Most major aquifer outcrop areas are in the drier parts of the UK - predominately the English Lowlands where groundwater is the principal source of public water supply. In water resources terms the Chalk, which outcrops in eastern and southern England, is the major aquifer; the Permo-Triassic sandstones are regionally important - in the Midlands and North-West especially. Limestone aquifers are also regionally significant and a number of minor aquifers (e.g. the Norfolk Crag) are of local water supply importance.
Away from the more westerly aquifer outcrop areas, groundwater replenishment (or recharge) in a typical year ranges from 500 mm to less than 100 mm in the most easterly outcrops. Recharge is normally concentrated in the November-April period when evaporation losses are modest. Evaporation losses, which exhibit limited year- on-year variability, result in a non-linear relationship between rainfall and aquifer recharge; a 20% reduction in annual rainfall can result in a reduction of 50% or more in groundwater replenishment for the drier, eastern outcrop areas. Consequently recharge variations tend to be much greater than those for rainfall. This volatility has been well illustrated over the last decade. Depressed groundwater levels in the 1995- 97 period contrast with remarkably healthy groundwater resources (and sustained groundwater flooding) during the winter and spring of 2000/01. The generally high groundwater levels over the first three years of the 21st century and particularly, the abundant groundwater resources early in 2003 helped limit the impact of the drought conditions during the ensuing summer.
Aquifer recharge patterns and groundwater level variations in 2005 exhibited the normal seasonal contrasts in most western and northern aquifer outcrop areas. By contrast, the major aquifers in the English Lowlands showed muted seasonal variations and, in most areas, a substantial decline in groundwater resources between the beginning and end of the year. The spatial and temporal distribution of the 2005 rainfall was unhelpful from a groundwater resources perspective. Many outcrop areas of the major aquifers recorded annual rainfall totals of less than 85% and the greater part of the deficiency was associated with the winter and early spring months when the bulk of groundwater recharge normally occurs. January and February rainfall totaled less than 50% of average in large parts of the South East and both November and December were also relatively dry. Local variations in seasonal rainfall totals were unusually large; this is reflected in substantial sub-regional variations in the health of groundwater resources during much of 2005.
2005 began with groundwater resources in the normal range, albeit mostly below average seasonal levels, in most limestone aquifer units but in parts of the Permo- Triassic sandstones and, particularly, the Chalk the groundwater resources outlook was relatively fragile. To the north and west of London, January groundwater levels in the Chalk (e.g. at Stonor in the Chilterns) were typically at their lowest for eight years, although substantially above early winter drought minima (e.g. those for 1991, 1992 and 1997). Healthy recharge early in 2005 was required if a belated seasonal recovery in groundwater levels was to take place. In the event, most late-winter and early spring low pressure systems followed tracks remote from the major aquifer outcrop areas. Correspondingly, the 2004/05 recharge season was limited in length and the overall volume of recharge was modest; commonly less than half the long term average.
Rainfall deficiencies continued to build into the early spring - only in 1975/6 has the November-March period been appreciably drier in the last 60 years - triggering an early onset of seasonal recessions in 2005. This is confirmed by Figure 1 which shows groundwater level hydrographs for a selection of index wells and boreholes across the UK. The broken line indicates the long term monthly average and the upper and lower shaded envelopes delineate the highest and lowest monthly levels on record. A 5-year period is featured because groundwater levels in many areas reflect recharge over a number of winter/spring periods. For some index wells, the featured hydrographs embrace a remarkable range of groundwater levels. At Stonor, groundwater levels in the spring of 2001 were well above previous maxima in a series from 1965 but by the autumn of 2005 they were approaching long term minima. Broadly similar behaviour is exhibited by the more responsive Little Bucket borehole in the Kent Chalk and, in the Permo-Triassic sandstones of Devon (Bussels No. 7).
Very slow responding aquifer units apart, most index wells and boreholes reported their lowest 2005 levels in the autumn. In September groundwater levels were well below average throughout most of the country with especially depressed levels in parts of the South Downs where natural base levels were closely approached; elsewhere, levels were appreciably above drought minima (see Figure 2). A similar pattern is evident in December (Figure 3) except that seasonal recoveries, in southern England especially, had returned groundwater levels to within the normal range - but still mostly below average. At year-end, concern regarding the water resources outlook focused on those groundwater units, mostly in the South-East and the Midlands where notably low levels were still being recorded. In such areas widespread spring failures had caused a substantial contraction in the river network with an associated loss of aquatic habitat.
The majority of observation wells and boreholes for which data are held on the National Groundwater Level Archive monitor the natural variation in levels. However, in parts of the UK groundwater levels have been influenced, sometimes over very long periods, by pumping for water supply or other purposes. As a consequence, some local or regional water-tables have become substantially depressed. For instance, contemporary levels at a number of boreholes in the Permo- Triassic sandstones of the Midlands are indicative of a significant regional decline. In contrast, rising groundwater levels have been reported from a number of conurbations; leakage from water mains is considered a significant factor in some cases. The implications of rising groundwater levels extend beyond the potential improvement in water resources that the rise represents. Groundwater quality may be adversely affected as levels approach the surface and a number of geotechnical problems may result, for instance the flooding of tunnels and foundations.
Artificial influence on groundwater levels have been particularly pervasive in London where increasing groundwater abstraction through the nineteenth and the first half of the twentieth centuries led to a 70-metre decline in groundwater levels in the Trafalgar Square borehole. Since the 1950s, a much reduced abstraction rate has resulted in a recovery of around 40 metres with levels rising by 1-2 metres a year through the early 1990s (see Figure 4) The potential disruption and damage (e.g. to the stability of buildings) which would result from a continuation of this rise, stimulated the development of a strategy to control rising groundwaters below London. Implementation of this strategy has resulted in a modest decline in levels at Trafalgar Square over the post-2000 period.
After a notably dry end to 2004, January rainfall totals in 2005 were below 50% of average across many aquifer outcrop areas. Although snowmelt contributed significantly to above average groundwater replenishment in some eastern areas in February, a dry early spring (and above average evaporative demands) generated insufficient recharge to produce seasonal water-table peaks within the normal range in much of the drought affected region.
Soil moisture deficits increased briskly in early March signaling an early onset to the 2005 recessions across much of the English Lowlands. Importantly however, the large spatial variations in winter and early spring recharge ensured that, even within a single aquifer, the drought's impact would be relatively patchy as the summer recessions developed. A modest pulse of recharge over the second half of April was useful in moderating the rate of decline in water-tables but Chilgrove (in the South Downs) still recorded its lowest April level since 1976; generally groundwater levels were significantly healthier to the north.
Limited May and early June rainfall, together with above average evaporative demands, effectively terminated the 2004/05 recharge season. With recessions now well established June groundwater levels were exceptionally depressed in parts of the South Downs and notably low (mostly in the lowest quartile for the month) across southern England and the Midlands. However, in many eastern outcrops of the Chalk, - from the Yorkshire Wolds to Suffolk - groundwater levels remained within the normal range. July rainfall totals were above average across most aquifer outcrop areas but the dry soil conditions allowed only minimal groundwater replenishment. In Sussex, the recessions at Chilgrove and Compton confirmed the notable local severity of the drought - in the last 100 years, only in 1934 and 1976 have groundwater levels followed a lower recession. Elsewhere the degree of water-table depression was less exceptional but by late September the regional character of the drought had been reinforced.
A notably wet October generated brisk groundwater level recoveries in northern aquifers, e.g. in the Permo-Triassic sandstones outcrops in Dumfries and Galloway and in the Chalk of Northern Ireland. Generally however, the seasonally dry soils provided limited opportunity for infiltration and seasonal recessions continued. In the Permo-Triassic sandstones of Devon, Bussels reported its lowest October level on record and water-tables were very depressed across most of southern England - and well below average in a broad zone through central southern England and the Midlands. With a few exceptions (including Compton) however, levels remained above the minima reported in 1976 and during the groundwater droughts of the early and mid-1990s.
Heavy November rainfall in the west and north strengthened the seasonal recoveries in most limestone outcrops and the more westerly of Permo-Triassic sandstones and Chalk outcrops (e.g. Rockley). Rainfall total were much more modest to the east and recessions continued in the eastern Chalk and in the slow responding sandstones of the Midlands. With soil moisture deficits eliminated across almost all aquifer outcrops, December provided an opportunity for substantial recharge. In the event, rainfall was less than 80% of average across much of the English Lowlands and the failure of the late autumn and early winter rainfall for the 2nd successive year ensured that groundwater drought conditions would extend well into 2006.