Groundwater and surface water form a complex and interconnected system influenced by geomorphology, geology, hydrology, and climate factors. Human activities, particularly intensive groundwater exploitation, add further complexity to these systems, especially in arid regions. Understanding groundwater-surface water interactions is essential for effective management and conservation. Maar lakes are unique surface bodies specifically connected to groundwater flow systems. In this study, we developed a conceptual model of the groundwater-surface water interaction of a semi-arid maar lake over space and time. Our objective was to understand the changes in the system and the relative influence of external forcing factors. The study focuses on Lake Alchichica, located in a semi-arid, endorheic basin in central Mexico, which has experienced a decline in water levels over recent decades. We employed a multiproxy methodology that includes the characterization of space-time variability in effective precipitation, regional hydrogeochemical patterns, isotopic and physicochemical patterns within the lake, dynamic changes in phreatic levels, and an overall assessment of anthropogenic pressures relative to natural variability. The hydrogeochemical patterns indicate that the groundwater flow path feeding Lake Alchichica is part of the regional groundwater flow system. This shows hydrogeochemical evolution from the Sierra Madre Oriental to the regional base level at playa lake Tepeyahualco. Isotopic composition and hydrochemical patterns near the lake provide evidence of groundwater-lake interactions, with increasing concentrations of various ions along the flow path, likely due to intense evaporation within the lake. From 2017 to 2021, phreatic levels declined by approximately 37.8 cm, indicating a decrease in the total volume of groundwater entering the lake, which affects the lake's water balance and level. This decline was not directly attributed to meteorological changes, suggesting alterations in the regional system. Although no significant changes in groundwater chemical composition were observed, other studies have reported changes in the chemical composition and temperature of the lake, indicating potential threats to the lake's habitat due to modifications in the groundwater-surface water interaction system.