The Water Cycle

The Water Cycle: An Overview

Earth is sometimes referred to as the Blue Planet. Seen from space, our planet is a most striking blue color due to the abundance of water. Our planet is unique in the solar system because we have water in all forms; solid, liquid and gas. Most of the water on earth (97%) is stored in the oceans. Other stores are lakes, rivers, soil, snowfields, glaciers, groundwater and the atmosphere. Water is constantly changing state, and moving around between these reservoirs. We call this the water cycle or hydrological cycle.

Key processes:

  1. Evaporation: When the Sun heats the surface of the earth, water evaporates from the oceans, and from lakes and rivers. The liquid water turns to vapor and rises up into the atmosphere.
  2. Condensation: When the water vapor cools down as it rises through the troposphere (the lower layer of the atmosphere), it condenses and little droplets form, which we can see in the form of clouds.
  3. Precipitation: The droplets get heavier and fall downward out of the cloud as rain or snow. 91% of precipitation falls back into the ocean, completing a water cycle.

Some of the water that evaporated from the oceans will precipitate over land. The movement of water vapor through the atmosphere is called advection. The rain or snow that falls over land will take one or multiple longer routes back to the ocean.

  • Interception: Some rain is intercepted by vegetation. It may evaporate from the leaves again, or slowly drip down to the ground. Here it could be taken up by the roots of the plant and transpired from the leaves, where it will evaporate again. The evaporated water will condense and precipitate again, before it can go back to the oceans.
  • Surface run-off: Some rain will fall on the surface of the ground. If this surface is impermeable, like rock, or saturated with water already, the water will just flow across the surface of the ground into a drain, canal or stream and eventually back into the oceans.
  • Through flow or subsurface flow is the water that penetrates the soil and seeps slowly into streams. Some water will percolate into deeper soil and become part of the groundwater.
  • Snow fall may be intercepted by vegetation or stay on the soil for a period before melting and flowing into streams.

As the water cycles through all the water stores, they get replenished. For example, the water in the atmosphere gets completely renewed every 8 days. Other stores take much longer to get completely recycled; a river can take a few months, and a glacier can take 100 years.  Groundwater can take hundreds or even thousands of years to renew.

Human impact on the water cycle

Humans impact the water cycle in several ways, from building reservoirs and dams, to pumping of groundwater, and changing landscapes altogether through urbanization and deforestation.

We have spent a lot of time and money to divert water from places where we did not want it to places where we did want it. We’ve dug canals, built dams and reservoirs, and regulated river flows, all to control the movement of water. By diverting water upstream, the downstream areas are impacted in the form of erosion, changes in ecological systems, and land inundation.

 San Joaquin land subsidence
Ground water over-extraction has lead to ground subsidence, such as here in the San Joaquin Valley, CA.

About a third of the world population uses groundwater as a source of drinking water every day, but most groundwater is used for irrigation and industrial use. Some of that water is relatively young water found in the first few hundred meters under the surface. Overall, we do not pump more water from the ground than the daily natural recharge. However, the groundwater is not equally distributed. In places like California people are already using non-renewable water that is thousands of years old. In Egypt, people are now tapping into aquifers that were last replenished a million years ago. Overusing groundwater can result in lower subsurface water and reduced stream and river flow. Many areas of the U.S. already show significant depletion of ground-water storage. Pumping water out of the ground faster than it can be naturally replenished can also cause land subsidence, where the ground literally lowers over time.

Urbanization increases demand for water, which causes more water diversions and groundwater mining. Also, cities increase water run-off, because impervious surfaces like roads and buildings keep water from penetrating the soil replenishing the groundwater. Instead, this water gets transported away through drains and sewers, but the flow is often very high following a rainfall event, leading to flash flooding in some cases.

In urban areas water is typically piped directly into streams, often leading to flash floods.

Deforestation causes drying of the soil and increased surface run-off. Besides a reduction of through flow, evapotranspiration is also reduced or absent. Without trees releasing water vapor, weather patterns above the land will change. There will be less precipitation, for example.

Climate change

As the climate gets warmer, the water cycle intensifies. More water evaporates from the surface of the oceans and more water comes from vegetation through evapotranspiration. Higher temperatures also mean that the atmosphere is able to hold more water before it condenses in the troposphere. This results in more rainfall in coastal areas, causing floods and landslides. It can also produce more frequent strong tropical storms.

In other areas, the soil dries out through evaporation. When it does rain, the water cannot penetrate the baked soil and will turn into surface run-off.  Groundwater does not replenish and the soil gets drier and drier, causing severe droughts.

Sources

Alley, William. Sustainability of Groud-Water Resources

Nasa (1997). Human impacts on the global water cycle

Nasa, Earth Observatory. The water cycle and climate change

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