The hydrologic cycle is the central theme of the study of hydrology. Hence, it is important to know what hydrologic cycle is all about, and the various processes involved in the hydrologic cycle.
Understanding the hydrologic cycle therefore, is basic to understanding water supply and is a key to the proper management of water resources.
Definition of Hydrologic Cycle
The movement and endless recycling of water between the atmosphere, the land surface, and underground is called hydrologic cycle. This movement, driven by the energy of the sun and the force of gravity, supplies the water needed to supported life.
Similarly, Viessman and Lewis (2003) defined hydrologic cycle as a global sun-driven process whereby water is transported from the oceans to the atmosphere to the land and back to the sea.
The ocean is the earth’s principal reservoir; it stores over 97 percent of the terrestrial water. Water is evaporated by the sun, incorporated into clouds as water vapour, falls to the land and sea as precipitation, and ultimately finds its way back to the atmosphere through a variety of hydrologic processes.
The hydrologic cycle can be considered a closed system for the earth because the total amount of water in the cycle is fixed even though its distribution in time and space varies. There are many sub-cycles within the worldwide system, which are generally open-ended.
It is these subsystems that give rise to the many problems of water supply and allocation that confront hydrologists and water managers (Viessman and Lewis, 2003).
Processes in the Hydrologic Cycle
The entire process in the hydrologic cycle can be divided into five parts, which include: condensation, precipitation, infiltration, runoff and evaporation. The hydrologic cycle is very complex.
This is because all biological lives depend on water. However, it is necessary, in spite of its complexities to look at the major path-ways or processes in the movement of water through the cycle.
Starting from the ocean, which is sometimes termed as the reservoir of water, evaporation takes place due to solar heating. The evaporated water moves into the upper atmosphere, where it condenses and may fall back as rain into the ocean bodies.
However, where there is an onshore wind, the evaporated water could be blown to the land in form of moisture laden wind. Over the land, condensation can occur especially where the moisture laden wind is forced high up into the atmosphere due to topography. This may result in orographic rains.
Even right on the land evaporation could take place from stagnant water body, the soil and vegetation-in a combined processes known as evapotranspiration. Rain that falls on land is used in very many processes.
Plants and animals depend on it for survival. Some of it sinks into the soil through infiltration, flows underground into river systems. Where the soils are saturated, runoffs are generated. The runoffs could flow into river systems. Man can even tap water from the soils by sinking bore holes and wells. The rivers could be dammed for irrigation purposes, domestic and industrial processes.
Water could also be evaporated from dams and in other processes in which man uses it. The evaporated water moves up into the atmospheric system where it can also condense and fall as rain. The movement of water continues along the river systems.
As the rivers move into the ocean, evaporation continues to take place; until it eventually empties into the ocean.
It is generally observed that evaporation is the driving force in the movement of water in the hydrologic cycle.
The movement of water just discussed is a simplified model of the hydrologic cycle. In nature, the movement of water could be much more complex and complicated.
The hydrologic cycle is so fundamental to sustainable water supply because it continues to move water and keep sources fresh. Without this process, life on Earth would be impossible.
In summary, it is no longer in doubt that the hydrologic cycle is the major theme of the study of hydrology. Most of the basic concepts and processes studied in hydrology have their roots in the hydrologic cycle, as would soon be revealed in the following modules.
It can safely be concluded that without the hydrologic cycle that continue to purify our water supply sources, life on Earth would have been greatly uttered.
The hydrologic cycle is the endless movement of water between the atmosphere, land and underground, driven by the sun and force of gravity.
The entire process in the hydrologic cycle can be divided into five parts- evaporation, condensation, precipitation, infiltration and runoff.
Evaporation caused by the sun’s energy is the driving force in the movement of water in the hydrologic cycle.
The hydrologic cycle is so fundamental to sustainable water supply because it continues to move water and keep sources fresh.