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The cooling effect of greenery

Extreme cold or heat, rain, gusts of wind, severe thunderstorms, snow, ice, or long periods of drought… You’ve probably already noticed it yourself, we are increasingly experiencing extreme weather conditions. The societal challenge of keeping cities livable means that we need to adapt our cities to the changing climate. One way to do this is through greenery. Greenery is essential for a climate-resilient and sustainable urban environment. Greenery provides cooling, reduces waterlogging, and increases infiltration capacity, which also helps combat drought. But how do we create green cities? And how do we ensure that the costs of green management don’t skyrocket?

Author: Malon Gerrits

Evaporation processes have a cooling effect on air and surface temperatures. In a natural environment, trees provide shade, lower surface temperatures, and air temperatures. However, large percentages of buildings and other paved surfaces result in lower evaporation.

Material properties related to the reflective and heat-absorbing capacity of sunlight affect surface temperature and, therefore, the air temperature in the city. The reflective capacity of sunlight is called albedo.

Hard surfaces and building materials in urban areas generally have a lower albedo than vegetated surfaces. The materials in urban areas reflect less and absorb more sunlight, leading to higher surface and air temperatures. This also means more smog formation. Warm air carries pollutants and dust particles to higher atmospheric layers, creating a smog layer. Cooler air from outside the city is warmed up at the urban fringe and does not penetrate into the city center.

The most effective measure to limit surface temperatures is to reduce the percentage of paved surfaces.

10 air conditioners cool as much as 1 tree

Trees are natural problem solvers. Trees and greenery have a cooling effect on the urban climate because they reduce solar radiation on the Earth’s surface through shading and absorb heat through the evaporation of moisture from leaves.

Research even shows that in summer (depending on the tree species), only 10% to 30% of sunlight reaches the ground in the shade of a tree. Various studies in America have shown that walls in the shade of trees are on average 20°C cooler. The research “The Heat-Resilient City” by the Amsterdam University of Applied Sciences describes that a 10% increase in greenery leads to half a degree less heat in the city.

Reduced peak runoff during rainfall

Greenery is not only beneficial for limiting warming but also offers advantages for water management. Greenery reduces peak runoff during rainfall. Instead of flowing directly into the sewer, the water is first stored by leaves or branches. Only when these become saturated does the water flow into the ground. As a result, a portion of the rainfall reaches the ground with a delay, and another portion evaporates. This reduces the simultaneous discharge of rainfall and reduces waterlogging.

The evaporation of water by greenery, the ground beneath it, and its surroundings contributes to reducing heat stress by achieving a 2°C to 5°C lower air temperature in the immediate vicinity. Additionally, it enhances biodiversity and quality of life potentials.

Leaf canopies of trees or pergolas above parking lots, squares, schoolyards, and playgrounds help keep the city cooler. They also reduce airborne dust and other pollutants, improve rainwater retention, and enhance overall quality of life. Unfortunately, greenery in and around cities is still under significant pressure. More trees are being cut down than planted, and there is an increasing trend of urban green spaces being built upon and paved over.

Slowly, this is starting to change, and more and more people are realizing the benefits of greenery for a city. Urban planners are recognizing the importance of investing in sustainable development.

Vital importance

Trees are therefore vital, but they need to be healthy and mature.

Greenery also has a water demand. During times of drought and water scarcity, greenery can exacerbate the problem. Attention is therefore important; by the time you can see the problem on the tree, it is often already too late. If you give the tree too much water, it will receive little to no oxygen at the roots. This not only makes the tree “lazy” by preventing root development compared to the crown, but in the worst case scenario, the tree can die. You can prevent this by planting the right trees in the right places, storing and conserving rainwater for dry periods, and through monitoring. Monitoring is simply necessary to allow greenery to grow in less natural areas such as city centers, viaducts, or rooftops.

Cost-effective management

Watering is customized and dependent on the conditions. It is not possible to provide a fixed amount and frequency. This makes monitoring water levels even more important. That is why since 2017, we have been helping green space managers, municipalities, landscapers, provinces, growers, and water boards work smarter and more efficiently. We achieve this by combining knowledge about trees and plants with the “Internet of Nature.”

ConnectedGreen is a smart system for remote monitoring of green projects, trees, beds, containers, lawns, and roof and wall greenery. The system consists of wireless sensors, a smart cloud environment, and user-friendly apps for both clients and contractors. This fosters better collaboration, and the measurement data is valuable for use in asset management systems and work planning (data-driven work). At the same time, it leads to significant cost savings in replanting, watering, and project visits.

Over 1500 of our wireless sensors have already been installed in projects throughout the Netherlands and Belgium.

Significant savings

In the municipality of Veldhoven, around 300 trees were knocked down by storm damage in 2019. The 70 trees that were replanted are now being cared for based on the data from the sensors. A total of 840 new trees have been planted in the municipality of Veldhoven. “The investment in the sensors and the accompanying subscription pays for itself quickly,” says green advisor Ron Berben. “After three months, I calculated the difference. If we had cared for all 840 trees based on the moisture sensors, we would have saved €20,000 in water, labor, and material costs over three months. I’m genuinely surprised. You don’t waste water, but you also don’t give too little, so the tree is optimally cared for. And there’s no need for extra trips to check the status of the tree and the soil. That’s an additional internal saving that isn’t included in the amount. The execution can also be monitored. The moisture sensors spike when water is given.”

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