2001 was the fourth successive year for which runoff from the UK was above average; but the 1961-90 mean was only marginally exceeded, and regional differences in annual runoff anomalies were substantial. The influence of catchment geology on river flow patterns was particularly evident during 2001. Throughout most western and northern regions, where impermeable catchments predominate, river flows were well below average for the year as a whole. Runoff deficiencies were greatest in parts of Northern Ireland and western Scotland where, for example, the River Luss established a new annual minimum runoff total. By contrast, rivers supported mainly by groundwater - which are concentrated in eastern, central and southern England - registered very high annual average flows, (Figure 1). This was in part a result of the lagged response to the unprecedented rates of aquifer recharge over the October-December 2000 period (but heavy recharge continued into the early spring of 2001). Although river flow recessions were well established by the end of the year, many rivers registered their highest annual runoff on record - examples included the Lud, Itchen, Great Stour and Lee, the latter in a 123-year record. For many of these rivers, the 2000 runoff total ranks second highest and their high flow regimes have been substantially redefined over the recent past.
Daily flow hydrographs for 20 index gauging stations throughout the UK are shown in Figure 2; the 2001 hydrographs are illustrated by the solid trace and the shaded envelopes illustrate the maximum and minimum daily flows over the preceding record. In Scotland, daily flows remained within the normal range throughout the year in most catchments. But late spring flows were significantly below average over wide areas; in some western catchments minimum flows for the year were registered in May or June. The summer recessions were regularly punctuated by significant spates but the highest flow rates in 2001 commonly occurred in October. As in 2000, steep recessions became established in impermeable catchments towards the end of the year - when flows in the Tay, for example, approached its previous end-of-year minima. In Northern Ireland flows were also depressed at year-end; the Annacloy established a new end-of-year minimum daily flow. These seasonally depressed runoff rates contrasted with the widespread spate conditions which characterised the beginning of the year.
Most English lowland rivers were characterised by sustained high flows until well into the spring of 2001. High flows were most persistent in spring-fed rivers many of which recorded remarkably high flows throughout the first six months of the year. Runoff for the River Mimram over the February-April period in 2001was around 40% greater than for any pre- 2000 four-month sequence in a 50-year record. Similarly, flows in the Ewelme Brook, which drains the scarp slope of the Chilterns, exceeded previous maxima by a very wide margin during the late winter and early spring. The singular nature of this runoff episode is confirmed by the flow record for the Wendover Springs (also in the Chilterns) which provides a unique insight into spring flow variability through the mid- and late-nineteenth century; no flows of a similar magnitude to those experienced in the spring of 2001 have been recorded in an (incomplete) flow series which begins in 1841.
The exceptional spring flows in rivers draining permeable catchments were primarily a response to extreme recharge over the preceding autumn and winter, but in many areas also reflect above average recharge through the previous three winters (see 'Groundwater'). In rivers reliant primarily on baseflow, the 2001 recessions began late, and often from unprecedented flow rates. Flows generally remained above average into the autumn but the seasonal recovery - which began in October in many western catchments - stalled late in the month; thereafter flows in many rivers dependant on baseflow remained relatively steady until the turn of the year - this implies a considerable decline in runoff rates relative to the monthly average.
For the second year in succession, river flow patterns across the UK departed substantially from those experienced in a typical year. The unusual nature of the 2001 flow regimes - in southern Britain particularly - is evident from the flow duration curves featured in Figure 3. These curves allow the proportion of time that river flows fall below any given threshold to be identified. Generally, the flow exceeded 95% of the time (a commonly used index of low flows) was greater than the long term average, remarkably so in many spring-fed lowland rivers. In most western and northern rivers the range of flows was appreciably narrower than in a normal year but the most notable duration curves were found in the English Lowlands where, commonly, flows exceeded the long term average throughout the full flow range - confirming the degree to which regimes have been refashioned over the recent past.
January began with flood alerts in many catchments across southern Britain and the risk of flooding remained high as outstanding runoff totals were established for the fourth successive month. In contrast protracted recessions characterised many western and northern rivers; from mid-month particularly, low flows were recorded in frozen catchments in northern Scotland. Frozen conditions produced further seasonally depressed flows during February in northern Britain. To the south however, late-winter flows remained close to, or above, bankfull for much of the month; further flooding occurred across the South-East during the second week. Winter (December 2000-February 2001) runoff totals throughout most of the English Lowlands eclipsed previous maxima - often by wide margins; for example the River Itchen established a new maximum 3-month runoff total (for any start-month) - substantially higher than any pre-2001 accumulations.
The regional runoff patterns of the late winter continued into the early spring of 2001. Notably low runoff totals were reported from parts of the Scottish Highlands and Northern Ireland - where the River Bush recorded its lowest March flow for 28 years. In contrast, the risk of flooding remained high across southern Britain and monthly runoff totals were again exceptional; many southern rivers established new maximum March runoff totals (including the Lymington and Otter with records of around 40 years). Outflows from many Chalk springs were also without recorded precedent.
Winter half-year (October-March) runoff totals for 2000/01 exceeded previous maxima for the majority of gauging stations in southern Britain. The River Thames established a new October-March maximum in a series from 1883 and previous maxima were commonly exceeded by 50% or more for gauging stations with flow records of less than 40 years. Catchments generally remained close to saturation in April but brisk recessions typified most western and northern rivers. Outstanding flow rates continued in many eastern groundwater- fed streams and rivers, and many further runoff records were established. Some south-eastern rivers (e.g. the Mole and Great Stour) reported October-April runoff (Figure 4) equivalent to around twice the annual average runoff - a remarkable circumstance in a UK context.
Dry and warm conditions in May enabled the seasonal river flow recessions to gain momentum but storms in mid-month produced minor floodplain inundations e.g. in the Midlands. Generally, mean flows for May were substantially below those of April across most of the country but some stark regional contrasts were evident by the end of the month - reflecting geological as well as rainfall differences. In most of northern Britain and Northern Ireland flows were relatively depressed at month end; the Tay reported its second lowest May runoff in a 49-year record. However, in southern England the Mimram and Itchen were amongst a substantial number of Chalk rivers establishing new maxima May runoff totals. Many rivers across southern Britain also established new maximum spring (March-May) runoff totals.
River flow recessions continued through June but were commonly interrupted by short-lived spates often associated with thunderstorms - localised urban flooding was common in mid- month in the English Lowlands (e.g. in High Wycombe and Norwich). Healthy flows continued to characterise baseflow dominated southern rivers but late June flows were depressed in some neighbouring impermeable catchments - triggering flow augmentation measures in a few rivers (e.g. the Sussex Ouse). In Scotland and Northern Ireland, mean flows in some rivers were below average for the sixth successive month and a few rivers (including the Luss and Bush) established new minimum January-June runoff totals.
Although the seasonal flow decline was maintained in July, substantial summer spates produced significant local flooding. During the first week exceptional rainfall intensities associated with thunderstorms overwhelmed some urban drainage systems, particularly in Strathclyde and Wales - Llandudno and Bala were badly affected; the peak flow at the New Inn gauging station on the River Dee was the highest in a record from 1971. Local flooding was also common in mid-month in the Midlands and East Anglia - mostly following thunderstorms. Flows in catchments that missed the convective events were substantially below average in most impermeable catchments, e.g. in rivers draining from the Pennines.
This pattern was repeated in August but with a significantly greater frequency of summer spates - generated both by further thunderstorms and sustained frontal rainfall. In Wales, the River Tawe reported its second highest August flow on the 12th and notable spates were common in East Anglia. August runoff totals were mostly well within the normal range but spatial variation was again considerable - flows in the Taw (Devon) were well below average whilst the Mimram - still showing the benefit of the remarkable groundwater replenishment over the winter of 2000/01 - eclipsed the previous August runoff maximum. Summer (June- August) runoff totals were close, or greater than, the previous highest on record for many Chalk rivers.
Thunderstorm-induced runoff events were again common in eastern England during September but in most UK catchments the summer recession stretched into the latter half of the month. Thereafter - and particularly following sustained and widespread frontal rainfall on the 30th - a brisk autumn recovery was initiated in most regions. Although September runoff totals were well below average in most catchments away from the English Lowlands, runoff totals for the water-year (October-September) were the highest on record for most gauging stations in England and Wales - testimony to the extraordinary flows rates maintained throughout the autumn and winter of 2000/01.
Sustained rainfall and the associated decline in soil moisture deficits during October encouraged brisk runoff recoveries across most of the country; these were most noticeable in the north where they were generated from a relatively low base. Spate conditions were common by the second week when the South Tyne recorded its highest October flow since 1967, and more extensive on the 20-22nd when flood warnings were triggered in many catchments. East Anglia was particularly badly affected - the Cam basin especially - and record river levels were reported for several lowland rivers (e.g. the Essex Colne and the Stort and Quin in Herts). At month-end exceptional October flows were recorded in parts of Highland Region and some mudslides (e.g. at Lochcarron) were reported. In a normal year many spring-fed rivers register annual minimum flows during the late autumn but in 2001 autumn flows remained very healthy - more typical of the late winter across southern Britain.
With catchments close to saturation and late October river flows well above average, the risk of flooding was high in early November across large parts of the UK. This vulnerability declined as the month progressed; the very limited rainfall triggering steep recessions in most river basins. With the exception of some rivers draining permeable eastern catchments, November runoff totals were well below average - only around 25% of average for a few southern catchments (e.g. the Kenwyn and Lymington). In Northern Ireland, the Annacloy registered its lowest November runoff since 1983. Autumn (September-November) runoff totals were generally in the normal range but notable longer term runoff deficiencies were developing, in the South-West particularly.
Minor spates occurred across much of the country in early December before the recessions resumed. These were particularly steep in northern Britain where frozen headwaters further restricted tributary inflows. Combined November-December runoff totals were depressed over wide areas (Figure 5) and by year-end flows in some Scottish rivers (e.g. Tay and Luss) approached late-December minima. The continuing absence of any strong seasonal recovery in spring-fed southern streams (e.g. the Lambourn) resulted in the monthly mean flow falling appreciably below average for the first time in around three years.