An overview of the 2010-12 drought and its dramatic termination
The 2010-12 drought in summary
Across most of the UK, 2010-12 has been remarkable inclimatic terms – characterised by exceptional departures fromtypical seasonal rainfall patterns.
Drought conditions
developed through the first half of 2010 impacting most
severely on Northern Ireland, western Scotland and, especially,nd1
northern England. 2011 was the 2 warmest year on record
for the UK and witnessed an extreme exaggeration in thenormal north-west to south-east runoff gradient across thecountry. With most rain-bearing weather systems following
northerly tracks, the drought became focused on south-west
England and, later, the English Lowlands where concentrationsof population, intensive agriculture and commercial activity
generate the highest water demand.
The drought intensified through the winter of 2011/12 andparts of England recorded their lowest 18-month rainfall (forperiods ending in March) in at least 100 years (see Figure 1);
the rainfall deficiencies were disproportionately concentrated
in the October-March periods.
Correspondingly, winterreplenishment to most reservoirs has been meagre and wasfollowed, in 2012, by the largest March decline in overall
reservoir stocks for England & Wales since 1993. Early April
stocks were the lowest on record for a number of major
reservoirs and many farm reservoirs had also failed to refill
through the winter.

Figure 1 October 2010 to March 2012 rainfall as a% of the 1971-2000 average

At a national scale, runoff for March was the lowest in a series from 1961 (see Figure 2) and, in many rivers, flows
during the final week fell below those recorded at the same time during the extreme drought of 1976. As more springsources continued to fail, the associated contraction in the stream network was as severe, for the time of year, as any
experienced in at least the last 50 years. This, together with the associated loss (albeit temporary) of aquatic habitat,
the desiccation of wetlands, low oxygen levels, limited effluent dilution and the appearance of algal blooms
underlined the environmental and ecological stress that was a defining characteristic of the 2010-12 drought.

3Flow (m3 s-1)

-1

Figure 2 Estimated daily outflows (m s ) fromEngland & Wales 2011/12 (the blue and pinkenvelopes indicate pre-2012 daily maxima andminima; the grey trace is the long term dailyaverage)

Generally, the drought was most intense in those parts of England where groundwater is a major source of waternd
supply. The failure of wells and springs increased through 2011 and, following England’s 2 lowest rainfall for
successive winter half-years, groundwater levels in many Chalk and limestone outcrop areas were approaching, orbelow, seasonal minima in early April 2012. For the Chalk, the country’s most important aquifer, estimated overall
storage, for the time of year, was lower than in 1976 and, in a series from 1951, only 1992 has registered marginallylower aquifer storage.
1

Unless otherwise stated all climatological comparisons are based on data provided by the National Climate
Information Centre (Met Office); the rainfall and temperature series extend back to 1910.
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The drought was of a very severe magnitude through the early spring of 2012 and hosepipe bans affecting 20 million
2
consumers were introduced in the first week of April. With soils at their driest on record for the time of year
(Figure 3), the drought continued to impact severely on agriculture and the environment. Crucially, with evaporation
rates set to climb through the late spring and summer, no early recovery in runoff and aquifer recharge rates could beexpected. Correspondingly, the water resources outlook was fragile across large parts of central, eastern andsouthern England.

SMD (mm)Figure 3 Variations in monthly soil moisture deficits (mm)for the English Lowlands (the blue and pink envelopes
indicate pre-2012 monthly maxima and minima; the grey
trace is the long-term monthly average). Data source:
2MORECS

A remarkable hydrological transformation
Early April 2012 witnessed a decisive change in synoptic
patterns and, with the Jet Stream adopting a persistent
southerly track, a sequence of very active Atlantic frontal
systems crossed the UK. April rainfall totals were thehighest on record across most of the country and forEngland & Wales it was the wettest April for at least 230
3years .
The extreme rainfall initiated a dramatichydrological transformation which, at a critical time forwater resources, rapidly reversed the normal seasonal
decline in runoff and recharge rates. As a consequence,
the focus of hydrological concern switched rapidly fromdrought stress to flood risk.
Cyclonic synoptic patterns continued to dominate
through the late spring and early summer, and rainfall
over the April-July period for England and Wales (Figure4) was the highest in the long England & Wales series
which extends back to 1766. The agricultural drought
may be considered to have terminated as soils wetted-up
through April in most areas. However, the subsequentpersistence of near-saturated soil conditions resulted inlimited access to farmland, damaged crops and restricted
harvesting opportunities through the late summer. It
also made most rivers very responsive to the summer
deluges and, very unusually, allowed sustained recharge
to most major aquifers.

Figure 4 April-July 2012 rainfall as a % of the 1971-2000
Differing catchment and aquifer characteristics average
substantially influenced the rate and magnitude of therunoff and recharge responses to the exceptional rainfall. In April, runoff rates increased smartly in impermeable
catchments and, by the final week, flood alerts were widespread – affecting rivers from Cornwall to north-east
Scotland. For England & Wales as a whole, late-April outflows eclipsed the previous maximum in a series from 1961.
After a respite in May, exceptional runoff rates continued well into the summer The Environment Agency reported
that over 4000 properties in England & Wales had suffered fluvial or flash flooding by the end of August. The
2Based on the MORECS time series from 1961 (Hough, M. and Jones, R. J. A. (1997) The Meteorological Office Rainfalland Evaporation Calculation System: MORECS Version 2.0 an overview. Hydrology and Earth System Sciences, 1(2),227-239.)
3
Alexander, L.V. and Jones, P.D. (2001) Updated precipitation series for the U.K. and discussion of recent extremes.
Atmospheric Science Letters, doi:10.1006/asle.2001.0025.
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www.ceh.ac.uk[2]seasonally remarkable runoff rates helped replenish parched wetlands and extend the drainage network into thepreviously dry headwater reaches of many rivers, but they also presented further problems for wildlife (e.g. theinundation of nesting areas in the Ouse Washes).
By contrast, the impact on water resources was clearly beneficial. Contrary to the normal seasonal pattern, reservoir
stocks increased, albeit erratically, through the late spring and summer and, for early September, overall stocks forEngland & Wales were the highest on record. More remarkably, average stocks through the summer of 2012
exceeded the December-February average for all but the wettest of winters. In the drought-afflicted regions thewater resources recoveries were remarkable. At Rutland, a major reservoir in East Anglia, stocks registered theirgreatest two-month increase in a series from 1988 (Figure 5) and generally all index reservoirs with the exception ofBewl (East Sussex) were within 10% of capacity at the end of August.

Figure 5 Variations in monthly stocks (% of capacity) for
Rutland Water (the blue and pink envelopes indicate pre2012 monthly maxima and minima; the grey trace is thelong-term monthly average)% ofcapacity

Facilitated by the remarkably wet soil conditions, aquifer recharge – normally very meagre in late spring and summer– was both substantial and sustained in 2012. However, differences in local rainfall patterns, the depth to thedepressed water-tables and the storage characteristics of individual aquifer units made for marked differences in thetime of onset and the rate of groundwater level recoveries. Rapid recoveries characterised most limestone outcrops
and much of the southern and western Chalk, leaving July groundwater levels above, or close to, previous summer
maxima (see Figure 6a); in a few areas (e.g. Dorset) local instances of groundwater flooding were reported. However,levels remained depressed in parts of the slow-responding Chalk (e.g. in the Chilterns) and, particularly, in the PermoTriassic sandstones of the Midlands where high storage capacities result in very sluggish groundwater level recoveries;
at Heathlanes (Figure 6b), even in September levels continued to track close to the lowest on record and the full
impact of the spring/summer recharge may not be evident until early 2013.

Level (m)Level (m)

(a)

(b)

Figure 6 Monthly mean groundwater levels (m) for: (a) Tilshead (Wiltshire) in the Chalk; and (b) Heathlanes(Midlands) in the Permo-Triassic sandstone (the blue and pink envelopes indicate pre-2012 monthly maxima andminima; the grey trace is the long-term monthly average)

How rare was the transformation?
There is no close modern parallel to the hydrometeorological conditions experienced over the first half of 2012. Rapidth4
drought terminations are not particularly rare – examples in the 20 century include 1992 , 1989, 1976, 1959, 1929
and 1922. Unlike 2012, however, none of the runoff recoveries occurred during the late spring and early summer
when, in many of the areas recently afflicted by drought, evaporation rates normally exceed rainfall, and seasonal
declines in river flows and groundwater recharge continue with only short-lived interruptions.

4In 1992, there was a substantial lag between the sustained rainfall through the late spring and early summer and theassociated recovery in runoff and recharge rates.
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In 2012, the April-June rainfall for England was nearly three times that for the preceding three months; Figure 7
illustrates the rarity of such a disparity. Using the extended historical series for England & Wales, the degree ofcontrast in 2012 has not been approached since 1830 and has been exceeded only in 1779 when extraordinary
climatic conditions characterised the late winter and early spring; the January-March rainfall in 1779 was easily thelowest in the 246-year national rainfall series.

Figure 7 Ratio of April-June toJanuary-MarchrainfallforEngland.
Data sources: NCIC, Met Office

In runoff terms, the singularity of runoff patterns over the first half of 2012 was even more evident. In a series from1961, the May-July runoff for England & Wales has never exceeded 65% of that for the preceding January-March; in2012, it exceeded 150%. A longer historical perspective is provided by the 138-year flow record for the River Thames.
In 2012, flows were very depressed entering April but increased dramatically to exceed previous daily maxima for late
April. Flows over the May-July period have exceeded (modestly) those for the preceding January-March in only twoyears and on average the runoff is around 40%; the corresponding figure for 2012 is over 220% (see Figure 8). Someparallels may be drawn with 1903 when, following a three-year drought, May-July runoff for the Thames actually
exceeded that in 2012 but the terminal phase of the drought was well underway by the late winter of 1902/03.

Figure 8 Ratio of mean Thames
naturalised flows for May-July tothose of January-March.
Data source: Environment Agency

Acknowledgement
This note provides an overview of the 2010-12 drought and the remarkably wet spell that terminated it. Moredetailed coverage of the development and rapid decay of the drought can be found at:
http://www.ceh.ac.uk/data/nrfa/index.html
http://www.metoffice.gov.uk/climate/uk/
This report has been compiled with the active cooperation of the principal measuring authorities in the UK: theEnvironment Agency, the Scottish Environment Protection Agency and, in the Northern Ireland, the Rivers Agency.
These organisations provided the great majority of the required river flow and groundwater level data. The Met
Office provided almost all of the rainfall and climatological information featured in the report and the reservoir stocks
information derive from the Water Service Companies, Scottish Water and Northern Ireland Water. The provision ofthe basic data, which provides the foundation both of this report and the wider activities of the National HydrologicalMonitoring Programme, is gratefully acknowledged.
Note: A fuller report on the drought and its terminal phase is currently being compiled. The report will be published in
the National Hydrological Monitoring Programme series in early 2013.
Please visit: http://www.ceh.ac.uk/data/nrfa/nhmp/nhmp.html
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