6. River Flows in NW and SE Britain
Annual average flow in rivers in NW and SE Britain expressed as deviations from the 1961 - 90 average
The flow of water in rivers is measured daily at over 1400 locations by the Environment Agency, the Scottish Environment Protection Agency and, in Northern Ireland, by the Rivers Agency. Data are collated in the National River Flow Archive maintained by the Institute of Hydrology, Wallingford.
The specific indicators chosen are the annual average flows in rivers in the north-west and south-east regions of Britain expressed as percentage departures from the 1961-90 average. Thus, positive deviations indicate above average flows for the year and negative deviations indicate below average flows. The rivers chosen to represent the northwest are the Ewe at Poolewe, Tay at Ballathie, Clyde at Blairston and Nith at Friars Carse*; those in the southeast are the Mimram at Panshanger Park, Great Stour at Horton, Lambourn at Shaw and Ock at Abingdon.
[Source: National River Flow Archive, Centre for Ecology and Hydrology, Wallingford, OX10 8BB]
Annual mean river flows provide a measure of the overall water resource for each region and represent the total annual freshwater outflow to the sea. Average river flows are important in providing water resources (e.g. reservoir replenishment), affecting effluent dilution, navigation, leisure activities and the maintenance of a healthy aquatic environment. Regional river flows reflect regional rainfall and evaporation patterns and provide an important benchmark against which to identify and appraise water balance changes.
Sensitivity to climatic and other factors
River flows are affected by a number of factors - including land use change, abstractions and water transfers - but they are principally sustained by that proportion of rainfall which remains after evaporation losses; these are greatest in warm, sunny conditions. In some catchments in the Scottish Highlands, evaporation accounts for less than 15% of the annual precipitation, whereas it can account for more than 80% of annual precipitation in low-lying parts of eastern England. Thus, any increased evaporation will have most effect on river flows in the south and east.
Change over Time
Over the 1961-2002 period regional flows for NW Britain ranged between 70% and 130% of the average. In a region where evaporation losses make a relatively modest contribution to the water balance this variability closely reflects annual precipitation. Post-1980 flows have, on average, been around 15% greater than for the preceding 20 years. However, the considerable year-on-year variability and the commencement of the regional series in a notably dry period - the 5 years ending in 1973 is the driest 5-year sequence for Scotland in the last 60 years - implies considerable caution is essential in interpreting and projecting the apparent trend.
In SE Britain, the greater sensitivity of mean river flows to evaporation losses is reflected in the greater year-on-year variability in annual average flows. No clear trend is evident over the last 40 years but the particular volatility of the post-1987 period is notable. Protracted low flows during the droughts of the early and mid-1990s have been succeeded by an unprecedented recovery in flow rates - culminating in the remarkably sustained high flows which characterised the autumn, winter and spring of 2000/01; for much of this period the groundwater contribution to river flows in the South East was the highest on record.
If UK rainfall increases and/or rainfall patterns become more continental in character - with a greater proportion of the total rainfall falling in the winter - average flows may increase. However, increased evaporation may be a counterbalancing factor, of particular importance in the drier regions. The exceptionally high annual flows recorded in Scotland over the 1988-2000 period and in the English Lowlands during 2000/01 have extended the range of recorded variability but it is too early to say whether this is a portent of the future.
* Substituted for the Level at Linnbrane - unsuitable for the indexing of the persistence of low flows