Hydrology is the science that deals with the waters above and below the land surfaces of the Earth; their occurrence, circulation and distribution, chemical and physical properties, and their interaction with their environment, including their relationship to living beings (NRC, 1991). Owing to its central focus on water, the science of hydrology holds a unique place in the field of earth system science, intimately intertwined with other water-related disciplines such as meteorology, climatology, geomorphology, hydrogeology, and ecology. As an applied science, hydrology is highly relevant to the management of the world’s water resources and water quality and for the prediction and mitigation of water-related natural hazards such as floods and droughts. Thus, hydrology is an exciting field of study principles.
There are different concepts for different parts of the hydrologic cycle and different spatial and temporal scales. The various branches of hydrology, however, do show remarkable parallels. The nature of hydrological variability is remarkably similar for different processes and the measurement techniques available to probe them have similar characteristics as well. Both have distinctly shaped the descriptive and predictive methods that have evolved in this discipline over the years and they ultimately control the accuracy of hydrological predictions. The common threads of the various hydrological subdisciplines may hence be a useful starting point for a presentation of fundamentals in the hydrological sciences.