Given ongoing climate changes, the need to conserve existing water resources through water/wastewater reuse and reclamation is becoming urgent. The general concept, Zero Liquid Discharge (ZLD), is a realistic option to counteract the increased evapotranspiration and decreased precipitation caused by climate-change-driven temperature variations and overconsumption of original freshwater resources. The studied Büyükçekmece Lake Dam is a reservoir supplying about 17% of Istanbul's population with drinking water. The elements of the shifting water budget for the Büyükçekmece Lake Dam were estimated for the middle and the end of the twenty-first century and compared with the future projections of climate change models for the worst-case scenario. Outcomes from those models anticipated that seasonal temperature variations could surge from the current 2.0 °C to 3.2 °C in mid-century and 4.0 °C to 6.0 °C at the end of the century. According to this scenario, rainfall in Istanbul is predicted to decrease by 12% compared to 1986–2005 rainfall data. According to the 2017 report by the Istanbul Metropolitan Municipality (IBB), the average monthly precipitation is expected to decrease by approximately 5% in winter and up to 33% in summer by the end of the century. Even if there were no significant variations in the annual precipitation, the drop in annual runoff would reach 15% due to increased evapotranspiration. As a result, annual streamflow to the lake would drop about 22% in the mid-century and 56% at the end of the century compared to the period between 1968 and 2005. These estimates revealed that the water capacity of Büyükçekmece Lake Dam would drop to 90 million m3 in the mid-century and to 45 million m3 at the end of the century. One of the solutions addressing incoming shortages is employing the LZD approach, which focuses on reusing/reclaiming already spent drinking water instead of over- drafting fresh resources.