Wednesday, 19 December 2012

November 2012 overview

Groundwater levels - November 2012
Groundwater levels in many areas have continued to rise during November.  High levels this early in the winter mean there is a strong possibility of localised groundwater flooding in the coming months.  Groundwater levels in areas of the western Chalk (e.g. South Winterborne in Dorset, Tilshead and the River Og in Wiltshire, King's Somborne and Bishop's Sutton in Hampshire and the Lamborne in Berkshire) have already risen to the Environment Agency's flood alert trigger levels.

Levels in the Chalk outcrops are generally above average for the time of year, rising by about 10 m at Chilgrove, Compton and Tilshead.  Previous monthly maxima have been exceeded at Ashton Farm and West Woodyates during November.

For more information, see the hydrological summary for November 2012 [PDF].

Friday, 16 November 2012

October 2012 overview

Map of groundwater levels in the UK, Oct 2012
Groundwater levels - October 2012.
We usually expect water levels to be around their lowest in early October, but this year has been very different, and aquifers have already been well topped up by the exceptional spring and summer rainfall. The water resources situation is therefore healthier than usual for this time of year. However, going into the late autumn and winter (the time of year when groundwater recharge normally occurs) with groundwater levels that are already anomalously high means an increased likelihood of groundwater flooding in susceptible areas.

Groundwater levels are particularly high in the Chalk of the south of England were groundwater flood warnings were already issued earlier in the autumn. For more information, see the hydrological summary for October 2012 [PDF].

Monday, 15 October 2012

2012 so far: an exceptional year

Hydrogeologists got just as excited about the big national events of this year - the London Olympics, the Jubilee - as everyone else. But we were even more excited by the unusual pattern of rainfall which has led to some very interesting responses in groundwater levels.

What we expect

Late autumn is usually when UK groundwater levels start to rise again. Plant growth slows at this time of year, and as the plants are taking up less water, soils 'wet up' so that rain can once again infiltrate down to replenish aquifers.

The unexpected

The dashed black line shows the monthly average water level for this borehole. The red ellipse highlights the winter of 2011/12 when groundwater levels did not recover as usual.  The blue ellipse shows how groundwater levels rose significantly in the summer of 2012, a time when we would expect to see levels fall.

Across much of the UK the pattern has been different this year. Rainfall was lower than usual last winter, so groundwater levels did not rise as expected. Droughts were predicted and eventually declared in many areas. Exceptional rainfall between April and July then ended the drought, causing remarkable rises in groundwater levels. Groundwater hydrographs were practically inverted across the winter-spring-summer 2012 period, as illustrated by the example in the image.

An incisive overview of the hydrological events of this period has been published by CEH: "An overview of the 2010-12 drought and its dramatic termination".

Whatever next?

We should now begin to see 'normal' autumn/winter recharge on top of the 'unusual' spring/summer recharge. This puts us in a good position in terms of water resources, but some areas will be more prone to groundwater flooding than at this point in most years.