e-Digest Statistics about: Inland Water Quality and Use
Rainfall
Water use, water quality and the health of the aquatic environment are all affected by climatic conditions. Water resources, in particular, are greatly influenced by the interplay of rainfall patterns and strongly seasonal evaporative losses.
Table 1 shows the national and regional annual rainfall totals for 1990-2005 expressed in millimetres and as a percentage of the 1961-1990 average. Figures are given for the ten former National Rivers Authority (NRA) regions rather than the eight Environment Agency (EA) regions in England and Wales, because the former provide useful extra geographical details of the rainfall patterns in the North East and South West regions.
Following drought conditions in the early and mid-1990s (see Water supply and use), the UK experienced its wettest 5-year sequence on record (in a series from 1900). The 1998-2002 period was characterised by abundant water resources but an increased frequency of flooding, which culminated in the extensive and protracted flooding of the autumn and winter of 2000/1. Notable flooding also occurred in April 1998, December 2002 (in Scotland) and January 2003 when flows in some rivers in southern England were the highest since the snow-melt floods of March 1947. Thereafter weather patterns changed and substantial rainfall deficiencies built up through the spring. These were reinforced during heatwave conditions in the late summer of 2003, by which time a severe drought extended across much of Europe. In the UK, the drought intensified in the autumn - contributing to the lowest February to October rainfall total since 1921.
The drought terminated in the early winter of 2003, heralding a two-year period during which the normal north-west to south-east rainfall gradient across the UK was heavily accentuated. Whilst much of northern Scotland was very wet, rainfall deficiencies again began to build in southern Britain through the dry and exceptionally mild winter of 2004/5. By the following summer, drought conditions had developed once more - particularly in the Southeast where the margin between water availability and demand is narrow. A second successive very dry winter and early spring then intensified drought conditions creating substantial water resources and environmental stress. By the summer of 2006, 13 million consumers were subject to water use restrictions.
Although the UK's resilience to within-year drought episodes was well demonstrated in 2003, our continuing vulnerability to extreme weather conditions has been re-emphasised over the last decade which has seen an extension in the recorded variability of river flows and groundwater levels in many areas.
'Key Fact (figure 1)' gives a scatter plot of the rainfall and temperatures for England and Wales over the summer and winter periods since 1845. The plots for the summer (June-August) and winter (December of previous year to February) periods for the last 30 years, compared to earlier periods, show a tendency for hotter, drier summers and milder winters, a number of which have been notably wet. However, the last two winters have been dry across much of southern Britain - an unusual circumstance in the context of the recent past but dry winters were appreciably more common in the 19th century.
Weather patterns over the last 20 years are broadly consistent with most recent climate change scenarios, but the inherent variability of the UK climate has also been underlined.; This, together with the substantial uncertainties associated with the climate change scenarios (in relation to future rainfall patterns in particular) implies that any apparent recent trends need to be viewed with caution, they may appear much less compelling when viewed in a longer historical perspective.
Changes in seasonal patterns of rainfall and temperature have important implications for water resources and flood risk. An increase in rainfall over the winter, when evaporation losses are modest, could increase flood frequency but would generally be beneficial for water resources. On the other hand, lower summer rainfall can, as in 1995 and 2003, result in pressure on water resources (e.g. increased demands for irrigation and garden watering during hot summers). Hot, dry summers also result in exceptionally dry soils. Autumn and winter rainfall must then restore soil moisture before water becomes available for the recovery of river flows and the replenishment of reservoirs and aquifers (water-bearing underground strata).
Rivers are sustained, and reservoirs and aquifers replenished, by the balance between rainfall and evaporation losses. Evaporation may occur directly from the soil, from open water surfaces or as transpiration from plants. For the UK as a whole about 40 per cent of rainfall evaporates but this proportion varies greatly across the country. As a result, runoff (the total outflow of rivers and springs) constitutes more than 80 per cent of the rainfall in the Scottish Highlands but less than 20 per cent in parts of the English lowlands. Table 2 gives countrywide estimates for long-term annual rainfall in millions of cubic metres together with the corresponding average runoff; the difference between the rainfall and runoff figures provides a measure of average evaporation losses. The figures in Table 2 for the individual countries and for the UK as a whole have been estimated independently and so the country figures do not necessarily sum to the UK total.
Figure 2 illustrates annual runoff variations for England and Wales, Scotland and Northern Ireland (where a province-wide gauging station network has operated only since the mid-1970s); the annual runoff assessments are based upon flows for about 25 major rivers across the UK. The five year period ending in 2002 was characterised by exceptionally high runoff in much of the UK - southern Britain especially. By contrast, runoff in 2003 was the lowest on record for Scotland and Northern Ireland and, at the national scale, only in 1973 has a lower annual runoff total been recorded (in a series from 1961).
Annual runoff totals returned to the normal range in 2004, albeit well below average in Northern Ireland. The volatility displayed by annual runoff totals in recent years is unusual but substantial year-on-year variability is to be expected. This, together with a tendency for high and low runoff years to form clusters, implies that any apparent trends should be treated with considerable caution. Above average runoff has been a common circumstance in Scotland over the post-1980 period. Correspondingly, mean runoff shows a significant increase over the 42-year national series - however a contributory factor is the notable dryness of the early 1970s, which tends to exaggerate the apparent steepness of the trend.
UK runoff for 2005 was considerably below average but regional variations were substantial. Scotland added a further year to a notable cluster with above average runoff since 1989; only four have been below average in this time span. By contrast, some catchments in southern England (the Southeast especially) reported their lowest annual runoff on record. For England and Wales as a whole, 2005 registered the 4th lowest annual runoff total in the last 30 years. Runoff for Northern Ireland was also below average, appreciably so in some eastern catchments.In Detail:
Further Information:
Data Tables:
- References, further reading and links to other resources:
- [1] Data holdings of the National River Flow Archive and National Groundwater Level Archive
- [2] Lessons learned: Autumn 2000 floods. Environment Agency, March 2001.
- Internet Links:
- For national and local information see:
- England and Wales: Environment Agency
- Scotland: Scottish Environment Protection Agency
- Northern Ireland: Environment and Heritage Service
- Department of the Environment in Northern Ireland
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Page last modified: 26 January 2007
Page published: 24 January 2006