4 main stages of Earth's water cycle: Here's all you need to know
- The water cycle is the path that water follows while moving around the Earth in different states.
- There are four primary water cycle steps: evaporation, convection, precipitation, and collection of water.
- This cycle is vital to all of Earth's life forms- both directly and indirectly. Without it, life as we know it will cease to exist.
Water is one of the primary natural elements, covering about 70 percent of Earth's surface. This liquid is essential for life as we know it, mediating our day-to-day activities.
Water sustains life by tying the land, atmosphere, and oceans into an integrated system. The Earth's water needs to purify, replenish, and circulate continuously to perform its functions. Nature carries out this entire operation through a process called the water cycle.
In this article, Interesting Engineering (IE) discusses the water cycle, its different steps, why it is essential, and some facts about it. So, let's get started.
What is the water cycle?
As per the standard water cycle definition, the water cycle is the continuous movement of water on, above, and below the Earth's surface. In simpler words, the water cycle is the path that water follows while moving around the Earth in different states.
Also known as the hydrologic or hydrological cycle, this phenomenon describes the water movement through three phases over four spheres to complete an entire cycle. The three phases are solid, liquid, and gas. And the four spheres? These refer to the atmosphere, lithosphere, hydrosphere, and biosphere.
Solid water, or ice, is found in snow, glaciers, and the North and South Poles. Think Arctic ice sheets and the snow that blankets the Himalayan mountain range.
Liquid water is present in lakes, rivers, and oceans, while water vapor is water in its gaseous state which can be found in the atmosphere - it's also a greenhouse gas.
Earth's water continues to change from one state to another while retaining the total number of water particles.
The water cycle stages
There are four primary water cycle steps, including evaporation, convection, precipitation, and collection of water. These preliminary steps are accompanied by different secondary stages, forming a never-ending cycle with no beginning or end.
The Sun evaporates liquid water sources, forming water vapor, which forms into clouds. These water vapors condense into droplets and fall out of the clouds as precipitation, such as rain, after merging enough droplets. And the cycle continues.
Below, here are the four basic steps of the water cycle in detail.
One major step of the water cycle is evaporation. This occurs when water molecules present at the surface of water bodies rise into the air after turning to vapor or steam. Being the ultimate energy source, the sun powers most of the evaporation on Earth. So, in simple words, the sun heats the water in the lakes, rivers, and oceans, turning it into steam or vapor.
Another process, called transpiration, is responsible for evaporation from plants, through their leaves. The water absorbed by the soil replenishes the roots and moves toward the leaves. After photosynthesis, the extra water moves out of the leaves as water vapor, entering the atmosphere.
Convection is the transfer of heat from one place to another due to the movement of fluids (which includes liquids and gasses). When fluids are heated, the hotter fluids rise while the colder fluid sinks at the bottom.
Convection happens because hot substances are less dense than cold substances. Better yet, have you noticed when running a bath that colder water sinks to the bottom? Well, this is the same concept of how convection works. Suppose you unplug the bath. In that case, you'll find that the colder, more dense water escapes first too!
Convection in the water cycle occurs when hot liquids or gases rise and cooler liquids or gases sink. When the air near the water's surface is heated, it will take the heat with it when it rises. When the water vapors in the air get cold and retransform into liquid, forming clouds and precipitation.
Condensation is a process by which a gas such as water vapor is changed into liquid water. When moisture cools and reaches saturation point, the tiny particles of water condense into larger drops of water. These droplets are inside the clouds and keep combining to form bigger ones. When the drops are heavy enough, they fall to the Earth as rain. If a cloud is colder, the water droplets may freeze to form ice.
Colder air can hold less water vapor. When saturation occurs, moisture becomes visible water droplets in the form of fog and clouds.
On the other hand, super low temperatures, like those below 0 degrees, force these droplets to fall as drizzle, snow, sleet, or hail. For example, sleet is a frozen rain that forms when rain droplets encounter cold air and freeze into ice before falling, while snow is formed when tiny ice crystals in clouds stick together to become snowflakes. When enough crystals stick together, they become heavy enough to fall to the ground. The precipitation that falls then fills streams and rivers, restarting the process.
Another part of the water cycle is the collection or storage of water. Not all of Earth's water is involved in the water cycle at one time - some of it remains stored in three primary locations. These locations include the atmosphere, the Earth's surface, and below the ground.
The water stored in the atmosphere can quickly move from one part of the planet to another. Alternatively, surface and groundwater storage depends on the geological features of the storage location.
About 96 percent of the Earth's total water is stored in the ocean. We cannot drink salty ocean water, but freshwater is stored in lakes, rivers, glaciers, snow caps, and groundwater storage.
What is condensation in the water cycle?
Condensation is one of the water cycle stages, the opposite of evaporation. This process converts water vapors in the air into liquid water. The vapors accumulate in the atmosphere cool down over time due to the low temperatures at high altitudes. As a result, they transform into tiny droplets of water or ice.
This is essential for the water cycle because it plays a vital role in cloud formation. Later, these clouds produce precipitation to help water return to the Earth's surface.
During condensation, water changes between gaseous, liquid, and solid forms. While transforming from one state to another, the water molecule arrangement also changes.
Typically, water molecules in the vapor form are more disorganized and arranged randomly compared to liquid water or ice. When vapors transform into liquid water, the water molecules become more organized and release heat into the atmosphere.
What is transpiration in the water cycle?
Transpiration is one of the steps of the water cycle responsible for releasing water vapor from plant leaves. This process has three primary steps.
Firstly, roots absorb the water from the soil. Next, the absorbed water travels through plant tissues, performing physiologic and metabolic functions in the plant. Leaf stomata then release water vapor into the air.
Why is the water cycle important?
The water cycle is vital for the environment as it helps determine the global climate. It is an integral part of biogeochemical cycles and affects all life processes on the Earth in one way or the other. Some reasons why the water cycle is essential are as follows:
The water cycle tremendously impacts the climate. Water vapor is actually Earth's most abundant greenhouse gas. It's responsible for about half of Earth's greenhouse effect, and helps keep the planet livable. The greenhouse effect raises the temperature, and if there's no evaporative cooling effect of the water cycle, it will lead to a drastically high temperature on the Earth.
The water cycle also cleans the air. During precipitation, water vapors attach themselves to dust particles, picking up dust, pollutants, and water-soluble gas while falling from the clouds as raindrops.
This cycle is vital to Earth because it affects all life forms directly or indirectly. It is responsible for providing water to plants, animals, and humans. The water cycle also moves pathogens, nutrients, and sediment into and out of aquatic systems.
While it is tempting to focus on water constantly circulating above, across, and even below the Earth's surface, it's important to consider that this same cycle is important for fuelling our homes, hydrating our bodies, irrigating our crops, and breaking down our waste.
As such, water, food, and energy are closely related and reliant on one another. Increasingly, we are being compelled to reconsider how we handle and use our water supply due to our growing requirement for these three essential resources.
Professor Gretchen Benedix is an astrogeologist and cosmic mineralogist who studies meteorites and figures the forming stages of the solar system.