Sports fields could cool cities via rainwater stored beneath turf

Artificial-turf sports and playground fields may be more durable and require less maintenance than those covered in natural grass, but they also get very hot during warm weather. Scientists have devised a natural method of cooling them – and the cities they're in – using rainwater stored beneath the turf.

Natural grass fields typically stay much cooler than their artificial turf counterparts, due mainly to the fact that water stored in the soil (and in the grass itself) gradually and continuously evaporates when exposed to hot ambient temperatures. As it does so, it produces a cooling effect.

By contrast, artificial turf tends to keep rainwater from reaching the underlying soil. As a result, that water just runs off the field as it falls, leaving the soil relatively dry. The turf can be sprayed with water to produce a short-lived evaporative cooling effect, but the thin layer of liquid on the turf evaporates quickly, plus it makes the playing surface slippery while it's still there.

Led by Dr. Marjolein van Huijgevoort from the KWR Water Research Institute, a team of Dutch scientists recently set out to develop a more effective alternative.

Building upon successful indoor experiments, the researchers created four different 5 x 5-m (16.4-ft) outdoor test plots. One of these had a layer of special open-backed 50-mm artificial turf on top, followed by a 30-mm layer of sand, a 20-mm layer of porous shock-absorbing foam, and an 85-mm-tall rainwater reservoir.

The latter took the form of a lattice-like polymer structural material called Permavoid 85S, which is typically used to store water beneath soil and grass in energy-efficient "green roofs" on buildings. A waterproof liner surrounding the reservoir kept the water from leaking out.

The idea is that when rain falls, it passes through the artificial turf and other layers, ultimately collecting in the reservoir. An array of rock-wool-stuffed vertical cylinders within the Permavoid then draw some of that water back up to the top of the reservoir via capillary action.

As hot outdoor temperatures heat the turf and other materials, water continuously evaporates up out of the cylinders, passing through the foam, sand and turf, and into the atmosphere. This produces an evaporative cooling effect much like that produced by natural grass and soil.

In tests performed during a heatwave in June of 2020, the turf on the surface of the reservoir-cooled test plot reached a temperature of 37 ºC (98.6 ºF).

By way of comparison, another plot consisting of standard latex-backed artificial turf over a shock-absorbing crumb-rubber substrate got all the way up to 62.5 °C (144.5 ºF). A third plot, consisting of natural grass and soil, was only slightly cooler than the reservoir-cooled turf plot, coming in at 35.3 °C (95.5 ºF).

What's more, temperatures recorded 75 cm (29.5 in) above the test plots showed that the air over the rainwater-cooled turf stayed significantly cooler than air over the standard turf. It is hoped that the technology will ultimately not only reduce the temperature of artificial-turf recreational fields, but also decrease the "heat island" effect which occurs in cities throughout the world.

And as an added bonus, once the cost of the technology is brought down, fields utilizing it will store rainwater that would otherwise just end up going down storm sewers. At times when there isn't enough rainwater to fill the reservoirs, they could be filled from the city's municipal water supply.

"People in urban areas, especially children, have a growing need for sport and play facilities," says van Huijgevoort. "With this work we show the benefits of the subsurface water storage and capillary irrigation system without negative effects of artificial turf fields."

A paper on the research was recently published in the journal Frontiers in Sustainable Cities.

Source: Frontiers