Measure rain runoff at your house

RainGardenWhen we get one inch of rain, 0.62 gallons of water fall on every square foot of surface area.

For example, if you have a vegetable garden that is 10 ft x10 ft = 100 square feet, and you want to know how much water falls on it in a one inch rain storm, you would calculate:
0.62 gallons/square foot x 100 square feet = 62 gallons of water falling on that section.

When rain falls on natural surfaces like gardens and forests, almost all of it sinks into the ground. But when rain falls on impervious surfaces like typical roofs, parking lots, roads and driveways, the rain can’t sink into the ground and runs off into storm sewers or combined sewers.

You can calculate the amount of stormwater that runs off your home or school if you know the dimensions of impervious surfaces on the property.

  1. Make a sketch of the property.
  2. Measure the length and width of any buildings, driveways, sidewalks, patios and parking lots, recording them on your sketch. You may need to break odd shaped spaces into rectangles or triangles you can more easily measure and calculate.
  3. Calculate the surface area of each section using the following formulas:
    • rectangle: length * width
    • circle: 3.14 * radius2
    • triangle: 1/2 * length of base * height
  4. Add up the surface areas for each type of surface, for example: driveway, roof, patio.
  5. For each type of surface, calculate the amount of rain falling on the surface. You can calculate based on the total rainfall for the year (average of 42″ in Cincinnati), the amount of a specific rain fall (last night it rained 0.6″) or a rain fall amount of your choosing.
    • x inches (you choose number) * 0.623 gallons/square foot/inch * y square feet of surface area = number of gallons falling on that surface area
  6. Add the rainfall amounts together for each type to get a sense of how much stormwater your property contributes to the combined or storm sewer.

You’ve just completed the first step in figuring out how you can use your property to collect stormwater and keep polluted water out of our rivers!