How Long Droughts Make Flooding Worse

On Wednesday, August 17, Hannah Cloke—a hydrologist at the University of Reading—was sitting in her home office when the rain started coming down. It was a welcome sight. Much of southern England had been baked bone dry during successive heat waves and the worst period of drought in almost 50 years—satellite images showed the country’s green and pleasant land turned a sickly yellow.

But as she watched with her expert eye, Cloke noticed things that others might not: how the water was pooling on the lawn rather than soaking into the ground, how the areas with the best drainage were under a tree in her garden. The prolonged period of drought had changed the nature of the soil, hampering its ability to absorb water.

It’s a pattern that’s repeated itself across much of Europe recently, as long-awaited rains have sparked flash floods. “Soil starts to act like concrete or tarmac,” says Cloke. “When we get any rainfall on it at all it just runs straight off—it’s classic soil physics.”

Soil is, counterintuitively, at its most absorbent when it’s a little bit damp. “When it’s really dry and when it’s really wet it’s actually difficult to get water into the soil,” Cloke says. A little bit of water changes its porosity—creating holes and pathways that allow more water to be absorbed. That’s partly because of surface tension—the way water molecules stick together to form droplets, which may then be too big to filter through the gaps in the dried-out soil. In slightly damper soil, the moisture breaks the surface tension of those same droplets, allowing them to combine with what water is already in the soil and more easily find a path to flow down.

Plus, in dry earth the gaps between the soil particles are full of air, which has no way of escaping—blocking the water from moving down into the earth. Soil particles themselves can also become hydrophobic, meaning that they repel water, as microbes close to the surface release waxy substances when they die off from heat or lack of water. To make matters worse, extremely dry ground can also form an impermeable crust—a phenomenon that can be worsened if it’s then compacted by footfall or farming equipment. It’s the combination of these factors that makes extremely dry soil so bad at soaking up rainfall.

“Soil moisture is very low because 2022 has been very dry—driest January to July since 1976,” says Simon Parry, a hydrologist at the UK Centre for Ecology and Hydrology. “In addition, the protracted very hot conditions throughout summer—including two heat waves—have acted to bake the ground surface. This means that the surface turns into an almost impermeable barrier, which further limits the amount of rainfall the ground can absorb.”

Cloke’s University of Reading colleague Rob Thompson provided a compelling illustration of this in a viral tweet. He upturned three glasses of water on three areas of ground where the grass had been watered to different levels: wet grass, the grass of a normal summer, and the dry grass from during a heat wave. The first two glasses slowly drained their contents into the soil, but the third glass remained almost full until the end of the video.

Essentially, drought turns flash flooding from a predominantly urban phenomenon—made worse by human-made surfaces—into something that can strike anywhere. “Drought and heat waves may mean that natural surfaces, both in urban and rural areas, may temporarily respond to rainfall in a similar manner,” says Parry.

There may be a vicious cycle at play here too, because droughts and floods could make subsequent droughts and floods more severe. Anything that scours the land—whether that’s a wildfire fuelled by drought or a flash flood that sweeps trees from the ground—will make future flash flooding more likely and raise the chance of landslips and mudslides.

Intense drought kills plants, leaving the landscape bare and reducing the soil’s ability to hold water. Heavy rain can also create a hard crust on top of the soil so that the next time it rains water runs off even faster. These effects are surprisingly enduring: German researchers have found evidence that forest soils with a history of drought are more likely to repel water, regardless of the contemporary condition of the soil.

Climate change will make periods of drought like the one Europe has experienced more frequent, but there are ways to mitigate the risk of potentially devastating flash flooding. Planning regulations in many countries already require developers to take into account the flood risk of replacing vegetation with concrete, for instance, but avoiding flash floods may also require changes to farms, parks, and gardens. Undulating fields with dips and ponds where water can collect and slowly seep back into the soil will stop it from running off too quickly.

Plant diversity is important too—a mixture of flowers and trees will absorb more water than a flat dry lawn. In her own garden, anticipating rain, Cloke went out and pricked holes in the lawn to give the water somewhere to go. “It’s about making the landscape a little bit uneven,” says Cloke. “It’s the same principles that you have for sustainable drainage systems around new houses; you get swales and berms [ditches and earthen walls], and you make the landscape lumpy, basically, so you can store all the little pools of water.”

In a way, the best way to manage the risk of flash floods after droughts is to try and unwind some of the changes that humans have made to the land so that we’re less affected by the changes we’ve made to the climate. “It’s really important to remember that we’ve always had floods, we’ve always had droughts, and the main problems come because of the way we’ve changed the landscape, the way we’ve changed our rivers, the way we’ve abstracted water, and we’re overusing the water that’s there,” Cloke says. “It’s going to get worse with climate change, but there are ways to live with these phenomena.”