Worksheet for Estimating Rainwater Harvesting Potential
Your final equation will use the following figures, which are explained below;
Catchment Area [ft. 2] x Rainfall [ft.] x 7.48 [gal/ft.3] x Runoff Coefficient = Net Runoff [gal]
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.)
Area Equations for Rectangles and Triangles
Rectangle Length [ft.] x Width [ft.] = Area [ft.2]
Triangle 1/2 (Base [ft.] x Height [ft.]) = Area [ft.2]
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.
This is the volume of one cubic foot of water. No calculations needed here!
“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.” (129) 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 Absorptions 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