The Hydrologic Cycle

The hydrosphere is a term that is used to describe all the water on the Earth’s surface. And the cycling of this water is called the hydrologic cycle, or the water cycle.The water within the hydrosphere is found in the atmosphere, as surface water, and as ground water.

During the hydrologic cycle, the water cycles through the earth via evaporationtranspirationrespirationcondensationprecipitationinfiltration, and runoff. As the water moves through this cycle it changes state between liquid, solid and gas phases.

Day 32.1

Evaporation is the process of water changing from a liquid to a gas. Most people understand that the energy from the sun causes water from the surfaces of oceans and lakes to evaporate. But many don’t know that water loss from plants (transpiration) and animals (respiration) contribute to the evaporation stage of the water cycle.

Condensation is the process of water changing from the gaseous state to the liquid state. As the water evaporates and begins to cool high in the atmosphere, it begins to turn from the gaseous state to the liquid state. The tiny water droplets collect around small particles of dust and form collections of floating water molecules. This is most commonly observed as clouds or fog. As the water molecules continue to collect, they increase in size and mass and begin to fall from the sky back to the earth in the form of precipitation. Water precipitates in the form of rain, sleet, hail, or snow.

Day 32.2

As water reaches the ground, some water is absorbed into the soil in process referred to as infiltration. The speed by which the water is absorbed in the soil is based upon the moisture level of the soil and the type of soil involved. Most of the water travels across the surface of the land in a process known as runoff. This includes surface runoff as well as water flowing as streams and rivers.

Day 32.3

Each phase change of the water cycle requires energy and in most cases, this energy is provided by the sun. In fact, the sun provides most of the energy on Earth. It is the sun’s energy that drives the wind, ocean currents, and allows plants to grow.

It is also important to understand that the flow of water on the Earth is a cycle. Therefore, the water in the hydrologic cycle remains constant. If we were to look at the amount of water that falls as precipitation in a particular area, it would be roughly the same as the amount of water that is evaporated or stored as ground water in that same area.

Water Average rate (103 km3/year)
Precipitation over land 107
Evaporation from land 71
Runoff & groundwater from land 36
Precipitation over oceans 398
Evaporation from oceans 434

(data retrieved from

So where is the Earth’s water now?

97% of the water is found in oceans and will remain there for 3,200 years! 2% of the water is frozen in ice caps and glaciers and will remain frozen for anywhere between 20 to 100 years. The remaining 1% is found as groundwater, lakes, streams, and rivers in the atmosphere. The water will remain in these places for as long as 10,000 years or as little as 9 days!