New White Paper Proposes a Calculation to Determine Optimal Irrigation Volumes for Recycled Water Users in California

Feb. 14, 2018

WateReuse California and the Southern California Salinity Coalition (SCSC) are pleased to announce the publication of a White Paper entitled, “Accounting for Salinity Leaching in the Application of Recycled Water for Landscape Irrigation,” released in February 2018. The White Paper can be downloaded from the SCSC website at

The White Paper was designed to provide science-based guidance to the California Department of Water Resources (DWR) related to determining how much recycled water should be used for landscape irrigation to reduce the negative effects of salinity on plant and soil health. The authors of the White Paper are Dr. Amir Haghverdi and Dr. Laosheng Wu, both of the University of California, Riverside.

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“The White Paper is an important tool for determining policies that are appropriate for communities throughout California, including those that rely on recycled water to maintain healthy landscapes,” said Jennifer West, Managing Director of WateReuse California, an organization that promotes the responsible stewardship of California’s water resources by maximizing the safe, practical, and beneficial use of recycled water.

“SCSC was pleased to work with WateReuse California in producing and distributing this White Paper to the California DWR,” said Mark Norton, President of SCSC, a non-profit focused on managing salinity in Southern California water supplies. “This topic is important to SCSC member agencies and other stakeholders throughout the state that face challenges related to elevated salt concentrations in source waters and recycled water.”

California has used recycled water in a variety of ways for over 100 years. When applied to landscape irrigation, recycled water tends to be higher in salinity than potable water. These salts can then accumulate in soil unless additional water is applied to flush (or leach) them out. In the absence of leaching, excess salts can affect the health of plants, causing stunted growth, wilting, and other damage. Applying water to leach excess salts from the root zone is the accepted best practice for maintaining this balance.

Recently, DWR initiated a process to amend the 2015 Model Water Efficiency Landscape Ordinance (MWELO), which sets minimum standards that regulate the design and installation of landscaping in California. MWELO includes a calculation that allows recycled water users to apply additional irrigation water beyond what would be permitted if potable water was used for the same landscape; however, it does not fully account for natural variations such as the salinity of the water used for irrigation. Consequently, the water allowance may not be sufficient to protect salt-sensitive plants, such as turf grass, when highly saline recycled water is used.

DWR plans to complete the MWELO update in 2018. To more fully inform the process (and based on member agency feedback), SCSC and WateReuse California sponsored the development of the White Paper to provide a scientific accounting for the additional salts frequently present in recycled water. The White Paper, therefore, proposes the addition of an alternate equation to the MWELO’s existing allowable water use calculations in cases when the source water influent to a recycled water production facility consistently exhibits high salt concentrations.

“The White Paper proposes a science-based solution to ensure the continuing vitality of community landscape and recreational amenities that rely on recycled water for irrigation,” said Kevin M. Hardy, Administrative Director for SCSC. “Dr. Haghverdi and Dr. Wu’s research demonstrates that tailoring MWELO’s water efficiency formula to account for salty source water and the known physiological requirements for root-zone health can be accomplished within the existing MWELO framework and in a manner consistent with MWELO’s policy objectives.”

The mission of WateReuse California is to promote the responsible stewardship of California’s water resources by maximizing the safe, practical, and beneficial use of recycled water. WateReuse California has seven regional chapters representing geographically diverse regions in California: Central Coast, Central Valley/Sierra Foothills, Inland Empire, Los Angeles, Northern California, Orange County, and the San Diego Region. WateReuse California also supports the efforts of WateReuse, a national organization with headquarters in Alexandria, Virginia, that advocates for policies, laws, and funding at the state and federal level to increase the practice of recycling water. Visit for more information.

The Southern California Salinity Coalition (SCSC) was formed in 2002 to address the critical need to remove salts from water supplies and to preserve water resources in California. SCSC members include the following water and wastewater agencies in Southern California: Eastern Municipal Water District, Inland Empire Utilities Agency, Metropolitan Water District of Southern California, Orange County Sanitation District, Orange County Water District, San Diego County Water Authority, Sanitation Districts of Los Angeles County, and Santa Ana Watershed Project Authority. The mission of SCSC is to improve the management of salinity in our water supplies through activities such as: establishing proactive programs to remove salts in water supplies; preserving, sustaining, and enhancing the quality of source water supplies, and educating the general public on challenges associated with salinity. SCSC is administered by the National Water Research Institute (NWRI), a nonprofit organization located in Orange County, California. Visit for more information.

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