April was a generally mild and unsettled month but, north-western Britain aside, rainfall deficiencies increased in most areas broadening the drought's impact but also reinforcing its regional focus. For England & Wales, rainfall over the 18 months to April is the second lowest since 1975/76 but deficiencies over parts of central and southern England are the greatest since 1932-34. Overall reservoir stocks for E&W are marginally above average and very similar to early May 2005. Aided by drought mitigation measures (e.g. at Bewl Water), reservoir levels are generally healthy apart from a few southern impoundments (e.g. Weir Wood, Colliford). Modest spates in many responsive southern catchments helped raise April runoff totals well above drought minima but flows are very meagre in many spring-fed streams. Correspondingly, groundwater levels are very depressed in parts of the English Lowlands, albeit mostly above corresponding levels recorded during in the protracted droughts of the 1990s. In much of central and southern England, increasing evaporation rates and soil moisture deficits in early May very probably signal the end of the 2005/06 recharge season. Groundwater levels and river flows are set to decline through the summer with the prospect of exceptionally low flows by the autumn.
Westerly airflows predominated during April bringing abundant rainfall to western Scotland but most low pressure systems weakened crossing the UK. Some catchments in Kent and Sussex benefited from seasonally late snow (up to 15cm in places on the 10th) but light showers and drizzle were the primary precipitation types in eastern and southern Britain; the only sustained frontal rainfall in parts of the South being registered on the 30th. Reflecting a recurring pattern in the recent past, north-western Britain was notably wet - parts of the western Highlands reporting twice the average April rainfall. By contrast, much of southern England registered well below average totals with many catchments in the South West registering <50%. Much of north-eastern Britain was also notably dry - some localities in Fife reporting monthly totals of only around 5mm. Rainfall for E&W over the Nov-April period for both 2004/05 and 2005/06 ranks amongst the lowest five in the last 50 years. More significantly, the 2-year winter/spring deficiencies for the Thames catchment are, provisionally, the lowest since 1890-92. Rainfall deficiencies since October 2004 are the equivalent of 4-5 months average rainfall over much of southern and central England but local variations in drought severity are significant; parts of London, east Kent, the Test basin (Hampshire) and south Dorset constitute pockets of particular intensity.
April river flow patterns were characterised by steep recessions in most index rivers across the UK. However for a few of the most drought-affected rivers (including the Medway and Sussex Ouse), April runoff was close to average; a valuable recovery at a critical time. More generally in southern England minor spates in responsive catchments helped maintain flows above drought minima. April runoff totals testify to an exaggeration in the normal NW/SE runoff gradient across the UK. Whilst the River Naver registered its highest April mean flow in a 30-year series, flows in some lowland spring-fed rivers in the South East were among the lowest on record. Nonetheless, April runoff for most index rivers was in the normal range. This contrasts with runoff accumulations over the winter and early spring and, more significantly, over the last 18 months; these confirm the drought's focus on southern Britain. Index rivers establishing new 18- month (ending in April) runoff minima include the Kenwyn, Medway and Test; flows in the latter have been below average for >30 months and the April runoff was the 2nd lowest (after 1976) on record. Flows in many Chalk streams are already well below those of a typical late summer and set to decline further - with a substantial contraction in the stream network, associated habitat loss and an increasing risk of algal blooms and other water quality problems.
April was a further month with below average, and spatially very variable, rainfall across most of the main aquifer outcrop areas. In a few areas, late-season recharge substantially improved the groundwater outlook (e.g. in parts of Sussex and Kent). But generally, April's patchy rainfall produced very limited infiltration in most of the drought affected region where, after two of the least productive winter/spring recharge seasons in the modern era, the 2006 groundwater level recessions will begin from well below normal spring maxima. The health of groundwater resources currently displays large regional and more local variations but groundwater levels are generally less depressed than at the same time in 1997 or 1992. However, previous late spring minima in the Chalk have been eclipsed in parts of the North Downs and the Chilterns. On the Isle of Wight, levels in the Greensand (at Alverstone) are also at their lowest on record for April. Recharge to the limestone aquifers was erratic over the winter but moderate spring recoveries have generally left levels in index boreholes well within the normal range. Outcrops of the Permo-Triassic sandstones present a much less spatially coherent picture but with exceptionally low levels in many Midland index wells. Morris Dancers is at its lowest April level in a 37-year record, Weeford Flats is dry, and levels at Heathlanes are approaching late spring minima. Levels are also depressed in some eastern minor aquifers (e.g.Woburn Sands and Suffolk Crag). The incidence of spring failures and wells drying-up will increase as summer groundwater level recessions develop - these are likely to be relatively gentle as natural base levels are approached.