Understanding the circle of water: the hydrologic cycle

Posted: Thursday, July 31, 2008

Updated: Friday, September 19, 2008

Where does our water come from? The hydrologic cycle, the system by which water is continuously cleaned and reused by the earth’s systems.

Most people appreciate the value of a cold glass of water or a dip in the pool on a hot summer day. Water is one of the most fundamental necessities to daily life, and one of the most abundant resources on the planet. However, most of the world’s water is salt water, while much of the remaining fresh water is frozen in ice sheets and glaciers. Overall, less than one percent of the world’s water is actually available for human consumption. Most of this water is found in rivers, lakes, and groundwater, although some of it is in the air around us. We know that we all use water, but where does our water come from?

The answer is the hydrologic cycle, the system by which water is continuously cleaned and reused by the earth’s systems. As with any cycle, there is no set starting point; however, there are a few main components that make up the cycle.

A diagram showing the hydrologic cycle of water

  1. Water is absorbed into the ground by infiltration. Some of the water may pass (percolate) through holes and small cracks. Vegetation can help increase the amount of water that is absorbed. Water absorbed into the soil can collect underground, forming groundwater.
  2. Water that is not absorbed by the soil will wash off the surface. Runoff can help refill our lakes, creeks, and wetlands (surface waters), but it also can carry pollutants and increase the potential for flooding.
  3. Through evapotranspiration, clean water vapor is released into the air directly by plants (transpiration) or by heat from the sun on the water’s surface (evaporation).
  4. Warm air can hold more water vapor than cool air, which is why we often have humid days in the summer and dry days in the winter. As warm moist air cools, it forms small water droplets—condensation—that may appear as clouds or fog.
  5. As water droplets condense they may form larger droplets that eventually become heavy enough to fall as rain or snow, called precipitation.

Basically, you can think of the hydrologic cycle as a big water purification system that operates world-wide and free of charge. Any water that enters the system can be recycled. For example, the water molecules in your glass once may have been drawn up by the roots of an oak tree, swallowed by a fish, or resided momentarily in a drop of dew.

Of course it isn’t always that simple—other components can affect the cycle. In a native woodland or prairie, much of the rainfall is absorbed in the ground or may be captured by trees and other vegetation. Runoff is usually minimal and contains relatively low amounts of pollutants such as dissolved chemicals or sediment.

However, in developed communities such as Minnetonka, transpiration and infiltration are often reduced because much of the original vegetation has been removed and the soil has been covered for the construction of buildings and paved surfaces. Accordingly, surface runoff is higher. However, there are ways to help reduce runoff and pollutants and help absorb water into the ground. Follow these five simple tips to help keep the hydrologic cycle functioning smoothly.

  • Capture and absorb water into the soil with a rain garden or native vegetation. Trees and shrubs have extensive root systems that can help increase infiltration.
  • Use proper watering practices when needed. Consider installing a rain barrel to re-use water from your roof.
  • Fix or replace leaking plumbing or appliances. A dripping faucet wastes water and energy.
  • Avoid using chemicals and fertilizers in your yard where possible and apply them properly when needed to avoid waste and runoff.
  • Cover exposed soil, which tends to absorb less water and can add sediment and nutrients to surface runoff.