"Tucson is a really successful example of how rainwater harvesting can be used to bolster existing supplies and ease demand on the system without building new infrastructure," says Paula Randolph, associate director at the Lincoln Institute of Land Policy's Babbitt Center for Land and Water Policy. "There's a lot to learn here."

Drying supplies

Similar to many cities in the western United States, Tucson's municipal water supply is drawn from two sources. The first is surface water, pumped from the Colorado River through the Central Arizona Project (CAP), a massive, 336-mile (540km) canal and pipeline system that brings water to central and southern Arizona. The second is groundwater, pumped from the underlying aquifer. CAP water is by far the main source, making up 82% of the city's supply.

But the Colorado River, a lifeblood that sustains 40 million people and four million acres of farmland across seven states in the south-western US and two states in northern Mexico, is an increasingly stressed resource. Decades of over pumping and the worst drought in 1,200 years have severely depleted the waterway and the reservoirs it feeds. Last year, for the first time ever, the US government declared a shortage on the river, triggering widespread restrictions across the region.

And the situation is expected to get worse. In a 2021 study, scientists with the US Geological Survey found that the river may lose nearly a third of its flow in the next 30 years. Climate change is expected to cause snowpack at its headwaters to decline and brings hotter temperatures that further decrease streamflow.

Paradigm shift

A dry spell in the summer of 1974 offered Tucson a grim preview of what this water scarcity could be like. One afternoon during peak demand, the city – which then relied entirely on groundwater – ran so low on water that the city's water utility could not guarantee domestic service or adequate fire-protection to all customers in higher parts of the city.

Spurred into action, the city made a commitment to conservation, initiating a programme that implemented, among other measures, codes that required new buildings to install landscaping needing little to no irrigation (known as xeriscaping) and a tiered water-pricing system that hiked rates for higher users – both still in place today.

Little remembers that summer, and the conservation ethic that followed. "That crisis led to a whole new awareness of water in the community," she says. "After that most people changed their behaviour and never went back to their old habits."

Beyond limiting demand, there remained the challenge of boosting supply from other sources. One key intervention – rainwater harvesting – would take another three decades or so to take root in the Tucson area, and much of the drive behind the change goes to one person: Brad Lancaster.

A permaculture enthusiast, Lancaster started by cultivating his backyard in the mid-1990s. As he grew frustrated with what he calls "the city's wasteful water-management policies", he travelled to southern Africa in search of alternative solutions. Here, he had the chance to meet Zephaniah Phiri Maseko, a farmer who had turned his family's barren plot in Zimbabwe into a veritable oasis just by capturing rain with a system of basins, swales and stone dams.

"I couldn't believe he did that with so few resources," recalls Lancaster. "Phiri showed me what just one person could do."

Once home, Lancaster decided to put Phiri's lesson into practice. Using his property as a pilot site, he dug roadside soil beds, planted them with native trees and shrubs and built earthworks around the plants to drain stormwater. As the trees grew, he took a step further and cut small gaps in street curbs to route stormwater runoff into his sidewalk garden.

Lancaster knew the practice was unlawful – the municipal government had outlawed curb cuts due to concern about water rights of downstream users – but that first cuts worked so well that he started making more. Noticing how well plants responded, most of his neighbours got interested, and so Lancaster decided to approach the city to ask about legalising the practice.

At first, city officials balked at the idea. "They thought the streets were designed to drain water, and nothing convinced them otherwise," recounts Lancaster. But in 2007, after three years of lobbying – and "loads of bureaucratic footwork", as he describes it – the city eventually made the process legal, kickstarting a broader paradigm shift that has radically transformed the way Tucson deals with rainwater.

"The philosophy here had been for decades to treat runoff as a waste," says Rodney Glassman, a former Tucson councilman who helped spearhead efforts to legalise curb cuts. "What Lancaster's example made us realise was that stormwater is actually something we can use and benefit from."

Watering Tucson

In the wake of Lancaster's campaign, Tucson enacted several measures to take full advantage of stormwater as a resource. In 2008, it passed a first-in-the-nation ordinance requiring new commercial properties to irrigate half of landscaping using captured rain. In 2013, it adopted a Green Streets Policy mandating all publicly funded roadway projects to capture the first half inch (1.3cm) of rain during a storm. And with a more recent initiative, the Green Stormwater Infrastructure of 2020, Tucson began charging a small sum on residents' water bills to raise about $3m (£2.65m) annually to support public stormwater capture projects such as the city's million-trees initiative.

Taken together, explains Candice Rupprecht, water conservation manager for the city's public water utility Tucson Water, these measures mean that "whenever we build a road, put up a parking lot, or rip and replace public and private infrastructure, we do it in a way that works with nature to manage stormwater as close to its source as possible."

This kind of approach yields benefits that go beyond water conservation, she says, including reducing soil erosion, mitigating street flooding risks, and creating green spaces that cool surfaces and help reduce the urban heat island effect, whose consequences are more severe across majority Spanish-speaking and lower-income and neighbourhoods.

In 2012, Tucson Water began an ambitious incentive programme that rebates homeowners up to $2,000 (£1,820) for the purchase of rainwater-collecting equipment such as tanks or the adoption of landscape design systems that capture rain for indoor and outdoor use. Over 2,600 households joined the scheme so far, according to the utility.

In the first few years, despite the programme's success in terms of engagement, Tucson Water found participating households were not conserving any more water than a control group of homeowners who didn't get rebates. In fact, some rebate recipients even increased their overall water consumption as they added new landscaping that required traditional watering to get established.