A demonstration of water flows that has gone viral on social networks is denied by many netizens.
Question asked on Twitter on August 17th.
A video showing the effects of water runoff on the ground during an episode of drought has gone viral on social networks in recent days and was broadcast on France 2’s weather program on Sunday 14 August. The video, released by the University of Reading, UK, aims to show “how long it takes for water to seep into parched soil” in order to understand “why heavy rains after a drought can be dangerous and lead to flash floods”.
Three videos are lined up on the sequence. One glass filled with water, inverted on very wet, moderately wet or very dry ground (from left to right). When the plate – which allows the glass to be turned upside down so that the water comes into contact with the bottom – is removed in the three images, three distinct patterns are observed.
In the first, on the far left, where the grass is very green and the ground is therefore wet, the water contained in the glass runs off very quickly (10 seconds on the video). In the second, where the grass is less green and the soil less moist, the water penetrates the soil more slowly (55 seconds). On the other hand, in the last video, where the ground is very dry, the water stays in the glass.
In the comments below the video, however, several Internet users point out prejudices about this experience. “Dense grass creates a lot of air around the base, normal grass less, it distorts the demonstration,” hints at a first. “The vacuum factor in the cup is not taken into account,” he replies one secondor “the design of this experiment appears to be flawed, the grass should have been the same length in all scenarios to ensure a fair and comparable test”.
A “simplified” demonstration
University of Reading researcher Rob Thompson, contacted by CheckNews, explains behind the video that the demonstration was primarily intended to demonstrate a phenomenon “in a simplified manner” rather than through a “rigorous science”. “The intention was to demonstrate a problem in a visual way to highlight the dangers of flash flooding after a drought,” he says.
The weather forecaster says: “The demonstration is inherently simple and that’s part of its charm and why I think it was so successful. The equipment was what most people would find for free: a cup and an item that can be used as a lid that can be quickly removed.
To respond to criticism, Rob Thompson directs us to the longer version of the video available on YouTube. On this we see that after more than 4 minutes of video, the researcher empties the water glass on the lawn and the water remains on the surface. “The seal isn’t perfect (although all three jars have some seal – the jars rise when empty) and probably makes a bigger difference, but that doesn’t change the order of drainage velocity,” asserts the researcher.
“Good illustration of the phenomenon”
July 2022 was the driest month (9.7 mm) since March 1961 (7.8 mm) and the driest month ever recorded by Météo-France over the period 1959-2022. Despite some rainfall and thunderstorms since early August, the Bureau of Geological and Mining Research (BRGM) pointed out in a May 1 that water tables are particularly dry.
Laurent Longuevergne, research director specializing in field hydrology at Geosciences Rennes, analyzes the phenomenon as follows: “A persistent drought effect on the soil, like the one that is happening today after long periods of heat and the lack of rainfall in July, has the effect of making it hydrophobic , which means that the water first slides on its surface before penetrating very slowly, sometimes even pooling in one spot and forming puddles.” Conversely, when the soil is moist, water penetrates faster, so it is called a hydrophilic soil. “This video is a good illustration of this phenomenon,” adds the researcher.
Runoff can therefore cause significant damage during heavy rains after an intense drought due to the accumulation of water on the ground. In July 2021, Belgium and Germany experienced such a phenomenon. Deadly floods had devastated both countries. According to Laurent Longuevergne, “in the future, global warming could increase droughts and thus the occurrence of such phenomena”. Rob Thompson specifies that he wants his video to “draw public attention to a possible future danger”.