December was a cold and unsettled month with wintry conditions predominating after the first ten days; snowfall was substantial - in eastern regions and on high ground particularly. This Arctic episode, which extended well into January, resulted in extensive and sustained transport disruption with major impacts on many communities. The low frequency of Atlantic frontal systems contributed to December precipitation totals being mostly below average but many rivers were in spate early in the month and flood alerts were both common and widespread. Thereafter recessions became established, heralding particularly low flows around month end. Nonetheless, the abundant late autumn and early winter replenishment ensured that December reservoir stocks were generally very healthy. There was some local drawdown to moderate flood risk (e.g. at Clywedog) but, entering 2010, most major reservoirs were close to capacity; there were a few exceptions in eastern England (stocks in Rutland reservoir were below the early January average). Despite the seasonally late onset of groundwater recharge in 2009, groundwater levels were generally above the December average and notably high in some areas. The water resources outlook for 2010 is therefore very encouraging but the considerable (albeit spatially very variable) snowpack storage implies a moderately enhanced flood risk - particularly in the event of a rapid thaw.
A succession of active frontal systems which dominated weather patterns in the late autumn continued into December but from the 2nd week, with high pressure established to the north of the British Isles, northerly or north-easterly winds brought very cold conditions with accompanying snow episodes across much of the country. On the 18th, accumulations of up to 20cm were reported (e.g. on the North York Moors) as bands of snow moved in from the North Sea. Further significant falls were reported later in the month; in Scotland fresh snow on a consolidated snowpack increased the risk of avalanches (one on Ben Nevis on the 30th caused two fatalities). The limited Atlantic influence on weather patterns is reflected in the December precipitation totals (the high proportion of snowfall implies that these are likely to be underestimates). The UK rainfall total was a little below the December average with meagre totals characterising much of central and western Scotland; Northern Ireland was relatively dry also. Conversely, totals exceeded the average, commonly by appreciable margins, across much of eastern Britain (reaching 200% in a few parts of Kent). Notwithstanding a preponderance of relatively dry months, provisional 2009 rainfall totals are close to, or above, average in all regions. Initial data suggest that the UK registered its 11th wettest year in a series from 1914 - but six of the wetter years cluster into the post-1997 period (within this timespan Scotland has been exceptionally wet).
Most rivers registered a wide flow range in December with high flows early on - more than 100 flood alerts were in operation on the 1st across England & Wales - followed by sustained recessions but with moderate spates in southern Britain around month- end; snowmelt being a contributory factor in most cases. With catchments saturated and many rivers already flowing close to bankfull, modest floodplain inundations were common but the rapid passage of most low pressure systems helped moderate storm rainfall totals and, as a consequence, the risk of severe flooding. Following notably high flows around the 8th, weather patterns became more settled and recessions became widely established - heralding notably low runoff rates particularly where frozen conditions gripped the headwaters. In western Scotland, flows in the Luss fell below previous end-of-year minima; flows were also depressed in parts of Northern Ireland. However, with considerable snowpack storage (10-30cm across many upland areas), milder conditions (accompanied by rain in some areas) triggered further spates around the 30th. December runoff totals were boosted by the lagged impact of the late November rainfall (and increasing baseflows in permeable catchments) but exhibited wide spatial variations. Monthly totals generally exceeded the average, and were among the highest on record in some southern catchments (e.g. the Dorset Frome) and the Yorkshire Derwent. However, limited precipitation combined with frozen catchments resulted in low runoff totals in some regions (e.g. western Scotland and Northern Ireland) - the Carron and Lagan both recorded their 3rd lowest December runoff in series of 31 and 38 years respectively. Nonetheless, 2009 annual runoff totals for the great majority of index rivers also exceeded the long term average; a few including the Nith eclipsed previous annual maxima.
Across the outcrop areas of most major aquifers December precipitation totals were above average and, with soil moisture deficits generally eliminated, heavy groundwater recharge would normally be expected. However, infiltration rates will have been significantly reduced by frozen ground conditions over the latter half of the month. In part due to access problems, the December levels for a few index boreholes are unavailable and, for the others, the reporting dates vary considerably - this can be particularly important when groundwater levels are changing relatively rapidly. Nonetheless, on a countrywide basis groundwater resources are clearly healthy - a reflection of the exceptional late autumn recharge. This, together with some early December infiltration, triggered a 15 metre rise in groundwater levels at Tilshead (in the western Chalk outcrop). Levels across the Chalk are generally within, or above, the normal early winter range but relatively low in some eastern areas where, for some index boreholes (e.g. Washpit Farm), the seasonal recovery is still awaited. Above average levels also characterise most index wells in the Limestone and Permo-Triassic sandstones aquifers with notably high levels at several locations - including Skirwith which benefited from the extreme November rainfall across Cumbria.