Indian Summer conditions characterised the first half of September but, thereafter, weather patterns were much more autumnal. September rainfall totals were considerably below average across most of the country. Correspondingly, reservoirs stocks generally followed a normal seasonal decline - but from elevated late summer levels in southern Britain. Early October stocks in some large Scottish impoundments (e.g. Loch Thom) were significantly below average but mostly very healthy elsewhere. For England & Wales as a whole, stocks were the 4th highest in a 20-year series; reflecting abundant runoff in most gathering grounds over the last 12 months. September river flows were typical of the early autumn except in some spring-fed rivers where they remained close to seasonal maxima. The return to more normal soil moisture conditions in the early autumn saw groundwater level recessions re- established but levels in most index wells were considerably above the September average. The recent dry spell has moderated, but not eliminated, the enhanced risk of groundwater flooding in vulnerable parts of the Chalk outcrop this winter.
The high pressure, which dominated synoptic patterns during the first half of September, receded in mid-month allowing frontal systems to bring significant rainfall to some areas, mostly in the western uplands; Lusa (Skye) reported 46.4mm on the 15th and Capel Curig (N. Wales) 55.6mm on the 24th (a day when several tornadoes were reported in southern Britain, e.g. in Farnborough). Of greater hydrological significance was a notable dry spell which had extended beyond 30 days by mid-September. Some areas (e.g. in the Lower Severn basin) registered no measurable rainfall during this episode, and much of central England reported <5mm. This very settled interlude is reflected in the September rainfall totals. A few isolated localities (e.g. in the northern Pennines) exceeded the monthly average but parts of eastern Scotland registered their 4th driest September in 35 years. Many catchments in the English Lowlands reported less than half the monthly average rainfall - adding to a cluster of dry Septembers in England since the mid-1990s. As in August, particularly modest rainfall was reported for those areas worst afflicted by the summer flooding. After record May-July rainfall, the Severn-Trent Region registered its 2nd lowest Aug-Sept rainfall in 16 years. Nonetheless, rainfall totals - over timespans of 4-12 months remain well above average. The Oct-Sept period was the 2nd wettest since 1929/30 for England & Wales and unprecedented for Scotland in a 94-yr series.
eptember runoff patterns exhibited substantial regional differences and a strong geological influence was evident, with clear contrasts between rivers draining impermeable catchments and those sustained principally by groundwater. In the former, some moderate spates were registered around the 24th (e.g. in northern England) but lengthy recessions were more typical. Entering October, flows in the responsive Annacloy (Northern Ireland) had fallen close to the monthly minimum. By contrast, in some spring- fed rivers (including the Coln and Lambourn) flows - although in recession - established new runoff maxima for September. Such rivers aside, September runoff was generally well within the normal range, albeit low in some sheltered eastern catchments in Scotland and Northern Ireland. The exceptional summer runoff is clearly reflected in runoff accumulations for the May-Sept period. Many new period-of-record maxima were established in a broad zone from the North-East to Devon; previous maxima for the Gt Ouse and Warwickshire Avon were eclipsed by considerable margins in records of >70 years. The widespread extension in the range of recorded summer flows is well illustrated by the River Coln (in the Cotswolds) where flows remained above previous daily maxima for three months until the end of September. Longer term runoff totals remain outstanding in much of Scotland where new water-year (Oct-Sept) runoff maxima were established from the Naver to the Nith.
Soil moisture deficits, which had largely been eliminated by the summer rainfall, were re-established through September and, by early October, were within the normal range across most aquifer outcrop areas. Soil conditions are thus exercising a normal seasonal constraint on aquifer recharge. Correspondingly, infiltration in September was minimal and groundwater levels recessions have become re-established except in the slowest responding aquifer units. Nonetheless, September groundwater levels were generally above average; notably so in many outcrops of the Jurassic Limestone (where September levels were still rising at the confined New Red Lion well), and in the western and northern extremities of the Chalk. In the latter, despite recent recessions, new September maximum levels were established at Aylesby and Rockley (in a series from 1933). Levels were also seasonally very high at Wash Pit Farm in Norfolk. Levels are still increasing in many of the slow responding Permo-Triassic outcrops in the Midlands; a particularly brisk recovery has been registered at Nuttalls Farm since the early summer. Spatial variations in soil moisture conditions are considerable but, in most outcrop areas, deficits are the equivalent of 6-10 weeks autumn rainfall. It is likely therefore that the winter recharge season will commence with groundwater levels above the seasonal norm and, in the event of a wet winter, maximum groundwater levels could be notably high, particularly in parts of the Chalk.