Runoff

The water cycle describes how water goes from the air to the land and back to the air. To do this, water sometimes is in liquid form, other times it is water vapor. Water can flow over the land in streams and rivers. Other times it flows underground through soil and rocks.

As water moves from one location to another and changes from vapor to liquid, scientists describe what happens as different processes. The water cycle has five main processes. These are:

• Precipitation
  
• Runoff
  
• Infiltration
  
• Groundwater Flow
  
• Evapo-transpiration
  

Precipitation is the process where water vapor in the air becomes liquid or solid water and falls to the land. Precipitation can take the forms of rain, snow, sleet, or hail. Precipitation occurs when areas of high pressure and low pressure air meet, or when areas of hot and cold air meet.

Runoff is the process of water running off the land surface. Water flows downhill from high places like mountains and hills into streams, then rivers, and finally into the ocean.

Infiltration is the process where water seeps into the ground. Just like a sponge soaks up water, so does the soil. When it rains, some of the water will infiltrate or soak into the soil. When the soil is full of water or saturated, then the rain will become runoff.

Groundwater Flow is the process where water in the ground comes back to the land surface. The term groundwater describes water that is stored underground in the rocks or geologic formations. When water infiltrates into the ground, it moves to the groundwater and is stored. Over time, the water stored in the groundwater flows (like a very, very slow river) back to the land surface further downhill. Usually this groundwater flows into the bottom of a creek or river or the ocean.

Evapo-transipration is the process where liquid water either on the land surface, in the soil, or in the ocean goes back into the air as water vapor. The term evapo-transpiration is actually two processes: evaporation and transpiration. Evaporation occurs when liquid water is heated and becomes water vapor, like boiling a pot of water. Water also evaporates when the air does not contain much water vapor. Transpiration describes how plants take liquid water from the soil and turn it into water vapor, which is released through leaves. The water is used to move nutrients from the soil into the leaves. In the leaves, sunlight and water and nutrients all play a part in photosynthesis, where plants store energy in sugars and starches. Evaporation and transpiration are grouped together, because it is hard to tell the difference between them.

 

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