A notably dry June added significantly to accumulated rainfall deficiencies (in the 6/7 month timeframe) across much of the country. Provisionally, the UK registered its 2nd lowest Jan-June rainfall in a series from 1914 with particularly exceptional deficiencies characterising much of northern England and western Scotland. The recent intensification in drought conditions is confirmed by the estimated June outflows from Great Britain: the lowest in a 50-yr series. Correspondingly, last month saw the 2nd largest June decline in overall reservoir stocks for England & Wales since 1989 and, entering July, stocks in a group of index reservoirs in north west England were only marginally above half full (heralding the introduction of a hosepipe ban affecting 6.5 million consumers). Stocks were also depressed in western Scotland (Loch Katrine especially) and in parts of Wales. Fortunately groundwater levels in most major aquifers (which outcrop extensively in eastern, central and southern England) generally remain within the normal range. As a result, flows in many spring-fed rivers are typical of the early summer. By contrast, flows in many rivers draining impermeable catchments are exceptionally low, causing significant environmental and ecological stress. With extremely dry soils across much of the country (which will absorb most of the summer rainfall) this stress is likely to increase - underlining the need to moderate water usage in the drought-affected regions.
June was a warm month with exceptionally hot interludes triggering increased water demand, high evaporation losses, health stress in vulnerable groups and causing problems for farmers and growers. Persistent high pressure again prevented rain-bearing Atlantic frontal systems from crossing the UK throughout most of the month. There were some showery episodes and a few storms (44mm was recorded at Manston in Kent on the 10/11th) but lengthy periods with little appreciable rainfall were much more typical. At Wallingford only 2.6mm was recorded in the four weeks from the 13th June; the 2nd longest sequence needed to reach this modest threshold since April 2003. June rainfall totals exceeded the monthly average in a few, mostly eastern, pockets (e.g. in Kent), but much of the country reported less than 60% of the 1971-2000 average. Provisionally, it was the driest June since 1995 for the UK, reinforcing rainfall deficiencies which began to build in December 2009. For Scotland, the last seven months have been the driest since 1964 and England & Wales reported its 2nd lowest Jan-Jun rainfall since 1956. Some western catchments registered appreciably below average rainfall in each month and the North West region (of England) received around 40% less than the average for the first half of the year; with the exception of 1929, this is substantially below any other Jan-Jun period in the 96-yr regional rainfall series. Exceptional deficiencies also characterise much of Wales and western Scotland where the Clyde basin recorded its 2nd lowest Dec-Jun rainfall in a series beginning in 1914. Longer term deficiencies are moderated by the extreme wetness of November 2009 across much of the UK.
Early June saw a continuation of the spring river flow recessions followed by a modest increase in runoff during the second week and, thence, a reassertion of recessions through into July. The June runoff totals provide a convincing general guide to the spatial variations in the intensity of the hydrological drought. Across much of the English Lowlands June flows were below average but well within the normal range - with flows in many Chalk rivers (e.g. the Itchen and Lambourn) tracking close to the average early summer recession. By contrast, flows in many western and northern rivers (which drain impermeable catchments and respond more quickly to rainfall deficiencies) are very depressed. Index rivers registering new minimum June runoff totals were widely distributed - from the Nevis (western Scotland) to the Cynon (South Wales) and previous daily minimum flows (for June) were eclipsed in a number of major rivers e.g. the Ness, Clyde, Tweed and Lune. Correspondingly, environmental drought stress has become increasingly evident, e.g. loss of habitat as the river network contracts, decreasing dilution available for effluents, and low oxygen levels (high water temperatures being an exacerbating factor) necessitating fish rescues. The deteriorating water resources outlook reflects the exceptional meagre runoff since the early winter. In western Scotland, some index rivers (e.g. the Nevis and Luss) have recorded less than 50% of average runoff since last November (a remarkable deficiency for western Scotland) and new minimum Jan-June runoff accumulations have been reported for a number of rivers in north-west England and north Wales. With soil moisture deficits close to early July maxima in many areas (see page 3), very depressed late-summer and autumn flows are in prospect (given normal summer rainfall patterns).
The generally hot and dry June weather produced notably large increases in soil moisture deficits across much of the country. Month-end smds were the highest on record (in a 50-yr series) for Great Britain as a whole, and exceeded previous maxima in much of northern England. Correspondingly, aquifer recharge was minimal during June and 2010 groundwater level recessions continued. The lack of rainfall since the early winter is clearly reflected in some responsive north-western index boreholes, e.g. Newbridge, where early summer levels only marginally exceeded the June minimum. Well below average levels also characterise most limestone wells (e.g. Ampney Crucis and Alstonfield), and many minor aquifers. However, even in a normal year, summer rainfall has little impact on levels in the major aquifers and current levels throughout most of the Chalk and Permo-Triassic sandstones outcrops (the Magnesian Limestone also) remain well within the normal summer range. The particular value of groundwater during spring-summer rainfall deficiencies is being well demonstrated but the exceptionally dry soils may be expected to delay the seasonal onset of recharge in the autumn and fuel concern about the groundwater resources prospects for 2011 in the event of a dry winter.