Rainwater Harvesting Supply Calculator
Rainwater harvesting calculators are important for several reasons. They help to better balance the supply and demand so that a system can be sized correctly and so the user has adequate storage to get through long periods without rainfall. There are countless rainwater harvesting calculators available on the internet. The calculator included below was developed by the Texas A&M AgriLife Extension service. It allows the user to determine stored water volume over the course of three years and whether supplemental water is needed.
The inputs include:
- Catchment area (ft2)
- Collection efficiency (%)
- Initial tank volume (gal)
- Tank size (gal)
- Plant water use coeff.
- Irrigated area (ft2)
- Monthly indoor demand (gal)
- Average monthly rainfall (in.)
- Average PET (in.)
- AC condensate (gal)
There is also an area to input a “yearly percent average rainfall.” This accounts for years of drought when the annual rainfall is lower than average. It can also be used as a safety factor.
The output is shown in both graph and table form. The graph shows storage volume as a green line and supplemental water as a purple line. When the supplemental line is above the the tank volume water line, more water is needed.
This Excel spreadsheet is for educational purposes.
Hazen Williams Pressure Loss Calculator
When working with piping in a rainwater harvesting system, friction can cause water to back up, especially in a wet-line conveyance system. To determine how much water will back up, the Hazen Williams pressure loss formula can be used to determine pressure loss, both in feet and psi. The calculator below was developed by the Texas A&M AgriLife Extension Service and can be used for educational purposes.
The inputs include:
- Inner pipe diameter (in.)
- Friction coefficient
- Flow rate (gpm)
- Length of pipe (ft)
Charts of provided below the inputs to explain the numbers involved. The output is shown as pressure loss in feet and psi.