In contrast to the previous year, when drought conditions extended across much of the country, rainfall totals for 2004 were appreciably above average in almost all regions of the UK with the highest anomalies in northern and eastern Britain - Scotland added a further year to a notably wet cluster. The inherent variability of the UK climate tends to produce substantial month by month rainfall variations but rainfall patterns in 2004 were particularly volatile. A very wet January helped reinforce the water resources recovery following the 2003 drought and well above average April rainfall ensured that reservoir stocks were generally close to capacity before the onset of the summer drawdown. May was relatively dry but very little water resources stress was experienced during the ensuing summer which was very unsettled. The June-August rainfall total was the 2nd highest (for the UK) since 1958, largely due to an exceptionally wet August. Countrywide, it was the wettest August for 48 years, causing significant crop damage and delays in harvesting. Severe but mostly localised flooding was also widely reported and several extreme events, the most notable at Boscastle (north Cornwall), underlined the UK's continuing vulnerability to very intense rainfall events. October was also wet, with further flooding but the sustained rainfall initiated a substantial seasonal recovery in most reservoir stocks and some unusually early recharge to a few eastern aquifers. Thereafter however the water resources outlook deteriorated considerably during the driest November/December for England and Wales since 1953. Regional rainfall deficiencies of 40% or more over the two months heralded a notable winter/spring drought across southern Britain - foreshadowing considerable water resources and environmental stress in the summer of 2005.
Mean temperatures for 2004 were only modestly below those in 2003 and, once again, well above average. The mean Central England Temperature1 for 2004 mean was around 1.0°C above the 1961-90 average, ranking among the warmest 10 years on record in a series from 1659. The late winter (of 2003/04) was especially mild and July was alone in registering a below average monthly temperature. In most areas, the warm conditions contributed to moderately above average potential evaporation losses for 2004 as a whole. Due to the relatively moist late summer soils transpiration rates in most areas were inhibited for only short periods. Correspondingly, annual (MORECS) actual evaporation losses were >15% above average throughout much of the English Lowlands and established a new annual maximum for England and Wales as a whole - in a series from 1961. Soil moisture deficits varied rather erratically through the year, particularly during the late summer and autumn. Following an unusual decline in August, and contrary to the normal seasonal trend, deficits increased again through September, exceeding the average across much of the Chalk outcrop at month end. A steep fall in October then facilitated the onset of groundwater recharge across most northern and western aquifers. By contrast, infiltration was limited across much of the English Lowlands where modest soil moisture deficits remained at year end in some eastern areas.
Figure 1 provides a guide to overall reservoir stocks for England and Wales for the 1988-2004 period, based on a representative network of major reservoirs. Reservoir stocks were generally very healthy throughout the 1998-2003 period but during the latter stages of the 2003 drought they fell well below the seasonal average across much of the UK. In southern England stocks in a few reservoirs (including Ardingly and Clatworthy) were still below 30% of capacity in early December. Fortunately, stocks recovered very briskly over the late autumn and winter of 2003/04. Overall stocks for England and Wales increased by 40% from early November 2003 to early February 2004 - the largest increase in any 3-monthe period in a series from 1988. Replenishment declined substantially during the late winter but, by early April, stocks were close to capacity in most major reservoirs - exceptions included Colliford and Roadford in the South West, and Silent Valley in Northern Ireland. For England and Wales as a whole, stocks were modestly above average at the onset of the seasonal decline in May (see Figure 2). However, despite significant replenishment to pumped storages in the English Lowlands, stocks generally fell more rapidly than normal through the early summer and overall stocks were a little below average entering August. Very unusually, overall stocks increased appreciably over the month (the last such occurrence was in 1992) mainly reflecting healthy inflows to western and northern reservoirs and a moderation in the rate of decline in many southern impoundments. Further increases in reservoir levels during October left overall stocks at their 3rd highest on record by early November. The atypical replenishment pattern continued with an appreciable decline of stocks into December but, by year end, overall stocks were still appreciably above average at the national scale. Significantly however, water levels were considerably below average in a number of important southern reservoirs (e.g. Bewl and Colliford).
Due in part to seasonally very healthy runoff over the August-October period, total outflows from the UK were modestly above average in 2004. A guide to the variation in annual runoff for England and Wales, Scotland and Northern Ireland - expressed as percentage departures from the long term mean - is shown on Figure 3. The runoff assessments are based on representative networks of gauging stations monitoring outflows from major river basins. The estimates for the first few years featured on each plot are less reliable due to the relatively sparse monitoring network at the time (prior to 1981 there were too few operational gauging stations to monitor total outflows from Northern Ireland). After a notably low runoff year (2003 which registered the lowest total since 1973) and a cluster of years with abundant outflows (1998, 1999, 2000 and 2003) each rank in the highest five in a series from 1961), runoff totals for Great Britain returned to well within the normal range. England and Wales reported outflows around 3% above average, the first time since 1995 that they have been within 15% of the long term average. The total for Scotland was about 16% above average and Northern Ireland registered a similar percentage below - the third time in four years that runoff has fallen well below average (but the 2001 and 2003 totals were considerably lower than 2004).
Seasonally typical river flows at the beginning of 2004 were in sharp contrast to the floods in early January 2003 and heralded a year when flows in most rivers remained within the normal range. Seasonal contrasts were also more muted than normal. Although annual runoff totals were generally not exceptional, flow patterns displayed considerable spatial and temporal variations through the year. River flows approached long term monthly minima in many responsive catchments during April, and again in June, but recessions in most rivers were reversed during the late summer. Widespread moderate flooding occurred in late January and early February, and again in October, but the events with the greatest impact were concentrated in August when extreme rainfall (over a range of timespans) generated a number of very damaging floods and landslides (from Cornwall to the Scottish Highlands). The estimated peak flows associated with the most outstanding events (at Boscastle2 and Loch Ogle3 in Perthshire) have important implications for understanding flood generating processes, flood management and the development of improved engineering design procedures. As a consequence of the exceptional late-summer runoff, most 2004 daily minimum flows were well above drought mimima. Seasonal runoff recoveries were generally strong in October in western and northern Britain, but relatively weak in the English Lowlands. This was particularly notable in spring-fed streams and rivers which exhibited remarkably stable flows over the latter half of 2004, implying well below average flow rates by the end of the year. The very limited November and December rainfall also resulted in seasonally low flows in most responsive catchments across the rest of the country.
Aquifer recharge patterns and groundwater level variations in 2004 were substantially different from those in a more typical year. Generally the rainfall distribution through the year was unfavourable in relation to groundwater recharge, with a number of the wettest episodes occurring in the April-October period - when evaporation losses are at their greatest. Infiltration was erratic throughout much of 2004 but seasonal contrasts in recharge, which were exaggerated in 2003, were less evident and contrasted with infiltration patterns which have typified much of the post-1997 period. In 2004, abundant infiltration characterised January and October (in many areas); much more unusually, significant and widespread infiltration was reported during late August. Recharge was very modest in the late winter/early spring (2003/04) and, particularly, during the last two months of the year when soil moisture deficits lingered in the outcrop areas of many eastern and southern aquifers. As a consequence, groundwater levels were at their lowest for seven or eight years over wide areas by the end of 2004. More generally, groundwater resources, which had provided an important buttress against drought conditions in the previous year were, by year end, much more poorly placed to counterbalance the impact of any substantial rainfall deficiencies in 2005.
2004 followed a period with very notable seasonal - and longer term - variations in river flows and rates of aquifer recharge. The preceding decade saw widespread drought conditions in 1995-97, in northern Britain and Northern Ireland during 2001, and again across much of the country in the summer of 2003. By contrast, severe floods were a feature of the spring of 1998 (across the Midlands), throughout most of southern Britain in 2000/01, and again in early 2003. Existing maximum recorded flows were widely eclipsed, although mostly by modest margins, and groundwater levels - responding to unprecedented rates of aquifer recharge (especially in the winter of 2000/01) - exceeded previous maxima for extended periods in many outcrop areas.
Figure 4 shows 5-year running means of annual rainfall for the 1914-2004 period for England and Wales, Scotland and Northern Ireland. Overall, the relative dryness of much of the 1970s has served to exaggerate the apparent increasing trend over the last 40 years; when considered within the context of rainfall records extending back to the late 18th century the main features may be seen as perturbations about a relatively stable mean. Nonethless, the recent past has been notably wet. The 1998-2002 period is the wettest 5-year sequence on record for England and Wales. As notably, the last 15 years have been, on average, 9% wetter in Scotland than the preceding average (4% wetter for Northern Ireland). North-western Britain has been especially wet over this timespan with winters contributing most to the additional rainfall. However a proportion of this increase may be attributable to a reduction in snowfall as a proportion of total precipitation (snowfall is normally underestimated). As winter rainfall has increased, there has also been a tendency towards a clearer partitioning between winter and summer rainfall across much of the UK. This is in marked contrast to the 19th century when many summer half-year rainfalls totals exceeded those for the preceding winter half-year.
A common climatological feature of the recent past has been the notably warmth, over the last decade particularly. The annual average Central England Temperature has, with the exception of 1996, been above the long term mean for every year since 1987 and the 1995-2004 mean is around 1.0°C greater than temperatures of a century ago. This very appreciable warming is evident in Figure 5 which illustrates winter and summer temperature and rainfall anomalies (relative to the 1850-1974 average) for England and Wales. The red diamonds show the plotting positions for the most recent 30-years; a high proportion have registered positive temperature anomalies. The winter (November-April) and summer (May-October) periods for 2003/04 both plot in warm/wet quadrant, as do most years since 1997; for the preceding eight years the summers tend to group in the warm/dry quadrant.
The warm conditions have encouraged increasing evaporation demands over the last 40 years. Figure 6 shows the 5-year running mean annual (MORECS) Potential (PE) and Actual Evaporation (AE) losses for England and Wales and for Scotland. PE losses for England and Wales have increased erratically but recent annual totals have been around 30mm greater than in the 1960s. AE losses, however, exhibit less of a trend. This is largely due the warmer (and, often, drier) summers with correspondingly very dry soil conditions, inhibiting transpiration rates and moderating overall AE losses in the English Lowlands especially. As a result, changing evaporative demands have had only a limited impact on the water balances of most catchments. For Scotland, transpiration rates are normally restricted for only very short periods across most of the country. As a consequence, countrywide annual AE losses closely approach PE totals in most years. Evaporation losses have increased by a similar margin to those for England and Wales since the 1960s but the impact on water balances in most areas has been more than counterbalanced by a larger increase in annual precipitation totals.
Rivers, reservoirs and groundwater reserves are sustained and replenished not by rainfall directly but that proportion which remains after allowing for evaporative demands. Runoff therefore provides the best index of the health of water resources. Figure 3 provides evidence of substantially greater runoff from Scotland over the post-1980 period than in the preceding 20 years. Notwithstanding, the enhanced evaporative demands, total runoff over the 1998-2002 period for England and Wales matched rainfall as being unprecedented over the post-1960 period and, very probably, throughout the instrumented era. A common feature of the annual histogram plots is the notable variability over the last decade with wide departures from the preceding average; seasonal departures have also been substantial in recent years also. Statistically significant increases in runoff and flood magnitude have been identified for some rivers (mostly in Scotland) and examples of both positive and negative trends in low flows can readily be found. However, given the natural range of hydrological variability and the pervasive impact of artificial influences (e.g. abstractions, river regulation, inter-basin transfers), any attribution to climatic change can be only tentative. Of particular significance in the UK is the fact that the great majority of UK river flow and groundwater level data has been collected over the last 40 years. In relative terms, the early part of this period can be broadly characterised as quiescent - with below average runoff and a low frequency of major flood events across much of the UK. As with many hydrological series the timeframe over which any trend analysis is undertaken can be very influential in determining the sign and magnitude of any apparent trend. When considered in the context of available long river flow records and curatorially appraised historical material, most apparent trends appear as perturbations about a relatively stable mean. Compelling evidence for an increase in the frequency or magnitude of UK hydrological extremes is still awaited. However, there are few modern parallels for the volatility of the recent past and this enhanced variability has substantial implications for water and environmental management.
2. Fenn C. R., Bettes, R., Farquharson, F. A. K., and Wood, T. (2005) The Boscastle flood of 16 August 2004: characteristics, causes and consequences. In: 40th Defra Flood and Coastal Management Conference, University of York, 5-7 July, 2005. Defra
3. Black, A., Cargill, A., Gilvear, A., Hoey, T., MacConnachie, M., Perrett, J., Rowan, J., and Werritty, A. (2005) Extreme precipitation and runoff, SW Perthshire Scotland, August 2004. Report for the Scottish Environment Protection Agency (a collaboration between SEPA and a group of Scottish universities)