For its size, the UK exhibits very large regional rainfall differences even in an average year. This inherent spatial variability was considerably enhanced in 2005. In parts of the western Highlands of Scotland annual precipitation totals exceeded 5000 mm whilst some localities in the South East recorded an order of magnitude less - rainfall totals falling below 450 mm in a few areas adjacent to the Thames estuary (Figure 1). Relative to the 1961-90 average, this represents about a 20% shortfall and 2005 rainfall totals were more than 15% below average across much of the English Lowlands. Figure 2 serves to underline the north-south contrast in rainfall anomalies for 2005. For much of the South East and the east Midlands, 2005 was the 2nd driest year since 1933; by contrast, rainfall was close to, or above, average throughout almost all of Scotland with the most notable anomalies north-west of the Great Glen.
Table 1 gives monthly and half-yearly national and regional rainfall totals (in mm and as a percentage of the 1961-90 average) for 2005. The general pattern of rainfall through most of 2005 was established during the December 2004-February 2005 period (see Figure 3), for which Scotland reported its 7th highest winter rainfall since 1920. Rainfall over England & Wales however was the lowest for 30 years (although the winters of 1991/92 and 1996/97 produced similar totals) with deficiencies exceeding 40% in parts of the South and the Midlands. Catchments in the South East were especially dry and the regional focus of the drought was reinforced through the spring of 2005. In the drought-affected regions summer, (June-August) rainfall totals were generally well above drought minima (e.g. those of 1990, 1995 and 2003) but did little to moderate the increasing hydrological stress.
In a water resources context, the November 2004-September 2005 period provides a better insight to the severity of drought conditions experienced in 2005 (see Figure 4). Within this 11-month timeframe England & Wales registered its 3rd driest such sequence since 1976, with parts of the South East reporting their 3rd driest November- September period since 1944. October was wet throughout the UK and, by a substantial margin, the wettest month of the year across most of the country. However, anticyclonic weather patterns - and modest rainfall totals - again predominated in southern Britain during November and December. This second successive relatively dry late autumn and early winter ensured the continuation of severe drought conditions into 2006 - and a very fragile water resources outlook in parts of southern England and the Midlands
Spatial contrasts in rainfall anomalies were particularly marked during the winter of 2004/05. Following a dry December in 2004, large parts of England recorded less than half the average rainfall in January whilst Scotland registered its 5th wettest January in a series from 1914. In the first fortnight, appreciable precipitation was recorded on almost every day, resulting in some extreme rainfall accumulations e.g. 341 mm at Strathyre (north of Callander) by the 10th January (the estimated return period exceeds 50 years). Exceptional precipitation totals were also recorded in the Lake District and North Wales.
Generally mild conditions through the late winter meant that rainfall constituted much the greater part of the precipitation even in northern Britain but substantial snowfall was experienced in northern England and the Scottish Borders during January and February. Snowstorms were less common but more widely distributed in March when, early in the month, drifting snow contributed to severe transport disruption and some school closures, as far south as Kent and Sussex. Such wintry interludes aside, the drought continued to intensify with some catchments in the South East reporting March rainfall totals below 50% of average; in London a number of raingauges registered monthly totals of 15 mm or less.
By the end of March, accumulated rainfall deficiencies from the beginning of November 2004 exceeded 40% of the 1961-90 average in parts of southern England. For England & Wales, the November-March period was the driest since 1975/76 and in parts of the South East the 5-month total was the 2nd lowest in over 60 years, firmly establishing the regional dimension of the 2005 drought. Although April provided some respite in much of the drought-affected regions, dry conditions resumed in May - contributing to a remarkable sequence of months with below average rainfall in some parts of the English Lowlands. Parts of the Thames Valley recorded negative monthly rainfall anomalies for 11 successive months beginning in November 2004. In this timeframe, most southern catchments registered only two or three months with above average rainfall. By the early summer, rainfall deficiencies were outstanding for many catchments in eastern and southern England over the post-1975/76 period.
In southern Britain particularly, the summer was characterized by extended dry periods punctuated by intense convective storms. Some very notable events were reported e.g. 69.4 mm in 3 hours at Hawnby (North Yorkshire), which included just over 50 mm in one 30-minute period (estimated return period exceeds 300 years). Radar estimates indicate storm totals of up to 125mm in 3 hours at Sutton Bank. Notwithstanding the dry start, summer (June-August) rainfall totals were within the normal range in almost all regions and, in England, most of the drought-affected areas reported only moderate negative anomalies. However, Northern Ireland registered its 2nd lowest summer rainfall since 1984. July was unusual in the context of 2005 as a whole: warm and dry conditions typified most of northern Britain with above-average rainfall across much of England & Wales. The July rainfall was very useful agriculturally but dry soil conditions ensured that it had little impact on water resources (apart from modestly reducing water demand).
Rainfall in August was spatially very variable with localized flash flooding and many lightning strikes with associated power loss (e.g. in North Tyneside and parts of Northern Ireland), whilst other areas (e.g. the South West), were notably dry. An unsettled late-September heralded generally wet conditions in October when successive pulses of frontal rainfall in the north and west and, especially in the Scottish Borders, triggered high spate conditions in many rivers. Nearly all the UK experienced above average rainfall but the impact on accumulated rainfall deficits was modest.
November saw significant snowfall in Cornwall and Wales which led to widespread transport disruption but, generally, weather conditions reverted to a familiar 2005 pattern with northern Scotland very wet whilst much of eastern and southern England reporting well below average rainfall. Autumn (September-November) regional rainfall totals were mostly well within the normal range but deficiencies of 15-25% typified much of the South East, further strengthening the drought's regional focus. Drought conditions intensified again in December when, despite significant snowfall towards the end of the month (e.g. in Yorkshire and Kent), all regions reported below average precipitation. In Northern Ireland, rainfall was only marginally below average but Great Britain registered its 4th driest December since 1975. The dry end to 2005 heralded the continuation of severe drought conditions into the following year.
Click here for a more detailed commentary on rainfall patterns in individual months of 2006.