7. Frequency of Low and High River Flows in NW and SE Britain
TJ Marsh

Number of days per year when rivers have low water flows (below those occurring 90% of the time)
Number of days per year when rivers have high water flows (above those occurring 90% of the time)

Specific Indicators
The specific indicators are derived from the same daily flow records as used for indicator 6. But in this case, the interest is in the prevalence of notably low and high river flows, providing a guide to the frequency of drought and flood episodes.
The specific indicators are: the number of days per year when the flows in the representative set of rivers in NW and SE Britain (given in Indicator 6) are (i) below those occurring 90% of the time before1990(dry periods), and (ii) above those occurring 90% of the time before1990 (wet periods).
[Source: National River Flow Archive, Centre for Ecology and Hydrology, Wallingford, OX10 8BB]

Relevance
Low and high river flows are obviously important. Low flows not only threaten water supplies, they also reduce effluent dilution, increase the likelihood of algal blooms and damage wetlands and aquatic habitats. High river flows present a risk of flooding to about seven million people in the UK - one of our most damaging and costly natural hazards. Any increase in the frequency and/or magnitude of river flooding would have major economic implications, particularly at a time when further and widespread floodplain development is anticipated.

Sensitivity to climatic and other factors
River flows are not related to rainfall alone. Flows in most rivers draining impermeable catchments in NW Britain decline rapidly when there is little rain, whereas, natural groundwater inflows sustain many rivers in SE Britain; however, when groundwater levels are low, river flows may be depressed for lengthy periods. In many catchments, climate-driven changes in the duration of low flows can be difficult to distinguish from changes resulting from land use change, inter-basin transfers, abstractions and effluent returns, and low flow augmentation programmes.

Throughout the UK most floods are associated with the passage of low pressure systems during the winter. Snowmelt can be a factor in northern catchments whilst thunderstorms can trigger summer flood events in the English Lowlands - in urban catchments especially. Also flood frequency can be affected, locally in particular, by land-use change (e.g. agricultural drainage, removing forests or urbanisation), or by river engineering works and flood alleviation schemes.

Change over time
(i) Low River Flows
Low river flows can be especially protracted in SE Britain where - in permeable catchments - they are often the result of very low groundwater levels. Records for the River Thames and elsewhere reveal no long-term trends but show extended periods of very low flows (e.g. 1890-1910 and in the 1930s). Low flow episodes were also very prevalent in the 1990s but exceptional autumn and early winter rainfall in 2000 heralded an exceptionally sustained period during which flows remained above the low flow threshold.

In NW Britain, where groundwater makes only a modest contribution to flows, the number of low flow days show less variability from year to year. Again no compelling trend is evident but low flows have recently been less prevalent than in the in dry 1970s.

If the UK climate becomes drier and warmer, the frequency of low flows may be expected to increase. However, a similar outcome could result if rainfall increases modestly but summers become drier and evaporative demands increase. In the latter circumstances, substantially different responses may be expected in rivers draining permeable catchments (where summer flows will be sustained by increased aquifer recharge in the winter) compared with those draining impermeable catchments.

(ii) High River Flows
The cluster of widespread flood events over the 1998-2003 period may suggest a rising trend in flood magnitude or frequency where flood records are of relatively short duration (less than 40 years). Certainly the prevalence of high flows over the recent past has been notable, but long term historical flood series for the UK show no significant overall trend although systematic fluctuations and strong climatic links exist.

The sustained nature of high flow conditions in the South East over the recent past (2000 and 2001 especially) has altered the visual impression of trend from a slight decline (to 1997) to a modest increase through time. This illustrates the sensitivity of apparent trends to the timeframe over which the data are examined, and the caution necessary when interpreting signals from relatively short records.

In NW Britain, the frequency of high flows has been greater over the last 20 years than in the drier 1970s but the increasing prevalence which can be identified up until the early 1990s - when there were a number of damaging floods in rivers draining from the Scottish Highlands - has not continued. There is no compelling overall trend.

If climate change results in higher rainfall and/or a greater proportion of rainfall occurring in the winter half-year, we may expect an increase in the frequency of high flows days. The impact of higher temperatures on snowpack storage and melting rates would also be influential in NW Britain. However, the incidence of notable storm events is likely to be of greatest significance, thunderstorm frequency being especially important in urban catchments.