Skip to Content

Water Harvesting Calculations

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

Rainwater Harvesting Nomogram for Tucson, Arizona
This diagram and a ruler can determine optimal sizes of rainwater tanks and/or catchment (roof) surfaces needed to meet desired amounts (in gallons) of available stored rainwater per day throughout year. What’s more, it can do the reverse — determine how much stored rainwater you can have available per day depending upon the size of your tank and/or catchment surface. Reprinted with permission from the Office of Arid Lands, University of Arizona, Tucson.

Rainwater Harvesting Calculator
This calculator helps estimate water needs of landscapes and potential cistern sizing based on your catchment area, water needs of plants, available water storage, and square footage of the landscape.

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 the Excel file 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)

Greywater HarvestingRainwater HarvestingStormwater & Street Runoff Harvesting