Study region: The Bakhtegan-Tashk Lake (BTL) system in southern Iran has experienced severe desiccation. Prolonged drought, declining surface inflows, and intensive groundwater extraction have changed the hydrological regime and disrupted groundwater-surface water (GW-SW) interactions. Study focus: This study investigates the groundwater-surface water (GW-SW) interaction as a key factor influencing the desiccation of BTL. Hydrogeological assessments, water-budget analysis, and statistical cross-correlation were employed to evaluate the dynamics between lake water bodies and groundwater levels from 87 observation wells during 2009-2020. The lake water balance indicated that surface inflows from precipitation and rivers were nearly offset by evaporation losses, emphasizing the lake's dependency on groundwater inflows for its long-term sustainability. New hydrological insights for the region: Results revealed an annual groundwater outflow of approximately 10.5 MCM from the lake toward adjacent aquifers in recent years, signifying a reversal in the natural flow regime. Cross-correlation analysis confirmed temporal lags and weakened hydraulic connectivity between the lake and groundwater system, driven by over-exploitation and reduced recharge. These findings demonstrate that human-induced hydro-geological regime changes have disrupted natural GW-SW exchanges, accelerating lake desiccation. The study enhances understanding of groundwater-lake interactions in semi-arid environments and provides essential insights for improving GW-SW modeling and sustainable groundwater management in the BTL basin and similar regions.