For the UK as whole, rainfall in 2005 was close to the long term average but its spatial distribution testified to a very notable exaggeration in the normal north-west/south-east rainfall gradient across the country. Whilst Scotland and parts of Northern England experienced a generally wet year, much of southern Britain saw an intensification in the drought conditions which began in the autumn of 2004. The associated water resources and environmental stress demonstrated the UK's continuing vulnerability to sustained periods of low rainfall - particularly when the deficiencies are disproportionately concentrated in the winter and spring. It is over these periods, when evaporative demands are modest, that replenishment of surface and groundwater resources is normally concentrated. In broad terms, the drought achieved its greatest severity in those parts of the country where rainfall is most limited and high population densities generate the highest water demand. Careful water management was therefore essential to reconcile the competing demands on dwindling water resources. Hosepipe bans were introduced in the early summer and low flow augmentation schemes were widely activated across southern Britain. A wet October provided a valuable respite but the very meagre late-autumn and early winter rainfall across much of the English Lowlands (for the second successive year) fuelled concern about the water resources outlook for 2006, particularly in relation to groundwater.
2005 was a year of volatile weather conditions, with a marked contrast between the synoptic patterns influencing northern Britain and those which predominated across the rest of the country. Most rain-bearing frontal systems followed tracks relatively remote from the English Lowlands resulting in above average annual rainfall for most of Scotland. Parts of the Highlands registered over 130% of average and some of the Inner Hebrides (e.g. Skye) were exceptionally wet also. Overall, Scotland was appreciably drier than in 2004 but 2005 still added to a cluster of wet years since the mid-1980s. Most of southern Britain was characterised by sustained periods of below average rainfall - but punctuated by summer thunderstorms in many areas. The 2005 England & Wales rainfall total was 91% of the 1961- 90 average but regional, and more local, variations were exceptionally large. A few, mostly north-eastern localities, registered over 120% of average but much of the South East reported notably low annual totals; locally, rainfall deficiencies exceeded 35%, e.g. in a few parts of London and north Kent. For much of the South East it was the 2nd driest year since 1933. Notable annual rainfall deficiencies also extended across large parts of the South West, east Midlands and the southern Pennines. For Northern Ireland, the 2005 rainfall total was very close to average but the western hills were relatively wet whilst parts of Antrim and Down recorded less than 80%.
The late autumn and early winter of 2004 was notably dry across most of southern Britain - the combined November/December rainfall for England & Wales was the 3rd lowest since 1953. This dry spell initiated drought conditions across much of eastern and southern England. These intensified through 2005 as some parts of the English Lowlands registered 11 successive months (up to October) with below average rainfall. By the end of September, rainfall deficiencies exceeded 25% over wide areas; the Southern and Thames regions recorded, respectively, their 2nd and 3rd lowest November-September rainfall since 1933/34. In the drought-affected regions, above average rainfall in October heralded a dry end to the year and accumulated rainfall deficiencies for the 14 months to the end of December 2005 were between 15-25% over much of southern Britain. In the same timeframe, rainfall across northern Scotland was well above average. This marked regional contrast established a template for the intensification of drought conditions in 2006.
2005 was another warm year: mean temperatures in 2005 were 0.8-1.2°C above average across the UK. Although unremarkable in the context of the recent past, the 2005 potential evaporation (PE) total for Great Britain still ranks 7th highest in the 45-yr MORECS1 series with PE losses 5-15% above average across much of the country. Throughout a significant proportion of the English Lowlands, modelled PE losses for 2005 exceeded the corresponding annual rainfall totals. More significant from a water resources perspective were the notably high actual evaporation (AE) losses which exceeded the average over around 90% of the country; anomalies exceeded 15% in many, mostly low-lying, areas. Dry soil conditions inhibited transpiration losses during the summer for a more limited period than during most recent drought episodes but soil moisture deficits increased briskly across much of the drought-affected region in late September. This was an important factor in delaying the onset of the seasonal recovery in river flows and aquifer recharge across much of eastern and southern England.
The last 15 years have witnessed large variations in overall reservoir stocks for the UK. Figure 1 provides an index of overall stocks for England & Wales - based on a representative network of major reservoirs. Stocks remained very healthy throughout the 1998-2002 period but declined steeply during the latter stages of the 2003 drought, thereafter recovering to near normal levels during 2004. In 2005, stocks in most Scottish reservoirs began the year close to capacity and, due largely to substantial replenishment in the early autumn of 2004, overall stocks for England & Wales were also appreciably above the 1988-2004 average at the beginning of the year. Boosted by substantial replenishment in most regions during April, overall stocks then remained well within the normal range throughout the year, falling below 70% of capacity in September (see Figure 2). Very healthy reservoir inflows in October helped ensure that, at the national scale, stocks ended 2005 modestly above average. Regionally however, reservoir stocks displayed very substantial departures from the seasonal average throughout 2005 - a direct result of the drought in southern Britain.
Stocks in many southern reservoirs remained well below average throughout 2005 with a number of important impoundments falling well short of capacity in the spring (when most reservoirs are normally full). In the South West, Roadford and Colliford Reservoirs were only around 75% full and, in Sussex, Ardingly Reservoir failed to reach capacity in the late spring for the first time in a series from 1988. A surge in water demand during heat-wave conditions in June triggered the introduction of hosepipe bans in the South East. Subsequently, with algal blooms an exacerbating factor in some areas, more widespread restrictions on water use were introduced. By September, a number of southern reservoirs including Bewl, Colliford and Stithians had fallen to around 50% of capacity. At Weir Wood reservoir (Sussex) stocks continued to decline and by October were at their the lowest (for the month) since the reservoir's construction in 1953. Stocks were also seasonally very low in several other reservoirs in southern England at year-end. By contrast, overall stocks in the major pumped storage impoundments which service London's water needs had closely approached the early winter average.
Despite the persistent drought conditions in southern England, totals outflows from the UK for 2005 were only around 7% below average. This reflects the balance between above average runoff for Scotland and considerable runoff deficiencies for Northern Ireland (for the 3rd successive year) and England & Wales which recorded its 4th lowest annual runoff since 1976. Figure 3 shows time series of annual runoff anomalies at the national scale, the assessments are based on representative networks of gauging stations monitoring outflows from major river basins. The estimates for the first few years of each plot are less reliable due to the relatively sparse monitoring network (prior to 1981 there were too few operational gauging stations to monitor total outflows from Northern Ireland with any precision).
Apart from a major flood event in north-west England in early January when the River Eden registered the 2nd highest daily mean flow for England & Wales on the National River Flow Archive, there were few extreme flood events in 2005. The lack of spate conditions throughout the English Lowlands was especially notable. For the River Thames, the maximum daily gauged flow in 2005 at Kingston was lower than for any other year apart from 1973 in the 123-year daily flow series. The dearth of high flows is reflected in the depressed annual runoff totals across southern Britain where a significant number of new annual minimum runoff totals were established, in Sussex and Kent especially. Despite the many notable accumulated runoff deficiencies in 2005, annual minima flows in individual rivers were unremarkable except in some, mostly spring-fed, rivers and streams in the English Lowlands. In the worst affected catchments the contraction in the stream network and associated loss of aquatic habitat was exceptional by the autumn.
The spatial and temporal distribution of rainfall in 2005 was generally very unfavourable for groundwater recharge. Annual rainfall totals were less than 80% of average across most major aquifer outcrop areas and, importantly, a large proportion of the deficiency was associated with the winter and early spring, when groundwater replenishment is normally most abundant. As a result of the 3rd driest November-April period since 1953/54 (for England & Wales), replenishment over the 2004/05 recharge season was less than half of the long term average over wide areas. Correspondingly, the spring maxima in groundwater levels were appreciable below average in most outcrop areas of the major aquifers, and barely discernible in a parts of the Chalk (e.g. the Chilterns) and the Permo-Triassic sandstones (e.g. the in the East Midlands). By the late summer, protracted recessions in groundwater levels left water-tables well below the seasonal average across much of the country, but the degree of groundwater level depression varied markedly, even within individual outcrop areas. This reflects differences in both rainfall patterns and aquifer properties; generally, the combined impact on water-tables was most evident in those aquifers which are the most important in water supply terms. Sustained October rainfall helped initiate the 2005 seasonal recoveries in many western and northern aquifers and groundwater levels were mostly in the normal range by year-end. By contrast, in much of eastern, central and southern England, the early autumn rainfall was insufficient to overcome soil moisture deficiencies. Groundwater level recessions therefore continued and the very limited November and December rainfall served, in some areas, only to moderate the rate of decline in groundwater levels. The failure of the seasonal recoveries to gain any momentum by year-end foreshadowed continuing water resources and environmental stress in 2006.
1. Hough, M., Palmer, S., Weir, A., Lee, M. and Barrie, I. 1995. The Meteorological Office Rainfall and Evaporation Calculation System: MORECS Version 2.0. An update to Hydrological Memo. No. 45, Met Office