The water cycle is one of the most fascinating examples of the earth’s ability to sustain and replenish its natural resources.
Here, we’ve shared everything you need to know about all the stages of the global water cycle, explained in easy-to-understand language.
📌 Key Takeaways:
- The term “water cycle” describes the repeated circulation of water on the earth and in the atmosphere.
- Some of the key stages of the water cycle are evaporation, transpiration, condensation, precipitation, and runoff.
- The water cycle operates as a cyclical system, meaning it has no start or end – although evaporation is typically labeled as the starting point because it often makes the most sense to begin with this process.
💧 The Water Cycle Explained
So, what is the water cycle?
The water cycle, which is also known as the hydrologic cycle, is the continuous movement and recycling of water on the earth. This distribution of water across the earth’s land surface provides the water that all humans, animals, and plants need to survive.
The earth’s water cycle is essential to ensure the constant availability of freshwater resources and maintain ecosystems.
View this water cycle diagram for a visual interpretation of the different processes involved in the hydrologic cycle.
🔄 Processes Of The Water Cycle
Below, we’ve explained in detail the different processes of the water cycle.
Water Vapor And Evaporation
Water doesn’t only exist in liquid form – it’s also a gas that’s present in the atmosphere.
The liquid water actually only occurs locally and momentarily, while water vapor is constant.
There are two important functions of water vapor:
- It helps trap heat and keeps the earth’s surface warm
- It’s the so-called beginning phase of the water cycle
Water evaporation is when liquid water is converted into water vapor, which rises in the atmosphere. Evaporation requires energy input, usually from heat from the sun’s rays.
When warmed, surface water bodies and areas of moisture gain kinetic energy, and this increase in movement causes some water molecules to escape and transition into a vaporous state. When water has reached this kinetic energy threshold, it’s known as the “vapor pressure“.
The rate of evaporation is affected by the following factors:
- The size of the water body (the larger the surface area, the faster the rate of evaporation)
- Humidity levels (the higher the humidity, the slower liquid water evaporates)
- The temperature (the higher the temperature, the faster the rate of evaporation)
- Airflow (the more airflow and movement, the faster the evaporation rate)
After evaporation, water is dispersed across the earth’s atmosphere in a process known as transportation (discussed later in this guide).
Transpiration
Transpiration is similar to evaporation, except it’s a process that takes place on plants, not surface water bodies.
Transpiration occurs when plants absorb water in the soil through their roots, then release this water in vapor form back into the atmosphere.
Plant roots have hairs and other specialized structures that facilitate the absorption of soil moisture, in a process known as osmosis. Water molecules are pulled up through the plant’s xylem and into the leaves, where water vapor exchange is regulated by the stomata (small pores that open to allow gases and water vapor to leave the plant).
When the stomata are open, water vapor in the leave’s cells evaporates into the surrounding air, and more water flows from the plant’s roots to the leaves.
The factors affecting the rate of transpiration are similar to the factors that influence evaporation – temperature, air movement, and humidity.
Light intensity and plant factors, including the stomatal density and leaf surface area, also affect the rate at which water evaporates from plants. The larger the leaves and the denser the stomata, the faster the rate of transpiration.
How do plants avoid too much water loss? Plants only lose so much water during transpiration. They have mechanisms that can regulate water loss and are able to control the opening and closing of the stomata to adjust how much water is released. This prevents excessive water loss in the changing climate.
The main purpose of water transpiration is to cool the leaves and aid nutrient intake, but it also plays a role in the water cycle and influences weather patterns because the evaporated water vapor ends up in the earth’s atmosphere.
Transportation
The evaporated water molecules from plants and water bodies rise, driven by convection currents and wind, and are dispersed in the atmosphere. This process is known as transportation.
The wind plays a big role in transportation, allowing newly formed water vapor to travel away from the water source below, so more water molecules can evaporate. Global wind patterns are influenced by differences in air pressure, which, in turn, are caused by different temperatures and solar heating across the surface of the earth.
The air has a certain water vapor holding capacity, depending on factors including atmospheric pressure and the climate. The air reaches 100% relative humidity once it can no longer hold any more moisture.
Water vapor in the atmosphere has a lower density than the surrounding air, which causes it to rise. However, water vapor doesn’t leave the earth and end up in space. Eventually, the upward movement of water through the atmosphere results in cloud formation due to condensation. We’ve discussed this process in more detail below.
Condensation
Condensation occurs when water vapor transforms back into its liquid state. The process happens when air temperatures surrounding the water vapor drop, which reduces the water vapor’s energy.
There are a few reasons why air temperatures may drop, including:
- The rising of air to higher altitudes
- Natural cooling when the sun sets
- Contact with a colder surface
When air cools, it’s not able to hold as much moisture as warm air. Once the air can’t hold any more water vapor, condensation occurs.
For condensation to take place, there needs to be an opportunity for water vapor to change into its liquid state. Nucleation sites provide surfaces for this process to take place. Examples of nucleation sites are dust and pollutants.
Here, water vapor condenses into liquid droplets. In the atmosphere, water vapor molecules join together, forming lots of suspended liquid droplets – clouds.
Clouds don’t immediately lose their water content. More and more water vapor condenses onto the liquid droplets that already exist on the cloud’s surface, causing them to collide, merge, and grow in size.
Eventually, the tiny liquid water droplets have all converted into large droplets that are too heavy to continue to move upward with the air, and that’s when precipitation falls (discussed in our next point).
Condensation doesn’t only occur up in the atmosphere. It also occurs closer to the earth’s surface, which is why you might notice condensation on your car windows or your lawn.
Dew happens when the temperature of a particular surface falls below the dew point temperature of the air, causing water vapor to condense back into a liquid on these surfaces. This water will end up evaporating back into the air as the atmospheric temperature increases later in the day.
Fog occurs when the air near the ground’s surface is saturated with water, forming a low-lying cloud.
Precipitation
Precipitation is when condenses water in the atmosphere falls back o the surface of the earth. This stage of the water cycle is essential in replenishing freshwater resources.
When water condenses, it forms tiny droplets of solid ice. As we know, colliding water droplets cause clouds to grow, eventually leading to precipitation (rainfall).
What initiates precipitation? There are a few possible catalysts:
- Collision and coalescence – Occurs in warm clouds. Larger water droplets merge with smaller droplets, forming raindrops. These rainwater droplets eventually become heavy enough to fall back to the land surface.
- Ice crystal formation – In cold clouds, below-freezing temperatures cause water vapor to freeze into solid ice crystals. During the deposition process, water vapor merges with the ice crystals and freezes, causing them to grow larger. These crystals may eventually collide, producing snowflakes.
The speed of rain or snowfall depends on the size, shape, and air resistance of the droplets.
The most common form of water to form from clouds is liquid water (rain). This occurs when the atmosphere’s temperature is above freezing, so the falling droplets don’t freeze.
Snow occurs when the atmosphere has below-freezing temperatures, so the precipitation falls as snowflakes or ice crystals. Sleet is a little different – it’s when liquid water freezes on its journey to the ground, as a result of passing through a freezing atmospheric layer.
Extreme weather events, like hailstorms, are formed when thunderstorm updrafts cause water droplets to be lifted above the atmosphere, causing them to freeze into ice. The size of the hailstones depends on how long they’ve had to grow while in the atmosphere.
Infiltration And Percolation
Once liquid or frozen water reaches the ground, there are a few different avenues that it may take.
Infiltration is one possible outcome. It’s used to describe the process in which water seeps through the surface of the ground. The infiltrated water then travels through layers of soils and rocks in a process called percolation. Infiltration and percolation is another essential stage of the water cycle because it replenishes groundwater supplies, like underground aquifers and springs, helping to balance water distribution on earth.
Water’s ability to infiltrate the ground depends on the local geology and surface conditions. Factors affecting infiltration rates include:
- The soil type (some soils are more porous and permeable than others)
- The slope gradient (steeper slopes have a lesser soil depth and have a reduced infiltration capacity)
- The amount of vegetation present (higher plant uptake reduces the amount of water that seeps underground)
- The presence of certain substances, like asphalt and concrete (which are impervious and will have a greatly reduced infiltration rate)
- The intensity and duration of rainfall (the higher the rainfall intensity, the faster the initial infiltration rate, until the soil becomes saturated)
If you live in a region with porous, permeable soil, the rate of infiltration (and therefore groundwater recharge) will be higher. Sandy soil can transmit water more easily than clay soil, which is less porous and permeable.
The moisture content in the soil will also affect infiltration. If the soil is already saturated with water, its infiltration capacity will be lower, and excess water may simply run along the surface until it reaches a surface water source. Dry soil will more readily absorb water and typically has a high infiltration capacity.
How does infiltration and percolation happen?
The process occurs largely due to the combined forces of capillary action and gravity. Water is pulled downward by gravity, so it seeps through soil rather than sitting on its surface. Capillary action is when water moves against gravity in soils with finer textures and smaller pores.
Percolation isn’t only the downward movement of water through the earth – it simply describes the movement of water through soils and rocks in any direction.
The maximum amount of water that soil can hold is known as its field capacity. At this stage, the soil moisture is too high for any more water to infiltrate the earth, and excess water will travel along the surface of the soil in surface runoff.
The eventual outcome of infiltration and percolation is the recharge of groundwater. Water enters groundwater reserves, which are used to supply water in springs and wells.
Note: Infiltration and percolation are often used interchangeably, but the two are slightly different. Infiltration is when water seeps through the surface of the ground, and percolation is when infiltrated water travels underground through rocks and soils.
Collection
In some parts of the world, the liquid water that falls from the atmosphere in precipitation ends up being collected in surface waters, like the ocean, lakes, reservoirs, and rivers. Water may also enter surface supplies as a result of runoff (discussed below), as well as snowmelt from higher ground.
Collection has similar benefits to infiltration. The difference is that this stage of the hydrologic cycle helps to replenish and maintain our surface waters, rather than our groundwater supplies.
Seas, lakes, and rivers are natural water collection zones. There are also man-made reservoirs and ponds that collect liquid water, and many reservoirs are built specifically for storing drinking water prior to treatment.
The size of the water body determines the amount of water it can store. Lakes and reservoirs may store hundreds, even thousands, of gallons of water, while the ocean is estimated to hold more than 352 quintillion gallons of water.
It’s normal for surface water supplies to fluctuate with the seasons, but due to global warming, water scarcity is becoming a real issue. This is exacerbated by the fact that many of our surface waters are polluted, meaning that much of the remaining water is unfit for use.
Even with the natural water cycle continuing as normal, experts predict that the global demand for fresh water will outstrip supply by 40% by as early as 2030.
Precipitation isn’t only collected in water bodies on the earth’s surface. Businesses and individuals also collect liquid water in above-ground and below-ground tanks, which can be used for a variety of purposes. Rainwater harvesting systems are commonly used in off-grid homes with no access to a municipal water supply.
Runoff
As we mentioned earlier, runoff occurs when water travels over saturated soils, or when it travels over impermeable materials.
Runoff is when water travels along the earth’s surface to another location. Water may pool in puddles in low-lying areas, where it eventually evaporates when exposed to sunlight, or it might form rivulets and run into a drain, a lake, a stream, or another surface water resource.
The direction in which water travels depends on the natural topography and slope of the terrain. Water flowing downstream forms pathways called channels, which transport the water towards lower-lying areas.
When water runs into rivers and streams, it’ll increase the water volume in these resources. This water is carried into larger water bodies, helping to replenish their water levels.
Runoff is one of the primary reasons why the earth’s surface continues to evolve in appearance. As water flows over the land surface, it detaches soils, rocks, and other debris, shaping the landscape.
Water from surface supplies gradually evaporates, causing the water to return to the atmosphere and prompting the water cycle to begin again.
🔚 Final Word
Hopefully, you now understand more about the movement of water from earth to the atmosphere, and back again.
Water is a vital resource. Knowing where it comes from is our first step towards conserving and protecting our precious water supplies.
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❔ FAQ
What are the 4 stages of the water cycle?
The 4 key stages of the water cycle are evaporation, condensation, precipitation, and collection. These processes describe the cycle in which water evaporates from surface waters, then vaporizes and collects in surface and groundwater supplies due to infiltration/percolation and runoff.
What are 5 facts about the water cycle?
5 facts about the water cycle are:
- The water cycle is also called the hydrological cycle. Both terms refer to the same main processes of evaporation, condensation, precipitation, and collection.
- The water cycle is continuous, with no starting and ending point. Water evaporates, condenses, returns to the earth in precipitation, and eventually evaporates again, repeating the cycle.
- Energy from the sun drives the water cycle. The earth’s surface is heated by solar radiation, which causes water to evaporate- the beginning of the water cycle.
- The water cycle is essential for sustaining life on earth. Without the water cycle, our natural water sources wouldn’t be replenished. Plants, humans, and animals can’t survive without water.
- There are multiple pathways within the water cycle. Precipitated water may infiltrate into the ground to replenish groundwater supplies, or it may collect in, or travel via runoff to, surface waters.
Why is the water cycle so important?
The water cycle is so important because humans, plants, animals, and marine organisms rely on water to survive. If water escaped the earth’s atmosphere rather than condensing into clouds and precipitating back towards the earth, we’d quickly run out of water resources. Water also carries nutrients and sediment within aquatic ecosystems, supporting aquatic life.
Does the water cycle ever lose water?
No, the water cycle doesn’t gain or lose water. Instead, water is constantly recycled, meaning that the water in the atmosphere eventually ends up back on the ground, and vice versa. However, there’s a problem known as glacial retreat, which is when glaciers melt at a faster rate than the ice can be replaced by precipitation, which means more water is in the atmosphere while our available fresh water supplies deplete.
