Principles of Life 3e Student Resources is no longer available and it was replaced by Biology and Life Sciences.
Animation 42.1 The Global Water Cycle
INTRODUCTION
Water cycles from oceans, to land, and back to oceans in a flow known as the hydrologic cycle. The sun drives the hydrologic cycle by providing the energy to evaporate surface waters. Most of the evaporation occurs from the oceans, and a portion of the evaporated ocean water then falls on land. Gravity completes the hydrologic cycle by driving water from the land back to the oceans via rivers, coastal runoff, and subterranean flows.
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Textbook Reference: Key Concept 42.3 Certain Biogeochemical Cycles Are Especially Critical for Ecosystems, p. 1036
CONCLUSION
The hydrologic cycle operates because more water is evaporated from the oceans than is returned to them as precipitation. The excess water is transported over land, where it falls as precipitation. Gravity then delivers water back to the ocean through rivers, coastal runoff, and groundwater flows. Water that falls on land renews water reservoirs, with water renewal being fastest (about 4 years) in lakes and rivers, important sources of our fresh water. In a sense, the hydrologic cycle is a continually operating distillation machine, evaporating water from the salty oceans and dropping fresh water back on land.