The continuous movement of water on and below the earth’s surface, and between the earth and the atmosphere, is known as the water cycle or hydrologic cycle. Water exists in many forms as it circulates. Water on the earth’s surface is known as surface water. Water beneath the earth’s surface is called groundwater, while water vapour is called atmospheric water (WSC 2006). The hydrologic cycle includes evaporation and transpiration, condensation of water vapour in the atmosphere leading to precipitation, and the movement of surface water and groundwater on the earth. Water from the atmosphere reaches the earth’s surface as precipitation: rain, snow, hail, and sleet.
The Water Cycle can also be explored through the interactive component on the right.
The hydrologic (water) cycle.
Source: Federal Interagency Stream Restoration Working Group 1998.
( click to enlarge )
After reaching the earth’s surface, water can:
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Filter into the soil, percolate through the soil to become soil water, or seep further down to replenish groundwater
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Collect as water in lakes, wetlands or oceans, or as snow or ice, as in glaciers
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Run off as surface water moving downstream in a path through streams, rivers, lakes, and wetlands
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Evaporate from soil and water surfaces, or transpire from vegetation (collectively evapotranspiration), to become atmospheric water, available to form precipitation as it cools and condenses, thereby completing the hydrologic cycle
Water percolating down to the groundwater table is known as groundwater recharge. Recharge, an essential component of the water cycle that often goes ignored, replenishes the groundwater resources, allowing continued use of the resource. To learn more about groundwater and recharge in the Limpopo River basin, please refer to the Groundwater section of Hydrology of the Limpopo River basin.
Water that falls on the ground and does not evaporate or is not absorbed is known as run-off. Surface water run-off follows gravity, flowing downhill to collect in streams and waterbodies. This is the water that we see on a daily basis and consider when we think about water in a river basin context; the most readily available water in the water cycle. It can be diverted or extracted from streams and rivers, stored in reservoirs when rivers are dammed and the potential energy in the gravitational flow of the water can be used to generate hydro-power.
The surface of standing and flowing water in the water cycle is exposed to the atmosphere. In the warm, dry conditions, a portion of this water can evaporate, rising up into the atmosphere as water vapour and forming clouds, falling back to earth as precipitation.
