Calculate your stormwater runoff

A stormwater activity you can do at home

What is stormwater runoff and why is it a problem? When rain falls or snow melts in Bothell, where do you think the water goes?  Does it soak into the ground?  Does it sit in a puddle until it disappears? If you answered "yes," then you are partially correct. Some water soaks into the ground or evaporates into the air.

Where does the rest of the water go?

Do you know what happens to the rainwater that doesn't evaporate or soak into the ground? Some of it flows over hard surfaces (like roofs, parking lots, driveways, sidewalks, roads, etc.) and into the nearest storm drain, stream, or ditch. This water is called "runoff" or sometimes "stormwater runoff." Storm drains look like big grates in the road where water "disappears" when it rains. These storm drains carry water underground directly into your local streams without being treated.

As this water flows over our hard surfaces, it picks up harmful pollutants that we have left behind. Pollutants are anything harmful to our natural resources. Sometimes the pollution is something you can see like trash, oil, soap, and dirt. Other times you can't see the pollution at all, like when chemicals, bacteria, pesticides, and fertilizers are washed into a nearby creek.

BS RainFallingInPuddle

This activity is meant to help you identify some sources of pollution you can see or not see around your home, and help you calculate your impacts. For more information or help with the answers, visit the Puget Sound Starts Here website.


  1. Name three sources of water pollution that can start at home.
  2. How can we reduce each of these types of pollution?
  3. Calculate your stormwater impacts using the three steps below.

Step 1:

 Calculate the area of your roof (width x length =area): 

  • Width of building (in feet)=
  • Length of building (in feet)=
  • Width x Length = area of roof (square feet)

Step 2:

Calculate the volume of water (gallons) running off your roof in a 1-inch rain storm: 

  • For every 100 square feet of roof surface, one inch produces 62 gallons of water
  • Area of your roof x .62 = volume of water

Step 3: 

Calculate your annual runoff:

  • Take the area of your roof number x inches of rain per year (Bothell is around 36 inches)  x .62 =gallons per year
  • Imagine all that water rushing off of your roof, across your property and street, picking up pollution, then delivering those pollutants to the nearest stream without any treatment.