Why rains after droughts can be so dangerous

After weeks of oppressive heat and days without a drop of rain – and after the official declaration of drought in parts of England, the United Kingdom – prepares for the arrival of copious rains.

But although at first glance this may seem like a relief, the torrential rains (predicted by the Met Office, the national weather office), actually represent a danger.

Scientists warn that this situation could cause Flash floods and they are unlikely to allow dry soils to replenish.

These consequences concern not only the United Kingdom, but also other countries in Europe hit by drought.

Why rains can lead to floods?

In addition to two heat waves and record temperatures this summer, many parts of the UK have seen much less rainfall than usual.

This has, in fact, baked the soils, leaving them dry and hard with very low moisture levels, notes the UK Center for Hydrology and Ecology.

If rain falls in large amounts and at high speed, as occurs with thunderstorms, the ground cannot absorb humidity.

Instead, it accumulates on the surface. On sloping surfaces, water runs off quickly and causes flash flooding.

The effect is like pouring water at high speed on concrete, explains Rob Thompson, a meteorologist at the University of Reading, to the BBC.

“The grounds of our gardens, parks and farmlands are now potentially as dry as asphalt and concrete. Non-asphalt areas will behave like asphalt when the rain hits them,” he says.

This video, recorded by Thompson, clearly shows how the earth reacts to contact with water.

The main effect that drought has on the soil is something called hydrophobicityexplains soil scientist John Quinton of Lancaster University.

When water hits a waterproof jacket, the jacket material repels it, causing beads to form on top and eventually run off.

Something similar happens when organic matter in the soil dries out, forming a layer of material that prevents the entry of water.

“Instead of the water moving down to the ground, it stays on the surface,” Quinton says.

soil structure

The drought has burned grass and other plants, turning parks and fields yellow.

This vegetation usually covers the ground and protects it from heavy rains.

“The vegetation breaks down the drops of rain from large thunderstorms into smaller drops. Without that protection, the large droplets damage the soil structure, which means even less water can infiltrate,” Quinton explains.

While the UK has many different types of soil, if it rains hard enough, the entire country is susceptible to flash flooding, Thompson notes.

any place with steep terrain and mountainous, where water can move very quickly, is at particularly high risk.

While the floods are unlikely to be as devastating as those that occurred in Germany and Belgium last summer, the potential is always there, Thompson says.

Thunderstorms can bring large amounts of rain, but usually over a small area and over a short period of time. That doesn’t give the soil enough time to recover.

The ideal after a drought, scientists explain, is a light rain for several hours and days so that the soil recovers its normal moisture levels.

But it will probably take weeks of above-average rainfall to end the drought.

According to researchers, the record temperatures observed in July would have been “virtually impossible” without the climate change man-made, and heat waves and droughts are likely to become more extreme and common.