Hydrology is that the study of the movement, distribution, and quality of water on Earth and other planets, as well as the hydrologic cycle, water resources and environmental watershed sustainability. It has evolved as a science in response to the requirement to understand the complex water system of the earth and help solve water issues. The areas of analysis among hydrology concern the movement of water between its various states, or among a given state, or just quantifying the amounts in these states in a very given region. Hydrologists apply scientific knowledge and mathematical principles to unravel water-related issues in society: issues of quantity, quality and accessibility. Parts of hydrology concern developing strategies for directly measuring these flows or amounts of water (Hydrological processes), whereas others concern modeling these processes either for scientific knowledge or for creating prediction in sensible applications (Hydrology Modelling). On the whole, the process that governs the depletion and replenishment of water resources of the earth is called Hydrology. Hydrology is subdivided into surface hydrology and marine hydrology. Domains of hydrology include hydrometeorology, surface hydrology, hydrogeology, drainage basin management and water quality, where water plays the central role. The areas of research within hydrology concern the movement of water between its various states, or within a given state, or simply quantifying the amounts in these states in a given region. Parts of hydrology concern developing methods for directly measuring these flows or amounts of water, while others concern modelling these processes either for scientific knowledge or for making prediction in practical applications. Hydrological research can inform environmental engineering, policy and planning. The branches of Hydrology include: Hydrogeology, Chemical hydrology, Ecohydrology, Hydroinformatics, Isotope hydrology, Surface hydrology, Hydrometeorology, Drainage basin management and Water quality.