An assessment of trends in UK runoff and low flows using a network of undisturbed catchments

In many parts of the world attempts to discern climatic-driven changes in river flow patterns are hindered by the combined impact of other, more direct anthropogenic disturbances such as abstraction and impoundments. This study capitalises on a newly defined 'benchmark' network of natural catchments in the UK to discern natural variability in flow regimes. Trend tests were applied to time series of runoff and indicators of low-flow magnitude and duration for two study periods to assess the sensitivity of trends to the frame of reference over which tests were conducted. Notwithstanding the volatility of the recent past, the results demonstrate a general stability in runoff and low flows since the early 1960s. The strongest signal to emerge from the study is a positive runoff trend for catchments in Scotland, which was resilient to the effect of the study periods. There was also some indication of increasing runoff in maritime western areas of England and Wales. These increases in maritime areas are likely to reflect the dominance of westerly airflows in the recent past, associated with an increase in the North Atlantic Oscillation (NAO) Index. For low flows, there were no compelling trends - significant positive trends over the 1973-2002 period are influenced by a sequence of notably dry years at the start of the period and were not observed over a 40-year time-frame. There are some indications of a tendency towards decreasing low flows in some eastern catchments, but this is supported by few significant results. The results of this study provide a baseline against which to assess longer-term change from historical flow time series and to monitor future change in the benchmark network. Copyright (C) 2006 Royal Meteorological Society.