Recently, Prague has been named the greenest capital in Europe. Not thanks to its particularly ecologic design or low footprint, but thanks to lots of trees and greenery that decorate it. I think it’s a good sign and an inspiration to other cities, as trees are not only beautiful, but they also play an important role in the ecosystem and quality of living.

See for yourself how green Prague is!

Of particularly high importance, in my opinion, are trees in the inner city on squares and streets. The role I would like to discuss now is the cooling effect they have on the surroundings. Trees keep our cities cool by two mechanisms. Firstly they provide shadow, and secondly, thanks to transpiration, they also actively cool the surrounding air by the evaporation of water which they pump from the ground. 

Comparable with a household air-conditioner

For the second effect to take place, water needs to get sucked out from the ground and then evaporated at the leaves. When water changes from liquid to gas, it absorbs heat. This so called latent heat of vaporisation amounts to 2,4 MJ/kg of water at 20oC. Some trees can transpire around 100L of water every day. Since there is hardly any evaporation at night, let us for simplification assume that a tree cools for 12 hours per day,hence evaporates 8l of water every hour, absorbing 20MJ of energy and creating a cooling power of 5,5 kW. That is comparable with a household air-conditioner.

In city conditions, with paved and concrete covered streets, it’s hard to get hold of such vast amounts of water, especially in the conditions of drought, which Prague and the region experiences lately (June-July 2018).

Tree trunks surrounded by paving and compacted soil – not good!

As the trees give us such massive service, we should ensure at least a sufficient supply of water and moist soil. In the past, the trend was to pave all the way to the tree trunk, but like that, no water can get to the roots with the occassional rain. Moreover, in case of a strong rain, no water is retained and flash flooding occurs. So, as explained in the blog about decentralized rain water management, small trenches should surround trees to allow water to infiltrate the ground and prevent deadly flash flooding.

Local flooding after a thunderstorm in Prague on 10.6.2018


Infiltrating cleaned waste water into top soil?

Add decentralised waste water treatment with soil infiltration and the effect might be further pronounced.

Prague’s 1.2 million inhabitants produce daily 140 million liters of waste water, 1.6 m3 per second, if all that got used for cooling, it would amount to a 3,8 GW cooler. That’s a comparable amount of water as gets evaporated by a 1 GW powerplant!

Trees are great air conditioners, they provide shelter and home to birds and all they need is soil, water and occasional maintenance. Let’s value them and keep them happy in our cities.

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