January was a dull and notably dry month across almost the entire country; the UK rainfall total is the 3rd lowest since 1964. An especially arid episode, from the second week until well into February served to intensify drought conditions in most areas. Consequently most reservoirs reported meagre storage increases but at month-end overall stocks for England and Wales were still only 2% below average, and well above corresponding stocks in 1997 and 1992. Stocks are, however, seasonally depressed in a number of southern reservoirs - the lowest on record (for January) at Bewl Water. The drought's clear regional dimension is further emphasised by the exceptionally low long term runoff accumulations for many rivers in central and southern England; flows in many spring-fed streams are more typical of the mid-summer. In parts of the South-East especially (e.g. the Chilterns), the continuing lack of any substantial winter recharge has left groundwater levels approaching long term minima. Two successive notably dry winters have made for a fragile - but spatially variable - water resources outlook with the prospect of very depressed summer flows and associated stress on the aquatic environment. The window of opportunity for the necessary groundwater replenishment is becoming narrow and significant drought stress is now almost inevitable. To moderate its extent and magnitude, rainfall in excess of 120% of average (about a 1 in 5 chance) is needed over the next 10-12 weeks.
High pressure dominated synoptic patterns throughout January. A few frontal incursions brought significant rainfall, e.g. on the 10th when Capel Curig registered 62mm. Thereafter, most areas reported remarkedly dry conditions - with rainfall totals of <5mm in parts of central southern England over sequences of 30, or more, days. Correspondingly, January rainfall totals were <40% across much of the country, with totals below 20% in a few areas (e.g. the Isle of Wight). More importantly, the last three months have been dry in all regions. For E&W the Nov-Jan period (on average, the wettest 3-month sequence in the year) was the 2nd driest since 1976 (with some eastern and southern catchments reporting only around 50% of average). Last year, the same period was only marginally wetter and, taken together, they closely match 1962-64 as the driest successive Nov-Jan periods since 1932-34. In many catchments the drought began during November 2004 and rainfall deficiencies over the last 15-months exceed 30% in the most severely affected areas - a zone from east Dorset to northern Kent (and extending into the Thames basin). Substantially drier 15-month periods have recorded for E&W (e.g. in 1975/76 & 1933/34) but a distinguishing feature of the current drought is the disproportionate contribution of the winter months to the overall rainfall deficiency. Correspondingly, the impact on reservoir and aquifer replenishment, and on river flows, has been severe in many areas.
Modest spate conditions characterised many responsive rivers early in January but sustained and steep recessions better typified flow patterns during the month. With baseflows very moderate and field drains running weakly if at all, flows in many rivers (including the Trent, Cree, Luss and Faughan) were approaching seasonal minima by early February. January runoff totals were well below average in almost all index rivers. In northern Scotland, the Naver reported its lowest January runoff on record but depressed runoff totals were most widespread in southern Britain - below 50% of average in many catchments with river flows more typical of mid summer in the worst affected areas. Importantly however, only in a few cases (e.g. the Mimram and Soar) was runoff less than in January 1997. More revealing in terms of drought severity are the accumulated runoff totals since October 2004. Across much of E&W (parts of Northern Ireland also) 15-month runoff accumulations are notably low, and unprecedented in this timeframe for a significant minority of southern rivers (from the Medway to the Kenwyn; also the Soar in the Midlands). For the Sussex Ouse, runoff since the beginning of 2005 matches 1975/76 as the lowest for any 13-month sequence in a 46-year record. Particular concern focuses on spring-fed streams and rivers where flows, in some cases, have been below average for more than 30 months and should, in a typical year, be approaching their maximum flow. In the absence of a sharp, and much belated, seasonal recovery, very depressed flows are likely in the late summer (with a much diminished stream network).
Rainfall to most outcrop areas was only 30-40% of average in January and, with soil moisture deficits yet to be satisfied across much of the English Lowlands, infiltration rates were again very modest (minimal in many eastern areas). As a consequence, the late autumn seasonal recoveries in groundwater levels, evident in many western and northern index boreholes, have faltered - with declines at some sites reporting late in January (e.g. at West Woodyates and Alstonfield). Of particular concern is the continuing failure of any significant seasonal recovery to be initiated in substantial parts of the central and eastern Chalk outcrop. Horizontal groundwater level hydrograph traces for some index boreholes confirm that, thus far, winter recharge only just matches natural outflows from the aquifer; at other index sites (in both the Chalk and Permo-Triassic sandstones) recessions have continued from 2005. Levels in many Chalk boreholes are seasonally depressed - closely approaching the long term minimum at Stonor, in the Chilterns - but overall storage in the Chalk is greater than in early 1997 and early 1992 (both very severe groundwater drought winters). Groundwater levels in most Limestone and Permo- Triassic outcrops are seasonally low but above drought minima; Morris Dancers in the East Midlands is an exception - reporting its 2nd lowest January level on record. As usual, accelerating evaporation rates are expected to curtail lowland recharge during (or before) the late-spring; this underlines the importance of sustained rainfall in the interim.