Have river flow droughts become more severe? A review of the evidence from the UK – a data-rich temperate environment

Abstract. When extreme hydrological events (floods and droughts) occur, there is inevitably speculation that such events are a manifestation of anthropogenic global warming. The UK is generally held as a wet country, but recent drought events in the UK have led to growing concerns around droughts becoming more severe – for sound scientific reasons, given physical reasoning and projections for future. In this extended review, we ask whether such claims are reasonable for hydrological droughts in the UK, using a combination of literature review and extended analysis. The UK has a well-established monitoring programme and a very dense body of research to call on, and hence provides a good international case study for addressing this question. We firstly assess the evidence for changes in the well-gauged post-1960 period, before considering centennial scale changes using published reconstructions. We then seek to provide a synthesis of the state-of-the-art in our understanding of the drivers of change, both climatic and in terms of direct human disturbances to river catchments (e.g. changing patterns of water withdrawals, impoundments, land use changes). These latter impacts confound the identification of climate-driven changes, and yet human impacts are themselves increasingly recognised as potential agents of changing drought regimes. We find little evidence of compelling changes towards worsening drought, apparently at odds with climate projections for the relatively near future and widely-held assumptions of the role of human disturbances in intensifying droughts. Scientifically, this is perhaps unsurprising (given uncertainties in future projections, and the challenge of identifying signals in short, noisy records, and a lack of datasets to quantify human impacts) but it presents challenges to water managers and policymakers. We dissect some of the reasons for this apparent discrepancy and set out recommendations for guiding research and policy alike. While our focus is the UK, we envisage the themes within will resonate with the international community and we consider ways our findings are relevant more broadly, as well as how the UK can learn from the global community.