The Hydrologic Cycle

The hydrologic cycle describes the perpetual processes of motion, loss and recharge of the earth’s water. The never-ending exchange of water from the atmosphere to the oceans and back again is known as the hydrologic cycle. The processes can be visualized as shown in Figure. This cycle is the source of all forms of precipitation (hail, rain, and snow), and thus of all water.The water from rivers, ponds, ocean and soil evaporates while water stored in plants transpires to form clouds which store the water in the atmosphere. The vapour in the clouds condenses to result in precipitation. Like all other phenomena in nature, this cycle also derives its energy from the sun. Explanations of various terms used in the figure are given below.

i. Evaporation

Solar energy falling on earth heats up the surface water molecules. These energized water molecules break free from water surface to evaporate and rise as invisible vapour in the atmosphere.

Hydrologic Cycle

ii. Transpiration
All the plants emit water vapour through leaves. This process is named as transpiration. Actively growing plants transpire more vigorously. It could be as high as 5 to 10 times the water it can hold. It is physiological requirement of the plant.

iii. Condensation

The water vapours from evaporation and condensation rise. As temperature falls at higher levels, the water vapour cools and eventually condenses with tiny dust particles present in atmosphere as nuclei. These nuclei grow and agglomerate into clouds. These clouds are mobile and are driven by air currents in the atmosphere.

iv. Precipitation

If these clouds further rise in atmosphere, they get supersaturated with water and that water falls down as rain. If the temperature of surrounding is too low, then hail or snow fall results. All these forms of water getting condensed and falling on earth is called precipitation. The rainfall is more near the mountain ranges because they cause the clouds to rise in air.

v. Run off

Excessive precipitation or snowmelt during summer result in flow of water in treams to rivers or ponds. This visible flow of water on land surface depends on topography of watershed and is called run off. We will discuss watershed in next section.

vi. Infiltration

As the land surface is permeable, part of the precipitation or run off will infiltrate through the surface downwards. This infiltration water reaches the groundwater.

vii. Groundwater

Depending on geology of soil below the surface, the infiltration water is stored as ground water. This groundwater is brought to surface by digging wells for our use.Sometimes, the groundwater can flow into streams or into ocean. Groundwater hydrology is the science that studies the storage and dynamic movement of groundwater.