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

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 to be 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 the 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 the basis for 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 influences are themselves being 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 with widely held assumptions regarding the role of human disturbances in intensifying droughts. Scientifically, this is perhaps unsurprising (given uncertainties in future projections; 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 that the themes within this study will resonate with the international community, and we conclude with ways in which our findings are relevant more broadly, as well as how the UK can learn from the global community.