By Jay Landers

With California in the grip of yet another severe drought, water managers across the Golden State must find ways to protect and conserve existing supplies as much as possible. Against this backdrop, the advocacy organization the Pacific Institute estimated in a recent report the extent to which increased water-efficiency measures could reduce California’s urban water use. The organization also has quantified the potential boost to local water supplies from expanded water reuse and stormwater capture efforts in the state.

Greater reliance on these three measures could reduce urban water use in California by millions of acre-feet annually, the report notes, while expanding local water supplies to a similar extent.

“Programs to tap this potential would tremendously help solve California’s long-standing water problems,” according to the April report from the Pacific Institute titled The Untapped Potential of California’s Urban Water Supply: Water Efficiency, Water Reuse, and Stormwater Capture.

Focus on urban water

Urban water use accounts for approximately 20% of water use in California, while the remaining 80% represents agricultural use, the report notes. During 2017 through 2019, California’s estimated annual total urban water use amounted to 6.6 million acre-ft. The Pacific Institute developed this estimate based on data provided by water suppliers in their electronic annual reports to the state.

Of this amount, residential use made up nearly two-thirds of urban water use, divided almost evenly between indoor residential use (32%) and outdoor residential use (31%). The remainder consisted of commercial, industrial, and institutional uses, at 16%; large landscapes, at 11%; nonrevenue water, at 8.1%; and other uses, at 1.5%. Between 2017 and 2019, the statewide urban per-capita water use averaged 152 gal. per day.

Further efficiencies

Although California has been a national leader in terms of improving the efficiency of its water use, much room remains for improvement. Ultimately, further efficiency improvements could reduce the state’s urban water use by 2 million to 3.1 million acre-ft per year, equivalent to reductions in use of 30-48%, according to the report.

To estimate potential water-efficiency improvements, the Pacific Institute developed moderate and high estimates for additional efficiencies that could be realized by replacing inefficient appliances and fixtures, replacing lawns with climate-appropriate plants, and reducing water losses in distribution systems.

By far, most of the potential water savings — 1.2 million to 2.1 million acre-ft per year, or 61-67% — are associated with the residential sector, the report notes. Within this sector, indoor and outdoor water use have major potential for improved efficiency. Efforts to improve efficiency among the commercial, industrial, and institutional sectors constitute 0.65 million to 0.9 million acre-ft per year, or 28-31% of the estimated potential savings. Finally, reducing losses in water distribution systems could save an estimated 0.11 million to 0.13 million acre-ft per year, or 5.5-6.2%.

Reexamining reuse

The past several decades have seen a steady increase in the volume of wastewater that is treated and reused for beneficial purposes in California. “Since 1970, the quantity of water reused in California has increased by more than 300%, with most of those gains occurring over the past 30 years,” the report notes.

During 2020, the most recent year for which data are available, California generated 3.1 million acre-ft of municipal wastewater, the report notes. Of this total, 728,000 acre-ft was reused, while another 286,000 acre-ft of treated wastewater was dedicated to instream flows and other environmental purposes.

Reuse in California has room to grow. Another 1.8 million to 2.1 million acre-ft per year of municipal wastewater potentially could be reused, according to the report. The lower estimate excludes treated wastewater that is discharged to inland surface waters, while the upper estimate includes such flows that are not allocated for instream flows or natural systems.

Of the 2.1 million acre-ft per year potentially available for reuse, approximately 1.5 million acre-ft, or 73%, is discharged to marine environments, the report notes. Another 283,000 acre-ft, or 14%, is discharged to land, while the remaining 259,000 acre-ft, or 13%, is discharged to inland surface waters and not currently reserved for instream flows.

Of course, just because wastewater is available for reuse does not mean it makes sense to do so. For example, the report notes the presence of “important local contextual factors, such as demand for recycled water and local water rights, that affect the degree to which the (reuse) potential can be fully realized.”

Meanwhile, greater implementation of water-efficiency measures could affect how much wastewater is reused. “Continued reductions in indoor per capita use can reduce the amount of water available for reuse, although population growth and increased economic activity could offset those reductions,” the report states.

Using urban stormwater

Capturing and using stormwater offers another way for California to stretch its water resources further. Although the practice is conducted to a certain extent in some locations, particularly in Southern California, no current estimates exist for how much stormwater is captured in California, according to the report.

That said, the potential for capturing stormwater in California amounts to 580,000 acre-ft in a dry year to as much as 3 million acre-ft in a wet year, according to the report. Notably, these estimates only include urban stormwater that could be captured above public supply aquifers.

The decision to include only the stormwater that could be captured physically above existing public supply aquifers was made to “put a more realistic bound on” the estimated volume of stormwater available for capture, says Heather Cooley, the director of research at the Pacific Institute and the lead author of the report. In this way, the estimate does not include stormwater that would have to be conveyed far distances before it could be recharged to an aquifer.

Ultimately, capturing anywhere near the total estimated volumes of stormwater will require greater use of decentralized and centralized approaches, according to the report. “This potential exists across scales — at the community, neighborhood, and even parcel or household scale — each of which will be essential for successfully capturing the full potential of this local water supply,” the report notes.

Multiple benefits

Each of the three options — water-efficiency improvements, reuse, and stormwater capture — also potentially confer “co-benefits” beyond simply adding to water supplies, the report notes. Such benefits can include greater water resilience, reductions in energy use and carbon emissions, improvements to the environment, more livable communities, and lower water and wastewater bills for utility customers.

Meanwhile, some of the water-efficiency improvements essentially pay for themselves after factoring in the savings associated with lower energy costs and reduced wastewater generation resulting from their use, Cooley notes. “Many of these efficiency measures have a negative cost because their savings exceed the additional cost of buying the more efficient device when you look over the lifetime (of the device),” she says.

Although the report focuses exclusively on the potential benefits to California, other areas also can benefit from greater use of water-efficiency improvements, reuse, and stormwater capture, Cooley says. “The numbers may be a little bit different, but I think (the report) does highlight the opportunities that can exist in” other locations, she says.

The report includes multiple recommendations for boosting efforts to save water through improved efficiency measures and increase supplies through reuse and stormwater capture. For example, the report calls on local agencies to expand programs to incentivize customers to adopt water-efficiency measures, including high-efficiency appliances and fixtures. Among the other recommendations, state or local agencies are called on to ban nonfunctional grass — that is, grass that isn’t used for recreational purposes — at businesses, institutions, and large housing developments and implement ordinances to require residential and commercial buildings to install water-efficient devices when undergoing alterations, improvements, or sale.

An ‘exuberant’ assessment

Although the report correctly concludes that more can be done in California in the areas of water efficiency, reuse, and stormwater capture, “the degree that we can do those things and do them economically is probably overstated,” says Jay Lund, Ph.D., Dist.M.ASCE, a co-director of the Center for Watershed Sciences and a distinguished professor of civil and environmental engineering at the University of California, Davis.

Because it does not offer details regarding the relative costs of the three options, the report fails to address the question of their cost-effectiveness, Lund says. The report’s authors have “painted with a very broad brush about what (is) the maximum you could do of everything,” Lund says. “In a few cases it just won’t be effective. In quite a few other cases, it will just be very, very expensive relative to other ways you could accomplish the same objective of providing a good water supply and saving the environment and things like that.”

While not dismissing the value of the report’s conclusions, Lund thinks they need to be viewed from a more realistic vantage point. The report is “a little bit exuberant about the potential of” the three options it assesses, he says. “Certainly many of these things do have useful potential, and I think that potential is moving forward over time. But I don’t think it will ultimately be as big as these numbers are.”