By Jay Lund
Imagine capturing some of the heavy rain that has been draining off Northern California roofs lately to water yards this summer, for what will likely be a fourth year of drought.
The drought has generated interest in household cisterns commonly known as “rain barrels” that collect and store rooftop runoff for when it is most needed – during the dry season – to irrigate landscapes and replenish community groundwater supplies. Advocates of these rainwater collectors point to their prevalence in Australia following its decade-long Millennium Drought.
But how cost-effective are rain barrels for individual home and business owners, compared with the more communal approach of adding storage capacity behind a dam upstream?
Here are some back-of-the-envelope calculations:
The cost of household cisterns includes the storage tank, installation and connection to a roof, maintenance and the value of land for the cistern’s footprint. A 50-gallon rain barrel costs about $100, and a 300-gallon tank runs about $600.
The cost of the storage tank or barrel alone amounts to $652,000 an acre-foot of storage capacity (summed over many households). This compares with about $2,000 an acre-foot for expanding storage capacity at large upstream dams.
But storage capacity for water supply is useful only to the extent water is available to capture. Most recent proposals for expanding upstream reservoirs in California yield annual water deliveries of only 5 to 20 percent of the additional storage capacity. In California’s climate, the additional storage space can refill only every few years, implying water delivery costs of $500 to $2,000 per acre-foot of water delivered (annualizing the initial cost at a 5 percent interest rate).
Rainwater cisterns in California might be drained several times during the wet season to replenish groundwater or even out stormwater flows, and once or twice during the spring for landscape irrigation.
For a typical home in coastal California, the annual pattern of storms might allow filling and emptying a 50-gallon cistern one to three times (with considerable overflow possible each time), yielding 50 to 150 gallons a year – less than 0.1 percent of a household’s annual water use in California. For inland homes, the actual water produced would be much less because the rain barrel is capturing runoff that likely would have been used by others downstream anyway.
Indeed, if a rain barrel’s installation removes 8 square feet of a highly watered lawn (1 to 2 acre-feet a year), the gallons saved from reducing the irrigated area would be similar to the water provided by the rain barrel.
So the cost of water supplied by household cisterns in California for landscape irrigation or groundwater recharge could be $11,000 to $32,600 an acre-foot. This is 10 to 20 times the wholesale cost of water in Southern California and 5 to 10 times the cost of desalinating seawater.
While the economics of household cisterns for water supply in California are unattractive, cistern collection systems do provide some environmental benefits. Evening out stormwater flows reduces the costs of managing it downstream. And the prominent display of rain barrels at homes and businesses serves as a constant reminder of the scarcity of water in California, perhaps increasing water conservation more generally.
Jay Lund is a professor of civil and environmental engineering and director of the Center for Watershed Sciences at UC Davis.
Portland’s Regional Water Providers Consortium has a nice primer on rain barrels
American Rainwater Catchment Systems Association has a large collection of additional information.
San Diego posts a Rainwater Harvesting Guide for homeowners.
Alliance for Water Efficiency provides an overview on the history and effectiveness of rain barrels and other useful resources, including:
- Harvesting, storing and treating rainwater for domestic indoor use (Texas Commission on Environmental Quality, 2007)
- A review of applicable policies and permitting requirements for non-potable use of cisterns (University of Florida IFAS Extension, 2007)
- The Value of Green Infrastructure: A Guide to Recognizing Its Economic, Environmental and Social Benefits (Center for Neighborhood Technology, 2010)