The experiment was conducted by Dr. Rob Thompson from the Department of Meteorology at the University of Reading in Great Britain. He selected three spots on the lawn – shaded and damp grass, a sunny lawn, and a completely dry area – and placed inverted cups of water on them. The effects were surprising: in the dried soil, water was hardly absorbed at all, while in other cases it disappeared.
Drought, followed by a sudden downpour, increases the risk of a flash flood
The recording shows that the rainfall immediately follows the period dries they will not improve the soil situation at all. This is because its top layer is as hard as concrete and cannot absorb a lot of water. It takes a long time for this dry and hard shell to soften enough to let the water flow into deeper layers.
Sudden excess water in extremely dry soil accumulates on its surface and overflows, resulting in flash floods such as we have seen in the last few years in Italy, Germany or Belgium. Sudden rainfall after a long period of drought is also a greater risk of long-term flooding, because the water not absorbed by the soil flows to rivers, raising their level.
When the rainy seasons alternate with drier seasons in the summer months, the green grassed soil takes in water much faster – as seen in two cases in the Thompson experiment. Rainwater is then able to reach deeper soil layers, although this happens more slowly. This shows how important the presence of green spaces in cities is – not only (rarely mown) lawns, but also trees and shrubs.
Currently, many European countries are struggling with drought, including the United Kingdom (you can see the drought even on satellite photos), France and Germany, where the Loire and Rhine channels are drying up. Soil drought persists in the west and north-west of Poland. Humidity in some places in our country ranges from only 15 to 25 percent.
Source: twojapogoda.pl, University of Reading