Historic hydrological droughts 1891-2015: systematic characterisation for a diverse set of catchments across the UK

Hydrological droughts occur in all climate zones and can have severe impacts on society and the environment. Understanding historical drought occurrence and quantifying severity is crucial for underpinning drought risk assessments and developing drought management plans. However, hydrometric records are often short and capture only a limited range of variability. The UK is no exception: numerous severe droughts over the past 50 years have been well captured by observations from a dense hydrometric network. However, a lack of long-term observations means that our understanding of drought events in the early 20th century and late 19th century is limited. Here we take advantage of new reconstructed flow series for 1891 to 2015 to identify and characterise historic hydrological droughts for 108 near-natural catchments across the UK using the standardised stream-flow index (SSI). The identified events are ranked according to four event characteristics (duration, accumulated deficit, mean deficit and maximum intensity), and their severity is reviewed in the context of events of the recent past (i.e. the last 50 years). This study represents the first national-scale assessment and ranking of hydrological droughts. Whilst known major drought events were identified, we also shed light on events which were regionally important, such as those in 1921 and 1984 (which were important in the southeast and north-west of the UK, respectively). Events which have been poorly documented, such as those of the 1940s in the post-war years or the early 1970s (prior to the landmark 1975-1976 event), were found to be important in terms of their spatial coverage and severity. This improved knowledge of historic events can support improved long-term water re-source planning approaches. Given the universal importance of historical drought appraisal, our systematic approach to historical drought assessment provides a methodology that could be applied in other settings internationally.