Because of the important role water plays in human survival, it has always been a subject of great interest, and the entire history of humankind can be written in terms of our need for water. From the very beginning, it was realized that water is essential for the satisfaction of basic human needs, and hence early civilizations flourished on lands made fertile by the major rivers: The Tigris and Euphrates in Mesopotamia, the Nile in Egypt, the Indus in India, and the Huang-He and Chang Jiang in China.
The magnitude and complexity of water resources development and management problems in the early days were not great. Population pressure, both in terms of numbers and concentrations, was not high; per capita demand was low and water was plentiful. When there were water related problems such as droughts or floods, people simply migrated to a better location. Pollution loads were low, and were primarily of an organic nature. Hence, until the early twentieth century, the demand for water, its efficiency of use, and its quality were generally secondary issues for most parts of the world.