May began dry and warm in most areas but a very moist westerly airflow predominated thereafter. Correspondingly, England and Wales registered its wettest May for 27 years with most regional totals exceeding 150% of the monthly average. Despite increasing evaporation losses, reservoir stocks generally increased, reaching around 4% above average for E&W as a whole by early June, with typical early summer stocks characterising most of the drought-affected region. The wet May was agriculturally welcome but notable storm events triggered a number of flood alerts and reversed, albeit briefly, the seasonal decline in river flows. May runoff totals were spatially very variable but mostly within the normal range. Exceptions included some groundwater-fed streams where the May runoff was close to period of record minima. Modest, but very useful, late spring aquifer recharge has reduced the spatial extent of the groundwater drought - now largely confined to a broad zone from the Midlands to the South East with especially depressed groundwater levels in the Chilterns and parts of the North Downs. A particularly arid episode, extending well into June, with an associated increase in water demand, has underlined the fragility of water resources in such areas. In a groundwater context particularly, the possibility of a third successive dry winter - a rare occurrence in the last 100 years but more frequent in the 19th century - has become an influential factor in the implementation of drought mitigation measures.
Apart from the beginning and end of the month, cyclonic conditions dominated synoptic patterns in May. Damaging thunderstorms in north-western Britain (e.g. Glasgow) on the 4th and sustained frontal rainfall across southern Britain on the 6/7th heralded a notably wet three-week period with measurable rainfall recorded on all but a few days. The resulting media refrain of 'the wettest drought on record' failed to acknowledge that such circumstances are not rare in the English Lowlands where groundwater resources status is a crucial factor; there are many more spring/summer episodes more deserving of such an accolade. Dry conditions at month end extended beyond a fortnight - with some localities in central southern England reporting <1mm of rain. May rainfall totals were below average in some catchments in eastern Scotland but most of the drought-affected region registered > 150%; some areas were especially wet (e.g. the Cotswolds, south London and coastal areas of eastern England; the Isle of Man also). The wet May contributed to the third wettest spring for E&W in the last 20 years but for parts of the drought-affected region (including the Thames basin), May was the only the second month with above average rainfall since October 2004. Correspondingly long term rainfall deficiencies remain high in much of eastern and southern England. Rainfall over the Nov. 2004 - May 2006 period is >25% below average for some (restricted) areas of southern England and for the Thames basin the 19-month total is the 2nd lowest (after 1995-97) since 1944.
In most responsive catchments river flows exhibited an exceptional range in May. Flows in the Ribble (Lancashire) closely approached early May minima but recovered dramatically with high spate conditions in the third week. In York, the Ouse exceeded bankfull on the 23rd and Flood Watches were common in the latter half of the month - by which time flows in many field drains had recommenced (e.g. in Essex). Flow recoveries were particularly valuable in the English Lowlands, allowing continuing abstractions to supplement pumped-storage reservoir stocks. The Yorkshire Derwent, Exe, Dart and Nevis were among a significant proportion of index rivers registering new maximum daily flows for May and notably high monthly runoff totals were common - in Wales and northern England especially. In Northern Ireland the River Camowen registered its highest May runoff in a 35-yr record. Flows in rivers draining permeable catchments (in the Chalk particularly) showed a much more muted response to the May rainfall but the modest recoveries were often sufficient to register the highest daily flows of 2006 thus far (e.g. on the Lambourn and Itchen). Nonetheless, May runoff totals were notably low in a number of spring-fed rivers and accumulated runoff totals (for the duration of the drought) remain very low across much of central and southern England.
Across most major aquifer outcrop areas, May rainfall totals exceeded the average by 50-80% but spatial variations in soil moisture conditions, land use and evaporative demands produced a very spatially heterogeneous response in terms of recharge. Not all of the May infiltration will yet have reached the more depressed water-tables but it triggered appreciable groundwater level increases in some responsive limestone aquifers - particularly in the Oolitic Limestone of the Cotswolds (see Ampney Crucis ), and useful seasonally late upturns some minor aquifers (e.g. the Upper Greensand on the Isle of Wight and the Essex Gravels). The wet spring has been beneficial across much of the Chalk also - particularly the more southerly and westerly outcrops - where levels are currently well above early summer drought minima (e.g. those of 1997, 1992 and 1976). Accordingly, there has been an appreciable reduction in the area afflicted by severe groundwater drought conditions - this now embraces some Permo-Triassic outcrops in the Midlands (see Morris Dancers) and, especially, the Chalk of the Chilterns and parts of the North Downs (extending into Kent). At Stonor (in the Chilterns), the May groundwater level was the 2nd lowest (after 1997) in a 46-year record whilst Well House Inn (North Downs) reported its 2nd lowest (after 1944) in a 65-yr record. In such areas summer recessions are expected to approach natural base levels with a continuing failure of lower level springs and an associated further contraction in the stream network.