08 Aug Well, is it a drought or isn’t it?

Whilst the UK’s famously variable weather is always a topic for conservation, it is clear that conditions so far this summer have been exceptional.  However, whilst there have been frequent comparisons with the infamous 1976 drought (when there were widespread water shortages across the UK), there are currently no indications of significant disruption to public water supplies in 2018.  So, the question we might ask is, is this really a drought?

 

Well, is it a drought or isn’t it? Pinning down what exactly a drought is can be surprisingly difficult. At the end of this piece we’ve outlined the different ways in which droughts can be defined. So how is summer 2018 shaping up against these?

Meteorological drought

Clearly the last couple of months have been exceptionally hot and dry.  In June, many parts of south and east England received hardly any rainfall, breaking the record for the driest June in the south and the 2nd driest June in the Thames and Wessex regions.  The first three weeks of July were similar:  cumulative rainfall totals to 24 July ranged from 4% of long-term average in east England to 26% in north west England1.  In the south east widespread rainfall in the last week of July alleviated the situation, accounting for the majority of the monthly total. The rain on the 26th ended a 58 day period of zero rainfall for a number of raingauges across the South-east of England.2

June was also exceptionally warm and was recorded as the 3rd warmest June on record for the UK as a whole (only 0.2°C behind 1976, the warmest)3. In some places previous June records were broken by a significant margin, e.g. in Wales and Northern Ireland. The late June heatwave also saw some very exceptional daily maximum temperatures – on 28th June, the highest temperature on record for Scotland (33.2°C) was recorded at Motherwell.

Hydrological drought

Because most streams rely on groundwater to keep flowing in dry periods (baseflow), the occurrence of hydrological droughts is very dependent on conditions over the previous winter (when most recharge to groundwater occurs).  The run up to summer 2018 has seen the climate swing between a wide range of wet and dry to hot and cold periods.  However, the net result has been a decent amount of recharge (rainfall over the last 12 months is slightly above average) and groundwater levels at the start of the summer were generally above average and are likely to remain so through to the autumn.  As a result, stream flows in most parts of the UK have been normal throughout the summer.  The only areas with significant concern on this front are the hard-rock areas in the north and west which have relatively little groundwater.

Looking forward, the key question on water resource managers’ minds will be, “What if we have a dry winter?”.  At the moment, the ground is exceptionally dry and so there will need to be more than average rainfall to wet this up and allow recharge to occur.  From a hydrological point of view, the impacts of this summer may well spill over into next year.

Agricultural drought

June was so dry in many parts of southern England that it was off the scale used to report rainfall deficits.  For the UK as a whole, end-of-June 2018 soil moisture was the driest on record (from 1961). It was also the driest soil moisture for any month since August 19954.  What the National Farmers Union describes as “tinderbox conditions” have severely reduced grass growth and depleted yields for many crops, leading to concerns that there will be a shortage of feed for livestock and dairy farmers later in the year.

As a result of a recent summit between farmers and DEFRA, farmers will be able to take up more groundwater to irrigate parched crops during the ongoing hot and dry weather after the Environment Agency announced it would relax some conditions linked to abstraction licences.  The arrangements will allow farmers to trade water allowances – as set out in their abstraction licence – on a short-term basis, without the need to change their licence. The Environment Agency will fast-track the process to enable farmers to act quickly and arrangements will be agreed locally where the Environment Agency is satisfied there will not be any adverse effects on the environment.

Socio economic drought

So far there has been no widespread impact of the hot and dry conditions on public water supply.  Because groundwater levels and stream flows have been OK, there has been plenty of water for supply (apart from the north and west as mentioned above).  However, the very hot weather and dry soils has led to a surge in demand that, in some places, has stretched some companies’ water treatment and distribution infrastructure to the limits.  Water companies are just putting their finishing touches on their draft business plans for the next five-year business planning cycle.  OFWAT has made it clear that it wants to see companies invest in ‘resilience’, but it is not yet clear whether any of the information from the 2018 hot weather event will be available to inform the finalised plans.

Whilst some farmers have been able to invest in on-farm storage reservoirs over the last decade, as discussed above, due to the exceptionally hot and dry conditions it is clear that 2018 will have a marked impact on the agricultural sector with potentially significant reductions in production.

 

Whilst in the UK summer 2018 looks very much like 1976 so far, it’s interesting to look at conditions more widely.  In 1976 the heatwave was very concentrated around the UK and Spain, whereas in 2018 the vast majority of the globe has been warmer than average so far.  According to the National Oceanic and Atmospheric Administration, 2018 is set to be the fourth hottest year on record5. Only three other years have been hotter: 2015, 2016 and 2017.  So, whilst we in the UK will need to keep a close eye on the rainfall over the winter, it is the widespread extent of the 2018 event and the global trend of hot years leading up to it that should be of most concern to us in the longer term.  Both the water supply and agricultural sectors will need to increase investment in resilience measures to cope with summers like 2018 in future.

 
What is a drought?

Whilst we might all understand the general concept of a drought (hot and dry weather), there are a range of different definitions. These generally focus on the impact on those experiencing the drought.  The four most common categories are:

Meteorological drought This focusses mainly on the climate: heat and wetness.  In the UK an absolute drought is defined as a period of at least 15 consecutive days when there is less than 0.2 mm of rainfall.  A 50% shortfall in rainfall over three months, or a 15% shortfall over two years could also be a ‘drought’.  The most well-known meteorological drought in the UK was 1976, when a dry 1975-76 winter was followed by one of the hottest and driest summers since records began.  In Devon and Dorset, some locations received no rainfall for 45 consecutive days and in some places, less than half the average rainfall was measured from October 1975 to August 1976.  Looking forward, most global climate models suggest that the UK will experience hotter and drier summers and that this type of drought will become more frequent.
Hydrological drought This is a prolonged (months to years) period of low stream flows.  Clearly meteorological droughts can lead to hydrological droughts, but the latter are mainly linked to longer periods of subdued rainfall/high evaporation and particularly to warm, dry winters when there is little or no recharge to stock up groundwater (which is what keeps most rivers flowing in the summer).  Hydrological droughts can be exacerbated by human activities such as abstraction and land use practices.  There was a significant hydrological drought in the UK between 1995 and 1998, when the warm, dry summers were followed by dry, cool winters and similar conditions occurred between 2003 and 2006 and 2009 and 2012.
Agricultural drought An agricultural drought occurs when climatic conditions mean that there is insufficient moisture in the soil for crops to grow normally.  What this actually means is hard to define numerically, as the susceptibility of crops varies during different stages of crop development. For instance, low soil moisture at planting may hinder germination, leading to low plant populations per hectare and a reduction of yield.
Socio economic drought This links to the previous three definitions and refers to a period when shortfalls in rainfall and water supply start to affect people’s lives and the economy as a whole.  Whilst the UK’s population is currently growing, demand for water for public supply and industry has generally dropped. Coupled with significant investment in water supply infrastructure over previous decades this should make the country less susceptible to this type of drought.  However, the predicted trends to hotter and drier summers are likely to lead to more demand and less supply and so this is perhaps not a time for complacency.

 

1https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/729921/Rainfall_and_river_flow_summary_18_to_24_July.pdf
2https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/732821/South_East_England_Water_Situation_Report_July_2018.pdf
3https://www.metoffice.gov.uk/news/releases/2018/june-end-of-month-stats
4https://www.ceh.ac.uk/news-and-media/blogs/uk-hydrological-status-update-early-july-2018
5http://www.noaa.gov/news/june-2018-was-5th-warmest-on-record-for-globe