Flash flooding risk during heatwave illustrated in simple video experiment

With nothing more than three cups of water, a UK scientist has clearly illustrated how drought conditions can heighten the risk of flash flooding once rain does start to fall.

In the experiment, Dr Robert Thompson from the University of Reading’s Meteorology Department, filmed himself at different times of year turning a clear plastic cup full of water upside down onto the same bit of ground.

The footage clearly reveals how weather conditions affect the absorption rate of water.

During wet but not waterlogged conditions, the water in the clear upturned plastic cup immediately sinks into the wet soil.

In the dry conditions of a "normal” British summer, in which some rainfall means the ground has been recently watered, the cup of water takes longer to penetrate the ground.

And in the final video – filmed during the current heatwave and near-drought conditions the UK is experiencing – the water remains in the upturned cup, unable to find its way into the hard dry soil.

Speaking to The Independent, Dr Thompson said: "Dry, parched, ground doesn’t let water in as effectively as already moist ground. There are a number of reasons that happens but [among the most important are] that the ground is compacted as moisture has been removed by evaporation and baking."

In addition, he said: "The soil particles become more ‘hydrophobic’, repelling water, and surface tension then helps hold the water from falling through microscopic gaps in the soil. Because the soil is resisting water entering, the water sits on the surface, to run off down slopes or simply sit in a pool."

He said he was "surprised by how big the effect was".

"I think part of the beauty of the experiment is it’s easy to repeat for anyone, you just need a glass and some water."

But it also highlights the enormous risk of flash flooding after dry weather – something seen in recent days in California’s famously hot and dry Death Valley, where a "one in a thousand years" deluge washed away roads, damaged infrastructure, carried away cars and left hundreds of people marooned.

All this occurred with just an inch and a half of rain in a single day – which officials said was near the region’s record rainfall for a single day.

Dr Thompson said: "Once you consider larger areas, and add the water as heavy rain, you have the effect [seen in the experiment] going on over a big area, and water running off, unable to get into the ground becomes a problem as it has potential to cause flash floods.

"It will also sit on the surface where it is quicker to evaporate, and heavy showers will often be followed by sunny spells to encourage this, so it will not help the current water shortages as effectively as it could."

He added: "However, heatwaves generally end in heavy showers and thunderstorms, rather than the day of drizzle that would really benefit the current situation. As 1976 often comes up, when the drought broke, the rain brought flooding to parts of the country."