Considering the UK as a whole, 2002 was another wet year - ranking 7th in the national rainfall series beginning in 1900 - and continuing a wet phase that began over the latter half of 1997. September was the only notably dry month whilst February, May-July and October/November were exceptionally wet - particularly in Northern Ireland which registered its wettest year in a 103-year series. Modest rainfall deficiencies did develop in parts of England and Wales in the early spring and early autumn but were insufficient to cause any significant water resources stress. In Scotland the temporal distribution of the rainfall was very unusual - exceptionally high rainfall accumulations characterised the January-July period but notable deficiencies began to build in the late summer across northern and some western catchments; the deficiencies were remarkably large by year-end.
2002 was another very warm year. The mean Central England Temperature was around 1.1°C above the 1961-90 average, making 2003 the fourth warmest year (after 1999, 1990, and 1949) in the 336-year CET series1. The combination of above average temperatures and above average rainfall (parts of northern and western Scotland excepted) contributed to relatively high actual evaporation totals - in the English Lowlands especially.
Nonetheless, water resources remained very healthy in most regions throughout the year. Drawdown releases were required (e.g. in February and December) in some major reservoirs to provide additional flood alleviation storage but overall stocks for England and Wales remained close to, or above, the monthly average throughout the year. Figure 1 shows the variation in overall stocks for a network of major reservoirs over the last eight years. Figure 2 compares the pattern of variation in 2002 with other recent years. As in the two preceding years, the seasonal decline in stocks began relatively late and overall stocks were close to capacity at year-end. Unsurprisingly therefore incidences of water supply stress were generally very localised - e.g. in January low reservoir levels in Silent Valley triggered appeals to moderate water use in Belfast; in July water was tankered to the Out Skerries (Shetlands) to augment dwindling stocks.
A guide to the variation in annual runoff for England and Wales, Scotland and Northern Ireland based on outflows from representative networks of major river basins is shown on Figure 3. For England and Wales, runoff exceeded the average for the fifth successive year; clustering is a feature of the series but runoff over the 1998-2002 period is substantially greater than in any other 5-year sequence in the 42-year national series - long term rainfall data suggest that there is unlikely to be any close precedents in at least the last 227 years. Although runoff in many western and northern catchments was relatively depressed, the 2002 runoff total for Scotland was considerably above average - ranking third highest after 1990 and 2000. In Northern Ireland, where a province-wide gauging station network has operated only since the mid-1970s, the 2002 runoff was the highest on record by a considerable margin, and around 85% greater than the series minima established in 2001.
River flows remained above the seasonal average throughout much the greater part of 2002 in most regions. Moderate flooding was widespread in February and notable summer spates were common especially in northern Britain. Thunderstorms produced many locally severe flood events as rainfall intensities overwhelmed urban drainage capabilities (e.g. in Glasgow on July 30th) but continuing flow recessions through September - and very dry early autumn soils - seemed to foreshadow notably low October flows. In the event, a very wet episode beginning in the second week resulted in the focus of hydrological concern switching decisively from the threat of depressed runoff rates to the risk of widespread floodplain inundations. The flood risk remained high in the late autumn (severe flooding was experienced in north-east Scotland in November) and in December when floodplain inundations were common approaching year- end. The flooding culminated in early January 2003 when flows in some rivers in parts of central southern Britain were the highest since the snowmelt flood of March 1947.
As during the previous four years, 2002 was characterised by very healthy groundwater resources throughout most major aquifers. The legacy of very healthy groundwater replenishment over the 1998-2001 period helped ensure that groundwater levels in the majority of index wells and boreholes remained above average throughout the year and, by late-2002, levels had yet to decline below pre-2000 maxima in parts of the slow-responding Permo-Triassic sandstones. In the Chalk, and most other aquifers across southern Britain, levels during 2002 rarely approached the remarkable maxima of 2000 and 2001 but still generally exceeded the seasonal average. This despite very low rainfall during November and December 2001 providing little impetus to the seasonal recovery. Fortunately groundwater levels in early 2002 generally showed a continuing benefit from the abundant recharge in 2000/01 and, following sustained and heavy infiltration in January and February 2002, late-winter groundwater levels were significantly above average in most areas. Although the normal seasonal pattern in groundwater level behaviour was readily recognisable in 2002, the sustained rainfall through May and June served to usefully extend the 2001/02 recharge season, and a rapid seasonal recovery - a feature of a number of recent years - was underway by late October in most areas. Thus, moderately depressed levels in 2002 were largely restricted to a brief period in the early autumn. By year-end, levels were still rising and a heavy pulse of infiltration over the latter half of December triggered localised groundwater flooding (e.g. in parts of the Chilterns), echoing the situation in late 2000 (however, the dryness of the late winter of 2002/03 prevented any repetition of the sustained groundwater flooding experienced in 2001).
Rainfall over the 1998-2002 period was around 13% above the 1961-90 for the UK and eclipsed the previous highest 5-year rainfall total (1875-79) for England and Wales (in a record from 17662). The last five years provide a remarkable contrast with the early and mid-1990s when drought episodes were both protracted and widespread, particularly in southern Britain. Over the same time period Scotland was subject to a number of damaging winter flood events. In large part these hydrological contrasts reflect the preferred tracks of Atlantic frontal systems crossing the British Isles. For lengthy periods over the recent past the high proportion of active depressions following abnormally southerly routes has brought sustained and heavy rainfall to the English Lowlands. Correspondingly, northern Scotland has been notably dry (as over the latter half of 2002). On many occasions the normally strong north-west to south-east rainfall gradient across the UK has been greatly moderated and even, for short periods (late-2002 included), reversed. This unusual synoptic backcloth has contributed to a recent extension in the range of recorded river flows and groundwater levels in many areas of the UK. The persistence of high flows has been especially notable, and record accumulated runoff totals (over a range of timeframes) have been registered for many spring-fed southern rivers.
Some consistency may be recognised between the recent past and current climate change scenarios - above average temperatures and evaporative demands, and the preponderance of mild, wet winters in particular (see Figure 4). A tendency for a higher proportion of the UK rainfall to fall during the winter half-year (October-March) has also been a feature of the last 25 years, and continued in 2001/02. If this more distinct partitioning of rainfall within the year were to be become an enduring characteristic of the climate it would have important implications both for flood frequency and water resources. However, the complex interactions between rainfall, evaporation and soil moisture conditions implies that extrapolations from rainfall patterns to runoff and recharge patterns need to be undertaken with caution. Over the last five years in the English Lowlands for example, above average evaporative demands have, generally, coexisted with above average summer river flows. Importantly also, the UK climate is inherently very variable and, as has been demonstrated, very sensitive to minor shifts in the preferred tracks of rain-bearing low pressure systems. The hydrological volatility of the last 10-15 years also tends to be exaggerated by the relatively quiescent conditions of the previous 20 years - when major floods events were relatively rare, in southern Britain particularly.