The OW is the most important among the seven watersheds that provide drinking water to Istanbul. These seven watersheds cover 2,353km2 (42% of the total area of 5,512km2) of land area in the Istanbul Province. The OW, on the Asian side of Istanbul, provides the largest supply of drinking water (35%) and faces the most acute urbanisation pressures. It has the highest population (371,400 inhabitants in 2000) and built-up area, which have increased through unplanned and irregular developments since the 1980s.