Drought terminations in the UK driven by high integrated vapour transport

Whilst the hydroclimatic drivers of drought are well-established, the forces which lead to the termination of drought conditions are less well understood.  An enhanced knowledge of the associations between these phenomena and drought termination is an important prerequisite for more robust operational forecasting of the end of droughts, a phase which assumes critical importance during protracted droughts which can span multiple years and substantially deplete water resources.

The influence of high integrated vapour transport (IVT) on drought termination has been established in the western USA, and atmospheric rivers have been linked to the ending of droughts in the mid-latitudes including North America and Asia.  Whilst high IVT has been demonstrated to be influential in major flood events in western Europe, a potential link with drought termination has not previously been identified.

This study systematically identifies drought termination events in river flow reconstructions for 302 catchments in the UK spanning the 1900-2010 period, and assesses the correspondence with high IVT values extracted from the ERA-20C reanalysis dataset spanning the same period.  Event coincidence analysis is used to quantify this association, with the Precursor Coincidence Rate (PCR) assessing the likelihood of high IVT preceding drought termination, and the Trigger Coincidence Rate (TCR) considering how often high IVT leads to drought termination.

PCRs were moderate to high across most of the UK, indicating that in most catchments a majority of drought terminations are triggered by high IVT.  TCRs were highest in the west of the UK, suggesting that in these regions a majority of high IVT episodes during droughts lead to its termination.  The combination of prevailing direction of landfalling high IVT with upland, wet and responsive catchments in the west (and vice versa) was supported by regression analysis.  In addition to determining occurrence, high IVT was also found to be influential on the characteristics of drought termination.

Metrics of PCR and TCR have the potential to inform management decisions in drought-impacted catchments, quantifying the likelihood of termination in instances of forecast high IVT events.  The importance of establishing associations between high IVT and drought termination is underlined by the higher confidence in IVT forecasts than direct rainfall forecasts over certain medium-range lead times.