The quantification of water inputs and outputs in closed basins is essential to obtain chemical, energy and water balances. In the case of saline endorheic wetlands, its environmental importance is intimately associated with its hydrological regime and the chemical characteristics of its waters. The endorheic basin of the Laguna de Pétrola is an example of closed basin where evaporitic enrichment of dissolved salts transported to the lake by surface water and groundwater. In this work a first quantitative approximation to water balance in groundwater discharge in Pétrola Lake for the hydrological year 2008/2009 is carried out (only year of the last 10 in which the lake was dry at the beginning and end of the period). For this, meteorological, groundwater and remotely sensed data have been used. The analysis was based on GIS operations. The hydrogeological balance elements of the endorheic basin and the lake were quantified separately. Based on the fact that the hydrological year begins with the lake completely dry, it has undergone a filling-emptying cycle throughout the studied period. The maximum volume of stored water is reached at the beginning of May (0.78 hm3), which corresponds to an estimated flooded area of 1.32 km2. Since then there has been a decrease in the surface area to minimum values (dry) at the end of September. The quantitative analysis of the components of the water balance revealed that the main source of inputs to the lake is the groundwater and surface runoff, almost double the direct contributions of precipitation. In general, the evaporation rate decreases with the increase in salinity. The annual values of evaporation in the lake for mean water salinity of 60 g/L TDS can reach 1.66 hm3. These values, together with the outputs resulting from the evapotranspiration of the surrounding vegetation, only represent 70% of the water inputs to the wetland. This would suppose a positive water balance that contrasts with the evolution of the surface of water in the lake. The volume of water that is missing is interpreted as being lost by leakage due to the effect of density-driven flow (DDF). The water of the lake has a density that can reach 1.29 g cm-3. However, the groundwater of the aquifer that feeds the lake has density values of 1.00 g cm-3. As a result of the difference in density between the lake's brine and the fresh groundwater, there is a DDF to the aquifer that can account for a considerable percentage of the lake's water outflows, up to 0.64 hm3.