High-flow and flood trends in a network of undisturbed catchments in the UK

This study presents the results of trend tests applied to high-flow and flood records from a network of catchments in the UK. These rivers have flow regimes which are relatively undisturbed by anthropogenic influences,enabling a characterization of natural, climate-driven trends. Trend tests were applied to indicators of flood magnitude and frequency, and also indicators of high-flow magnitude (10- and 30-day maxima) and duration (prevalence of highflows). Significant positive trends over the 30–40 years leading up to 2003 were identified in all indicators, and these were primarily found in upland, maritime-influenced catchments in northern and western areas of the UK. The results suggest a trend towards more protracted high-flows and more spate conditions in northern and western areas, but trends in flood magnitude were less prevalent. Few compelling trends were found in the English lowlands, where significant high-flow trends were influenced by a sequence of notable flood events through the exceptionally wet winter of 2000/1. The observed trends have parallels with some projections for extreme rainfall changes under climate change scenarios. However, highflow indicators for many northern and western catchments were correlated with the North Atlantic Oscillation (NAO),so recent increases are probably caused by a shift towards a more prevalent positive NAO since the 1960s. Six longer (>55 years) hydrometric records were used to place recent trends in a historical context; there was little evidence for trend in the longer time series, but fluctuations in the records suggest that recent trends may be influenced by multi-decadal variability, which has important consequences for trend detection as the majority of UK gauging station records begin in a relatively quiescent period for high-flows.