Hydrology is the scientific study of the movement, distribution, and quality of water on Earth and other planets, including the hydrological cycle, water resources, and environmental watershed sustainability. In this article, we shall discuss the water or hydrologic cycle and its general features. The major processes involved in the cycle will also be explored.
Understanding the Hydrologic or Water Cycle
The water or hydrologic cycle describes the circulation of water as it evaporates from land, water, and organisms, enters the atmosphere, condenses, and is precipitated back to the Earth’s surface. It then moves underground by infiltration or over land by runoff into rivers, lakes, and seas.
The water cycle is also the continuous process by which water is purified through evaporation and transported from the Earth’s surface and the oceans. The total amount of water on Earth remains about the same (>1,404 million km³ or 370 billion gallons) from year to year, but water moves from one compartment to another (e.g., seas, rivers, etc.).
The hydrologic process supplies fresh water to land masses and plays an important role in creating a habitable climate and moderating temperatures worldwide. Water movement back to the seas and glaciers are important forces that shape land masses and redistribute materials.
Plants absorb water from the ground and pump it into the atmosphere through a process called transpiration. This process involves the evaporation of water from plant and soil surfaces and is described as evapotranspiration. Water evaporates as water vapor into the air. Ice and snow can sublimate directly into water vapor.
About 75% of annual precipitation is returned to the atmosphere by plants. Solar energy drives the hydrologic cycle by evaporating water from surfaces. All the physical, chemical, and biological processes involving water as it travels in the atmosphere, over and beneath the Earth’s surface, and through growing plants are important in the hydrologic cycle.
There are many pathways that water molecules may take in the continuous cycle of precipitation and returning to the atmosphere. It may take millions of years when residing in ice caps or flow through rivers into the seas.
It may soak into the soils (infiltration) and later be evaporated from the soil surface into the atmosphere or indirectly through plants (transpiration) back into the atmosphere. It may percolate through the soil into groundwater aquifers or reservoirs or may flow into streams, rivers, or springs.
Water is tapped for use in homes and industries, and used water is returned to the cycle by discharging into streams or on the soil surface, from where it can evaporate or sink into the soil. The processes involved in the water cycle include evaporation, precipitation, groundwater flow, and other components.
The hydrologic cycle also involves the exchange of heat energy, which causes temperature changes. For example, during evaporation, water takes up energy from the environment, cooling the environment.
On the other hand, during condensation, water releases energy to its surroundings, causing warming. The water cycle is important in maintaining life on Earth.
By transferring water from one place to another, or one reservoir to another, the water cycle purifies water, replenishes the land with fresh water, and transports minerals to different parts of the world.
Water also reshapes the geological features of the Earth through erosion and sedimentation. The water cycle also influences the Earth’s climate due to heat exchange.
In the hydrologic cycle, water moves constantly between aquatic, atmospheric, and terrestrial compartments, driven by solar energy and gravity. The total annual runoff from land to the oceans is about 10.3 x 10¹⁵ gallons.
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Detailed Description of the Water Cycle
Rising air currents take water vapor up into the atmosphere, where cooler temperatures cause it to condense into clouds. Air currents move water vapor around the world; cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow or hail and can accumulate as ice caps and glaciers (storing frozen water for thousands of years).
Snow packs thaw, melt, and the water flows over land as snowmelt. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface runoff. Some runoff enters rivers and flows toward the oceans.
Runoff and groundwater are stored as freshwater in lakes. Part of the runoff flows into rivers, while some soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers (storing freshwater for long periods of time).
Some infiltration stays close to the land surface and can seep back into surface-water bodies (e.g., the ocean) as groundwater discharge. Some groundwater finds openings in the land surface and comes out as freshwater springs. Over time, the water returns to the ocean, and the cycle starts again.
The hydrologic cycle begins with the evaporation of water from the surfaces of the ocean and Earth. As moist air is lifted, it cools, and water vapor condenses to form clouds. Moisture is transported around the world and then returns to the surface as precipitation. On reaching the surface, three processes may occur:
- Evaporation of water back into the atmosphere.
- Infiltration of water through the surface into the groundwater. Groundwater may seep into the oceans, rivers, and streams or be released into the atmosphere through transpiration.
- The balance of the water that remains on the Earth’s surface is the runoff, which enters lakes, rivers, streams, and is carried into the oceans, where the cycle begins again.
Changes in the Hydrologic Cycle Over Time
The water cycle describes the processes that drive the movement of water throughout the hydrosphere. The storehouses for most of the water on Earth are the oceans. It is estimated that about 95% of the 1,386,000,000 km³ (the world’s water supply) is stored in oceans. The oceans supply about 90% of the evaporated water that enters the water cycle.
Ice caps and glaciers accumulate in cold seasons and reduce the amounts of water in other compartments, but the ice melts in warm periods, reducing ice storage and increasing the water content of other compartments.
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Impacts of Human Activities on the Water Cycle
Human activities that affect the global water cycle in significant ways include agriculture, industry, alteration of the chemical composition of the atmosphere, construction of dams, deforestation, afforestation, removal of groundwater from wells, water abstraction from rivers, and urbanization.
One of the main sources of atmospheric water is transpiration from the dense vegetation making up tropical rainforests. Deforestation changes the amount of water vapor in the air. This, in turn, most likely alters local and global weather patterns.
Another change in the water cycle caused by humans results from pumping large amounts of water from groundwater for irrigation and domestic purposes. This can increase the rates of evaporation over land, and unless this loss is balanced by increased rainfall over land, groundwater supplies will become depleted.
The water cycle drives the movement of water throughout the hydrosphere. Much of the water is stored in different compartments. The water cycle is important for the circulation of water on Earth among all the compartments. This cycle is driven by solar energy and makes water available to humans and other living organisms.
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