Hydrological Cycle

Hydrological Cycle – UPSC World Geography Notes

The water cycle, scientifically termed the Hydrological Cycle, delineates the path traversed by water molecules from the Earth’s surface to the atmosphere and back, intermittently penetrating below the surface. To delve deeper into this cycle, read on.

Hydrology, a scientific discipline within physical geography, is dedicated to investigating the water cycle. It concentrates on comprehending the worldwide genesis, distribution, and attributes of water.

This expansive system, energized by the Sun, sustains an unceasing exchange of moisture among the oceans, the atmosphere, and the land.

Hydrologic cycle

The hydrologic cycle encompasses the perpetual movement of water within the Earth-Atmosphere system.

Fundamentally, the water cycle delineates the journey of water as it cycles from the Earth’s surface to the atmosphere and returns.

The global water cycle can be characterized by nine fundamental physical processes that collectively define the continuous flow of water.

  • Water traverses diverse layers of the Earth’s surface, such as oceans, glaciers, and lakes, concurrently progressing (albeit at a slower pace) through the sub-surface soil and rock layers. These paths are intricate.
  • Subsequently, the water is released back into the atmosphere.
  • The fundamental characteristic of the hydrologic cycle is its lack of a definitive beginning or end.

Research indicates that over 90% of the moisture in our atmosphere originates from the evaporation of oceans, seas, and various water bodies such as lakes, rivers, and streams. Evaporation is the process whereby water changes from a liquid to a gas.

The remaining 10% is primarily released into the atmosphere by plants through transpiration. Plants absorb water through their roots, and it is subsequently expelled through tiny pores on the underside of their leaves.

Additionally, a minute quantity of water vapor enters the atmosphere through sublimation, wherein water undergoes a direct transformation from a solid state (ice or snow) to a gaseous form.

In cases where the temperature remains below freezing, sublimation contributes to the gradual reduction of snow banks.

Stages of the Hydrologic Cycle

Evaporation, condensation, precipitation, interception, infiltration, percolation, transpiration, runoff, and storage represent some of the key processes involved.


Evaporation is the transformation of a substance from a liquid to a gas. In meteorology, water holds a paramount significance.

The process of evaporation requires energy to take place, and potential sources of this energy include the sun, the atmosphere, the planet, or anything on Earth, including people. Most individuals have firsthand experience with evaporation.

For instance, when the body becomes hot due to atmospheric temperature or exertion, it sweats, releasing water onto the skin. The purpose is to utilize the body’s heat to facilitate the evaporation of the liquid, extracting heat and cooling the body. This effect is akin to the sensation experienced after stepping out of a shower or swimming pool.


Transpiration involves the evaporation of water from plants through stomata.

  • Stomata, tiny openings associated with vascular plant tissues, are visible on the underside of leaves.

The passive process of transpiration in most plants is regulated by the humidity of the atmosphere and the moisture content of the soil.

Merely 1% of the water lost during a plant’s transpiration contributes to the plant’s growth, with the remaining 99.9% being released into the atmosphere.


Condensation signifies the conversion of atmospheric water vapor into a liquid state. In the atmosphere, condensation manifests as clouds or dew.

Two distinct temperatures, the dewpoint temperature and the air temperature, must diverge for condensation to take place. The dew point, essentially the temperature conducive to dew formation, represents the point at which air becomes saturated upon cooling. Further cooling prompts water vapor to condense. When the dew point and air temperature coincide, foggy conditions often emerge.

Condensation serves as the antithesis of evaporation. As water vapor possesses higher energy than liquid water, condensation releases excess energy in the form of heat, a phenomenon contributing to the formation of hurricanes.


As small condensation particles collide and merge, if they grow too large for the ascending air to sustain, they coalesce into precipitation and descend to the ground. This precipitation can take various forms, such as snow, sleet, hail, or rain.

On our planet, precipitation serves as the primary supplier of fresh water. Both over land and oceans, the Earth receives an average of approximately 38 1/2 inches (980 mm) of precipitation annually.


When the Earth becomes saturated and experiences significant precipitation, runoff takes place as the land can no longer absorb additional water. Runoff contributes to the formation of rivers and lakes. While some runoff undergoes evaporation in the atmosphere, the majority of water in rivers and lakes eventually returns to the oceans.

If a lake is the ultimate destination for runoff without an outlet for water to flow out, evaporation becomes the sole method for the water to return to the atmosphere. As water evaporates, salts or contaminants are left behind, leading to the salinization of the lake, akin to the Great Salt Lake in Utah or the Dead Sea in Israel.

The evaporated runoff re-enters the atmosphere, initiating the restart of the hydrologic cycle.

Hydrologic Cycle and Water Storage

In the global water cycle, water is primarily stored in three main reservoirs: the atmosphere, the Earth’s surface, and the ground.

The movement of water stored in the atmosphere can be relatively swift from one part of the Earth to another.

Surface and subsurface storage depend largely on geological factors associated with the specific types of soil and rocks present in the storage location. Surface storage includes oceans, lakes, reservoirs, and glaciers, while subsurface storage comprises soil, aquifers, and cracks in rock formations.

Water follows unpredictable paths through the eight additional primary physical stages of the water cycle.

The water in the atmosphere undergoes replacement approximately every 16 days. Soil moisture is generally replenished annually, while wetlands experience replenishment about every five years, and lake water remains in one area for nearly seventeen years.

In places where societal development is limited, groundwater regeneration can extend over 1,400 years.

The uneven flow and distribution of water over time, as well as its spatial distribution in different geographic and geological locations, can lead to extreme events like floods and droughts.

FAQs on Hydrological Cycle

1. What is the water cycle and its importance?

  • The water cycle is the continuous process of water circulating between the Earth’s surface, atmosphere, and underground. It involves processes like evaporation, condensation, precipitation, and more. Its importance lies in sustaining life, regulating climate, and maintaining the Earth’s water balance.

2. What are the forms of the hydrological cycle?

  • The hydrological cycle comprises various forms, including evaporation, condensation, precipitation, infiltration, runoff, transpiration, and storage. These processes collectively contribute to the continuous movement and distribution of water on Earth.

3. Why is the water cycle important?

  • The water cycle is vital for several reasons. It ensures the availability of fresh water, regulates Earth’s temperature, supports ecosystems, and plays a crucial role in weather patterns. Additionally, the water cycle is essential for agriculture, providing the necessary moisture for plant growth.

4. What is meant by the water cycle?

  • The water cycle, also known as the hydrological cycle, describes the continuous movement and transformation of water on Earth. It involves processes like evaporation, condensation, precipitation, and runoff, maintaining a dynamic equilibrium in the distribution of water across the planet.

5. What is the difference between evaporation and condensation?

  • Evaporation is the process in which liquid water transforms into water vapor due to heat. It usually occurs at the surface of bodies of water. On the other hand, condensation is the phase change from water vapor to liquid water, often forming clouds. Evaporation involves the absorption of heat, while condensation releases heat back into the surroundings.

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