Understanding how hydrological processes affect the transport of dissolved and suspended constituents through catchments and aquifers is one of the main aims of studies of Earth surface processes. Water is “the universal solvent.” Water chemically weathers rocks and soils, carrying dissolved salts from the continents to the seas. It interacts with decaying vegetation and carries organic carbon seaward. Water readily carries chemicals that humans use to the sea, including fertilizers and other agrochemicals. The global water cycle is thus linked to other element cycles, for example, to carbon and nitrogen cycles, and is inextricably linked with a host of ecosystem functions. Moving water carries suspended solids as well as dissolved salts, so the water cycle is also closely tied to cycles of erosion and sedimentation.