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

Abstract

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 high-flows). 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, high-flow 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. Copyright © 2007 Royal Meteorological Society