May was a mild and sunny month with high pressure dominating for extended periods in much of southern Britain. Correspondingly, some southern catchments registered less than half the average May rainfall - contributing to the 3rd driest Dec- May period for England & Wales since 1976. Fortunately, the May rainfall patterns generally favoured the gathering grounds of many upland reservoirs and some seasonally notable increases in stocks were registered (e.g. in the Lake District and North Wales). All index reservoirs in Scotland and Northern Ireland reported above average early June stocks and overall stocks for England & Wales are appreciably above the early summer average. The apparent inconsistency with the scale of the recent rainfall deficiencies owes much to the record reservoir stocks registered in the autumn of 2008. River flow patterns were typical of the late spring but depressed runoff rates characterised some impermeable eastern and southern catchments by the end of May. Seasonally high evaporative demands contributed to above average soil moisture deficits across much of the country by month- end, terminating the 2008/09 aquifer recharge season across the greater part of the UK. Groundwater levels are following typical recession patterns across most major aquifers. Overall water resources status is well within the normal early June range but a hot dry summer could impact particularly on flows in responsive rivers.
High pressure extending from the continent was a persistent feature of synoptic patterns during May - ensuring that most Atlantic frontal systems followed relatively northerly tracks. This pattern was established early in the month: whilst Scotland was wet (Tyndrum, in Perthshire, recording >90mm of rain from the 6-8th), much of the English Lowlands experience breezy and dull conditions but with very little rainfall - some localities (e.g. south Oxon) reporting accumulations of <1mm over the first 13 days of May. A low pressure system brought intense downpours to much of the English Lowlands on the 14/15th but stable conditions then became re-established heralding notably dry and warm conditions around month end. The synoptic patterns are directly reflected in the May rainfall totals. In Scotland, parts of the western Highlands recorded >200% of the monthly average whilst rainfall totals were an order of magnitude lower in a catchments in southern and eastern England. The UK May rainfall was near- average but substantial medium-term rainfall deficiencies characterise large parts of the country; a few areas, mostly in eastern Britain, have recorded seven successive months with below average rainfall. Provisional figures indicate that England & Wales experienced its driest spring (March-May) since 1996 and, over the Nov-May period, rainfall accumulations are the lowest since 1989 in much of north east England, extending into eastern Scotland. In this timeframe the Welsh region reported its 2nd lowest rainfall since 1976. Importantly, particularly in a water resources context, 12-month rainfall accumulations are close to, or above, average in all regions of the country.
May witnessed typical late-spring river flow patterns - seasonal recessions punctuated by spates, particularly around mid-month. At the national scale, the monthly runoff was very close to the May average but spatial variability in runoff rates was large, reflecting contrasts in both rainfall patterns and the geological characteristics of individual catchments. Moderate spates were common; in Northern Ireland, the Mourne eclipsed its previous May maximum on the 5th and, on the 9th, the Ness registered its 3rd highest May peak flow in a 36-year series. Further spates occurred around the 15th mainly in southern catchments, but these were mostly very modest affairs. By month-end flows were approaching late-May minima in a number of responsive eastern rivers (e.g. the Whiteadder). May runoff totals exhibited wide regional, and more local, variability; generalising broadly, well above average totals typified many western catchments whilst depressed totals characterised much of eastern Britain. Flows generally held up well in most groundwater-fed streams and rivers but runoff totals were well below average in many impermeable catchments; the Tone registered its 2nd lowest May runoff since 1976. Estimated outflows for England & Wales were the lowest since 1990 for the spring (Mar-May) period and the focus of moderate hydrological stress is well captured by Jan-May runoff accumulations - runoff being relatively depressed in a broad swathe from Devon to south-east Scotland.
By the end of May soil moisture deficits were considerably above the late spring average across most major aquifer outcrop areas. Correspondingly, infiltration during May was minimal and groundwater level recessions were entrenched by month-end. The differing storage characteristics (both between and within aquifers) are reflected in the May groundwater levels. In most Chalk index wells levels were within the normal late spring range but relatively low in, for example, the fast-responding Chilgrove well. By contrast, the dry spring has yet to significantly impact on the deep and slow-responding Therfield well. Generally, late-spring groundwater levels were below average in the major limestone aquifers and, exceptionally, in the Middle Jurassic Limestone of the Cotswolds, Ampney Crucis reported its lowest May level since 1976. In the Permo-Triassic sandstones, groundwater levels are mostly in the normal range but healthier in the slow-responding outcrops across the Midlands than in the more northern outcrops; in southern Scotland, Newbridge reported its lowest May level (in a relatively short record). Only localised recharge is now likely to occur until the autumn - when the dryness of the soils will be very influential in determining the onset of seasonal recharge.