The role of al-Bīrūnī in the history of hydrology. A modern vision 600 years in advance.

In today's contemporary world, a walk through the epistemology of the hydrological cycle can help us to frame many current problems in a broader framework, reminding us to remain humble. Among all the scientific concepts that are taught in schools of all kinds and levels everywhere on the planet, the water cycle is one of the concepts that remain in the head even after a long time from the end of compulsory school. The ease in remembering this theory can be attributed to its relative simplicity: water evaporates from lakes, seas and oceans; the vapor forms the clouds; from the clouds come the rains and snows that feed the glaciers and Earth's surface and underground waterways, which in turn return to the oceans, and so on. However, the mechanism is only apparently simple. The trained hydrologist knows very well that studying and communicating the complexity hidden behind hydrological science requires patience and dedication. Modern hydrology, in the Western world, can be considered born at the end of the 17th century thanks to two members of the Académie Royale des Sciences: Pierre Perrault (1611-1680) and Edme Mariotte (1620-1684). The treatises of these two scholars – De l'origine des fontaines (Perrault 1674) and Traité du mouvement des eaux et des autres corps fluides (Mariotte1686) – can be considered the first attempts to verify the actual ability of precipitation to supply rivers and sources by means of quantitative analyses. Anyway, it is clear that a modern vision of the hydrological cycle was present in the Islamic world at the time of al-Bīrūnī. Although he did not leave writings dedicated solely to hydrological sciences, al-Bīrūnī's "modern" vision of the hydrological cycle clearly emerges in some extant passages of his works. In this memoir, we will discuss how al-Bīrūnī's vision of the hydrological cycle mirrors a theoretical framework widespread among the Islamic scholars of his age, and how its fundamental concepts anticipate the modern hydrological vision that took shape in Europe in the mid-1600s.