable isotope analysis (δ 18 O, δ 2 H) and a Bayesian mixing model (MixSIAR) to quantify the relative and absolute contributions of three recharge mechanisms: diffuse, focused, and mountain‐front recharge (MFR) derived from Cordilleran precipitation, springs and snowmelt. Results indicate that total recharge to the lowland alluvial aquifer varies seasonally from ~8.5 to ~27.2 mm/month in the dry and the rainy seasons, respectively. Diffuse precipitation‐driven recharge dominates in winter (45%–61%), focused recharge prevails in summer (60%–65%), and MFR provides a stable 11%–17% contribution throughout the year. Irrigation return flows contribute approximately 8% of total diffuse recharge in the valley zone, partially sustaining aquifer replenishment during dry months. The combined SWAT–MixSIAR framework reveals that the Andes Mountains act as a perennial source of groundwater from mountain‐front and, potentially, mountain‐block processes. These findings underscore not only the role of mountainous areas as a water reservoir for lowland aquifers, but also the relevance of integrating different approaches when estimating groundwater recharge.