How prolonged droughts make floods worse

on Wednesday, Aug 17, Hannah Cloke – a hydrologist at the University of Reading – was sitting in her home office when it began to rain. It was a welcome sight. Much of southern England had been baked bone dry during successive heat waves and the worst drought in almost 50 years – satellite images showed the country’s green and pleasant land turning a sickly yellow.

But as she watched with her experienced eye, Cloke noticed things others might not notice: how water pooled on the lawn instead of seeping into the ground, how the areas with the best drainage were under a tree in her yard. The prolonged drought had altered the soil’s composition and affected its ability to absorb water.

It’s a pattern that has been repeating itself across much of Europe of late, as long-awaited rains have triggered flash floods. “The ground starts to behave like concrete or asphalt,” says Cloke. “If it rains at all, it just drains away — that’s classic soil physics.”

Contrary to intuition, soil is most absorbent when it’s a little damp. “When it’s really dry and when it’s really wet, it’s actually difficult to get water into the ground,” says Cloke. A little water changes its porosity – creating holes and pathways through which more water can be absorbed. This is partly due to surface tension – the way water molecules stick together into droplets that may then be too large to filter through the gaps in the parched soil. In slightly wetter soil, the moisture breaks the surface tension of those same droplets, allowing them to combine with the water already in the soil and more easily find a way to flow down.

Also, in dry soil, the gaps between soil particles are full of air that cannot escape, preventing water from flowing into the soil. Soil particles themselves can also become hydrophobic, i.e. repel water, since microbes near the surface release waxy substances when they die off due to heat or lack of water. To make matters worse, extremely dry soil can also form an impermeable crust – a phenomenon that can be exacerbated if it is then compacted by footsteps or agricultural implements. It is the combination of these factors that makes extremely dry soil so poor at absorbing rain.

“Soil moisture is very low because 2022 was very dry – the driest January to July since 1976,” says Simon Parry, hydrologist at the UK Center for Ecology and Hydrology. “In addition, the prolonged very hot summer conditions – including two heat waves – have helped to heat up the soil surface. This makes the surface a nearly impermeable barrier, further limiting the amount of rainfall the soil can absorb.”

Cloke’s colleague Rob Thompson from the University of Reading illustrated this in a viral tweet. He tipped three glasses of water over onto three areas of the ground where the grass had been watered at different rates: wet grass, the grass of a normal summer, and the dry grass of a heat wave. The first two jars slowly drained their contents into the earth, but the third jar stayed almost full until the end of the video.

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