Would a rainwater cistern save you money?
The Biological and Agricultural Engineering Department at North Carolina State University has produced a free computer model that simulates a cistern for your garden based on historical rainfall data and and calculates how long it would take to recoup your initial cost in water savings based on the amount of city water usage replaced by your cistern. I capture water from 500 square feet of roof, and my cistern can store up to 500 gallons of water. I ran the model so that I watered my 600 sq ft garden 1" a week or 53.4 gallons a day, with waterings at 8am and 6pm during the months March through September.
Because my cistern was absolutely free, I am able to garner savings right away. However, if I had paid the $500 that systems of its size typically cost, it would take me 15 years to pay it off in water savings. The cistern replaced 57% of my city water usage. This calculation is based on whether there was water in the cistern when I was scheduled to do the twice daily waterings.
Out of curiosity, I switched over my design to a 100 gallon storage system (typical of a rain barrel or two small linked barrels) and assumed a cost of $100, I was able to start saving money on water in 6 years. The cistern replaced 31% of my city water usage.
So, before you jump the gun and ride the newest green fad, do the math!
This nifty model has rainfall data files and municipal water costs for most areas of North Carolina. I ran the model with "custom usage" rather than "irrigation" which schedules irrigation by demand based on historical PET estimates. I figured this was more representative of my watering habits.