Study finds replacing spray irrigation with drip irrigation can eliminate runoff

SCCWRP and the County of San Diego have completed a three-year study that found that replacing spray-irrigated turf with drip irrigation and drought-tolerant landscaping can reduce or eliminate the volumes of irrigation and rain water running off these surfaces – a finding that has helped convince the County to expand its turf-replacement program to encompass 13 residential and commercial properties and counting.

The study, described in a SCCWRP technical report published in October, found that turf replacements – considered a type of non-structural stormwater BMP (best management practice) – can successfully eliminate irrigation-derived runoff during an estimated 85% of all storms.

Wet- and dry-weather runoff from residential grass areas can become a significant source of pollution in storm drain systems; discharge permits in the County of San Diego require stormwater managers to eliminate dry-weather runoff from these areas.

Because the study was conducted in phases over a three-year period, researchers began sharing initial results two years ago. The early findings helped inform the County’s decision to expand its Waterscape Rebate Program, which compensates residential and commercial property owners who voluntarily replace spray-irrigated turf areas with drought-tolerant landscaping and drip-irrigation systems.

Since 2022, the County has funded turf replacement projects in 12 residential communities and one commercial property spanning 354,830 square feet of spray-irrigated turf. Five additional turf-replacement applications are pending, with each applicant committing to replace at least 20,000 square feet of turf.

The turf-replacement study began in 2021, when the County replaced turf and traditional spray irrigation with drought-tolerant landscaping and drip irrigation in a privately managed, inland San Diego County residential community.

To assess whether the newly installed BMP was retaining drip-irrigation water, researchers installed soil moisture sensors and then ran the drip irrigation system for two hours. They visually inspected for evidence of irrigation running off the site, then cross-referenced the inspections with soil moisture measurement data. A year later, they repeated testing with limited replication to confirm conditions hadn’t changed.

Other Southern California municipalities, including South Orange County, have been closely following the study and are considering replicating San Diego County’s financial incentive program for turf replacements.

Also as a result of the study, San Diego County invested in building a computer model that can quantify the water-saving and runoff-reduction benefits associated with individual proposed turf replacement projects. SCCWRP will begin working with the County to validate the model’s performance in winter 2024.

For more information, contact Dr. Elizabeth Fassman-Beck.