Understanding the hydrological functioning of the scarce freshwater bodies of semiarid regions is crucial, especially in those areas affected by anthropic activities involving land-use changes. In the dry western edge of the Argentina Pampean plains, a system of more than 100 shallow lakes of remarkable stability occurs. These lakes exhibit low salinity compared to those located in the more humid belt. This system has constituted the main water resource for humans from prehispanic times to the present. Stable isotopes were used to establish the seasonal surface-water/groundwater interactions and the hydrological conditions in a lake of the Dry Pampean Plain (DPP), i.e., Lake Los Pocitos, to understand the mechanism that guarantees such a resource. Results indicate that evaporation mainly controls the isotopic composition of lake water, overwhelming the effect of higher rainfall inputs during the wet (but also most evaporative) season. The delta O-18 mass balance model indicates greater groundwater inflow to the lake during the dry season (similar to 0.4 m month(-1)) compared to the wet season (similar to 0.2 m month(-1)). Lake level decreased in the wet season due to the lowest groundwater inflow and the greatest evaporation rate. Based on the proportion of water entering a lake that leaves through evaporation, Los Pocitos corresponds to a throughflow lake with a short water residence time (similar to 0.47 years). These hydrologic conditions, along with freshwater inputs from a dune located at the western margin of the lake, determine the existence of this relatively stable and freshwater lake in the DPP where high evaporation rates are registered.