The end of the drought?

Andy McKenzie (left) with BBC Meteorologist Nick Miller during filming Drought fears continue in England despite recent rain; filmed using a BGS hydrogeological map in our 3D visualisation suite at Keyworth.

Dr Robert Ward, Head of Groundwater Science at BGS, said,

'Because the current drought has developed over a long period of time and groundwater levels are now so low, it will take four to five months of above average rainfall, like we’re experiencing now, to fully replenish affected aquifers.'

'A groundwater drought can be likened to a bank account that is overdrawn.
If we keep spending (abstracting water) the debt will increase (groundwater levels get lower).
If the debt gets too bad, then when we pay in our wages (rainfall recharge) it won’t cover the debt.
It’ll get even worse if our wages are cut (dry winters).
If we are going to get out of debt we need reduce how much we spend (hose pipe bans and other restrictions) and keep paying in our wages — however much we get.
It may take a long time until we’re back in the black just like it will take us a long time to overcome the current drought.'

Caption BBC Inside Out film crew filming in the dry bed of the River Lambourn, April 2012.

Just as the first Swallow heralds summer, jokes about the 'wettest drought on record' herald the end of a drought — but is it really the end?

In late April 2012 scientists from the BGS worked with the BBC to provide scientific insight into the drought:

With wet and gloomy weather over much of Southern Britain, many rivers in flood, and with April rainfall breaking records — more remarkably for having been concentrated in the second half of the month — it is hard to conceive that we will still have water resource issues this summer.

But the drought we have been in, and still are in, has its roots over a two year period rather than just two weeks and it affects groundwater more than it affects rivers.

Low winter rainfall

A dry spring in 2011, followed by a dry winter in 2011/2012 seriously reduced recharge to the groundwater aquifers that, in Southern England, supply the majority of water supply.

At the end of March this year groundwater levels over much of the Chalk aquifer, our most important, were at or near record lows for the month.

Soils were dry at the end of March 2012; as dry as we would have expected them to be in May in a normal year.

When rain falls on dry soils the soils soak up the water, and plants use this water for growth, and there is little opportunity to recharge groundwater reservoirs. We confidently predicted that, unless there was exceptional rainfall, drought conditions would worsen.

Exceptional spring rainfall

Now that the exceptional rainfall has happened, does this mean the drought is over?

The simple answer is no!

The slightly longer answer is no, but it may not be as bad as we feared.

The rainfall in April has wetted up the soil, and it's probably approaching its 'normal' wetness for early May.

But we don't normally expect there to be much, if any, groundwater recharge in a normal May — plant growth and evaporation will probably use all the water.

In fact when soils are particularly dry heavy rain can run off rapidly rather than soak in — which has made local flooding worse.

Aquifers will recharge... slowly

With so much rain there will be some recharge to aquifers.

Limestones in the Midlands, should respond well, and even in the Chalk there will be some effect, although it will take several weeks before it is detectable by BGS and Environment Agency monitoring.

However any effect has to be put in the context of the recharge we lost over the last two years — which has been equivalent to three or more months of winter rainfall; two weeks of heavy rain will make some difference, but as plant growth picks up and uses more water it will be limited.

Unless the really exceptionally wet weather of the last part of April continues through much of May, it's likely that groundwater levels will be still very low, and will fall further over the summer.

The opportunity for significant recharge will await cooler and wetter weather in the autumn.

What about the hosepipe ban?

The water restrictions and hosepipe bans that have been put in place were largely a response to the expected low groundwater levels over the summer, and as we have seen, we are not expecting much change in levels from this rain.

However, continued high rainfall will mean that storage reservoirs will fill further; there is more water in rivers to abstract, and farmers and gardeners will not have to look to irrigation as urgently as before.

These factors should, at least, give water resource managers some breathing space, but drought probably won't be declared over unless we receive autumn rains that can seriously start to recharge our major aquifers.

Of course the lesson of the last two dry winters is that recharge is unpredictable!