Earth Sciences

What Is The Hydrologic Cycle?

What Is The Hydrologic Cycle?

The hydrologic cycle is a sun energy-powered process in which water moves between the sky, the oceans and the land. The sun causes the water on the ocean’s surface to evaporate and these water vapors rise and condense into very small droplets which then cling to particles of dust. These droplets then form into clouds. Typically, water vapor will stay in the atmosphere for a short period of time, until it becomes precipitation and falls to the Earth as snow, hail, rain, or sleet.

Absorption and Surface Runoff

Some precipitation will fall to land and will either be absorbed, or it will become surface runoff. Surface runoff will gradually flow into streams, rivers, lakes or gullies. The water in the streams will flow seep into the ground, flow to the oceans, or evaporate back into the atmosphere. Transpiration will transfer the water in the soil absorbed by plants back into the atmosphere. The water from the soil will then evaporate into the atmosphere. Together these processes are referred to evapotranspiration.


Some of the soil’s water will seep downward into a porous rock zone which contains groundwater. An underground permeable rock layer is capable of transmitting, storing, and supplying large amounts of water is referred to as an aquifer.

Precipitation and Evaporation

Precipitation mostly occurs on land, but the majority of the Earth’s precipitation (78%) and evaporation (86%) occurs over the oceans. Throughout the world, evaporation and precipitation are balanced. While some areas of the world have less evaporation and more precipitation than others, the reverse is also true, over a few years period on a global scale, everything balances out.


Scientists have determined that the hydrologic cycle consists of five main processes. The hydrologic cycle begins as condensation, turns into precipitation, then infiltration begins, resulting in runoff, which eventually leads to evapotranspiration.

The hydrologic cycle is something you likely learned about in school. It is an interesting process that has a major impact on all of the water that you see in your daily life, including what falls from the sky.

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Jamie is an amateur astronomer and every day space geek.