About 70% of the earth’s surface is covered with water.  This water evaporates and condenses into clouds.  The clouds cause precipitation and this water falls back to the earth’s surface, ready to start the cycle again. 

The water on Earth now is the same water that’s been on Earth since the beginning.  The rain that falls on us is the same water that rained on the dinosaurs, King Tut, and George Washington.  What makes that awesome feat possible?  The water cycle.  The water cycle is the process that re-circulates water so we can have bodies of water as well as clouds and precipitation.

The first step of the water cycle is evaporation. About 85% of the water vapor in the air comes from water that evaporated from the oceans.  The other 15% comes from evapotranspiration, which is a catch-all term for water that evaporates from over land. This includes water vapor produced by plants during transpiration, water from lakes, streams, puddles and soil moisture, direct evaporation of snow and even water vapor from the breath of animals. 

The second step of the water cycle is condensation. Now that the atmosphere is full of water vapor, that water vapor condenses into water droplets.  Sometimes, like early in the morning, the water vapor condenses on the grass as dew and seeps back into the soil, ready to be evaporated again. But most of the water vapor condenses higher up in the air and forms clouds. Once the water droplets are in a cloud, two things can happen.  Either the cloud will dissipate and the water droplets will become vapor again, or the cloud will grow and it will begin to precipitate.

The third and final step of the water cycle is precipitation. Precipitation includes all water that falls from the sky, both in liquid and frozen form, which reaches the ground. Once the precipitation makes its way to the ground it can end up soaking into the ground, run off into streams and lakes, become snow cover, be used by plants, be inhaled by animals or fall directly back into the ocean. Then the water cycle can begin again and continue for millions of years to come.

Aside from the above steps of the water cycle, there are also ways that water can be stored on Earth that play a role in the water cycle at various times throughout the year. Water can be stored in lakes, streams, glaciers, icebergs, and the ground.

As precipitation falls towards the earth, some of the water seeps into the ground, a process known as infiltration. The amount of water that seeps into the ground varies with several factors such as duration, type, and strength of precipitation, type of soil, slope of the land, and land cover. Duration and strength of precipitation play a role in the amount of water that the ground can hold and whether the water will seep into the ground or travel across the ground surface. Certain types of soils hold water better than others and can absorb more water. As the slope of the land varies, so does the amount of water that will be able to seep into the ground. If there is a steep slope, the water will be more likely to just run down the hill rather than get absorbed by the ground. If the ground has no slope, the water will be more likely to seep into the ground or remain above the ground (as over a flat road) and potentially cause flooding. The amount of water that the ground absorbs also will depend on the land cover. Vegetation impacts the speed of water that will move across a surface. More vegetation leads to slower flowing water.

The factors that impact infiltration also impact surface runoff. Runoff occurs when water flows across the ground surface. If no water is able to seep into the ground, the water will flow across the ground surface. This occurs many times near mountains as water flowing quickly down a mountainside is unable to be absorbed by the ground. If a certain type of soil does not absorb water very well or if the soil is already saturated, then the water remains on the surface. Clay is an example of a soil that absorbs water slowly in comparison to sandy soils. More runoff will occur over land with clay soil rather than land with sandy soil. Runoff can also occur during snowmelts from mountainsides.

Activities to further explain this module: