The Water Cycle
Without water, there would be no life on Earth. Water is continually moving around, in the ocean, rain and rivers, but Earth and its atmosphere comprise a closed system: no new water arrives from outer space – rather, the water that exists on earth today is the same water that existed billions of years ago. Within this system, water is continually recycled, in a process known as the hydrologic or water cycle.
This cycle is important to all species on Earth, including humans. One of the most pressing issues relating to the water cycle is the supply of drinking and irrigation water. Only three percent of all Earth’s water is freshwater and less than half of this three percent (0.1% of all the total fresh water) is accessible in lakes, rivers and shallow groundwater. The rest is locked up in polar ice sheets or deep underground, and is not available for drinking or irrigation.
What happens in the water cycle?
When water falls to the earth as precipitation (rain or snow), one of three things may happen to it:
- it may return to the atmosphere;
- it may soak into the ground; or
- it may flow over the surface into a stream, lake, wetland or the ocean.
Evaporation from these water bodies and from the soil returns water to the atmosphere once again.
- Before contacting the earth, precipitation may be intercepted by vegetation, mountains or human-made structures, from where it either evaporates into the atmosphere again, or flows to the ground.
- Whether or not the water soaks into, or infiltrates, the ground depends on numerous factors such as the permeability of the soil, the rate of precipitation, and the amount of organic debris and vegetation on the surface.
- Water that infiltrates the ground may be taken up by plant roots for use in photosynthesis. The excess water diffuses through the openings in the leaves; this process is called transpiration. Because it is difficult to measure evaporation and transpiration separately, the combined processes are often referred to as evapotranspiration.
- Within the soil, water that is not taken up by plants percolates through the spaces between soil particles, generally moving downward until it encounters a fully saturated zone at the level of the water table. At this point, the water is called groundwater. Groundwater flows according to gravity and hydraulic pressure, taking the path of least resistance through the most permeable soils and rock. Significant collections of groundwater are called aquifers, and these are sometimes tapped with wells for drinking or irrigation. If water is extracted from an aquifer faster than it is replenished, the source can dry up.
- Groundwater may eventually flow back to the surface as a spring or into surface water bodies. The portion of a stream’s flow that is nourished from groundwater is called baseflow; unlike surface flow, this amount usually does not vary much during the year.
- When the rain falls or snow melts too fast to infiltrate the ground, or when the ground consists of pavement or hard clay, water travels across it as surface runoff (also called overland flow). This type of flow is relatively uncommon in forested areas, where leaf litter and other organic material create a spongy layer that soaks up water. In this case, rain water flows just under the surface as subsurface flow. This flow differs from groundwater in that it moves quite fast and therefore has a more immediate and variable effect on stream flow.
Understanding the water cycle is important for appreciating how these processes are affected by human activities and developments. Drinking water in many parts of the world is becoming more scarce due to growing demands, alterations to watersheds and unsustainable water consumption. Even in Victoria and other places in British Columbia, seasonal water shortages are increasingly common.
Important Terms When Thinking About Water Cycles
a geologic formation that stores or transmits water; usually refers to those formations that can yield enough water to be useful to people.
Groundwater that flows into streams and rivers.
The change of state of a liquid to a gas.
The combined processes of evaporation and transpiration.
Subsurface water that is contained and flows within the saturated zone.
The process of water seeping into the ground.
Surfaces that do not allow water to soak into the ground; usually refers to urban structures such as paved roads, roofs or parking lots.
Precipitation that contacts a solid surface and is detained there; some of it may then evaporate back to the atmosphere.
The movement of water through pore spaces in the ground.
The ability of a material to allow the passage of a liquid, such as water. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas impermeable material, such as clay, don't allow water to flow freely.
Condensed or solid water that falls from the sky, i.e. rain or snow.
The subsurface area in which all the pore spaces between rock and soil particles are filled with water.
The detention of water in soils and surface water reservoirs.
Water that flows through large pores (animal burrows, roots holes, organic debris, etc.) beneath the surface, and reaches streams relatively quickly.
Rainwater or snowmelt that flows over the surface of the ground instead of infiltrating the soil.
Diffusion of water through the pores (stomata) of plant leaves.
The highest level at which groundwater has accumulated to form a saturated zone in the ground.