Rainfall over the outcrop areas of most major aquifers during 2002 was generally 15-30% above the 1961-90 average. Because of the non-linear relation between rainfall and groundwater replenishment, this implies that recharge to some eastern aquifer units was more than 50% above the annual mean. This, together with the notable high recharge over the 1997-2001 period, helped ensure that in almost all areas groundwater levels were above the seasonal average throughout most of 2002.
In 2001 the seasonal recovery in groundwater levels began later in the autumn than is normal, and replenishment to the major aquifers was modest until the middle of January 2002. Thereafter however, infiltration was heavy and the wet spring served to extend the recharge season into May (and, in some areas, the early summer). The unusually late recharge episode helped to maintain generally healthy groundwater levels through the summer but with very dry soil conditions in September the autumn recover in groundwater levels was expected to be sluggish. In the event, infiltration rates were remarkably high by late October and regular heavy pulses of recharge fuelled a very steep rise in levels through the late autumn; some parallels could be drawn with the dramatic recovery of autumn 2000. Infiltration was especially abundant over the last two weeks of 2002 and although the heavy pulse of recharge had not all reached the water-table by the end of December, localised groundwater flooding triggered concern that early 2003 could witness a repetition of the early-2001 groundwater flooding in southern Britain.
Figure 1 shows 1998-2002 groundwater level hydrographs for a selection of index wells and boreholes throughout the UK. Five-year plots have been used because groundwater levels in many areas show considerable persistence - reflecting groundwater replenishment over a number of recharge seasons. The groundwater level trace is shown together with the monthly maximum and minimum levels for the pre-1997 record. The normal seasonal variation in levels is clearly evident in the hydrographs for most monitoring sites in the Chalk, limestone and in the more responsive Permo-Triassic sandstones units. The spring maxima were generally within the normal range in the Chalk and limestone aquifers - albeit well below the 2001 peaks. In the generally much slower-responding Permo-Triassic sandstones outcrops of the Midlands and northern England, levels continued to reflect the exceptional recharge over the autumn and winter of 2000/01 and for some index wells levels remained above pre-2000 maxima throughout 2002. Such aquifer units aside, minimum levels during 2002 were generally healthy (at Redbank, groundwater abstraction has a significant affect) and the steep rise in water-tables towards year end resulted in notably high groundwater levels across most outcrop areas (see Figure 2). Overall groundwater resources were very healthy entering 2003.
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 London, 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 30 metres; levels rose by 1-2 metres a year through the early 1990s (Figure 3). Level data for the 1998-2002 period are under review but 2003 data confirm that control measures (principally abstraction by Thames Water instigated in the late 1990s) have reversed the increase - other wells in central London confirm a decline in levels since around 1999. Rising groundwater levels have also been reported from other 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.
Groundwater levels began 2002 within the normal early-winter range in most major aquifers, albeit well above average in much of the eastern Chalk and still close to seasonal maxima in much of the slower-responding Permo-Triassic sandstones outcrops. Generally, the distribution of rain in the first half of January did not favour the major aquifers but, thereafter, successive pulses of moderate intensity rainfall produced abundant late-January infiltration.
Sustained rainfall and near-saturated soil conditions in February were conducive to further very substantial groundwater replenishment - infiltration across much of the South-East was 200-300% of the monthly average. This fuelled a brisk increase in groundwater levels and a corresponding increase in spring outflows. Levels throughout most of the Chalk were well above average but still considerably below the outstanding maxima registered during 2000/01. New February maximum levels were reported in the Permo-Triassic sandstones for a number of western index wells and boreholes (e.g. Yew Tree Farm); this reflects both the late-winter rainfall and the abundant recharge over the three preceding winters.
Many index wells register their highest levels in the late winter and early spring. This pattern was followed in the faster-responding aquifer units in 2002, but the mild and windy conditions during March encouraged seasonally high evaporative demands and infiltration rates during the latter half of the month were modest. With natural outflows (from springs and seepages) exceeding recharge, groundwater levels began to fall in some index boreholes (e.g. Rockley and West Woodyates in the Chalk); the seasonal decline began from typical spring maxima across much of the country.
In April, high temperatures combined with record sunshine hours (in some areas) produced evaporative demands more than 25% above the monthly average. Steep increases in soil moisture deficits confirmed the termination of the recharge season in many eastern aquifer units. Groundwater levels were in steep decline in all but the slowest-responding wells and boreholes. Nonetheless, April mean groundwater levels throughout most of the Chalk and the limestone aquifers were close to, or a little above, the spring average. This was also true of most minor aquifers (e.g. the Norfolk Drift and Essex Gravels).
May rainfall totals were well above average across most aquifer outcrop areas in southern Britain. Unusually, end-of-month soil moisture deficits were lower than in mid-April. This provided the opportunity for some seasonally late infiltration in several areas (e.g. the North Downs) and some modest local increases in levels were reported (e.g. in the Jurassic Limestone of the Cotswolds) but this did not materially change the hydrogeological outlook - May was characterised by a typical late-spring recession. Despite this decline, levels in the Permo-Triassic sandstone outcrops in the Midlands, north Wales and northern England commonly remained close to, or above, pre-2000 maxima.
Accelerating evaporative demands and the associated dry lowland soils ensured that infiltration during June was restricted to intense local storm events and had minimal impact on overall resources. Groundwater levels declined steadily through the month in almost all index wells and boreholes. Generalising broadly, levels in the Chalk exceeded the monthly average in the more easterly outcrops but were well within the normal summer range elswhere. A similar picture characterised the limestone aquifers but the Permo-Triassic sandstones present a less spatially coherent picture - June levels were around average in the South-West but still seasonally very high in the Midlands and northern England.
Soil moisture deficits were unusually low across the outcrop areas of most major aquifers in July - but still sufficient to restrict infiltration to very localised (and modest) pulses following sustained storm rainfall (e.g. at Chilgrove in the South Downs). Predictably, the early summer situation was replicated as recessions continued with groundwater levels mostly well within the normal summer range (parts of the Permo-Triassic sandstones remaining an exception).
As usual in August aquifer recharge was minimal - confined to localities where heavy rainfall coincided with a thin soil cover. In broad terms, the rainfall patterns favoured the major aquifer outcrop areas - but served to moderate soil moisture deficits rather than arrest the seasonal recession in groundwater levels. After a period of two years or more with seasonally high levels, water-tables in much of the Chalk closely approached the average. The same is true of most minor aquifers (e.g. the Suffolk Crag). By contrast, levels in parts of the Permo-Triassic sandstones remained close to, or above, pre-2000 maxima (e.g. at Llanfair DC). Overall groundwater resources for the late summer were healthy.
Contrary to the normal seasonal pattern, soil moisture deficits increased through the latter half of September and by month-end were above average across most of the UK - raising the prospect that the autumn recovery in recharge rates would be delayed. As usual in September groundwater levels generally exhibited little change from the late summer. Levels in the great majority of index wells and boreholes in the Chalk and limestone aquifers (and most minor aquifers also) remained close to the long term average for the early autumn. In the Permo-Triassic sandstones levels had generally returned to within the pre-2000 band of variation.
Following a dry start, rainfall over the five weeks from October 9th was between two and four times the average across most of the major aquifer outcrops. This produced a very steep decline in soil moisture deficits - allowing infiltration to recommence by early November in all but the most easterly aquifer units. Brisk groundwater level recoveries were reported from the more responsive aquifers (e.g. the Jurassic Limestone and south-western Chalk) but levels continued to decline in some deeper Chalk wells where the lag between infiltration and water-table response can be several months (e.g. at Therfield).
In most aquifer outcrop areas November rainfall totals were in the 150-250% range, resulting in abundant infiltration - exceeding four times the November average in some, mostly southern, areas (including the Chilterns). As this substantial pulse of recharge reached the water-tables, the 2002 seasonal recoveries gathered momentum. Rapid rises in groundwater levels were reported in the Chalk - e.g. at Rockley and Ashton Farm where a new maximum November level was recorded. November levels were also well above average in most limestone aquifers, notably so in the Lincolnshire Limestone. The much higher storage in the Permo-Triassic sandstones made for a more sluggish recovery but levels were rising in most outcrop areas, and generally above, to well above, the late-autumn average.
Once again, rainfall patterns favoured the major outcrop areas during December. With soils saturated and evaporative demands modest, the rainfall was particularly hydrologically effective. Some localities in the eastern Chalk registered over three times the average infiltration for December. More significantly in relation to overall groundwater resources, three-month recharge totals were, in some areas, comparable with the record figures registered over the last three months of 2000. Correspondingly, groundwater recoveries accelerated - at Chilgrove levels had risen 35 m from their 2002 minimum and were close to the surface at year-end; overflowing wells and boreholes were reported from a few localities (e.g. the Chilterns). Entering 2003 overall groundwater resources were exceptionally healthy but the threat of extensive groundwater flooding was very real (in the event, much drier conditions from early January 2003 rapidly moderated the groundwater flood risk).