Trends in seasonal river flow regimes in the UK

A wide range of hydrological trend studies have been published for the UK, but there has not previously been a UK-wide assessment of changes in seasonal river flow regimes in a large number of catchments reflecting the diversity of UK rivers. This represents a gap in research, as climate change impacts are likely to vary regionally and seasonally, and seasonal river flows form the basis of many climate change impact assessments. This study attempts to fill this gap, by analysing trends over the 1969–2008 period in a network of 89 catchments from across the UK. Many UK catchments are heavily disturbed by human influences, so this study primarily focuses on catchments with near-natural flow regimes, to enable climate-driven trends to be distinguished from direct anthropogenic disturbances such as river regulation and abstractions. Trends are characterised for four standard seasons (December–February, March–May, June–August, September–November), for seven flow quantiles. Particular emphasis is placed on examining spatial patterns in observed trend magnitude for median, high and low flows. A set of eight catchments with long records (starting in the 1930s or earlier) are used to assess the representativeness of recent trends in a long-term context, via a moving window trend analysis. The results of this study suggest a much more complex pattern of regional and seasonal variation than revealed in previous work. Some findings resonate with observed rainfall changes, and also with potential future climate change – e.g. increased runoff and high flows in winter and autumn, and decreased flows in spring. The latter is a result which is sensitive to study period, and is not observed in longer records. In summer, there is no compelling evidence for a decrease in overall runoff or low flows, which is contrary to trajectories of most future projections. Overall, the results do not suggest immediate concern for current water resource management on the basis of observed trends alone; however, the differences between observations and model projections suggest these findings should not be viewed complacently, and greater reconciliation between data- and model-based assessments should be sought as a basis for informing water management decisions. The spatial heterogeneity of observed trends (in the lowlands of southeast England especially) suggests caution is needed in extrapolating from small catchments to large regions; understanding this heterogeneity is a major topic for future research.