Estimated Net Runoff

The Estimated Net Runoff function provides an estimate for how much rainfall can be realistically collected off a surface for an average year.

Catchment surfaces such as roofs or non-porus pavement are likely to lose 5% to 20% of the rain falling on them due to evaporation and minor infiltration into the catchment surface itself. To account for that potential loss, the runoff coefficient we use here is 0.80 (80%).

Example:

Calculate the net gallons of rain running off the roof in an average year from a home that measures 47 feet long and 27 feet wide at the drip line of the roof. Rainfall in this area averages from 0.875 ft a year. Assume that the loss of water that occurs on the catchment surface is at a high. This means that the runoff coefficient is at 80%.

Thus,

(length (ft) x width (ft)) x rainfall (ft) x 7.48 gal/ft3 x 0.80 = net runoff (gal) 

(47 ft x 27 ft) x 0.875 ft x 7.48 gal/ft3 x 0.80 = net runoff (gal) 

1,269 ft^2 x 0.875 ft x 7.48 gal/ft^3 x 0.80 = 6,644 gal

Answer: Net Runoff = 6644 gal

Resource:

• Lancaster, Brad, and Joe Marshall. Rainwater Harvesting for Drylands. Vol. 1. Tucson, AZ: Rainsource, 2006. Print.
• "Rainwater Harvesting | Sustainability Workshop." Autodesk Sustainability Workshop. Autodesk Education Community, n.d. Web. 31 May 2015.