Most major aquifer outcrop areas are in the driest parts of the country - predominately the English Lowlands where groundwater is the principal source of public water supply. In water supply 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 500mm to less than 100mm in the most easterly outcrops. Evaporation losses 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. Consequently year-on-year variations in recharge tend to be much greater than for rainfall. This volatility has been well illustrated over the last decade. Depressed groundwater levels in the 1995-97 period contrasted 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 resources in January of 2003 helped limit the impact of the drought conditions in the ensuing summer but except in the slowest responding aquifers, groundwater levels had returned to the normal range by early 2004.
Rainfall over the outcrop areas of most major aquifers during 2004 was within 10% of the 1961-90 average with significant positive anomalies over much of the eastern Chalk. However, the distribution of rainfall throughout the year was generally unhelpful for groundwater resources. The very limited rainfall in February and March, and then more notably in November and December, restricted recharge during what are normally four of the most productive months for infiltration. Recharge during 2004 was more than usually episodic with good replenishment in January and (in most areas) October. In addition, the appreciable April/early May infiltration was particularly beneficial in many areas, providing a late pulse of recharge at a time when the seasonal decline in groundwater levels is normally well established, at least in most eastern and southern outcrop areas. Despite this, total infiltration in 2004 was appreciably below average across most major aquifers. By year-end, groundwater levels had generally declined to well below average over wide areas, albeit substantially above drought minima.
The behaviour of groundwater levels throughout the year is shown in Figure 1 which features 2000-2004 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-2000 record. For 2004, the hydrographs confirm the rather erratic recharge pattern and provide a guide to the spatial variations in total replenishment to individual aquifer units.
Following notable declines in groundwater levels across most major aquifers during the 2003 drought, the 2003/04 winter recoveries needed to be generated from well below average autumn minima in many outcrop areas. In such circumstances the lag between infiltration and water-table response can be considerable but the sustained rainfall in December 2003 helped initiate recoveries in most aquifers, the recoveries did, however, vary considerably in their strength and persistence. In the southern Chalk (the Yorkshire Wolds also), recoveries were of a typical magnitude but in the Chilterns (e.g. at Stonor) they were much more faltering. With total winter recharge falling below average in most outcrop areas, the 2004 seasonal groundwater level recessions began from moderate spring maxima - April groundwater levels were mostly below average (Figure 2) and in the Chilterns the seasonal recession began at its lowest spring peak since 1998. Some compensation was provided by sustained April/early May rainfall which fuelled a modest upturn in groundwater levels during the late spring (e.g. at Chilgrove). Thereafter, groundwater levels mostly followed normal recessions through until the late summer when the impact of significant, and unusual, August infiltration was evident in the more responsive aquifers (e.g. the Carboniferous Limestone and Middle Jurassic outcrops; in the former - at Alstonfield - groundwater levels reached their highest early autumn level in a series from 1974 - see Figure 3).
The occurrence of groundwater level minima during 2004 showed limited spatial or temporal coherence but they were mostly within the normal range. However, the dry end to the year meant that no appreciable seasonal rises in groundwater levels were evident in some aquifer units in eastern, central and southern England. For some index sites (e.g. Nuttall's Farm) groundwater levels declined steadily throughout 2004 and the absence of an autumn/early winter recovery was especially notable in the eastern Chalk. By year-end levels at Stonor in the Chilterns levels had declined to their lowest since the end of the 1995-97 drought. Entering 2005, groundwater levels were mostly within the normal winter range (see Figure 4) but the lack of substantial infiltration in late 2004 implied that the normal January-February rise in groundwater levels would be considerably weaker than normal, and the groundwater resources outlook less healthy than in any of the previous seven or eight years.
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 5). 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.
Soils remained at, or very close to, saturation throughout January and with modest evaporative demands, infiltration rates were well above average throughout most aquifer units - exceeding 150% of the January average in parts of the Chalk. January 2004 levels confirmed that a strong, if belated, recovery was underway in almost all outcrop areas. In the southern Chalk (e.g. at Chilgrove and West Woodyates) levels had risen by 30 metres or more since the minima recorded during the terminal phase of the 2003 drought. Levels remained below average, but generally within the normal range, in the slower-responding eastern outcrops where the recovery had only recently been initiated. Steep winter recoveries characterised most limestone aquifers; in the Jurassic Limestone of the Cotswolds, levels at Ampney Crucis, which had been at their lowest since 1996 in the autumn of 2003 rose to above the January average. In contrast to the limestone and Chalk aquifers, the delayed recovery in most of the Permo-Triassic sandstones outcrops (the Bussels index borehole, in the South West, was an exception) began from relatively healthy levels - a reflection of recharge patterns over several years. Generally, the 2003/04 groundwater level recovery gathered momentum in January and levels in most index boreholes were appreciably above average at month-end; heavy late-January infiltration implied that groundwater levels would rise further in February.
The February rainfall distribution was unfavourable for groundwater replenishment, in both spatial and temporal terms. Most aquifer outcrop areas reported significantly below average monthly rainfall and, although infiltration rates remained high in the first week, the subsequent dry spell saw modest soil moisture deficits develop across southern England. Groundwater levels for index boreholes which reported in early February reflected the heavy antecedent recharge but some of those reporting around month end confirmed a continuing late-winter recession (e.g. at Ampney Crucis and Chilgrove). Nonetheless, February levels across almost the entirety of the Chalk aquifer were within the normal late winter range (Killyglen in Northern Ireland was an exception). Similarly, levels in most index wells in the limestone aquifers were around the seasonal norm. In the Permo-Triassic sandstones, below average February levels typified most of the more responsive wells (Llanfair DC in North Wales excepted) but healthy levels still characterised the slowest responding aquifer units, including most outcrops in the Midlands. Levels in the minor aquifers of eastern England were also typical for the time of year. The early 2004 recharge resulted in much healthier overall groundwater resources than at the corresponding time in many recent groundwater drought years (e.g. 1992, 1996 and 1997).
Although very moist soil moisture conditions were helpful for groundwater replenishment in March, rainfall across most outcrop areas was < 75% of the average and infiltration rates less than half the long term mean. As importantly, winter half-year (October-March) recharge totals were well below average across almost all aquifer outcrop areas. This implied that, in the absence of further significant recharge, the spring maxima in 2004 would be the lowest for seven years over wide areas, with most recessions commencing from below average levels (but see below). Recessions were well established in late March across much of the southern Chalk (e.g. at Chilgrove and Rockley) but levels continued to rise at the deep Therfield well (near Royston). Levels were also falling in the Limestone aquifers but rising slowly in the slowest responding Permo-Triassic sandstones outcrops. Overall groundwater resources for the early spring were substantially lower than at the corresponding time in 2003 (and in the preceding five years). Nonetheless, most March 2004 groundwater levels were within the normal spring range; generally below average but well above the levels which characterised the droughts of the early and mid-1990s, and much of the 1970s.
April began with soil moisture deficits building across many eastern aquifer outcrop areas and, with evaporation losses accelerating, the prospect for further significant recharge was poor. In the event, the April rainfall was generally well distributed through the month and 30%, or more, above average across most major aquifer units; in the east, the frontal rainfall on the 18th was especially beneficial. The substantial late-spring rainfall was sufficient to re-initiate infiltration and provide a seasonally late pulse of recharge which generated an upturn in groundwater levels (e.g. at Ampney Crucis in the Jurassic Limestone and Rockley in the Chalk). Evidence of a recovery is lacking in the slower- responding Chalk outcrops but April groundwater levels were well within the normal range for almost all index sites. Despite steep groundwater level declines in the late winter and early spring, this was also true across most of the limestone aquifers. Spatial variation was much more evident in the Permo-Triassic sandstones where notably high levels at Yew Tree Farm contrasted with the relatively depressed water-table in southern Scotland. Nonetheless, most Permo-Triassic sandstones index sites registered typical spring levels. Overall groundwater resources were only a little below average at month- end with, in some areas, the likelihood of further modest recoveries before the summer recessions become firmly entrenched.
The first week of May saw some modest, but very useful, infiltration before aquifer recharge was curtailed as soil moisture deficits increased rapidly over the rest of the month. Importantly, the early May rainfall succeeded in postponing the full onset of the summer recession in groundwater levels across most major aquifers (see the Chilgrove hydrograph for example). As a consequence, May levels remained above the monthly average in almost half the index wells and boreholes. Notwithstanding the erratic recharge patterns through the spring, groundwater levels across much the greater part of the Chalk outcrop were well within the normal range - exceptions included Northern Ireland where the very responsive Killyglen borehole registered its lowest May level in a 20-year record. Levels were also relatively depressed in the Permo-Triassic sandstones of the North West (see Skirwith) but near-average levels characterised most of the outcrop; in the west Midlands, levels had fallen close the average for the first time in five years. Levels in the limestone and minor aquifers were also generally around the normal level for late spring.
Soil moisture deficits increased rapidly in the first half of June and, notwithstanding an unsettled final week, end-of-month values remained considerably above average in southern Britain. Consequently, little or no infiltration occurred in the major aquifer outcrop areas. After the modest late spring infiltration moderated the seasonal decline in the more responsive aquifers, groundwater level recessions re-established themselves in June. By month-end levels were in decline in all but the slowest-responding aquifer units. June levels in the Chalk were close to the early summer mean in many index wells and boreholes but regional regional differences were appreciable (levels remained low at Killyglen in Northern Ireland). Groundwater levels within the normal June range also typified the major limestone and Permo-Triassic sandstones outcrops, but levels in the latter were relatively depressed in the Midlands (e.g. at Weeford Flats) and in the most northerly outcrops. Levels remained a little above average in the minor aquifers of eastern England.
As usual in mid-summer, very dry soil conditions across the major aquifer outcrop areas generally restricted infiltration to very localised events associated with convective storms. However, the impact of the exceptional early July rainfall in a zone from Cambridgeshire to the Humber was reflected in the (very modest) increase in levels in the Chalk at Aylesby and an inflection the recession for the New Red Lion borehole (in the Lincolnshire Limestone). Elsewhere, groundwater levels in the great majority of index boreholes were in well established seasonal recessions. In the Chalk, levels were relatively low in the more south-westerly outcrops (and in parts of the Chilterns) but levels closely followed the mean seasonal trace in most areas. This situation was broadly replicated in the limestone aquifers but the Permo-Triassic sandstones displayed much less geographical coherence. Levels were notably low in the most northerly outcrops but still very high in some of slowest responding aquifer units. Again however, levels at most index sites were in the normal range. The overall 'near-average' groundwater resources picture had remained fairly stable through the late spring and most of the summer.
August rainfall totals were 130-200% of average over most (but not all) major aquifer outcrop areas but local variability was large. In marked contrast to the normal seasonal pattern, soil moisture deficits declined in August - dramatically in some outcrop areas (e.g. parts of Cambridgeshire). Generally however, substantial deficits remained at month end, and were above average in parts of the south-western Chalk outcrop. Infiltration rates were high relative to the August average (which is negligible for most outcrop areas) but very modest in absolute terms. Many reporting dates were too early to capture any impact of the August rainfall but groundwater hydrographs for some of the more responsive aquifer units provide evidence of a very early seasonal upturn in groundwater levels e.g. at Ampney Crucis, Killyglen, Newbridge and Alstonfield where levels increased by over 10 metres. Elsewhere, the August levels confirmed the continuation of a typical summer recession in the Chalk. Late summer levels were close to average in the Lincolnshire Limestone also and remained within the normal range for most index wells in the Permo-Triassic Triassic sandstones (albeit with significant regional variations). The main benefit of the abundant August rainfall was the increase in soil moisture and the corresponding increased likelihood of a relatively early onset of the seasonal recovery in recharge rates (a notable contrast with 2003).
Most frontal systems followed tracks remote from the English Lowlands during September; as a consequence, rainfall across many major aquifers was less than 50% of average. In addition, evaporative demands were seasonally high throughout most of the UK - 20-30% above average across much of the southern Chalk outcrop. Thus, contrary to the normal seasonal pattern, soil moisture deficits increased in much of southern and eastern Britain; by month end they exceeded the average across the greater part of the Chalk outcrop. Recharge opportunities were therefore modest but early groundwater level recoveries (heralded by the exceptional August downpours) continued in some responsive aquifer units - notably in the Carboniferous Limestone where the September level at Alstonfield was the highest, by a considerable margin, in a 30-year record. Less dramatic recoveries were reported for other limestone index wells (e.g. in the Lincolnshire Limestone and the northern Chalk). Elsewhere in the Chalk, levels remained within the normal autumn range - but relatively depressed in the south-western outcrops. Levels in most of the Permo-Triassic sandstones outcrop areas were also healthy, notably so in some of the more northerly outcrops (Yew Tree Farm especially). More modest exceedance of the September average characterised most of the minor aquifers in East Anglia (e.g. the Norfolk Drift and Essex Gravels).
October rainfall totals were in the 150-200% range across many major outcrop areas and, by month end, significant (but generally below average) soil moisture deficits were restricted to the drier parts of eastern and southern England - encompassing a large part of the Chalk outcrop. Infiltration was substantial over the latter half of the month but at many index wells and boreholes the major proportion remained in the unsaturated zone when the October levels were measured. Nonetheless, recoveries were firmly established in parts of the Chalk aquifer (e.g. West Woodyates) and autumn levels were generally well within the normal range across the outcrop - and notably high in the Yorkshire Wolds. Steep recoveries since mid August left levels in most limestone index wells considerably above average and, as has been the case for much of the preceding two years, levels in the Permo-Triassic sandstones outcrop displayed large spatial variations; mostly above average (exceptionally so in some of the more north-westerly outcrops) but modestly depressed in a few areas. October groundwater levels in most minor aquifers were healthy and, with most outcrop areas close to saturation, the prospects for further substantial late autumn recharge were very good.
November rainfall totals exceeded the average across a modest proportion of the central Chalk outcrop but elsewhere many aquifer units received less than half the 1961-90 average. Correspondingly, some areas registered little or no infiltration (e.g. parts of the North Downs). The patchiness of the autumn recharge patterns also reflected the large spatial variations in late autumn soil moisture deficits across the English Lowlands where, at the end of November, significant deficits remained in the east. November groundwater levels in many aquifer outcrops showed little change from those in October (the late autumn is normally a time of brisk recoveries). Levels in the Chalk were generally well within the normal range but a distinction could be drawn between those where recoveries were underway (e.g. in the south-western and north-eastern extremities) and those slower responding units where the summer decline had yet to be arrested (see, for instance, Stonor). The generally heavy October recharge ensured that levels in most index wells in the limestone aquifers were above average entering the winter of 2004/05. Late autumn groundwater levels in the Permo-Triassic sandstones reflected the wide distribution of the outcrop areas and the substantial variations in response rates - nonetheless all were within the normal range.
Despite the low rainfall across most aquifer outcrop areas, residual soil moisture deficits were sensibly eliminated in all but a few parts of eastern England by the end of the year. However, infiltration totals for December were well below the monthly average, declining to less than 20% in much of the eastern Chalk. As a consequence, the 2004 groundwater level recessions for the eastern Chalk extended into the new year. Groundwater levels in parts of the Chalk (e.g. Stonor, Redlands) were at their lowest since 1998. However, the residual benefit of abundant recharge to the Chalk over previous winters can still be identified - thus levels remained substantially above winter drought minima (e.g. those for 1991, 1992 and 1997). Groundwater levels in the limestone aquifers had generally declined from very healthy early autumn levels but remained in the normal end-of-year range. This was true of levels in most Permo-Triassic sandstones outcrops also although levels in many index wells began 2005 at their lowest January level for around seven years. Overall groundwater resources for England and Wales remained close to average but the barely discernible seasonal recovery in the eastern (and parts of the southern) Chalk implied a need for substantial late winter and spring recharge to avoid depressed groundwater levels in the summer of 2005.