When I last wrote on the subject of drought, at the end of March 2012, conditions in Southern England were looking pretty grim: Accumulated rainfall was 200mm down on the climatological norm for the previous 12 months, the River Thames was running at 50% of normal, and the chalk aquifers to the north of Reading were at their lowest in the 51 years since records began. A ‘back of the envelope’ calculation suggested that to end the drought by the end of July would require roughly twice the climatological rain rate – and how we should be careful what we wish for… A shift of the jet stream to the south of the UK brought the wettest April in 100 years. Conditions remained unsettled throughout May and further heavy rain arrived in early June to mark the occasion of the Queen’s Diamond Jubilee. So where are we now?
The present rainfall deficit for the South East of England is 76mm. Whilst technically this is still a meteorological ‘drought’ departures of this size relatively common and the deficit could be made good by 3 or 4 days of heavy rainfall. The recent rain has translated into run-off that has more or less alleviated the stream flow droughts that were being experienced in many of the UK’s rivers.
There has also been an improvement in the groundwater situation. However, aquifers recharge slowly and the dry spring 2011 coupled with the dry winters 2010/11 and 2011/12 have resulted in deficits equivalent to several months of winter rain. Evapotranspirative losses throughout the summer mean aquifer levels are unlikely to see further improvement until the traditional recharge periods of the autumn and winter.
So is the drought over? In terms of meteorology and stream flow, yes; for groundwater, maybe. The major lesson to be learnt from what has been a very near miss is the continued vulnerability of the UK to drought conditions.