Water is indispensable for life. There are two types of water cycles in nature:
Global Hydrological Cycle:
It is the process of evaporation of water into the atmosphere from seas, oceans, lakes, and rivers. Water vapors cool and condense to form clouds. These precipitations come down on earth in the form of rainwater or snow or directly into the sea. Water falling on land either evaporates into the atmosphere or flows down through rivers into the oceans and seas.
Smaller Hydrological Cycle:
It consists of the entrance of water into the living system and its return to the environment. In an aquatic ecosystem, plants and animals take water directly from their surroundings and excrete it. After their death and decay, water returns to the surrounding medium.
In terrestrial ecosystems, plants absorb water from the soil through their roots. Animals take water directly from freshwater sources or in the form of water. Organisms retain a part of this water and excrete the rest. Plants get rid of this water in the form of vapors by transpiration. Evaporation of water from trees makes the surrounding air cool and determines the microclimate.
Steps in Hydrological Cycle:
The various steps of the hydrological cycle are-
(a) Evaporation- Water from oceans, lakes, rivers, and pods evaporates in the form of water vapor by the heat of the sun. The water vapors being lighter than air rise up and go into the atmosphere.
(b) Sublimation- Sublimation is a conversion of solid to gas, without passing through the intermediate liquid phase. Ice sheets and ice caps from the north and south poles, and icecaps on mountains, get converted into water vapor directly, without converting into liquid.
(c) Absorption and Transpiration by Plants- Plants absorb water continuously from the soil with the help of their roots. Part of this water is utilized for photosynthesis. The remaining water which is not used by the green plants is given off into the atmosphere by transpiration (the loss of water from the aerial parts of a plant, mainly leaves in the form of water vapor is called transpiration).
Transpiration is responsible for the huge loss of water from plants and indirectly from the soil. In fact, transpiration is the major source of water vapor in the atmosphere. Respiration by plants and the decay of dead plants also release water vapor into the atmosphere.
(d) Consumption and Release of Water by Animals- Animals consume drinking water from various sources like ponds, wells, lakes, rivers, etc. Animals also take in some water by feeding on plants directly or indirectly. Animals release water vapor into the atmosphere by breathing, evaporation of water from their body surface, excretion, and by decay of their dead bodies.
(e) Condensation- Water vapors being lighter than air, rise in the air and go high up in the atmosphere. As the vapors rise, they get cooled and condense to form tiny droplets to form the cloud.
(f) Ground Water- The tiny droplets of water in the cloud join together to form bigger drops of water. These fall on the earth in the form of rain. A major part of the rainwater falling on earth goes into the ponds, lakes, rivers, and oceans. Some of the rainwater is absorbed by the soil. In the soil, it is retained as soil water. In cold regions, during winters, water falls down in the form of snow. In this way, the water which was lost as water vapor from the earth returns to the earth (both in soil and water bodies) and the water cycle is completed.
Thus, in nature, there is a circulation of water and moisture that is maintained between living organisms, the atmosphere, and the earth. This is known as the water cycle or hydrological cycle. Water is an important as well as a significant environmental factor. Without a hydrological cycle, biological cycles could not occur, ecosystems could not function and life could not be sustained.
Regulation of Hydrological Cycle by Forests:
Forests contain a large number of trees and other plants. A large amount of soil water is transferred to the atmosphere by transpiration in the form of water vapor. More water vapor in the atmosphere brings down more rain on the earth. If forests are cut or destroyed in an area, less rain will take place in that area. Thus forests help in bringing more rainfall to an area and regulate the hydrological cycle.