Runoff patterns during 2005 were characterised by wide spatial contrasts associated with the exaggeration in the normal north-west to south-east rainfall gradient across the country and the influence of catchment geology on the river flow response to the developing rainfall deficiencies. Their combined impact on annual runoff totals is shown in Figure 1. Regional, and more local, differences in accumulated runoff over the year were exceptionally large. Whilst in northern Scotland, annual runoff totals were generally well above average, most catchments in southern Britain registered totals below 75%. Deficiencies were most notable in the South East where many rivers remained below the monthly average throughout the year. Annual runoff totals fell below 50% in much of Sussex and Kent: the Sussex Ouse and Medway (Kent) were among a number of rivers eclipsing previous annual runoff minima (in records of around 45 years). Runoff deficiencies for the November 2004-December 2005 period were even more notable; new 14-month minima were recorded for a substantial minority of rivers across southern England - from Cornwall to Kent (see Figure 2).
Figure 3 shows 2001-2005 hydrographs representing the total outflows from Great Britain, England and Wales, Scotland, and Northern Ireland - the hydrographs are based on flows for a network of large rivers which, taken together, provide a convincing guide to runoff patterns at the national scale. The hydrograph for Northern Ireland reflects, in part, the controlled flow releases from Lough Neagh into the Lower Bann - these constitute more than a third of the total outflows from Northern Ireland. The daily outflows are shown as a bold trace and a red infill is used to emphasise periods of below average flow; the use of a logarithmic scale also gives greater prominence to low flow episodes. The long term daily maximum and minimum flows are also shown - represented by the blue and pink envelopes. In 2005, at the national scale, notably high outflows were confined to early January and the late autumn with seasonally low flows characterising the June-September period, and December. The rarity of high outflows is a notable feature of the 2005 hydrograph for England & Wales which registered its 4th lowest annual runoff in the last 30 years.
A spatially more detailed breakdown of flow patterns in 2005 is provided by Figure 4 which shows annual hydrographs for 20 index rivers across the UK. The long term daily maximum and minimum flow envelopes are also shown. The very limited incursions of the 2005 hydrograph into these envelopes confirms the rarity of exceptional daily flows throughout the year. Examples of outstanding high flows are largely restricted to parts of north western Britain ( the Lake District especially) early in January and in south western Britain (Wales especially) in early November. Notably low late summer/early autumn flows also characterised many spring-fed southern rivers but the hydrograph for the River Mole serves to illustrate how, in responsive catchments, intense summer storms provided valuable surface runoff to alleviate the protracted recessions.
In rivers draining permeable catchments, the summer storms had far less impact and the very meagre replenishment to groundwater over the 2004/05 winter resulted in monthly flows for many rivers spring-fed rivers remaining below average throughout 2005. The River Itchen is included in this category and its September runoff total was the lowest in a 48-year record (see Figure 5). In Sussex, the ephemeral River Lavant flowed for only 7 days in 2005 (it remained dry throughout 1973 and 1989) and high level spring failures led to a progressive contraction in the stream network, and a considerable loss of aquatic habitat. Aside from a few rivers draining permeable catchments, daily minimum flows in 2005 were not exceptionally low, generally exceeding the minima recorded in a number of recent drought episodes (notable 1976 and 1995).
Flow duration curves provide a means of comparing the regime for a particular year with that for the previous record. They allow the proportion of time that river flows are above, or below, any given threshold to be identified. The 2005 duration curve for Scotland exhibits little systematic departure from the 1961-2004 curve (Figure 6). For both England & Wales and Northern Ireland however flows for each percentile below the 98th are appreciably below the corresponding period-of-record figure. A substantially greater measure of disparity is evident for a number of individual rivers (see Figure 7). The lack of spates together with a reasonably healthy baseflow contribution through the summer explains the exceptionally stable regime for the River Stringside in Norfolk. In Hertfordshire, where drought conditions were much more severe, the impact on the 2005 flow regime for the River Mimram is very much more evident.
The north-south disparity in runoff rates was particularly marked in January. While flows were below normal in the south, sustained heavy rainfall in Scotland and northern England on already wet catchments caused river levels to rise rapidly with widespread floodplain inundation. In Scotland, more than 70 Severe Flood Warnings were issued. The River Teith reached its highest level in 49 years; Loch Lomond its 2nd highest in a 27-year series and the Tay (at Caputh) recorded its 4th highest flow since 1948. Severe flooding in Carlisle on the 8th January necessitated the evacuation of several thousand people as the River Eden exceeded its previous highest flow in a 38-year record. Notable flood events also occurred on the South Tyne, in the Conway valley, and in Northern Ireland where the flooding was more localised. Flows then declined steeply into mid February. Meanwhile, in southern England, the winter drought intensified and accumulated runoff deficiencies continued to grow. February runoff for the Thames was the 3rd lowest since 1944; in Sussex, the Ouse registered its lowest February runoff in at least 40 years. In Northern Ireland the accumulated runoff since November 2004 for the River Annacloy ranks 2nd lowest in a 25-yearr record. February flows in many spring-fed streams (e.g. the Lambourn) were more typical of the late summer.
River flows remained depressed in March over wide areas but were interrupted by spates in mid month, most notably in northern Britain. Accumulated runoff deficiencies increased across much of southern Britain and March outflows from Northern Ireland were very moderate. The River Mimram is typical of many spring- fed southern catchments where the lack of a normal winter recovery is shown by the exceptional stability in flow rates through the winter and spring (as aquifer recharge struggled to match outflows). April flow patterns across most of northern Britain and Northern Ireland were typical for the time of year with significant spates in a few catchments (e.g. the River Leven, Cleveland and Vyrnwy in north Wales). In the south, flow recession continued apart from short spates arising from thunderstorms. Several rivers draining to the English Channel - including the Exe and Wallington (Hants), both with records of around 50 years, reported their second lowest November-April runoff. For the River Ouse (Sussex) and the Mole the accumulated runoff was the lowest on record, eclipsing the exceptional 1975/76 minima. A dry May led to a further intensification of the drought in southern Britain but in Scotland runoff over the November-April period was considerably above average in most catchments; the Spey registered its 3rd highest runoff in this timeframe in a 53-year record.
June saw high flows across much northern Britain, with thunderstorms causing remarkable runoff rates in parts of North Yorkshire - monumental flows were reported for the River Rye and the Cod Beck; at Boltby Reservoir, flows over the dam spillway were close to design capacity. Some localised urban flooding also occurred in the North East (e.g. in Newcastle). By contrast, across most of southern, eastern and central England, drought severity increased. Flow recessions continued into July triggering the activation of additional low flow augmentation schemes (e.g. in Dorset). Despite some flow recoveries over the final week, recessions were especially steep in Scotland where the rivers Luss and Clyde registered new late-July minimum flows. In August, convective activity led to further flash flooding from Tyneside to Dorset although these had little impact on runoff totals at the basin scale.
Newsworthy urban flooding (e.g. in parts of west London on the 9/10th) was a regular feature during September and a few fluvial spates were reported (e.g. in Yorkshire and western Scotland). Generally however, recessions continued and were particularly protracted in the most drought-affected areas. Away from western Scotland, almost all index rivers registered well below average runoff. Monthly runoff totals continued to decline; the Test and Itchen both reported their lowest September flows in records of 48 years. The warm conditions encouraged algal blooms (e.g. on the Itchen and Medway) and fish rescues were triggered by the shrinkage of ponds and wetlands. In Northern Ireland, outflows from Lough Neagh were the lowest in a 25-year series.
October saw rapid recoveries in most responsive catchments with significant flooding in many areas. On the 11th, the Teviot (at Hawick), exceeded its previous maximum flow (established in January 2005) in a 42-year record; around 200 properties were flooded. In Wales, Haverfordwest was flooded as the Western Cleddau recorded its highest level in 40-year record. More localised flooding was also common as rainfall intensities exceeded urban drainage capacities (e.g. in Carlisle) or soil infiltration capacities (surface runoff inundated vulnerable settlements near St Austell in Cornwall). Traffic disruption was severe and Flood Warnings were widespread, continuing into November. Some notable spates also occurred in impermeable lowland catchments (e.g. on the Wey in Surrey). However, depressed flow rates continued in many spring-fed rivers; following 29 consecutive months with below average monthly flows, the Lambourn closely approached its early October minima (in a record from 1962).
Further notable spates occurred in western and northern catchments in November with locally severe flooding and landslides in Wales. On the Severn, demountable flood barriers were employed at Bewdley and Worcester. More generally, storm debris and leaf-fall contributed to local drainage problems. From mid-month, recessions were particularly steep, aided by frozen upland catchments, and by month end, flows were approaching early winter minima (e.g. in the Forth) in many responsive catchments. Such impermeable catchments aside, November runoff totals were mostly within the normal range.
December began with very healthy flows in most rivers draining impermeable catchments and flood alerts were common. Steep river flow recessions then became established and daily flows were approaching late-December minima by the final week in many responsive catchments (from the Aberdeenshire Dee to the Mole in Surrey). Minor runoff recoveries were a feature of the final few days (snowmelt was a contributory factor in many areas) but December runoff totals were depressed over wide areas.
Click here for a more detailed commentary on runoff patterns in individual months of 2005.