Withlacoochee Permaculture Guild

Catchment Area – Runoff Calculator/Spreadsheet. Download and adapt this spreadsheet for your site’s average annual rainfall and/or surface material. Use as a dynamic computer spreadsheet or print out a hard copy to use as a quick reference guide for estimating runoff from variously sized catchments.

Note that the spreadsheet default is a 90% runoff coefficient, meaning 90% of the rain falling on the catchment will run off. The other 10% will be lost to evaporation, wind, leaks, infiltration into the catchment surface, etc. This is typical for an asphalt or concrete roof or hardscape such as a patio, road, or driveway. You can change the runoff coefficient in Excel for different catchment surfaces if needed. (Metal: 95%; Concrete/Asphalt: 80-95%; Tar Roof: 85%; Sonoran Desert Uplands: 20-70%; Bare Earth: 20-75%; Grass/Lawn: 5-35%; Gravel: use coefficient of material below gravel).

Water Harvesting Calculations from Water Harvesting Calculations, Appendix 3 of Rainwater Harvesting for Drylands and Beyond, Vol. 1.

via http://www.harvestingrainwater.com/rainwater-harvesting-inforesources/water-harvesting-calculations/