30 Apr 2012

Flood warnings in a drought? How?

There was a huge contrast in the weather across the UK through the weekend. Scotland and Northern Ireland were bathed in sunshine, whereas England and Wales were drenched in heavy rain, accompanied by gale force winds.

South west England has had the greatest amount of rainfall during the past three days, with Liscombe, Somerset and Dunkeswell, Devon having 76.6mm and 63.6mm of rain respectively. The rest of England and Wales has had around 25-40mm – a significant proportion of what would be expected during the whole of April.

The wet weather follows weeks of downpours and as of the 24 April, England and Wales as a whole had received 153 per cent of their average rainfall.

There were numerous problems caused by the wet and windy conditions at the weekend, with trees and power lines blown down. However, the thing that has got people scratching their heads the most is how can there be flood warnings in force when we are experiencing a drought?

Although it sounds unbelievable, there is an explanation. There are two main ways in which we resource water here in the UK. The first is from groundwater deep down below the surface. The second is from water at the surface, such as rivers and reservoirs. Whilst both of these are a valuable source of water, the speed at which each of these respond to rainfall patterns varies immensely.

The key to explaining why there are flood warnings in a drought lies in the speed at which surface water sources respond to rainfall.

During the past few weeks the soil has become increasingly saturated to the point whereby, with the exception of the Environment Agency’s Midlands, South East and Anglian regions, soils are more or less holding as much water they can.

This has meant that when rain has fallen during the past few days, in most places the soil hasn’t had the capacity to soak up any more water. As a result, the water very quickly flows through the water catchments and straight into rivers. This is why river levels have risen quickly and some places have suddenly become prone to flooding.

Whilst it may have become a water fest at the surface, it’s a very different story deep down in the aquifers where groundwater, our other main source of water lies.

Water here is desperately lacking with the latest Environment Agency water report stating that the majority of sites across England and Wales still have groundwater levels notably low or lower for this time of year.

So why isn’t rainfall at the surface percolating downwards into the aquifers where it is most needed? The problem at this time of year is that it is hard for rain to reach deep down because evaporation rates are higher and growing plants take up a lot of water through their roots.

If the wet weather continues, some rain will eventually percolate down to where it’s needed, but nowhere near enough to offset the lack of rainfall experienced in the past two winters – a time of year when groundwater recharge is greatest.

As you can see, it’s not quite the case of it being the wrong kind of rain, but more a case of rain accumulating in the wrong places.

The week ahead will bring further heavy rain at times to England and Wales, which will again cause river levels to rise at some locations, bringing a risk of flooding.

Don’t forget, you can get the latest flood warnings on the Environment Agency website and you can contact me at any time on Twitter – @liamdutton

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