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| Why is heavy rain after a drought a danger? |
After weeks of hot and dry conditions in most parts of the UK, drought conditions have also been declared in parts of England. In such a case, it seems that all we need is good rain.
But this week, the forecast of heavy rain with lightning and thunder by the Meteorological Department may cause more trouble than good.
In such a situation, scientists are warning that these rains can lead to flash floods and it is unlikely to replenish the dry soil.
This is the reason why our dry land does not need heavy rain at this time.
Flash floods
On top of two heatwaves and record-breaking temperatures this summer, many parts of the UK received below-average rainfall.
The UK Center for Hydrology and Ecology says this effectively cooked the soil, leaving it dry and hard with very little moisture.
If the rain is heavy and fast, as in thunderstorms, the soil cannot absorb the moisture. Instead, rainwater collects on the surface and runs off quickly on sloping surfaces, causing flooding.
Dr Rob Thomson, a meteorologist at the University of Reading, told BBC News that the effect is similar to that of pouring water at high speed on concrete.
He says: 'Our gardens, parks and farmlands are now probably as dry as tarmac and concrete. Areas that are not tarmac will behave like tarmac when it rains.'
Professor John Quinton, a soil scientist at the University of Lancaster, explains that the biggest effect of drought on land is called hydrophobicity.
When water hits the waterproof layer, it pushes back and forms a droplet or droplets that eventually run off.
A similar process occurs when the organic matter in the soil dries out, forming a layer of material that keeps water out.
'Instead of going into the soil, the water stays on the surface,' says Professor Quinton.
'Soil Texture'
It's not hard to see how the drought has wiped out grass and other vegetation, turning parks and fields yellow.
These usually cover the soil in a way and protect it from heavy rain.
'Vegetation (grasses, greenery) breaks up large raindrops into smaller droplets,' explains Professor Quentin. Without this protection, larger droplets damage the soil structure, resulting in less water being absorbed into the ground.'
Dr Thompson explains that although there are many different types of soil in the UK, if there is enough rain, the whole country can be prone to flash flooding.
Areas are particularly at risk wherever there are steep, mountainous areas where water can move very quickly.
Although the situation is unlikely to get as bad as the devastating floods in Germany and Belgium last summer, fears are always there, Dr Thompson says.
Sometimes when there is heavy rainfall in a small area and in a short interval, it does not give the soil time to recover.
Scientists say that the soil returns to normal levels after several hours and days of light rain.
But more than average rain weeks are likely needed to end the drought.
Scientists have said that the record-breaking temperatures seen in July were 'virtually impossible' without human-caused climate change and that heat waves and droughts are now likely to become more severe and common.
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