September was a month of contrasts. A remarkably wet start in many areas of England & Wales gave way to a period of high pressure and comparative stability in the latter half of the month. Early in the month, widespread flooding occurred in England & Wales; parts of Northumbria experienced their most severe floods on record. Thereafter, the settled conditions brought a welcome respite, ameliorating the immediate risk of further flooding. Nevertheless, soils in most regions remain wetter than average for the time of year, and many catchments remain vulnerable to further very wet interludes. The early onset of aquifer recharge, together with notably high catchment runoff accumulations (for 2008 thus far) contribute to a very healthy water resources outlook. Overall reservoir stocks for England & Wales eclipsed the previous maximum for early October by a substantial margin.
The cyclonic conditions responsible for a very wet August persisted into early September. On the 5th a low pressure system brought prolonged heavy rainfall to the South West and Wales before moving northeastwards on the 6th. 24hr rainfall totals in excess of 40mm were widely reported in western areas and in the Midlands, but in Northumbria the rainfall was extraordinary. Morpeth recorded 80mm, its highest 24hr total in a record from 1898 and, over 3 days, Goldscleugh (in the Northumbria National Park) recorded > 250mm. By mid-month, high pressure brought a period of stability and the rest of September was relatively dry in most areas; in Wallingford, less than 6mm of rain was recorded between the 12th and 30th. Nevertheless, September was a wet month overall in England & Wales - notably so in Northumbria, the fourth wettest in a 95-year series. Parts of Scotland were relatively dry throughout the month, continuing a 3 - 5 month spell of below average rainfall in the far north. Elsewhere however, rainfall totals over the last three months were exceptionally high (> 70% above average in the South West and North East) and many areas of the UK continue to register notable accumulations over timeframes up to 10 months. For the Jan - Sept period, the England & Wales rainfall total is the highest on record (in a series from 1914).
Following an exceptionally wet June - Aug period, there was an elevated, and seasonally unusual, flood risk across many areas of the UK during early September. This vulnerability was underlined following the severe rainfall on the 5-6th, when around 100 flood warnings were issued by the Environment Agency. Flooding was reported across a large swathe of Wales, south-west England, the Midlands and the North East, where the most severe flooding was concentrated. Around 1000 properties were affected in the Morpeth area as the Wansbeck registered a new maximum flow in a record from 1963 (estimated return period exceeds 100 years). New maximum flood levels were established in neighbouring catchments - a flood mark from 1948 in the Till catchment was exceeded by over 2ft. Elsewhere, the Yscir, near Brecon, registered a new period-of-record maximum, and new September maxima were recorded for many index rivers across the Midlands and Wales. Reports of geomorphological impacts included channel redefinition (e.g. in the Till catchment) and mudslides (e.g. in the Cotswolds). New September runoff records were established in a significant minority of index rivers; these contributed to the highest September runoff on record in the 48-year England & Wales series. New maximum Jul - Sept runoff totals were registered in the majority of index catchments in the South West, South Wales and the North East. In many rivers draining permeable catchments (e.g. the Coln, Itchen and Hampshire Avon), early autumn flows were notably high for the 2nd year in succession and, more generally, the last two years have seen significant redefinition of summer high flow regimes. In contrast, in northern Scotland, runoff has been below average (notably so in the Carron) over the last few months. Nonetheless, for the year so far, estimated outflows from Great Britain are the highest in a series from 1961.
The early September rainfall, on the tail of an exceptionally wet summer, ensured that soil moisture deficits were largely eliminated in central and western areas of the UK. In contrast, significant deficits persist in the eastern aquifer outcrop areas and most boreholes in the Chalk of East Anglia had levels in the normal range entering October. In the far north and far south west of the Chalk outcrop (where the early September rainfall was more significant), new September maximum levels were registered for Wetwang (E. Yorks) and Ashton Farm (Dorset). In other aquifers, levels were generally well above average, and a particular feature of the month was the range of major aquifers in which new September maxima were established. Seasonally early replenishment to the Permo-Triassic sandstones contributed to record levels in the more northerly outcrops, and in central England, new maxima were recorded in the Magnesian (Brick House Farm), Carboniferous (Alstonfield) and Jurassic (Ampney Crucis) limestones. Such early replenishment implies a lengthy recharge season, contributing to a very favourable water resources outlook, although in the event of a wet autumn vigilant monitoring will be required in some areas susceptible to groundwater flooding.