The Water Cycle

Evaporation Water on Earth's surface enters the atmosphere through evaporation. Heat energy from the Sun breaks the bonds that hold water molecules together. The free molecules spread out and become a gas.

Evaporation The Sun is the driving force behind the water cycle, which begins with evaporation. Nearly 90% of moisture in the atmosphere evaporates from oceans, seas, lakes, and rivers.

Evaporation On a global scale, the amount of evaporated water equals the amount of precipitation. This keeps the water cycle in balance. Although global evaporation equals global precipitation, it is not equally distributed over the earth. Evaporation is greater than precipitation over oceans. The opposite is true over land, where precipitation is greater than evaporation.

Evaporation Evaporation requires energy and water. It takes about 600 calories of heat to evaporate one gram of water. The warmer the climate is, the more heat energy is available for evaporation. Most of the world's evaporation happens in tropical areas near the equator.

Infiltration and Runoff Earth's surface acts like a giant sponge as precipitation seeps into soil and rock. This process is called infiltration. Infiltrated water replenishes groundwater, which can be deep underground. The amount of infiltration depends on several factors:

Infiltration and Runoff Precipitation intensity and duration—more water infiltrates when precipitation happens over an extended period. Soil properties—sandy soils absorb more water than clay soils. Soil saturation—dry soil absorbs more water than wet soil. Slope of the land—flat areas absorb more water than slopes.

Infiltration and Runoff Water that doesn't infiltrate flows over the ground as surface runoff. If you've ever seen a stream of water flowing downhill during a rainstorm, you have seen surface runoff. Rivers and streams also carry surface runoff. About 1/3 of runoff water returns to the ocean. The rest reenters the atmosphere through evaporation and transpiration. Eventually, this water will condense and fall back to the surface as precipitation.