We have used the formula at the bottom of this page to provide an example for calculating your potential for rainwater harvesting. In this example, we have a 2,500 square foot roof area that has a 90% runoff efficiency. In our example area (Iowa), we enjoy an average of 36 inches of rainfall in a year's time. That offers us 50,490 gallons of water over the course of a year.
You can also break that formula down into months to determine how much water storage you would need, depending on your usage for plant watering and irrigation.
We'll be delighted to help you estimate how much money you might save by using rainwater for your outdoor and indoor needs.
Worksheet for Estimating Rainwater Harvesting Potential
Your final equation will use the following figures, which are explained below:
Catchment Area: This refers to the amount of space from which you will be collecting rainwater, or the ‘footprint’ of your roof. NOTE: This is NOT the same as the surface area of your roof. It may help to imagine looking at your roof from straight above (as a raindrop might see it!) The dimensions you will need are those that you would measure from this raindrop-eye view. (For ease of measuring, you can use the dimensions of your house, adding to these dimensions to adjust for overhang.)
Catchment Area [ft. 2] x Rainfall [ft.] x 7.48 [gal/cu ft] x Runoff Coefficient = Net Runoff [gal]
Area Equations for Rectangles and Triangles:
Rectangle: Length [ft.] x Width [ft.] = Area [sq. ft.]
Triangle: 1/2 (Base [ft.] x Height [ft.]) = Area [sq.ft.]
Rainfall: You can use any amount for this, depending on what you want to know. If you want to estimate your annual rain harvesting potential, use the average annual rainfall for your area. If you want to know how much you can harvest in a single event, then use the amount of rainfall for that event. NOTE: Make sure to put the amount of rainfall in ‘feet’, not inches. So, for example, if you had 2 inches of rain, you would use 2/12 ft., or 0.167 ft. in your equation.
7.48 [gal/ft.3]
This is the volume of one cubic foot of water. No calculations needed here!
Runoff Coefficient: “Impervious catchment surfaces such as roofs or non-porous pavement can lose 5% to 20% of the rain falling on them due to evaporation, and minor infiltration into the catchment surface itself.” The Runoff Coefficient allows us to account for this lost water. It will not be an exact figure, but it can give you a more accurate estimate of your rain harvesting potential.
Roof Surface Types, Approximate Absorption Rates, and Consequent Runoff Coefficients Pitched Metal 5% .95 Concrete or Asphalt 10% .90 Built-up Tar or Gravel 15% to 20% .85 to .80 (respectively) Now determine your own rainwater harvesting potential.
_______ft.2 x ____ft. x 7.48 gal/ft.3 x __________ = ________ gallons!
(Catchment Area) (Rainfall) (Runoff Coefficient) Net Runoff!
This content is used by gracious permission of the author of Rainwater Harvesting for Drylands and Beyond, Brad Lancaster, available at our store!
