This report documents the 2004-06 drought in a hydrological framework with particular reference to its impact on water resources and the aquatic environment. Much of the data used is of a provisional nature and may be subject to future revision.
For more detailed documentation of the development and decay of drought conditions, please refer to the 'Water Watch' website. The locations of many of the monitoring sites referred to in this report please see the Location Map.
The last decade has been hydrologically volatile across much of the UK. Protracted drought conditions in the mid-1990s, with exceptionally depressed groundwater levels, were followed by the wettest 5-year sequence in the England and Wales rainfall series which begins in 17671. Whilst Scotland registered several periods of major rainfall deficiency, damaging flooding was a recurring feature in southern Britain. Outstanding winter rainfall in 2000/01 resulted in groundwater levels reaching unprecedented maxima over wide areas. Abundant water resources characterized most of England and Wales over this period, and continued into the winter of 2002/03.
The spring, summer and autumn of 2003 were exceptionally warm and dry. England and Wales registered its second lowest February-October rainfall in 83 years; as remarkably, Scotland reported its driest 9-month sequence (for any start month) since the 1955 drought2. By July severe drought conditions extended across much of western and central Europe. In contrast to mainland Europe, where the drought was of a lesser duration, more intense, and triggered a review of drought management in many European countries, the UK was subject to relatively minor water resources and environmental stress. The 2003 drought underlined the resilience of the UK to even major within-year rainfall deficiencies when the preceding winter has been sufficiently wet to leave surface and groundwater resources in a healthy state.
However, the very dry autumn soil conditions in 2003 delayed the onset of the seasonal recovery in runoff and, particularly, aquifer recharge rates throughout much of the English Lowlands. The continuing groundwater level recessions in parts of the Chalk (the primary source of water supply in much of eastern and southern England) left early winter levels at their lowest since 1997 in some areas. Correspondingly, the spring groundwater level recessions in 2004 began at below average levels and the failure of the seasonal recovery to gain any momentum through the autumn signalled the onset of sustained drought conditions, in the South East especially.
Taken together, 2005 and 2006 constitute the warmest two-year sequence in the 337-year Central England Temperature series3. The summer periods included several notably arid episodes (e.g. June 2006) but the water resources and environmental impacts of the 2004-06 drought largely reflected the lack of rainfall over two successive winter/spring periods. Drought conditions developed through the late autumn of 2004 across much of the English Lowlands and lasted, in most areas, until the early winter of 2006/07. Whilst the elevated temperatures in 2004-06 are consistent with a continuing warming trend, successive notably dry winters are unusual in the context of the last 30 years. Prior to the 1st World War however, clusters of dry winters were substantially more common with November-April rainfall totals in the English Lowlands often less than for the summer half year4. The 2004-06 drought has served to emphasise the continuing vulnerability of England and Wales particularly, to low winter rainfall. It also underlines the need to understand more fully the mechanisms which contribute to multi-decadal climatic variability in the UK.
The 2004-06 drought had a strong regional focus and was generally most severe in the driest parts of the UK where groundwater is the major source of water supply and a combination of high population density, intensive agriculture and commercial activity generates the highest water demand. Careful management was required to reconcile the needs of a wide range of water users with the requirements of the aquatic environment.
From the late autumn of 2004, persistent anticyclonic conditions across southern Britain resulted in most rain-bearing low pressure systems following tracks remote from the English Lowlands. This resulted in a notable exaggeration in the normal north-west/south-east rainfall gradient across the UK. Parts of north-west Scotland were very wet whilst large rainfall deficiencies became established across much of eastern, central and southern England.
Considering, the drought-affected region as a whole, rainfall deficiencies were most severe over the 21 months ending in July 2006. In this timeframe England and Wales reported its 3rd lowest rainfall since 1932-34 with the most exceptional regional deficiencies in the Thames and Southern regions (see Table 1). Figure 1 illustrates the 21-month rainfall deficiencies across southern Britain relative to the 1961-90 average. Maximum deficiencies exceed 25% - mostly in the South East but notable drought conditions also extended into parts of the Midlands and the South West, Cornwall especially. Figure 1 also testifies to the very substantial local variations in drought intensity due, in part, to the convective nature of much of the summer and early autumn rainfall.
Of primary significance from a water resources perspective was the disproportionate concentration of the overall rainfall deficiency in the winter and spring - when modest evaporation losses allow the bulk of reservoir replenishment and aquifer recharge to take place. The paucity of November-April rainfall during the 2004-06 period for the Thames basin is evident from Figure 2. This shows notable 2-yr November-Aprili rainfall deficiencies for the Thames basin over the last 125 years. For 2004/05 and 2005/06 the combined deficiency is the largest for over 100 years (marginally eclipsing 1944-46)4.
In most drought-affected regions, rainfall over the two summer half-years (May-October) was within the normal range, and close to the average across much of the South East where a relatively dry 2005 was balanced by above average rainfall in 2006. Temporal variations in summer half-year rainfall did, however, impact on the drought's severity in 2006: after a damp late spring, the drought re-intensified through June and July which, taken together, were the 2nd driest in the last 23 years for England and Wales. Correspondingly, soil moisture deficits increased steeply and the region subject to drought stress extended. Rainfall deficiencies began to moderate through an unsettled late summer and the drought's intensity weakened substantially through the autumn of 2006.
i The November to April period is used because it embraces the full aquifer recharge season for many outcrop areas in the English Lowlands
Throughout 2004-06, river flow patterns in drought-affected regions were notable for an exceptional lack of spates and outstanding long term runoff deficiencies. Generally however, annual minimum flows were less notable and, in most areas, remained significantly above recent drought minima.
In runoff terms the drought's duration varied appreciably across southern Britain but, generally, flows remained seasonally depressed from the late autumn of 2004 to the autumn of 2006 - later in many slow-responding spring-fed rivers in the English Lowlands. Of particular hydrological interest over this period was the virtual absence of high flow events. For the Thames at Kingston, no daily flow (naturalised) exceeded Q10 during the 29 months to October 2006; there is no closely comparable dearth of spate conditions in a flow series from 1883. In relation to water resources stress, the associated long term runoff deficiencies were of more direct relevance. Figure 3 shows November 2004-August 2006 runoff totals expressed as a percentage of the long term average for a network of index catchments across the UK. In this timeframe, runoff was below average across almost all of the country but the largest deficiencies - exceeding 30% - were mostly confined to central and southern England. New minimum 22-month runoff totals (for periods beginning in November) were established from Cornwall to Kent (see Table 2). Most of these minima represented the lowest runoff for any 22-month period and some, including those for the Medway and the Sussex Ouse, eclipsed previous minima by a considerable margin. In rivers with a high baseflow component, flows continued to decline well into the autumn. For the Thames, the November 2004-September 2006 runoff was the lowest since 1947-49.
Figure 4 illustrates monthly river flow patterns for a selection of rivers in central and southern England over the last decade. The periods of below average flow are emphasized by the red shading; the logarithmic scale also gives greater prominence to the low flow range. In impermeable southern catchments river flows were, typically, more seasonally depressed in 2005 than in 2006 when the drought's impact was generally more evident in spring-fed streams and rivers. In permeable catchments, monthly flows remained below average for very extended periods (over three and a half years in the case of the Lambourn) and approached long term minima in the final stages of the drought. For the Mimram, the September flow in 2006 was the lowest in a series from 1953. Similarly depressed flows characterized many spring-fed streams in the English Lowlands, and the associated contraction in the stream network was broadly similar to that experienced in the early autumn of 1992 and 1997.
Artificial flow enhancement (e.g. by groundwater augmentation or river regulation) helped to moderate low flows in some stressed areas of southern England particularly. However, even in rivers with sensibly undisturbed regimes, flows in 2005 and 2006 generally remained above drought minima. Figure 5 features flow frequency plots (annual 30- and 90-day minima) for the River Lambourn in the Berkshire Downs and Table 3 provides return period estimates for long duration n-day minima in 2005 and 2006 for three index rivers. The return periods associated with the 2005 and 2006 n-day minimum flows are moderate and, apart from the more responsive Gt Stour, similar to those associated with the 1989 and 1997 droughts; the 1976 minima were much more extreme. Table 4 ranks annual 7-day minimum flows and confirms that the short-term flow minima experienced during the 2004-06 drought were of an extreme magnitude only in a few cases (e.g the Mimram); again most exceed the corresponding minima for 1976 by a significant margin.
Over the 1998-2002 period, reservoir stocks across England and Wales remained very healthy but declined steeply through the intense spring and summer drought of 2003. Stocks generally recovered in 2004 and, despite the drought conditions over the succeeding two years, they remained mostly well within the normal range at the national scale. Figure 6 provides an index of overall reservoir stocks based on a network of major impoundments across England and Wales. This confirms the health of overall stocks through most of the drought period but a relatively steep decline through the spring and summer of 2006 left early August stocks at their lowest for a decade, but still within 5% of the late summer average.
The limited depletion at the national scale masks the degree of water resources stress evident in parts of southern Britain where the impact of the developing drought on reservoir stocks is much more evident. Figure 7 shows 2004, 2005 and 2006 monthly reservoir stocks for Bewl Water (on the Kent/Sussex border) and Colliford (Cornwall). For the former, stocks remained reasonably healthy until the summer of 2005. Thereafter, the absence of any appreciable seasonal recovery left stocks at their lowest on record entering 2006. Aided by a wet late spring and Drought Permits allowing additional pumping from the River Medway, stocks at Bewl Reservoir recovered briskly. Summer stocks were considerably higher than in 2005; a situation replicated throughout much of the drought-affected region but there were important exceptions. In parts of the South West stocks remained seasonally depressed, particularly in Cornwall where Stithians and Colliford reservoirs declined to below 50% of capacity in the late summer.
Stocks continued to decline into the late autumn of 2006 and at year-end Colliford was only marginally above half full - reflection of a long term rainfall deficiency (since 2001) and a small catchment area relative to the reservoir's capacity, implying that post-drought recoveries could extend over several years.
Depleted surface resources triggered the introduction of a range of drought mitigation measures (e.g. publicity campaigns to moderate demand, local water transfers, reductions in compensation flows and temporary switching of depleted reservoirs to non-consumptive mode). Sutton and East Surrey Water introduced a hosepipe ban in the late spring of 2005. A year later, with groundwater resources also seasonally depressed over wide areas, water use restrictions were extended across much of the South-East, affecting over 13 million consumers in total. Stocks in the major-pumped storage reservoirs which service London can decline rapidly through the summer but, aided by reduced demand, held up well in 2006. With most gathering grounds close to saturation by the late autumn, gravity-fed reservoirs reported healthy replenishment from mid-October and, generally, stocks had recovered to their normal seasonal range by early January 2007.
The drought achieved its most extreme expression in relation to groundwater resources in parts of the English Lowlands. In groundwater terms particularly, the drought's development can, in part, be seen as a legacy of the arid conditions experienced throughout much of 2003. Very dry late autumn soils substantially delayed the seasonal onset of aquifer recharge and (notwithstanding above average November-April rainfall) aquifer replenishment over much of the English Lowlands was insufficient to return groundwater levels to their normal spring maxima in 2004.
From the autumn of 2004 to the winter of 2006, the spatial and temporal distribution of rainfall was very unhelpful from a groundwater perspective. Winter and spring rainfall was well below average across most major aquifer outcrops and the seasonally dry soil conditions (a consequence of above average evaporative demands) again served to restrict the length of the recharge seasons. The 2004-06 period provides a very clear demonstration of the sensitivity of aquifer recharge to changes in winter half-year rainfall and the consequent vulnerability of groundwater resources to clusters of dry winters. Figure 8 plots the relationship between October-March rainfall and effective precipitation (EP) totals for the western Chilternsii. Winter half-year rainfall totals of less than 250 mm can be expected to generate very meagre recharge and a 30% deficiency in rainfall translates into a recharge deficiency of around 60%. Estimated EP totals for the October-March periods in both 2004/05 and 2005/06 were below 35% of average.
The effect of the paucity of recharge is illustrated in Figure 9 which features 1996-2006 groundwater level hydrographs for five index wells and boreholes in England. For the Chilterns (represented by the Stonor borehole), the residual benefit of exceptionally high groundwater levels in early 2003 is still evident in 2004 but the weakness of subsequent seasonal recoveries left levels approaching long term minima by early autumn 2006. As in other Chalk outcrops, the degree of water-table depression would have been more severe without a seasonally late pulse of recharge in April and May which moderated groundwater level recessions (and briefly reversed them in a few areas). A broadly similar recharge pattern is evident in the slow-responding Permo-Triassic sandstones of the Midlands where groundwater levels reflect recharge patterns over a number of years. At Heathlanes, barely an inflection in the recession can be recognised over the 2004-06 period and, by the autumn of 2006, groundwater levels had fallen below all but the minima recorded in the final phase of the 1995-97 drought.
The wells featured in Figure 9 are located in outcrops where the drought achieved its greatest severity. Away from these aquifer units, the depletion of groundwater resources was generally much more moderate. Levels in most limestone aquifers remained within the normal range and the drought's impact on minor aquifers (e.g. in Norfolk) was limited. Figure 10 maps September 2006 groundwater levels for a network of index wells and boreholes across the UK. It serves to emphasise the large regional and more local variations in the degree of water-table depression which characterised much of the 2004-2006 drought. Even within the Chalk aquifer, the degree of resource depletion varied substantially with relatively healthy resources in the more northerly outcrops. Figure 11 allows the drought's impact on overall storage in the Chalk to be considered in the context of groundwater resource variability over the last 37 years. The limited spatial extent of the most severely depressed water-tables in 2004-06 is confirmed by the lesser magnitude of the resource depletion relative to the protracted groundwater droughts of the early and mid-1990s. On limited evidence, the 1990-92 groundwater drought was probably the most severe in England and Wales for more than 80 years5.
ii Data kindly supplied by the Environment Agency
Many factors influence the health of river ecosystems including temperature, oxygen, light and discharge. All elements of the flow regime are important, including floods, average and low flows. Depressed groundwater levels and the associated widespread failure of springs contributed to very sustained low flows and a major contraction in the lowland stream network over the 2004-06 period. Limited inflows (from surface or groundwater) and exceptional evaporation demands led to many ponds drying up and fish rescues were needed as water levels became critically low. Low oxygen levels were an exacerbating factor in some water bodies but oxygen concentrations remained relatively healthy in most spring-fed streams. The lack of spates and drying up of headwater tributaries represented a particular risk to migratory fish that require sufficient flow to trigger upstream movement and to reach their spawning grounds.
Although it is difficult to predict the effects of droughts on fish populations, it is possible to estimate how low flows change the amount of suitable habitat available for certain species. Flow in the river interacts with channel morphology to create the patterns of depth, velocity and width that freshwater communities utilise. Prolonged periods of low flow can have adverse affects on river health through a lack of dilution and by altering the physical conditions in the river. During periods of low flow less wetted area may be available, depths may be shallower and velocities slower. This can be a particular problem for young salmonid fish, which prefer moderate velocities and avoid very shallow water whilst drift-feeding.
Figure 12 shows that during the prolonged low flow period in the summer of 2006 only around 5% of the River Lymington at Balmerlawn (Hampshire) provided conditions that were suitable for drift-feeding young trout. During 2006, there was a prolonged period of limited availability of suitable drift-feeding trout habitat when compared to a more typical year such as 2004. However, conditions were not as bad as in the summer of 1976: the most severe drought on record at this site. Low flows during 2006 had a less damaging impact on habitat in more steadily flowing rivers such as the River Kennet (Berkshire) which, unlike the Lymington, relies primarily on groundwater to sustain summer flows. During 2006, the Kennet provided levels of habitat similar to those in 2004 (Figure 12), and the availability of suitable habitat far exceeded that of the summer of 1976, when flows were the lowest in a 45-year record. Flows recorded across southern England in 2006 suggest that habitat loss was broadly similar to that which occurred during the summer/autumn droughts of 1992 and 1997.
Drought termination can be a protracted process in the UK with wet interludes, particularly during the summer half-year, providing a temporary respite but having little immediate impact on water resources (aside from a reduction in water demand). On the other hand, some recent droughts (e.g. 1976, 1984, 1995) have ended dramatically as prolonged sequences of vigorous low pressure systems brought heavy rainfall extending over a number of months.
In 2006, convective storms in the late summer helped, locally, to moderate soil moisture deficits but a more general amelioration of the drought awaited the onset of persistent cyclonic conditions during an exceptionally mild autumn. A high frequency of active frontal systems continued into the early winter. England and Wales registered its 4th highest August-December rainfall in the last 52 years and most, but not all, of the drought affected regions registered substantially above average rainfall in this timeframe (see Figure 13). Rainfall was again very spatially variable with some areas reporting only average rainfall (e.g. the Isle of Wight) and for a few catchments, overall rainfall deficiencies increased (e.g. in Cornwall).
Soil moisture deficits lingered into 2007 in a few areas close to the Wash but generally declined rapidly through October allowing runoff and recharge rates to recover smartly across much of the English Lowlands. River flows increased briskly through November in many impermeable catchments - most reporting above average flows by month-end. Flow response was characteristically more sluggish in spring-fed rivers but the onset of substantial aquifer recharge (see below) triggered a brisk recovery during the late autumn. In December, moderate floodplain inundation was widely reported in mid-month and, by early January, the focus of hydrological stress had (in most areas) switched decisively to the risk of flooding.
Notwithstanding drawdown in some major reservoirs for flood alleviation purposes during the early winter of 2006/07, stocks increased briskly across almost all of the UK. For England and Wales as a whole the November/December increase was the highest for any two-month period in the last 10 years. Entering 2007, most impoundments were close to capacity but there were several important exceptions in the South West.
Seasonal water-table recoveries in most western and northern aquifers gathered momentum in October and the elimination of soil moisture deficits across most of the English Lowlands triggered the most productive recharge episode for the Chalk in the last four years. This generated steep recoveries in the rapidly responding aquifers but more sluggish responses in the core of the drought-affected region. However, a very large pulse of recharge was descending through the unsaturated zone at year-end. Estimated EP totals for the October-mid January period strongly indicate that recharge in parts of the Chalk (e.g Chilterns) had already exceeded the combined replenishment for the full recharge seasons in 2004/05 and 2005/06. Recharge to the Permo-Triassic sandstones was abundant also but their characteristic very high storage implies that a full recovery, to above the seasonal average, may take several years.
In most drought-affected regions, the water resources outlook changed dramatically over the final three months of 2006. In early October, the possibility of a third successive dry winter carried with it the prospect of historically low groundwater levels by the summer of 2007, possibly the lowest since the Long Drought of 1890-1909 in parts of the English Lowlands, with an associated likelihood of severe resources and environmental stress.
Persistent cyclonic weather patterns through the autumn moderated the drought and the Met Office forecast for the winter period December 2006 to February 2007 (issued 5/12/06) indicated that average or above-average rainfall was the more likely outcome. At year-end, the updated forecast continued to reflect a weak indication for average or above-average precipitation through the remainder of winter. The substantial uncertainty associated with this forecast reflects in part the possible influence of developing El Niño conditions in the southern hemisphere.
At the end of 2006, rainfall deficiencies in the drought's core region since October 2004 remained the equivalent of more than four months average rainfall. It is unnecessary, however, to fully satisfy these deficiencies in order to secure a healthy water resources outlook - this has been demonstrated by the substantial surface and groundwater recoveries consequent upon a modest (typically 20%) reductions in the long term rainfall deficiencies over the latter months of 2006.
Entering 2007, reservoir stocks across southern England were much healthier than in early 2006 (Stithians and Colliford in the South West were exceptions). More tellingly, early January groundwater levels in the southern Chalk were considerably above those of a year ago, or set to become so as the heavy December infiltration reaches the depressed water-table. Model projections suggest that even with 80% late-winter/early-spring rainfall, groundwater levels should remain appreciably above those recorded last summer. Correspondingly, December 2006 flows in spring-fed rivers were typically 30-70% greater than those in the previous year. This is an encouraging sign but monthly runoff totals were still below average in many eastern spring-fed streams. This is true of some streams fed from the slow-responding Permo-Triassic sandstones in the Midlands also.
It should be stressed that the rainfall and recharge recoveries are not of a similar magnitude to those in 2002/03, which left groundwater resources in southern Britain sufficiently healthy to withstand the ensuing intense spring-autumn drought. A dry late winter and early spring could lead to a rapid deterioration in the water resources outlook, as happened in 1990 and 1995, for example. This could have particularly severe consequences for some smaller impoundments, agricultural reservoirs and shallow groundwater resources in western and northern Britain. As usual, the length of the aquifer recharge season will be important in the drought-stressed aquifers; a wet spring to delay the onset on the seasonal recessions in groundwater levels would be particularly beneficial.
This overview would not have been possible without the active co-operation of many organizations - the Environment Agency, Scottish Environment Protection Agency, Rivers Agency (Northern Ireland) and the Met Office in particular - which contribute data and information to the National Hydrological Monitoring Programme (run by the Centre for Ecology and Hydrology in collaboration with the British Geological Survey; both component bodies of the Natural Environment Research Council).
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