The Spatial Coherence of European Droughts – UK and European Drought catalogues

Abstract

The project, ‘The Spatial Coherence of UK and European Droughts’, investigates potential relationships in the occurrence, development and extent of droughts in mainland Europe with view to assessing whether UK drought forecasting can be improved. This report describes the methodology developed and applied to generate European and UK drought catalogues from 1901 to 2005. Two complementary drought definitions are considered: hydrological, relating to unusually low streamflows; and meteorological, relating to less-than-usual monthly rainfall totals. Building on previous research, objective drought definition criteria were implemented on 579 streamflow records across Europe and 0.5o gridded rainfall data covering the European land-surface. A Regional Drought Index (RDI) describes the extent and severity of hydrological drought in a given region by calculating the proportion of the region where the observed streamflow is lower than a daily varying low flow (90th percentile) threshold. A Regionalised Standardised Precipitation Index (RSPI) describes the extent and severity of rainfall deficiency in a given region, using a concept similar to that of the RDI. Twenty-three regions showing the same timing of hydrological drought were identified across Europe. These include four distinct zones within the United Kingdom. A two page summary is presented for each identified region which graphically displays the spatial extent, duration, and seasonality of major hydrological droughts for the period 1961-2005 and from 1901-2005, for meteorological droughts. The catalogue reveals a wealth of interesting detail such as the drought-rich period of 1975-76 in Great Britain and the drought-poor period of the 1980s in Eastern Europe. Only the 1975/1976 drought is coherent on a pan-European scale for a persistent period. Some major UK droughts do not appear to have had any equivalent impact in Europe – for example, the summer droughts in 1984 and 1995. In contrast, there are some droughts which manifest themselves over a wide area in continental Europe, but are not expressed in the UK – for example in late 1971/1972. Some long droughts (1962 – 1964; 1995 – 1997; 1988 – 1992) result from a combination of both winter and summer deficiencies