Unit operations in a hydroskimming refinery. Nonshaded portions show the basic distillation and recovery units that make up a simple topping refinery, which produces petrochemical feedstock and industrial fuels. Shaded portions indicate the units added to make up a hydroskimming facility, which can produce most transportation fuels. hydrosphere discontinuous layer of water at or near the Earth's surface. It includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour. Water is the most abundant substance at the surface of the Earth. About 1.4 billion cubic kilometres (326 million cubic miles) of water in liquid and frozen form make up the oceans, lakes, streams, glaciers, and groundwaters found there. It is this enormous volume of water, in its various manifestations, that forms the discontinuous layer, enclosing much of the terrestrial surface, known as the hydrosphere. Central to any discussion of the hydrosphere is the concept of the hydrologic cycle. This cycle consists of a group of reservoirs containing water, the processes by which water is transferred from one reservoir to another (or transformed from one state to another), and the rates of transfer associated with such processes. These transfer paths penetrate the entire hydrosphere, extending upward to about 15 kilometres (nine miles) in the Earth's atmosphere and downward to depths on the order of five kilometres in its crust. This article examines the processes of the hydrologic cycle and discusses the way in which the various reservoirs of the hydrosphere are related through the hydrologic cycle. It also describes the biogeochemical properties of the waters of the Earth at some length and considers the distribution of global water resources and their utilization and pollution by human society. Details concerning the major water environments that make up the hydrosphere are provided in the articles ocean, lake, river, and ice. See also climate for specific information about the impact of climatic factors on the hydrologic cycle. The principal concerns and methods of hydrology and its various allied disciplines are summarized in Earth sciences.

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