HYDROLOGIC (WATER (H2O)) SYSTEMS** Water originally formed within the Earth & was brought to the surface by a process of outgassing, which is the release of trapped gasses from rocks, forced out through cracks, fissures, & volcanoes from within the Earth. * Water began to collect on the Earth’s surface after it had cooled, about 3.8 billions of years before present (bybp). * Water attained its present amount (~ 1.36 billion km3) approximately 2 bybp. * Water covers approximately 71% of the Earth’s surface. * Other than gravity, water is the major agent of geomorphic change on the Earth’s surface * Distribution * 97.22% is in the oceans and other saltwater bodies, 2.78% is freshwater * Of all this freshwater: 22% is groundwater and 78% is surface water * Of the ~78% freshwater that is on the surface, ~99% in ice & glaciers, and ~1% is in freshwater lakes & ponds, rivers & streams * Properties of water: * It occurs in all 3 states (phases) of matter (solid, liquid, gas) at the T° found on Earth. * It undergoes phase changes, changing from solid to liquid to gas, etc. (melting, freezing, evaporation, condensation, sublimation, & deposition). * When water changes state, energy is either added or released.