Although practice of hydrology can be traced back to as early as 5000 to 6000 years ago in ancient Mesopotamia, Egypt and China, the study of hydrology emerged as a discipline in the 17^th century when Pierre Perrault, Edmé Mariotte and Edmond Halley conducted their experimental work on the hydrologic cycle. For quite a long period, hydrological education and research programs were offered primarily as an engineering discipline in universities in the United States and many East Asian countries. With our increasing understanding of the mechanisms of spatial and temporal distributions of water over large watersheds, and even the globe, and the capabilities of computer modeling of complicated hydrological processes, the study of hydrology has now extended to and interacted with meteorology, geophysics, environmental science, and mathematical statistics, and has established its own right as a branch of geoscience.