Water availability is the key factor when looking at thermal performances of natural soils and vegetation. Let’s have a look at the soil: If the soil begins to dry out, it loses its capability to “destroy” the incoming energy through evaporation. Of course, the energy is not really “destroyed”, but it is used to change water from the liquid phase into the gas/vapour phase. The energy required for this transition is no longer available to heat up the surface and the air.
Let’s have a look at the near-surface temperature (5 cm above ground) in the last 10 days of a 20-day heat spell like in the previous post, but this time above a grass surface in a park.
First, as we expect, the air temperature close to the grass surface is lower than its counterpart above the asphalt road (starting at 296 K = 22,85 °C). But we also see that, in contrast to the asphalt road, the peak air temperature is quickly rising from day to day reaching 302 K (=28,85 °C) on the 10th day. And still, the trend shows that the temperature is still rising and no equilibrium has yet been reached. In fact, dry and dark soils with dry vegetation can easily get as hot as a medium dark asphalt road and lose all of their cooling potential.
And we have not yet talked about the vitality of the plants growing on this drying soil.
To make sure, that parks and other green landscapes will not only stay green, but also not behave like sealed surfaces, it is important to ensure water supply during heat waves. The best solution for this are local rainfall harvesting systems, because one thing is for sure in any heat wave: the next thunderstorm will come and then we will have excess water to handle.