Developing a large-scale water-balance approach to seasonal forecasting: application to the 2012 drought in Britain

Seasonal hydrological forecasts, or outlooks, can potentially provide water managers with estimates of river flows and water resources for a lead time of several months ahead. An experimental modelling tool for national hydrological outlooks has been developed which combines a hydrological model estimate of subsurface water storage across Britain with a range of seasonal rainfall forecasts to provide estimates of area-wide hydrological conditions up to a few months ahead. The link is made between a deficit in sub-surface water storage and a requirement for additional rainfall over subsequent months to enable sub-surface water storage and river flow to return to mean monthly values. The new scheme is assessed over a recent period which includes the termination of the drought that affected much of Britain in the first few months of 2012. An illustration is provided of its use to obtain return-period estimates of the ‘rainfall required’ to ease drought conditions; these are well in excess of 200 years for several regions of the country, for termination within a month of 1 April 2012, and still exceed 40 years for termination within three months. National maps of sub-surface water storage anomaly show for the first time the current spatial variability of drought severity. They can also be used to provide an indication of how a drought situation might develop in the next few months given a range of possible future rainfall scenarios.