January 2014


Period: From January 2014      Issued on 13.01.2014 using data to the end of December

Summary

Following a dry start, the last two weeks of December were very wet in many areas, leading to a considerable change in the hydrological situation. The model-derived projections, based on data up to the end of December, are for January river flows and groundwater levels to be normal to above normal in most areas. However, the severe weather has continued into early 2014 and, in parts of southern Britain, the typical average rainfall for January was received in the first week. The upper range of projections is therefore more likely than the median projections, and river flows and groundwater levels are likely to be notably high or exceptional in the south. The rainfall since mid-December has led to widespread soil saturation, leading to a significantly elevated risk of both fluvial and groundwater flooding through January, particularly in southern England. In those areas prone to groundwater flooding, the risk of flooding will remain elevated until the end of the winter.

Rainfall

Latest predictions for UK-precipitation show a slight signal for near or just above average rainfall during January-February-March as a whole. The probability that UK precipitation for January-February-March will fall into the driest of our five categories is around 20% and the probability that it will fall into the wettest category is between 10 and 15% (the 1981-2010 probability for each of these categories is 20%).

River flows

Except in a few catchments in north-east Scotland and eastern England, the average flows for December were normal or above across the UK, and notably high across much of Scotland. Model projections suggest a continuation of near normal to above normal flows in January across Great Britain. However, given the widespread and sustained rainfall in the first week of January in southern Britain, it is likely that average flows for January and February will be notably high in this region, particularly in large, slowly responding catchments. More generally, across much of Great Britain, soils are very wet and river flows are already above normal, so even moderate rainfall amounts may trigger notably high flows – there is therefore an elevated risk of flooding in most regions. In contrast, the far north-east of Scotland and parts of eastern England are likely to see below normal flows persisting. The three-month outlook suggests flows are likely to be in the normal range across Great Britain, but the impact of recent rainfall will increase the likelihood for some time of higher flows in slowly responding catchments, particularly those fed by groundwater (see below).

Groundwater

The December rainfall caused remarkable increases in levels in faster responding boreholes and, as such, groundwater levels were above normal (and in many cases notably high) in southern Britain. In addition, above normal levels persisted in the Permo-Triassic aquifer of the north-west. In contrast, below normal levels were registered in the Chalk in Yorkshire. Generally, model projections are indicative of above normal levels, although exceptional levels are projected for some aquifers units given even average January rainfall (which has been received in the first week across parts of the south). Below normal levels in Yorkshire are likely to persist, given the relative dryness of this area in the recent past. The three-month outlook is for a broadly similar situation, with above normal levels in a majority of aquifers. The increased likelihood of high groundwater levels across the southern Chalk over this timeframe suggests that, in areas prone to groundwater flooding, there will be a continuing flood risk through the rest of the winter.