The Hydrologic Cycle, also known as the water cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth.
The diagram shows an overview of the process.
There are five basic processes that make up the hydrologic cycle: Condensation, precipitation, infiltration, runoff, and evapo-transpiration.
Condensation is the process of water changing from a vapour to a liquid. Water vapour in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warm air rises, the water vapour will lose energy, causing its temperature to drop. The vapour then has a change of state (liquid or ice).
Precipitation is water being released from clouds as rain. Precipitation begins after water vapour, which has condensed in the atmosphere, becomes too heavy to remain in atmospheric air and falls. Under some circumstances precipitation actually evaporates before it reaches the surface. More often, though, precipitation reaches the Earth’s surface, adding to the surface water in streams and lakes, or infiltrating the soil to become groundwater.
Infiltration: A portion of the precipitation that reaches the Earth’s surface seeps into the ground through the process called infiltration. The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water. The more openings in the surface (cracks, pores, joints) the more infiltration occurs. Water that doesn’t infiltrate the soil flows on the surface as runoff.
Runoff: Precipitation that reaches the surface of the Earth but does not infiltrate the soil is called runoff. Runoff can also come from melted snow and ice. When there is a lot of precipitation, soils become saturated with water. Additional rainfall can no longer enter it. Runoff will eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes.
Evapo-transpiration is water evaporating from the ground and transpiration by plants. Evapo-transpiration is also the way water vapour re-enters the atmosphere. Evaporation occurs when radiant energy from the sun heats water causing the water molecules to become so active that some of them rise into the atmosphere as vapour. Transpiration occurs when plants take in water through the roots and release it through the leaves, a process that can clean water by removing contaminants and pollution.