The Water Cycle

A diagram showing the water cycle.

As the cliché goes, water is life. Water is a naturally occurring element that is utilized by all living as well as non-living things. Water is in a perpetual state of movement and transition from one state to another. There are three states of water namely solid (ice), liquid, and vapour.

Water Cycle Definition

The water cycle, which is also known as the hydrological cycle or the hydrologic cycle, refers to the endless movement of water below, on, or above the earth’s surface. The total mass of water present in earth is relatively constant over time. Whether this water is present as salty, fresh or in the atmosphere is dependent on a broad range of weather variable. Whatever these variable are, the mass remains constant. For example, for the amount of water in the atmosphere to increase, then it means the amount of either saline water or fresh water has decreased. Water is matter. Basic science shows that matter cannot be created or destroyed, but it can be changed. There are various processes through which water moves from one state to another including evaporation, infiltration, both surface and underground flow, as well as condensation and precipitation. Water can change states (ice, liquid or vapour) through evaporation, condensation, and precipitation. Surface or sub-surface flow only serves to move water from one location to another.

Roughly 96% of the total amount of water on the earth is composed of saline water. Only 4% is left to make up fresh water. Approximately 68% of the 4% of total fresh water is tied up in glaciers and ice with another 30% underground.

Water Cycle Steps

The sun is the major source of energy for the cycle. When the sun is out, it heats any water that is out in the open. The bigger the water body, the bigger the effect. The water particles take up energy from the sun and in turn evaporate into the atmosphere in liquid form. For the water to evaporate, it does not have to be in liquid form. Ice and snow can also be evaporated. Snow and ice can either turn to liquid state first and then be evaporated, or the temperature can be high enough to sublimate instantly to vapour. Sublimation is a process where a substance changes from solid to gas directly without turning into liquid first. Another form of evaporation is known as evapotranspiration. Evapotranspiration refers to the water vapour that is obtained mostly from plants.

The water molecule is made up of two atoms of hydrogen, and one atom of oxygen and its chemical formula is H₂O. The H₂ represents the two hydrogen atoms, and O is for oxygen. Atoms can be simply defined as the building blocks of molecules. The main components of the air in the atmosphere are nitrogen and oxygen. Both of these elements are denser than the water molecule. As a result, the water molecule is capable of gaining height in the atmosphere as a result of buoyancy. It is common knowledge that heavier substances stay at the bottom with the lighter ones moving to the top.

It is common knowledge that, the higher you go, the colder it becomes. As the vapour gain altitude, the air pressure keeps on dropping. The air pressure is directly proportional to the temperature, so the temperature also drops. When the temperature drops sufficiently, the water vapour, full of energy from the sun, start transferring this heat to the surrounding atmosphere. When the water vapour transfers enough of its energy, precipitation (tiny water droplets) forms in a process called condensation. A massive concentration of the precipitation is visible from the surface of the earth as clouds. The higher the level, the darker the clouds. Sometimes, fog or mist can be formed when the condensation takes place close to the surface. Condensation close to the ground can be due to a sudden drop in air pressure or when a draft of warm moist winds collides with a cold wind.

Clouds full of precipitation can fall in the region they formed, or they can be blown away by winds to deposit their load in other areas of the world. Precipitation can fall in different forms depending on the temperature in the clouds. When the temperature is above 2⁰, then the precipitation will most likely be in liquid form, also known as rain. On the other hand, when the temperature is below 2⁰, then crystal particles will form which will fall as hail, sleet,or snowflakes that forms snow. When snowing takes a avery long period of time, then glaciers are formed, which are capable of storing water for aeons. Most of the precipitation will fall on the earth in the form of water , which creates periodical rivers as well as develop run-offs on the surface of the planet which can be drained off into seasonal bodies like swamps with most of it draining into seas and oceans. Some small amount of this water infiltrates into the ground to be used by plants. Some of it supplies some of the underground streams and rivers. Some of this precipitation also replenishes aquifers. Aquifers can be described as natural underground water storages whose primary purpose is water storage. The water also feeds springs and geysers. As time progresses, the majority of this water finds its way into oceans, and other water bodies and the cycle is repeated all over again.

Other Processes in the Water Cycle

There are two primary processes involved. The first one is precipitation. Precipitation refers to the water vapour condensed high up in the atmosphere which falls to the surface of the earth. Most of the precipitation falls as rain while some of it comes down in the form of snow, sleet, hail, fog, among others. Approximately 78% of the global precipitation falls over the ocean with most of the remaining percentage falling over land while a small portion forms snow. Some of the water vapour may change directly to ice in a process known as deposition.

The second one is evaporation. Evaporation is the process through which water turns from liquid to a gaseous state and rises into the atmosphere. When evaporation is mentioned, it also includes transpiration from vegetation. Collectively, the vapour from plants and water bodies is known as evapotranspiration. It comes as no surprise that most of the gas comes from oceans and large water bodies since they are more exposed to the effects of the sun. Evaporation from oceans accounts for a whopping 86% of global water vapour. Some water may sublime.

There are several minor processes, such as infiltration. Infiltration refers to the myriad ways that water traverses on or beneath land. As the water flows, some of it is drained into large water bodies while some of it may filter down into the earth. Infiltration also leads to a process known as sub-surface flow. As the name suggests, subsurface flow is the movement of water beneath the surface of the earth. Some of this water is deposited into aquifers, drained into oceans or comes back to the surface as springs.

Others include snowmelt, which is the water produced by melting ice or snow. Canopy interception is another process which refers to precipitation captured by plant leaves. The rain evaporates back without falling onto the ground. Water can also be influenced by gravity in the process of percolation, or it can enter the mantle through the process of plate tectonics.

Effects on Climate

Energy is a crucial element in the water cycle. One of the consequences of the sequence is changes in temperature. For instance, through evaporation, water takes up energy (in the form of heat) from nearby sources which in turn reduces the temperature of the surroundings. Conversely, when water is in the form of vapour, it is full of heat energy. For it to condense, then it must release this heat to the surroundings which in turn leads to a surge in temperature. The condensation in the atmosphere can lead to the formation of rain, or if the conditions are suitable, snowing will take place. These antagonistic processes play a significant role in influencing climate.

Aside from influencing climate, the cycle is also responsible for the purification of water when it evaporates. When dirty liquid water evaporates, only the water particles turn to vapour. The impurities that are in the water are left on the surface. When this vapour falls in the form of rain, the water is pure and clean for human consumption. The flow of water on the surface of the earth and beneath it also plays a crucial role in moving minerals from one region to another. Most living things require certain minerals that are found in water for their survival and well-being. Most of the underground flow also helps to shape some of the magnificent and breathtaking structures located in the world. An excellent example of such an arrangement is a stalactite. There are some negative impacts to surface runoff with the most common being erosion.


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