“Exploration for Hidden Water” by Mohammad Karaji—The Oldest Textbook on Hydrology?

Reviewing a classical Persian book on ground‐water problems of the 11th century, it is shown that Ground‐water hydrology developed absolutely independently of geology. The Arabic‐Persian scientists of that time knew about the hydrologic cycle and the existence of different types of water underground; and they knew that the quality of water depends on the character of the rocks through which ground water flows. Techniques of developing ground‐water resources in the Middle East, i.e., the digging of wells and qanats, were of such a high standard that they are still in use today. Generally, it can be stated that the oriental scientists of the Middle Ages knew of the basic principles of hydrology. Thus they were far ahead of their contemporaries in Europe.