Another rainstorm is soaking Los Angeles, and the think pieces are out examining the city’s river-turned-flood-control-channel. It prevents flooding but wastes water, the story goes. Two UCLA water experts weigh in below on granny flats, water recycling and more.

UCLA water expert Edith de Guzman describes the need to replenish groundwater, but points to the challenges in making Los Angeles more porous, even as the region continues to densify. “Accessory dwelling units,” known as ADUs or granny flats, help with housing but not with permeable surfaces, she notes, and are one of many ways the city is over-paved.

“Tired” is how UCLA water expert Greg Pierce describes the common post-rain laments that Los Angeles fails to capture stormwater. Stormwater capture comes third in effectiveness behind conservation and water recycling, he notes, and adds that the slow pace in building stormwater capture projects is part of an important compromise for neighborhoods that also want other benefits from open space.

UCLA water researcher Edith de Guzman
Expertise in urban sustainability and climate change adaptation

Cooperative extension specialist at the UCLA Luskin Center for Innovation

  • “The river was channelized because in its natural state, a storm like the one we had a week and a half ago would have forced the course of the river to change.”
  • “The river is now more efficient at quickly funneling large volumes of water, but the simultaneous urbanization of L.A. and channelization of the river means what was once a mostly porous watershed now exacerbates flooding risk throughout the watershed. This is because water has fewer spaces to sink into the soil, concentrating more stormwater into fewer spaces, while also making peak flows higher and making them happen more quickly and dramatically than they did pre-development.”
  • “In built-out urban areas like L.A., the challenge is retrofitting over-paved developments to be more porous.” 
  • “The state can look to L.A. County's Measure W, a 2018 ballot measure that passed, which now provides about $300 million per year in funding for water-quality and supply-improvement projects.”
  • “There are related issues, such as the push to build more ADUs to address the housing situation, which create denser and thus less porous neighborhoods. Passage – and enforcement – of low-impact development laws that require parcels to manage their own stormwater are critical, although notably, the type of storm we saw at the start of February far exceeds the capacity of just about any parcel to manage.”
  • “In recent decades, aquifers have been over-pumped – think over-drafting your bank account. When we are able to replenish them during wet years, we are helping address an ongoing issue of groundwater overdraft – though we would need many years like the last couple of years, updated infrastructure, and stronger efficiency measures to address that issue fully.”

UCLA water researcher Greg Pierce
Director of UCLA’s 
Human Right to Water Solutions Lab, and co-director of both UCLA’s Water Resources Group and UCLA’s Luskin Center for Innovation

  • “We’ve had two good water years – this is a positive story. We averted the acute water crisis that we were having through fall 2022 from the drought and Colorado River shortages, and we’ve bought time to continue making investments in local stormwater capture and supply.”
  • “I find the question of why Los Angeles didn’t save more rainwater kind of tired. Through big spreading grounds, the work of the flood control district, and stakeholders on L.A. County’s Measure W (now the Safe Clean Water Program), we are saving more water than we were six to seven years ago. It’s not increasing as quickly as people hoped, but we’re heading in the right direction. Yet criticism is so harsh that at this point it’s literally slowing progress and demoralizing the people working on it.”
  • “It’s worth noting that conserving the water we already have and increasing water recycling are the more important steps. Stormwater capture comes third in the list of effective ways to increase the local water supply.”
  • “Los Angeles’ Measure W projects have a strong public process for community involvement, and that’s good, even though it means we aren’t increasing water storage as quickly. Some of these projects turn into parks with greenery, which provide open space and heat reduction rather than necessarily maximizing stormwater capture. We can achieve multiple benefits, but we can’t pretend that we can maximize each benefit with each project. There are tradeoffs, and one of those is increasing water supply more slowly.”
  • “The good news is that everyone in L.A. is aware of the need to capture more storm water, and we have the money, so it’s mostly about building fast enough, and most importantly, about maintaining the capture infrastructure we build.”

Media are encouraged to quote from De Guzman or Pierce’s comments, or reach out on these and other environmental topics for additional context, research and analysis from UCLA climate experts.

Alison Hewitt
Senior Media Relations Officer
UCLA Strategic Communications | Office of Media Relations | @ashewitt | @ahewitt.bsky