Why Is Heavy Rain So Dangerous After A Drought?

It can be easy to reduce water to a simple matter of demand and supply, particularly when exploring the utility aspects of water infrastructure, including freshwater supply and wastewater and trade-effluent removal.

For businesses looking to save money on their water bills, often there needs to be little more thought than that, but exploring water as a matter of supply and demand is slightly reductive, at least in certain cases where more water does not help to meet the demand.

An example of this which has become increasingly common over the past few decades is the increased dangers of flash flooding after drought conditions, as the ground paradoxically becomes increasingly hydrophobic.

The reasons for this are relatively easy to demonstrate but complex to explain. However, it is important to understand why this happens in order to develop effective ways to mitigate it.

Baking The Earth

Flash flooding is a sudden, extreme level of downpour that can cause rivers to burst their banks or cause floods in situations where water cannot be either soaked into the ground or channelled into aqueducts fast enough.

Typically, flash flooding takes place predominantly in urban areas due to the use of impermeable materials to make roads, pavements and other solid surfaces. However, increased temperatures mean that flash flooding can affect essentially anywhere.

In 2022, Dr Rob Thompson of the University of Reading performed an experiment that drew widespread attention, comparing how long it took for a cup of water to be soaked into the ground on a wet day, a typical summer’s day and a heatwave.

The difference was exceptionally stark, with the heatwave-baked ground barely allowing any water to be absorbed, causing it to pool on the surface. This is a problem a lot of gardeners and landscapers can attest to.

On sloped ground, this water runs off as efficiently as it would on tarmac, flowing off the top, dry, hydrophobic layer of the ground.

If there is a period of light drizzle that lasts several days, this will eventually fix itself as the water gently soaks into the earth and restores the soil to its previous pre-parched condition, albeit probably with some long-term side effects caused by the drought.

The biggest concern is when there is a lot of rain falling very heavily to end a drought, as this can become exceptionally counterproductive.

Heavy rain pools, runs off and in sufficient quantities causes intense flooding and the exceptional damage that it can cause.

What Can Mitigate Flash Floods?

One of the biggest variables that can affect how much the ground is affected by drought conditions is the vegetation conditions in the area in question.

This is one of many reasons why cover crops are used in the agricultural industry to cover bare soil. Rainwater lands on the plants and is more easily soaked into the ground, reducing and potentially removing the effect of flash flooding.

Forests are a larger example of this effect in action, with water soaking roots and leaves on the ground, ultimately nourishing the forest floor instead of running off and contributing to a potential flash flooding situation.

This is the other reason why urban conditions flood more easily, as well. Alongside the fact that little flood protection is provided by planters in otherwise hydrophobic surfaces such as concrete and tarmac, most green spaces in urban areas tend to be dominated by relatively short grass.

Thanks to a combination of little surface area and the stress caused by the mowing process, lawn grass is often the first vegetation to die in drought conditions, and as a result, the earth easily develops that impermeable baked layer.

This can, however, be fixed by incorporating a more diverse array of plants, allowing lawns to grow and planting trees and flowers that will collectively absorb more water.

As well as this, the classic late winter trick of aerating a garden will provide places for water to flow into the soil.

Finally, the shape of the ground can matter as well. If there are divots and ditches in an area, it creates small crevices and ponds where water can pool and soak into the ground.

This approach is often used for creating sustainable drainage systems.

Positive Or Negative Feedback Loop?

Whilst the acute issues surrounding rain and drought conditions are well-documented, one question that has been asked by a growing number of people with a greater sense of urgency is whether these conditions will lead to more or less extreme flooding in the future.

One fear that has been widely expressed is that the devastation caused by a drought-induced flash flood is that the damage caused by both the dry ground and the fast-flowing water can cause long-term damage that reduces the area’s resilience against future flooding.

Part of the reason for this is that both floods and droughts affect soil conditions, which could make it more difficult and more resource-intensive to grow vegetation to mitigate it or damage natural forests which are an essential part of preventing floods in some areas.

In the short term, negative feedback loops have been reported and experienced, but it is difficult to ascertain if this is the case over the long term and if the soil had a chance to recover from the effects of the initial drought.

Alternatively, is it possible that there is a negative feedback loop largely caused by human intervention? After droughts and flash flooding, more resources are typically invested in a stricken area to ensure that this does not happen again.

This is something seen recently after the extreme weather in Europe over the Summer of 2023. Northern Europe faced extreme heatwaves and drought conditions, whilst Greece and other parts of South Europe and the Middle East were affected by unexpected flash flooding.

Since this point, the former has started to build up drought resilience infrastructure, such as the construction of reservoirs, a greater use of water recycling and desalination, as well as methods to improve the replenishment of groundwater sources.

Ultimately, water infrastructure is exceptionally complex, to the point that there are situations where lots of water can be a problem for a parched area.