Using captured rainwater for irrigation so that regular tap water need not be used is an excellent way to save water. The amount of rainwater captured and available for irrigation is a function of the rainwater harvesting system itself and how much water can be stored.
A single rain barrel holds about 55 gallons of water. If this barrel can be filled and used 20 times over the course of an irrigation season, the savings would amount to 1,100 gallons – assuming 100% of the water can be used to replace standard irrigation.
Savings from rainwater harvesting systems depend primarily on four factors:
- How much rainwater can be captured
- How much rainwater can be stored
- How much rainwater can be used to replace standard irrigation
- The amount of rainfall
Any of these four factors can potentially limit the water savings from a rainwater harvesting system. The amount of rainfall and the amount of storage are often the key limiting factors. Storage is critical because you usually don’t want to use harvested rainwater while it is raining. Ideally the harvested rainwater is stored and then used on days when there is little or no rain.
In Austin, Texas some residential rainwater harvesting systems have storage tanks ranging in size from 1,500 – 2,500 gallons (Gregg, 1999). If you have a collection area of 1000 square feet (quite small) and can collect 80% of the rain that falls it would take approximately 4 inches of rain to fill a 2,000 gallon tank. This is the type of calculation that must be done to determine potential water savings from rainwater harvesting systems.
For detailed information on sizing and designing rainwater harvesting systems download (free) the “Texas Guide to Rainwater Harvesting”.