Factors such as the changes in the climatic conditions, urbanization as a result of the population growth, etc. have caused a negative pressure on water resources at present. Being an outcome of the climate change, water scarcity has been intensely experienced at some region, whereas it is expected to cause worse problems in other regions in the near future. Being one of the most significant techniques to deal with the water scarcity, rainwater harvesting method, which has been used by various civilizations in history, has been neglected for the last decades, whereas it has gained its importance again in recent years. Currently, governments have been researching on the techniques to use water resources effectively so that the waste of water could be decreased, or the relevant groups, seeing the possible risks in the future, expect the responsible bodies to take proper decisions in this regard. This study was carried out to reveal how much of the water needed by the public green areas in could be obtained from the rainwater collected from the building roofs. Building roofs and green areas in Buyukcekmece District were calculated by using the AutoCAD and ArcGIS as well as the maps. Also, the district precipitation and evapotranspiration data for 20 years of were obtained from the 1st Regional Directorate of Meteorology in Istanbul. The results indicate that 2.266.146 m3 can be collected in a year by rainwater harvesting from the roofs of the buildings in the district, and the amount of water need for the irrigation of public green areas is 471.802 m3, if irrigation is carried out daily. Therefore, 100 % of the water required for the irrigation of the public green areas in the district could be provided with the rainwater harvesting.