Continuous Recycling of Water

The water cycle is a continuous circulation of all the water (H2O) contained on the surface of planet Earth. Water is in constant movement between different water reservoirs, such as lakes, rivers and the ocean. This uninterrupted circulation of H2O is driven by natural processes occurring on the planet, including evaporation, condensation, precipitation, filtration and runoff. During this dynamic movement, H2O undergoes several transformations from one state to another, consequently, water, which is abundant as a liquid, may change from liquid to gas (vapor) or to solid (ice) form. The circulation of H2O is very important at maintaining the ecosystems of the planet. 

The Hydrologic Cycle

The Hydrologic Cycle
Credit: John M. Evans/USGS-USA Gov

Evaporation, Condensation and Precipitation of Water

One of the most important natural processes that occur between the planet and the atmosphere, comprising H2O, is the evaporation, condensation and precipitation of this fluid. Water stored in the planet´s water reservoirs, such as lakes, rivers, plants and oceans, is moved into the planet´s atmosphere as water vapor. Once vapor has reached a certain altitude, it condenses into clouds and falls to the ground as precipitation. Air currents circulating at high altitudes in the atmosphere carry these clouds to different regions of the Earth, where their content drops to the Earth´s surface as rainfall, refilling rivers, lakes and oceans. Once liquid droplets reach higher altitudes, they turn into ice crystals, which eventually will fall to the ground as snow on top of tall mountains and ice caps at the poles.

Why is the Water Cycle so Important?

The importance of the water cycle stems from the fact that this natural process maintains water continuously circulating over the Earth, keeping its reservoirs filled up all the time. This happens when precipitation falls on mountainous valleys, forming lakes, eventually finding its way down through streams between the mountains, creating water springs and filling up underground aquifers. Some of this liquid is extracted and utilized by man for agricultural processes, and some other, will continue flowing down in water streams and through fractures in rocks. Eventually, all this fluid will reach the oceans, where a new water cycle will begin.

Water Reservoirs
Credit: Andrew Bowden

Benefits of the Hydrologic Cycle

The distribution of H2O around the world influences the geological and biological characteristics of the ecosystems on the planet, including forests, deserts and aquatic ecosystems. The circulation of H2O transports nutrients and minerals, depositing them on the Earth´s surface. Flows of the elemental liquid on the ground and underground are purified by filtration, recharging the Earth´s water reservoirs, such as dams, with fresh liquid. This water is distributed between different locations by irrigation. The flow of water also reshapes the surface of the planet through erosion and deposition of rock particles and minerals.

Global Climate

It is believed that climate change will affect profoundly the hydrologic cycle, in the future. It has been predicted that some regions in the world will be affected more than others, with some regions becoming drier than others due to global warming. This could not be possible if it were not for human intervention. Humans are affecting the hydrologic cycle by engaging in activities, including agriculture, deforestation, manufacturing industry, urbanization and the emissions of CO2 into the atmosphere, among many other activities.

Global Climate

Global Climate
Credit: NOAA