Water is one of the key ingredients to life on earth. About 75 percent of our planet is covered by water or ice. The oceans hold about 97 percent of the water on earth. About 1.7 percent of earth’s water is stored in polar ice caps and glaciers. Rivers, lakes, and soil hold approximately 1.7 percent. A tiny fraction—just 0.001 percent—exists in the earth’s atmosphere as water vapour.
The earth’s water cycle began about 3.8 billion years ago when rain fell on a cooling earth, forming the oceans. The rain came from water vapour that escaped the magma in the earth’s molten core into the atmosphere. Energy from the sun helped power the water cycle and earth’s gravity kept water in the atmosphere from leaving the planet.
Water cycle is the endless process that connects all that water. In its three phases (solid, liquid, and gas), water ties together the major parts of the earth’s climate system — air, clouds, the ocean, lakes, vegetation, snow, and glaciers.
The water cycle explains how the earth’s fixed supply of water collects, purifies, distributes, and recycles. Through the water cycle the water remains unchanged and is transformed from one physical state to another. The water cycle is powered by energy and gravity.
The sun enables water to evaporate into the atmosphere while gravity allows water to be drawn back onto the earth’s surface as precipitation. Precipitation is any liquid or frozen water that forms in the atmosphere and falls to the earth.
Water cleans itself through evaporation and condensation
Water is naturally filtered and purified as it flows above ground through streams and lakes and below ground aquifers. Soil filters water naturally by physically removing large debris and particles as water percolates down through the soil layers. Bacteria and microorganisms in the soil further purify water by breaking down chemicals and contaminants.
Earth’s water is always moving
The changing of water into water vapour is known as evaporation. Approximately 84 percent of water vapour in the atmosphere comes from the ocean. The rest comes from land.
Transpiration is the process of evaporation through plants. The water evaporated from plants comes from two different sources. Water falls on plants as rain, dew, or snow. Plants also absorb water from the soil. The water moves from the roots through the stems to the leaves.
Once water reaches the leaves, some of it evaporates from the leaves, adding to the amount of water vapour in the air. Water vapour is a greenhouse gas. Greenhouse gases such as water vapour and carbon dioxide insulate the earth and keep the planet warm enough to maintain life as we know it.oo much of these greenhouse gases can cause earth’s atmosphere to trap more and more heat. This causes earth to warm up.
Clouds are an important function in the water cycle. Clouds move water from one place on earth to another. Clouds alter the temperature of the air and earth’s surface. They influence how much of the sun’s energy is absorbed and trapped in the atmosphere.
Condensation is the process where water vapour becomes liquid. When the water droplets in clouds combine, they become heavy enough to form raindrops to rain down.
Air can only hold a certain amount of water vapour, depending on the temperature and weight of the air – or atmospheric pressure – in each area. The higher the temperature or atmospheric pressure, the more water vapour the air can hold.
When a certain volume of air is holding all the water vapour it can hold, it is said to be saturated. When a saturated volume of air cools or the atmospheric pressure drops, air is no longer able to hold all that water vapour. The excess amount changes from a gas into a liquid or solid (ice).
How water comes back as rainfall
Precipitation is the process by which atmospheric water under gravity is returned to the ground. Clouds are created when water vapour turns into liquid water droplets. These water droplets form tiny particles, like dust, that are floating in the air.
In the cloud, with more water condensing onto other water droplets, the droplets grow. When they get too heavy to stay suspended in the cloud, water is released from clouds in the form of rain or snow.
Some of the precipitation stays on the surface until it evaporates marking a start to the water cycle. The rest flows downhill as runoff. Runoff is surface water that moves across the surface of the land and enters streams and rivers and eventually to the ocean. Runoff keeps rivers and lakes full of water and changes the landscape by the action of erosion.
Infiltration occurs when water moves into the ground from the surface and begins to soak into the soil and rock layers underneath. Soil temporarily stores water, making it available for root uptake, plant growth and habitat for soil organisms. Percolation is the movement of water through the soil itself and into the ground to replenish earth’s groundwater.
The precipitation that does not infiltrate the ground or return to the atmosphere by evaporation (including transpiration from plants) is called surface runoff. Surface water is any body of water above ground, including streams, rivers, lakes, wetlands, reservoirs, creeks, and ocean.
The region from which surface water drains into a river, lake, wetland, or other body of water is called its watershed or drainage basin. Surface water participates in the water cycle, precipitation, and water runoff feed surface water bodies.
Freshwater can also be found underground
Water table is an underground boundary between the soil surface and the area where groundwater saturates spaces between sediments and cracks in rock. Water pressure and atmospheric pressure are equal at this boundary. The water table falls in dry weather or when we remove groundwater faster than its replenished and rises in wet weather. The water table is located at the top of the zone of saturation.
Deeper down are geological layers called aquifers – porous, water saturated layers of sand, gravel, and bedrock through which groundwater flows. Aquifers are further categorised into confined aquifers and unconfined aquifers.
Unconfined aquifers allow for water to seep from the surface directly into the aquifer. Its upper surface is open to the atmosphere through permeable material. Confined aquifer has a layer of impermeable rock which prevents water from the ground surface to seep in. Instead, the water must seep in from a further location where the layer of rock or dirt does not exist.
Unconfined aquifers form at a quicker rate compared to confined aquifers. This is because they are closer to water sources from rain, streams, or rivers. Confined aquifers are fed by underground tributaries. Confined aquifers, replenishment can take a long time, as its sources of water are underground systems that must travel long distances.
Confined aquifers are sometimes referred to as fossil aquifers. They are found deep underground and were formed many years ago. Some have long since been cut off from replenishment sources. Accessing them as a source of water supply equates to mining because they will eventually be completely used up.
How do human activities modify the water cycle?
Clearing forests and vegetation from land for agriculture, mining, road and construction. Forest and vegetation cover play an important role in evaporation and transpiration. Reducing plants leads to declining rainfall, subjecting the area to drought.
On the other hand, tropical forests catch huge volumes of rainfall falling from the sky. Rainforests soak up rainfall brought by tropical storms by intercepting rainfall and allowing slow infiltration into the soil. This regulates floods and river levels. Without forest cover infiltration rates are affected and more overland flow occurs. This means more destructive flood and drought cycles can occur when forests are cleared.
Agriculture increases soil erosion (surface water runoff). Post-harvest fields are bare of vegetation. Farmed crops compared to natural vegetation cover are poor in controlling soil erosion. Agriculture often reduces vegetation cover and soil compaction from machinery affecting the ability of water absorption by soil and rocks underneath the earth’s surface. Drainage patterns are changed too. Farmers intentionally dig drainage ditches within and around their fields to prevent water logging of plants. This means that water moves initially via overland flow and then via small channels into rivers, affecting annual patterns of those rivers. Water drainage patterns are changed too.
Water pollution. Herbicides and pesticides are soluble in water hence can enter the water cycle and contaminate water at every stage of the cycle. Herbicides and pesticides interfere with the balance of the ecosystem and water bodies harming plants, animals and microorganisms that are important to the cycle. Excess nitrogen that drains from the soil as runoffs into the rivers introduces additional nutrients leading to unnatural enrichment of water bodies. These organic and inorganic nutrients lead to imbalance in marine life.
Other human activities changing water cycle
Construction of dams for agriculture or electricity affect water cycle. Dams are often built across rivers to store water that would naturally find its way to the lower reaches of the river and into the sea. The presence of the dam upsets the natural balance of the river, affecting the animal and plant life in and around it.
Groundwater abstraction is the process of taking water from a ground source. It is often pumped through boreholes and wells from underground aquifers, as a source of freshwater. Over abstraction can lead to surface rivers drying up or the level of groundwater aquifers and the water table reducing.
Groundwater abstraction may have two effects. 1) Saltwater intrusion occurs in coastal regions where saline water can migrate inland and upward when freshwater is pumped out of the ground affecting water quality. 2) Land subsidence occurs when large amounts of groundwater have been withdrawn from certain types of rocks. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rocks fall on themselves.
Positive feedback loop. Human activities don’t directly cause the increase of water vapour in the atmosphere. When greenhouse gases like carbon dioxide and methane increase, the earth’s temperature rises. Rising temperatures increase evaporation by plants, soil, and water. More water evaporates from their surfaces ends up in the atmosphere as water vapour.
Because warmer air holds more moisture, its concentration of water vapour increases. This happens because water vapour does not condense and precipitate out of the atmosphere as easily at higher temperatures.
The water vapour then absorbs heat radiated from earth and prevents it from escaping out to space. This further warms the atmosphere, resulting in even more water vapour in the atmosphere.