How do you water a million trees in a desert?

In Riyadh, a good year brings about 101 millimetres of rain. In the same year, the sun and wind can evaporate roughly 1,971 millimetres of water from an open surface, according to Turenscape's project notes. That is nearly twenty times more water leaving than falling. Now plant a million trees.

This is the arithmetic King Salman Park has to beat. The answer its designers chose is neither magic nor a miracle: it is a loop. Water that Riyadh has already used is cleaned, polished and sent back out to feed shade.

No drinking water for the lawns

Parsons, the engineering firm that runs the project management office, says the park's irrigation demand is planned to be met entirely by treated sewage effluent, or TSE. That means recycled municipal wastewater, not the potable water that homes and offices rely on. The Foundation's chief executive has said trees along roads and near buildings are meant to absorb carbon dioxide, add humidity and ease the urban heat island, as quoted in the Parsons announcement.

According to RIBAJ's visit to the site, the treated water arrives through the Green Riyadh network, a pipe system of about 1,350 kilometres that distributes effluent across the city, and an on-site plant raises its purity further for delicate plants. Most of the watering is then done underground, with subsurface irrigation, because water that never touches the surface cannot evaporate.

The plant that cleans it

At the centre of the loop is the water treatment complex, which Omrania designed. The practice reports a design capacity of up to 192,000 cubic metres of treated water a day, on a complex of 137,736 square metres, using ultrafiltration and reverse osmosis. It won an Architizer A+ Award in 2025.

You will see other numbers, and they are worth untangling, because each measures something different:

• 192,000 m3 a day is the design capacity of the complex, per Omrania.
• About 150,000 m3 a day is the treated wastewater throughput quoted by Gerber Architekten, which describes it as polished to near-drinking standard, in an Arab News feature.
• 29,850, 160,000 and 40,700 m3 a day are three components of the water package that MEED reported when Al Bawani and Wetico were selected in March 2024: a sewage treatment plant, a TSE facility and a water-treatment component. MEED's articles are paywalled, so we rely on summaries of those figures.
• About 50,000 m3 a day is what a temporary plant was running in September 2026, according to MEED. That is about 26 percent of the 192,000 design figure.

So these are not rival answers. They are a design capacity, a throughput, a package of components and a snapshot of today.

Soil that holds on to water

Water is only half the trick. The other half is the ground. RIBAJ describes a matrix about two metres deep laid over bedrock, made of loose site material, red regional soil, compost and a mulch topping, to improve water retention. Arab News, quoting the design team, puts the regenerative soil layer at 2.5 metres and mentions biochar and pumice. The two sources describe the layer slightly differently, so we give the range: about two to two and a half metres. More than 13 million cubic metres of earth were reportedly excavated while the landscape was shaped.

Where the project stands

On 28 September 2026, acting CEO Dale Chadwick told a MEED conference that irrigation had been installed for both Phase 1 and Phase 2. Landscaping was restarting for the season, with about four million plants due this year at 50,000 to 60,000 a day. The Foundation puts the wider target at about one million trees, and Parsons also cites 20 million plants, while RIBAJ cites 30 million, so plant counts differ by source and date. We have not found a published count of what is already in the ground.

What we do not know yet

Two things matter for the loop to hold, and we cannot yet confirm either. First, we have not found a published long-term TSE supply or offtake contract; the only trace is the 2024 selection of the water package contractors. Second, the plant is running at about a quarter of its design capacity while the park is still being built, so the real test comes when thousands of visitors, and the trees' summer thirst, arrive together. A desert park that depends on recycled water is only as dependable as the pipes and pumps behind it.

That is also what makes the idea impressive. The park does not pretend the desert is not a desert. It treats the city's used water as the raw material of a forest, and it puts the cleaning plant at the heart of the design. If it works, the greenery will be a kind of public plumbing, and the loop will be the quietest piece of infrastructure in Riyadh.