Drought in the United Kingdom

Droughts are a relatively common feature of the weather in the United Kingdom, with one around every 5–10 years on average. These droughts are usually during the summer, when a blocking high causes hot, dry weather for an extended period.^[1] However droughts can vary in their characteristics. All types of drought cause issues across all sectors, with impacts extending to the ecosystem, agriculture and the economy of the whole country in severe cases of drought. The south east of the country usually suffers most, as it has the highest population (and therefore demand) and the lowest average precipitation per year, which is even lower in a drought.^[2] Even in these areas in severe droughts, the definition, impacts, effects and management are all minimal in comparison to drought prone areas such as Australia and parts of the United States. In recent years however, the summers of 2007, 2008, 2009 and August 2010 were wetter than normal, 2007 being wettest on record.^[3]

Definition and comparison with other countries

A drought is usually defined as an extended period of weather (usually around 3 weeks) where less than a third of the usual precipitation falls.^[4]

In the United Kingdom an absolute drought currently defined as a period of at least 15 consecutive days or more where there is less than 0.2 mm (0.008 inches) of rainfall.,^[4] although before the 1990s a drought was defined as 15 consecutive days with less than 0.25mm (0.01 inches) rain on any one day. This previous definition sometimes led to confusion, as many argued that if less than 0.25mm of rain fell in 30 days, is that 2 droughts and if 0.26mm fell after 25 days, is the drought over? This led to the new definition but many believe hindsight is the best way to judge if a drought has occurred.^[5]

In the longer term, drought in the United Kingdom can also be defined as a 50 per cent deficit over three months, or a 15 per cent shortfall over two years.^[6]

Compared to other countries, the United Kingdom definition of a drought is much less severe. In Libya in the Sahel region, a drought is usually only recognized after two years without any measurable rainfall. If this were to happen in the United Kingdom, the consequences would be disastrous.^[4]

A different type of drought is the hydrological or agricultural drought whereby moisture is in the soil but little is getting to vegetation, either because it is frozen (which can occur in severely cold winters in the United Kingdom) or because of very high temperatures which means that the rate of evapotranspiration is exceeding the rate of uptake of water from the plant (which can be seen in the United Kingdom, on hot days, when plants wilt as their stores of water are depleted).^[6]

A hydrological drought can occur, after a relatively dry winter whereby the soil moisture storage, reservoirs and water table have not risen sufficiently to counteract the warm summer weather. These sort of conditions can go over several years, even with above average rainfall at the time as the rainfall only slowly percolates through the water stores and replenishes them.^[6]

Causes

The main cause for a long spell of dry weather in the United Kingdom is usually a blocking anticyclone (often the Azores high) system that forces other low pressure systems around it, usually to the northwest. This can happen any time of year, but brings hot sunny weather in summer and dry, cold and foggy weather in winter. This is why in dry spells the northwest of the United Kingdom actually often receives above average rainfall, as depressions and associated fronts are pushed towards the north.^[4]

A severe drought in the United Kingdom needs to have the high pressure in charge of the weather for an extended period, commonly for weeks or even months at a time. Most often sea surface temperature anomalies in the Atlantic and intensification of the mid latitude westerlies in the Pacific can bring a stable anticyclone, meaning that the pressure can remain above average for weeks, or even months at a time, allowing the heat to build and dry weather to continue during the anticyclonic system.

Notable events

Several notable droughts in the United Kingdom have occurred in recorded history, some of these in the 21st century. They can be divided into two categories, the meteorological drought where little or no rain fell over a relatively short period and the hydrological drought, where below average rainfall has occurred over an extended period.

Meteorological droughts

One of the most major meteorological droughts of recent times occurred in the year of 1976, where a dry 1975 winter was followed by one of the hottest and driest summers since records began. The drought effectively began in October 1975, but with low temperatures and therefore low evaporation rates during the winter, the below average rainfall did not present an immediate problem. As the dry winter ended and was followed by the hot and particularly dry summer of 1976, the drought became one of the most famous in United Kingdom history.

The drought become a serious problem in late spring. In April, no rain fell in parts of Cornwall and then in June, no rain fell over Devon and Kent. In August no precipitation was measured in North Wales. This meant that in some places, less than half the average rainfall was measured from October 1975 to August 1976. For example, in Kew, just 235mm (9.25 inches) of rain was measured over that period, which was 43% of the average, meaning the amount of rain that fell was comparable to a semi-arid climate. In Devon and Dorset, some locations received no rainfall for 45 consecutive days through July and August, another United Kingdom record. This was on top of another 3 periods of absolute drought, totalling 58 days with no measurable precipitation.

The lack of rain can be attributed to the high pressure system over the United Kingdom at the time, which meant average pressure was 5 millibars above the expected for the summer months.

This drought was intensified by the exceptional heat that occurred all through it. Records were set for the heat as well as the lack of rain, resulting in very high evaporation rates. From 23 June, temperatures were over 32 °C (90 °F) for two consecutive weeks around the United Kingdom, peaking at 35.6 °C (96.1 °F) in June on the 28th at Mayflower Park in Southampton. This still is the highest ever June temperature since records began. The highest temperature in the summer was 35.9 °C (96.6 °F) at Cheltenham on 3 July.^[7] The effects of the drought included substantial and long-lasting changes to the tree composition of forests. European Beech was severely affected by the drought and took around twenty years to recover. To date, their growth has failed to recover completely, reaching only about 75% of their pre-drought growth rates,^[8] even decades after the drought. Additionally, death due to drought resulted in the loss of around 15% of the mature beech trees in the forest. In contrast, sessile oak did not suffer any death or growth suppression due to drought, but instead benefited from reduced competition with beech. Sessile oak growth increased suddenly after the drought and remained unusually high until the 1990s, when beech recovered and regained dominance. European Beech dominates temperate forests over large areas of Europe, but is considered to be particularly drought sensitive and is predicted to be outcompeted by more drought tolerant species in many regions as the climate warms.

Another event like the 1976 drought was in the summer of 2003. This was also notable for the temperatures involved, with 100F being hit in the United Kingdom for the first time since records began, with a top temperature of 38.5 °C (101.3 °F) recorded in Faversham, in Kent and some places recording up to, or above, 40 °C (104 °F) unofficially. Also this drought and the associated heatwave affected the whole of Europe, not just the United Kingdom, as with the 1976 event. The average temperature was the highest since 1868, measured over February to September. This added to the severity of the drought, with high rates of evaporation. The summer ranked as the 4th highest for potential evaporation since 1961, and this evaporation alone exceeded rainfall totals across a third of the United Kingdom.

The United Kingdom had its driest February to October period since 1921, and in the drought period had conditions only comparable to the previous conditions of 1976. Over the whole of the United Kingdom, the rainfall totals were the 3rd lowest since records began in 1900 and in some regions only 25% of the average rainfall fell in this period. Scotland suffered during this period as it had its driest spell since the 1955 drought, and coupled with a dry winter here (England and Wales had relatively wet winters as depressions were forced south) this led to a hydrological drought here too. Also, with several local convective thunderstorms missing specific areas, these localities had exceptionally low rainfall totals, compared to the average, with some places having their driest February to October since 1697. The drought ended in October when a low pressure system finally arrived to bring substantial rain to the United Kingdom. Some areas in Southern England had more rain in this 6 day period than they had received in the previous 3 months. Fortunately the preceding winter of 2002-2003 was relatively wet, which reduced the severity of the drought greatly, unlike the dry 1975 winter which led into 1976.^[9]

Hydrological droughts

A significant hydrological drought occurred between 1995 and 1998 in the United Kingdom where the warm, dry summers were followed by dry, cool winters. This meant that over the 3 years, the lack of winter precipitation failed to counteract the dry summers, so slowly the water table fell, and reservoir water levels began to fall.^[6] Water levels were only replenished after some exceptionally wet years from 1999 to 2002. Similar conditions were felt between 2003 and 2006, with only the record breaking rainfall of 2007 and 2008 replenishing the water levels.^[7]

Impact

United Kingdom droughts have similar consequences to other droughts elsewhere in the world. The first of these is river and reservoir levels begin to drop as rainfall fails to counteract evaporation.^[4]

As drought conditions continue, groundwater levels drop and this provides excellent conditions for fires to develop. With hot, dry weather and no moisture underground, trees lose moisture and become very flammable in dry conditions. This leads to wildfires which usually is the main impact of drought in the United Kingdom, with moorland vegetation such as heather badly affected as the peat bogs dry out. Also, these fires can continue, even when seemingly put out, as the smoldering peat re-ignites the dry vegetation. However, during severe droughts, many trees can burn, and people's lives can be at risk, as in the 1976 drought when a fire encroached on a hospital, and only a wind direction change saved the patients' lives. As embers can be transported easily, and if drought is severe enough, fires can start miles away from their original position as they are transported by wind and even dust devils. With these situations, roads are often closed to prevent loss of life and further damage. These fires also can destroy wildlife habitats, and this can also threaten wildlife.^[7]

Often, in severe droughts, crops can fail as the soil does not contain sufficient moisture to keep them alive and this is usually the largest economic impact with £500 million ($830 million) worth of damage from failed crops in 1976. Another, more subtle impact is the insurance claims from damage to houses and businesses from fire and subsidence (caused by the dry and shrunken soil), sometimes amounting to over £50 million ($83 million) in severe droughts.^[7]

Responses and management

The responses to drought in the United Kingdom are managed by the Environment Agency. There are 4 stages to drought management, applying to both businesses and homes.

The first of these is a media campaign, urging people to save water and prevent specific measures to be taken. For example, use a water butt to collect any rainfall.

If drought continues, and water levels continue to decrease, further measures are brought in to save water. The first stage of this is a hosepipe ban, which prevents the use of hosepipes, and can be applied to hot tubs, pressure washers and other similar devices in a future drought. These measures were brought in, particularly the ban on hosepipes, in 2006 and 1976.

The second stage involves conserving any non-essential supplies of water. These are brought in when no foreseeable precipitation is forecast and water supplies are already very low. The management options including simply widening the hosepipe bans, to include sprinklers or drastic measures such as banning cleaning of buildings, vehicles and windows or the filling of swimming pools. These measures were also brought in during the 1976 drought.

The third and final stage involves drastic measures of water rationing to all businesses and homes in the United Kingdom in an emergency drought order. The measures are brought in only in exceptional conditions of extended periods with little or no rainfall. These actions include rationing of water, with no water supplies for certain times of the day, or allowing water for a specific amount of time. Also standpipes and water tankers can be used as a last resort to have only set amounts of water given to each household in a neighbourhood. Again, these measures were used during the 1976 drought.^[10][11]

Also, each area in the United Kingdom has its own drought plan, in the event of any future drought.^[12]

See also

• Sahel drought
• 2003 European heat wave
• 1976 United Kingdom heat wave
• 2006 European heat wave
• Climate Change

References

Bibliography

• The Great Drought of 1976. Evelyn Cox (1978). Hutchinson, Readers Union Group ISBN 0-09-133200-1