16 Apr 2012

Rain is falling but the drought area expands. Why?

The Environment Agency has declared today that another 17 English counties are now officially in drought just a few weeks after parts of Yorkshire were added to the drought zone.

Counties now officially in drought areas are Nottinghamshire, Leicestershire, Derbyshire, Staffordshire, West Midlands, Warwickshire, Shropshire, Worcestershire, Herefordshire and Gloucestershire, Cornwall, Devon, Dorset, Somerset, Bristol, South Gloucestershire, parts of Hampshire, and most of Wiltshire.

Whilst the Environment Agency stress that public water supplies are unlikely to be affected by the ongoing drought, they are once again calling for water to be used wisely.

During the past week, I’ve had quite a few people ask me on Twitter if the drought will end because of the rain that has fallen over the past week. The simple answer to that is no, but why not?

The answer lies in the lack of rainfall over southern and eastern parts of the UK in the past two winters – a time of year which is particularly important for recharging our water resources.

Taking England alone, rainfall during the past two winters has been just 82 per cent of average. However, this is only part of the story as there has been a huge variation in rainfall across different parts of England. When looking specifically at central, southern and eastern areas, there’s only been around three-quarters of average rainfall in winter 2011/12.

Effectively rainfall has been persistently below average for such a long time, that in order to counterbalance the dryness, rainfall would have to been well above average for many months.

You may have also read that in some parts of England currently in drought, reservoirs are actually more than 90 per cent full. You’re probably thinking that this means everything is fine and why is drought still being been declared?

There’s a less visible element to our water resources that needs to be remembered: groundwater.

Groundwater is the largest available reservoir of fresh water and lies deep underground in bodies of rock known as aquifers. Water moves around through these aquifers pulled by gravity and pushed by the force of water above it, with many impurities removed as it does so.

This makes groundwater a great source for public water supplies. However, groundwater moves very slowly compared to surface water, so there can be a lag in the response of groundwater levels to the pattern of rainfall being observed in an area.

The latest Environment Agency weekly water situation report indicates that groundwater levels are below normal or lower at the majority of sites in England and Wales. In fact, at thirteen sites, groundwater levels are exceptionally low.

On a more positive note, up until the 10th April most of England and Wales had had 30-50 per cent of the average April rainfall, with the Anglian region having had 67 per cent – this with still two-thirds of the month to go.

So what does the weather hold for the next few weeks? Well, there’s a continued signal for the weather to remain unsettled with low pressure bringing further showers or longer spells of rain – even to drought-hit areas.

During this week alone, there could be 15-30mm in eastern areas, 20-50mm in western areas, with as much as 70mm across some hills and mountains.

Whilst this week’s rain will no doubt be welcome, the wet weather will have to continue for some time to put the drought train into reverse.

Don’t forget a five day forecast is available on the Channel 4 Weather website. You can also contact me on Twitter – @liamdutton

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