What is rainwater harvesting?
Since San Diego County has little rainfall and limited collection of that rainfall, most of its drinking water is delivered via aqueducts and pumps. Compounded with an affinity towards large green lawns and backyard swimming pools, our water needs are both enormous and expensive. Rainwater harvesting is the concept that we can provide a portion of our irrigation needs through rainwater collection (and usually storage). The use of greywater (the water that goes down our drains) for irrigation also can assist these needs. These concepts are becoming popular in Southern California as a way to irrigate landscaping and gardens.
How to Harvest Water
- ROOF RAINWATER COLLECTION. In the City of San Diego less than 10” of rainfall for the average year on a 1,000 square foot roof adds up to 6,000 gallons per year of water that can be collected. Some regions of the county experience much higher rainfall and coastal areas can also collect morning dew from metal roofs. All this water is useful for irrigation and can be stored for dry spells, especially late summer droughts. Using gutters and downspouts to collection barrels, large tanks, or landscape collection basins are the common ways of collecting this water. Metal roofs are the cleanest surfaces to collect water from.
- RAIN COLLECTION BARRELS. Small collection barrels ranging from 50 -200 gallons are easy to acquire and fit on your property but hold so little water you would need to use many barrels to collect all the rain that falls on most homes. Generally larger barrels or tanks cost less per gallon than smaller ones.
- LARGE TANKS. It makes the most sense to size a rainwater collection tank to be able to hold as much water as you may desire for dry parts of the year. Since your roof likely collects at least 6,000 gallons a year you want a tank that can store as much of this as possible. Underground tanks cost more to install, but solve the problem of how to fit a large tank on your property.
- LANDSCAPE DESIGN. The most affordable way to collect rainwater for irrigation use may be to design your garden with depression areas or swales where water can collect. You can also dig deep trenches or small wells filled with gravel so that rainwater penetrates deep into the soil and stays there for an extended period. Consider grading your entire property so that water flow from hardscape and rooftops stays on site rather than being drained to the street. Plant large areas of your yard where watering would be wasteful with native plants or other drought-tolerant plants – this is often referred to as xeriscaping.
- GREYWATER RE-USE FOR IRRIGATION. Greywater is the water used for laundry, hand washing, and showering that can be re-used for irrigation. It is legal in San Diego to use laundry machine greywater to water plants without any permit needed! Laundry uses more water than any other function in most homes. If using laundry for greywater make sure to only use detergents that are safe for all plants – this means sodium and boron free.
Rainwater Harvesting for Drylands and Beyond
How Water & Energy are Related