The Water Cycle

What Is the Water Cycle?

The water cycle, or hydrologic cycle, describes the continuous movement of water through Earth’s atmosphere, surface, soil, groundwater, organisms, and bodies of water. Water can change location and physical state as it moves. It may exist as liquid water, water vapor, or ice.

Unlike energy, which flows through ecosystems and is eventually released as heat, water is matter. It is conserved and repeatedly recycled through Earth’s systems. The water molecule itself is not destroyed when water evaporates, condenses, freezes, or moves through an organism.

Earth’s Water Reservoirs

A reservoir is a place where water is stored for some amount of time. Major reservoirs include oceans, glaciers and ice caps, groundwater, lakes and rivers, soil water, the atmosphere, and living organisms.

Water does not spend the same amount of time in every reservoir. Some atmospheric water may move through the cycle relatively quickly, while groundwater or ice can store water for much longer periods.

Evaporation and Transpiration

Evaporation occurs when liquid water gains enough energy for some molecules to escape into the atmosphere as water vapor. The Sun is the major external energy source driving evaporation at Earth’s surface. Evaporation occurs from oceans, lakes, rivers, soil, and other wet surfaces.

Transpiration is the loss of water vapor from plants, primarily through small openings called stomata in leaves. Plants absorb water through their roots and transport it through their tissues; some of that water eventually exits the plant as vapor.

Condensation and Cloud Formation

Condensation occurs when water vapor changes into liquid water. As moist air cools, water vapor can condense onto tiny particles in the atmosphere, forming microscopic droplets. When enough droplets or ice crystals collect, clouds become visible.

Condensation is the reverse phase change of evaporation: evaporation changes liquid water into gas, while condensation changes gas into liquid. Cloud formation does not mean that water has permanently left the cycle. Clouds are another temporary reservoir.

Precipitation

Precipitation occurs when water falls from the atmosphere to Earth’s surface. It can occur as rain, snow, sleet, or hail depending on atmospheric conditions and temperature.

Precipitation does not always immediately become runoff. Some water is intercepted by vegetation, some infiltrates the ground, some becomes stored as snow or ice, and some reaches streams, lakes, or oceans. The pathway depends on temperature, soil, vegetation, slope, and other environmental conditions.

Runoff, Infiltration, and Groundwater

Runoff is water that flows over the land surface toward lower areas such as streams, rivers, lakes, and eventually the ocean. Runoff tends to increase when precipitation is intense, soils are saturated, slopes are steep, or surfaces are relatively impermeable.

Infiltration occurs when water enters the soil. Some infiltrated water is taken up by plant roots, while some moves deeper into the ground and can become groundwater. Groundwater may slowly flow through soil and rock and eventually discharge into springs, streams, lakes, or the ocean.

Human Impacts on the Water Cycle

Human activities can change how quickly water moves among reservoirs. Deforestation can reduce transpiration and change how rainfall reaches the soil. Urbanization creates impervious surfaces such as roads and buildings, which can reduce infiltration and increase surface runoff.

People also remove groundwater for agriculture, industry, and drinking water. If groundwater is withdrawn faster than it is naturally replenished in a particular system, groundwater levels can decline. Dams and reservoirs can alter the timing and location of water storage and downstream flow.

Common Misconceptions

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