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46.3B: The Water (Hydrologic) Cycle

  • Page ID
    14232
  • Water has a large effect on climate, ecosystems, and living organisms and is continuously cycled through the environment.

    Learning Objectives

    • Explain the path of the hydrologic cycle and its importance

    Key Points

    • Water cycling affects the climate, transports minerals, purifies water, and replenishes the land with fresh water.
    • Water with a longer residence time, such as water in oceans and glaciers, is not available for short-term cycling, which occurs via evaporation.
    • Surface water evaporates (water to water vapor) or sublimates (ice to water vapor), which deposits large amounts of water vapor into the atmosphere.
    • Water vapor in the atmosphere condenses into clouds and is eventually followed by precipitation, which returns water to the earth’s surface.
    • Rain percolates into the ground, where it may evaporate or enter bodies of water.
    • Surface runoff enters oceans directly or via streams and lakes.

    Key Terms

    • residence time: the average time a particular molecule of water will remain in a body of water
    • condensation: the conversion of a gas to a liquid; the condensate so formed
    • surface runoff: overland flow of excess water (with or without accumulated contaminants) that cannot be absorbed by the ground as infiltration
    • Evaporation: the process of a liquid converting to the gaseous state
    • sublimation: the transition of a substance from the solid phase directly to the vapor state such that it does not pass through the intermediate liquid phase

    The Water (Hydrologic) Cycle

    Water is the basis of all living processes. More than half of the human body is made up of water, while human cells are more than 70 percent water. Thus, most land animals need a supply of fresh water to survive. However, when examining the stores of water on earth, 97.5 percent of it is non-potable salt water. Of the remaining water, 99 percent is locked underground as water or as ice. Thus, less than 1 percent of fresh water is easily accessible from lakes and rivers. Many living things, such as plants, animals, and fungi, are dependent on the small amount of fresh surface water supply, a lack of which can have massive effects on ecosystem dynamics. Humans, of course, have developed technologies to increase water availability, such as digging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, the supply of fresh water is still a major issue in modern times.

    image

    Water availability: Only 2.5 percent of water on earth is fresh water. Less than 1 percent of fresh water is easily accessible to living things.

    Water cycling is extremely important to ecosystem dynamics as it has a major influence on climate and, thus, on the environments of ecosystems. For example, when water evaporates, it takes up energy from its surroundings, cooling the environment. When it condenses, it releases energy, warming the environment. The evaporation phase of the cycle purifies water, which then replenishes the land with fresh water. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the earth through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet. Most of the water on earth is stored for long periods in the oceans, underground, and as ice. Residence time is a measure of the average time an individual water molecule stays in a particular reservoir. A large amount of the earth’s water is locked in place in these reservoirs as ice, beneath the ground, and in the ocean, and, thus, is unavailable for short-term cycling (only surface water can evaporate).

    image

    Residence time of water: This graph shows the average residence time for water molecules in the earth’s water reservoirs. Most of the earth’s water is unavailable for use because it is locked in ice, beneath the ground, or in the oceans.

    There are various processes that occur during the cycling of water, which include the following:

    image

    Cycling of water: Water from the land and oceans enters the atmosphere by evaporation or sublimation, where it condenses into clouds and falls as rain or snow. Precipitated water may enter freshwater bodies or infiltrate the soil. The cycle is complete when surface or groundwater reenters the ocean.

    • evaporation/ sublimation
    • condensation/precipitation
    • subsurface water flow
    • surface runoff/snowmelt
    • streamflow

    The water cycle is driven by the sun’s energy as it warms the oceans and other surface waters. This leads to the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor) of frozen water, which deposits large amounts of water vapor into the atmosphere. Over time, this water vapor condenses into clouds as liquid or frozen droplets, which is eventually followed by precipitation (rain or snow), returning water to the earth’s surface. Rain eventually percolates into the ground, where it may evaporate again (if it is near the surface), flow beneath the surface, or be stored for long periods. More easily observed is surface runoff: the flow of fresh water either from rain or melting ice. Runoff can then make its way through streams and lakes to the oceans or flow directly to the oceans themselves. Rain and surface runoff are major ways in which minerals, including carbon, nitrogen, phosphorus, and sulfur, are cycled from land to water.