It wasn’t easy: finding a piece of parched ground after it had been raining madly for a week. But it had to. A test had to be repeated that received a lot of attention last summer.
In the middle of the summer, British researcher Rob Thompson decided to show the British public how periods of prolonged drought can increase the risk of flooding. Thompson is a meteorologist at the University of Reading, in the radar group to be precise, and in the course of his life he has become fascinated by the phenomenon of rain. He calls himself Dr. Rainman and also: Pluviologist.
At the beginning of August, England was still in an endlessly hot heat wave, but on August 9 the BBC was able to announce that the ordeal was over, that it was finally going to rain. Heavy showers were coming. You bastards, Thompson thought, heavy rains are the last thing we need. Here come flash floods because the dried out soil can hardly absorb water. Every meteorologist and soil scientist knows that.
To clarify his concern, Thompson took a explainer video in the University’s Harris Garden. He filled a plastic cup with water, closed it with a piece of cardboard, turned it over, set the thing on the floor and pulled the cardboard out from under it. The cup emptied on damp and semi-moist grass. On a dry and completely withered patch of grass, the water remained in the cup. The mind stood still, but the point was made.
Thompson put the film on his twitter account and it soon reached LinkedIn and TikTok and then the mainstream media. But suspicion and assertiveness have been brought to unprecedented fruition by social media and there was immediate criticism of the demonstration. There would have been cut in the video, so it was not at all clear on which surface the cup emptied the fastest. And the cup was so airtight on the dry ground that that alone prevented water from running out, because, of course, water could only get out if air went in. And if the cup apparently emptied quickly on damp grass, that didn’t mean that the water also got into the ground. It could also be somewhere among the blades of grass.
Thompson posted a director’s cut with more details but that didn’t help. Claims of scholars are no longer taken for granted. That is why the experiment was repeated last week, calculated on the day that one downpour after another swept over Amsterdam. Fortunately, dry ground was found under the concrete ramp of the bridge over the Amsterdam-Rhine Canal, in a place that, given the colorful paintings and abandoned items, is apparently used for pastime and the like.
Thompson’s results were measured with a glass glass shiny bestätigt. The water remained on the gray sand, even if the glass did not fit well with the substrate at all, in the damp grass next to the driveway it drained quickly. Incidentally, it did not take any glass at all to show that the dry sand did not absorb water. Check out the photo and also note the steep angle between water and sand at the edge of the spilled puddles. that steep ‘contact angle’ is characteristic.
The fact that dried out soil is often difficult to wet, and can even be downright water-repellent, has been previously described in this newspaper. Hobby gardeners and houseplant enthusiasts have known it for a long time. In 2013, the AW column discussed the difference in water repellency between dry dune sand and dry aquarium sand. During the drying out, organic substances can build up in the soil surface that make the soil hydrophobic. The substances are often formed under the influence of bacteria and fungi from other substances that plants brought into and onto the soil. Most often, water repellency is accompanied by a high content of organic matter. But the grain structure of the soil and the acidity also play a role. And of course the water content.
Water drop penetration time
Wageningen soil scientists have paid a lot of attention to this phenomenon over the years. Not so much for fear of that flash floods but because it is so difficult to provide agricultural crops or garden plants with sufficient water during and after periods of drought. Even if it rains again, it can take a long time for the water repellency to disappear naturally. The problem can be solved artificially, but it is best to prevent the build-up of a hydrophobic layer by not letting the soil dry out.
Thompson’s cup test is in fact a rough version of an elegant test that was developed long ago in Wageningen to quantify water repellency. It measured how many seconds it takes for a drop of distilled water to sink into a soil sample: the water drop penetration time (WDPT). If it went within 5 seconds, the bottom was considered wettable, but it could also take hours. A modern variant of the test measures how much alcohol you have to add to distilled water to bring the WDPT to exactly 10 seconds. If even brandy and gin remain in place for 10 seconds, the soil is badly wrong.