Apart from a few catchments, drained mostly by spring-fed streams in southern England, mean river flows in 2004 were unexceptional but runoff patterns through the year showed wide departures from normal. River flows were seasonally very low in the late winter, early summer and during much of November and December. By contrast, flows were seasonally very high in many impermeable catchments during the late summer, helping to ensure that annual minima were relatively healthy. Widespread floodplain inundations were relatively uncommon in 2004, particularly by comparison with the preceding five years. However, the heavy August rainfall, much of it convective, produced many flash floods which were especially disruptive in urban areas; transport disruption was very substantial. Storm runoff and sewage overflows also depleted oxygen levels in many rivers; in London, a large fish kill occurred on the Thames Tideway on the 3/4th. Many new maximum August flows were registered and some new period-of-record maxima were established in northern England and eastern Scotland. The exceptional runoff rates achieved an extreme expression in mid-month. On the 16th in north Cornwall when a rainfall total of 200mm was recorded in just over four hours at Otterham - generating remarkable flow rates in a number of steep streams which drain to the coast. At Boscastle, below the confluence of the Jordan and Valency rivers the peak flow was estimated at around 180 m3s-12; a major rescue operation was needed to prevent fatalities. Two days later, a similar peak runoff rate was estimated for Glen Ogle in west Perthshire (120 m3s-1 from a catchment of 11.4 km2).
In water resources terms the most significant features of runoff patterns in 2004 were the unusually high runoff, and corresponding reservoir replenishment, over the August- October period, and the ensuing sustained river flow recessions. These resulted in substantial accumulated runoff deficiencies over the final two months of the year, heralding drought conditions in 2005. Annual catchment runoff totals for 2004 were mostly within the normal range (see Figure 1) but with a clear tendency for above average runoff across much of north-western Scotland - the Ewe and Carron both registered their third highest annual runoff on record - and below average runoff in southern England, where many rivers registered less than 75% of the annual average. Runoff was particularly meagre in parts of Sussex, Kent and Devon.
Figure 2 shows 1999-2004 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 pink and blue envelopes. Generally outflows were abundant in January, with significant flooding around the turn of the month. Outflows then declined steeply and were notably low across much of the UK in the early spring, and again in the early summer but recovered dramatically in August when many rivers draining impermeable catchments registered new maximum monthly runoff totals (see Figure 3). Outflows were again exceptionally high in October when, with most catchments saturated by the third week, the risk of flooding was high. In the event, a dramatic decline in the frequency of Atlantic frontal systems contributed to November and December runoff totals which were substantially below average across most of the country. Figure 4 confirms that the December runoff totals were substantially below average across large parts of eastern and southern Britain; the Kenwyn (Cornwall) recorded its second lowest December runoff in a 38-yr series.
A spatially more detailed breakdown of flow patterns in 2004 is provided by Figure 5 which shows 2004 hydrographs for 20 index rivers across the UK. Whilst flow variability was considerable, significant extensions to the low and high flow envelopes were relatively rare. The Ness and Tay (in Scotland), Annacloy (Northern Ireland) and Lune (England) are among a number of rivers where monthly minima were approached or eclipsed in June. In relation to the typical seasonal pattern, flows in August were the most exceptional. The Tay and Tweed (not featured) were among a number of Scottish rivers reporting their highest August daily flow on record; as notably, the August daily maxima were the highest flows in the year on these rivers - an unprecedented circumstance in records of around 50 years.
The seasonally high late summer runoff was a primary factor in ensuring that, in in most areas, annual minima flows for 2004 were relatively healthy and well above drought minima. Pre-2004 daily minimum flows were eclipsed only in a few isolated cases, mostly urban rivers in the early autumn (e.g. in south London). However, in southern England especially the August spates failed to herald any sustained recovery of runoff rates through the autumn. This was especially evident in spring-fed streams and rivers (e.g. the Lambourn and Itchen) where flows remained remarkably stable over the latter half of the year. This stability represents a transformation from very healthy late summer flows to well below average flows approaching year-end.
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 2004 duration curves for Britain, England and Wales, Scotland and Northern Ireland are similar to those for the preceding record (Figure 6). A greater measure of disparity is evident for a number of individual rivers (see Figure 7). In northern Scotland, the 2004 duration curves for the Ewe and Ness both plot above that for the preceding record throughout the full flow range. The most revealing departures are found in spring-fed rivers in the English Lowlands where the relatively high late summer flows, and the failure of the seasonal recovery late in year, made for a narrow flow range throughout 2004. As a consequence, the flow duration curves for some rivers draining permeable catchments were remarkably flat (see, for example, the Mimram). For many more in southern Britain the absence of sustained high flows during 2004 is confirmed by the degree to which the 5%ile flow fell below the corresponding long term average - see the Thames, for example.
Most rivers experienced a wide range of flows in January but the overall runoff pattern was in marked contrast to the depressed flows which characterised most catchments through the autumn of 2003. By the second week of January, the focus of hydrological concern had switched decisively to the risk of flooding. Flood Alerts and Warnings were common in January and snowmelt at month end, together with heavy rainfall, triggered notable spates in many catchments. The peak flow on the Alconbury Brook in East Anglia was the highest in a 42-year record and the Dee (North Wales) closely approached its highest January flow on record - heralding severe flooding in early February. In north west Scotland, the River Ewe registered its second highest flow in the last 10 years and, generally, the late-January spates boosted runoff totals to well above average in the great majority of index catchments. Flows in some, mostly eastern, spring-fed rivers remained below average but a belated seasonal recovery gathered momentum over the latter half of the month. In many areas the January runoff terminated lengthy sequences of below average monthly flows - extending back to February 2003 on the Thames, and longer in many Scottish catchments. A measure of the contrast with antecedent flow patterns is provided by the Aberdeenshire Dee where above average January flows followed its lowest annual runoff total in a series from 1929.
With catchments saturated and snowmelt an exacerbating factor in northern Britain, February began with a high flood risk over wide areas. Flood Warnings were very common during the first week when extreme runoff rates characterised a number of western rivers. In North Wales, new maximum daily flow were established on the Dee (at New Inn) on the 3rd and on the Alwen (4th); the associated flooding was particularly severe in the Conwy Valley (e.g. at Llanwrst and Trefriw). The Kent (Lake District) was among several rivers in north-west England which exceeded previous maximum recorded flows; a larger number of western rivers (including the Wye and Severn) eclipsed previous February peaks. Thereafter, river flow recessions were exceptionally steep in many impermeable catchments. In some, mostly Scottish, catchments (including the Nith and Forth), period-of-record February minima were approached around month-end. The counterbalancing effect of these contrasting flow episodes resulted in monthly runoff totals that were well within the normal range throughout most of the UK. This was true of most winter (December-February) runoff totals also, but longer term runoff deficiencies remained large in many areas - in eastern Scotland especially. The belated seasonal recovery of flows in groundwater-fed streams in the English Lowlands continued in February, but flows remained well below average across most of southern England.
During the first week of March the continuation of the steep February recessions produced depressed flows in impermeable catchments across northern Britain; the Forth, Tweed, Ribble and Nith were amongst many index rivers eclipsing previous daily minimum flows for mid-March. A sharp recovery in runoff rates then produced significant spates around the 19th when localised flooding was reported in the Scottish Highlands. Steep recessions resumed thereafter and many rivers were again approaching seasonal minima by early April. March runoff totals were well below average (typically, < 70%) at almost all index gauging stations. The Tay reported its second lowest March flow in the last 20 years and, in England, the Trent registered its 3rd lowest March flow in a 47-year record. Many rivers (including the Sussex Ouse) reported their lowest March runoff since 1993. Flows in rivers draining permeable catchments are less responsive to a paucity of early spring rainfall, but seasonal recessions had begun in many Chalk streams by late March, typically from well below average spring peaks. This increased the expectation of low flows by the late summer. Importantly however, for most spring-fed rivers, the March flows were substantially above corresponding values in recent drought years (e.g. 1997, 1992, 1991 and 1976).
River flows in most catchments were seasonally depressed and in brisk recession entering April, but - contrary to the normal seasonal trend - they increased over the month. In a few areas modest floodplain inundations occurred during the latter half of April. Heavy runoff from the North York Moors on the 18/19th triggered Flood Warnings on the Derwent and contributed to notably high runoff in north east England. Localized, mostly urban, flooding was common on the 26/27th (e.g. in Worcester, Sheffield and London) as convective storms overwhelmed local drainage systems. Flow patterns in the Thames typified many rivers with runoff rates increasing from considerably below to appreciably above average through April. Correspondingly, April runoff totals were mostly well within the normal range but with significant variations reflecting both regional rainfall patterns and catchment geology. Local contrasts where particularly evident in the English Lowlands where rivers draining impermeable catchments responded much more quickly to the April rainfall than spring-fed rivers and streams. Runoff over the Nov-April period was appreciably below average across almost all of the UK and longer term accumulations - reflecting the impact of the 2003 drought - remained depressed over wide areas. In the period from February 2003, many rivers (including the Trent, Exe and Medway) registered only a single month with above average flows; in this timeframe, the 15-month accumulations for the Exe and Medway rank as the 2nd and 3rd lowest, respectively, in records of almost 50 years. The late-spring return to more typical flow rates was therefore particularly welcome.
In many regions, the recovery of runoff rates during April continued into May. After the first week however steep recessions became established in most index rivers draining impermeable catchments. By month-end, long term daily minima (for May) were being closely approached in many western and northern rivers, including the Ness, Nith and, in Northern Ireland, the Mourne. In parts of the English Lowlands, localised flooding was reported in response to thunderstorms (e.g. at Haywards Heath on the 19th) but the general pattern was epitomised more effectively by the Thames which mirrored the April flow pattern; flows were appreciably below, the long term daily average by late May. Runoff totals for the month were above average in a few responsive eastern catchments and a larger proportion of spring-fed rivers in the English Lowlands. Elsewhere, runoff totals were mostly well below average - typically in the 50-90% range - and the Lower Bann (which flows from Lough Neagh) registered its second lowest May runoff since 1984. The modest May runoff served to increase long term runoff deficiencies over wide areas. Amongst the rivers which established new 16-month minimum runoff totals (for sequences ending in May) were the Aberdeenshire Dee, Luss Water, Naver, Faughan and Annacloy.
Exceptionally dry soil conditions greatly limited the effectiveness of the early June rainfall and flows in most rivers exhibited steep recessions - a new minimum June flow was recorded for the Ribble (on the 15th) contributed to outflows from Great Britain as a whole falling marginally below previous minima (in a series from 1961) during the second week of June. The depressed runoff resulted in a number of flow augmentation schemes being activated (e.g. in south Wessex). By contrast, some localised - mostly urban - flooding was reported and more notable flow recoveries characterised the final week of the month. The River Whiteadder eclipsed its previous maximum June flow on the 24th having closely approached its minimum in the second week. South of the Moray Firth, a 48-hr rainfall total of 80.4mm (at Torwinny) triggered severe flooding in Elgin and Rothes on the 23/24th - precautionary evacuations were organized. The Isla at Grange reported its 3rd highest level in a 44-year record and the main Inverness-Aberdeen railway was closed. June runoff totals showed very wide regional and more local variations. In northern Scotland, the River Naver registered its highest June runoff in a 27-yr record whilst notably low runoff characterised much of south-western Britain; in South Wales, the River Tawe reported its lowest June runoff since 1975. In the English Lowlands the generality of runoff totals were in the 50-85% range but mostly well above drought minima. Geological control on flows rates were very evident; flows in many Chalk catchments were near-average whilst neighbouring streams draining impermeable catchments reported <35% of the June average.
Significant late-June and early July flow recoveries in rivers draining impermeable catchments were soon reversed as brisk recessions again became established - by month- end these had produced very depressed runoff rates in a few western rivers, including the Clyde, Eden, Ribble and Yscir, the latter registered its lowest July daily flow since the 1984 drought. Most rivers, however, followed a normal summer recession punctuated locally by short-lived spates (often thunderstorm generated) - a few were notable; the Soar reported its highest recorded July peak on the 24th. Natural groundwater support through springs and seepages, though declining, helped ensure that flows in rivers draining permeable catchments in the English Lowlands remained well within the normal range. Monthly runoff totals for a few index gauging stations were notable - the lowest July runoff for 20 years on the Yscir - but the generality of index stations reported typical July runoff totals, albeit mostly below average. Longer term accumulations testified to notable deficiencies, in the west and north especially. The August-July runoff for the Annacloy was the lowest for any 12-month sequence in a series from 1979 and a significant minority of other rivers in Northern Ireland - northern and eastern Scotland also - closely approached previous August-July minima.
August was an exceptional month in runoff terms - the second highest (after 1985) in the 45 year series of Great Britain outflows. Following sustained July recessions, flows in most rivers were considerably below the monthly average at the beginning of August. Widespread thundery activity produced many locally intense runoff events on the 2-4th - triggering severe urban flooding and generating massive transport disruption (e.g. in north-west London on the 3rd). Storm runoff and sewage overflows also depleted river oxygen levels; a large fish kill occurred on the Thames Tideway on the 3/4th. Notable urban flooding continued throughout the month (e.g. in Londonderry, Wycombe, Redruth, Sheffield and London, where the peak on the Wandle on the 18th was close to the period-of-record maxima). Widespread fluvial flooding is rare in August (some parallels can be drawn with 1961 and 1931) but, with rainfall intensities exceeding infiltration capacities and catchment headwaters becoming saturated, flood risk increased rapidly. Bankfull flows were exceeded in many rivers in eastern Scotland and northern England during the second week. The Braan was among a number of rivers in the lower Tay basin recording new maximum flows and, on the 11th, the Tay itself reported its highest August flow (>1000 m3s-1) in a 52-year record; other rivers establishing new August maxima included the Earn, Trent and Ribble. On the 16th, extreme runoff in the Valency and Jordan - estimated at around 180 m3s-1 below the confluence - resulted in severe structural damage at Boscastle on the north Cornwall coast (where there is a history of significant flooding - its location and topography make it particularly vulnerable to high- intensity storms). Many flood warnings (and Flood Watches) were in operation in mid- month; on the 18th the Bedburn Beck (Durham) exceeded its previous maximum flow, in a 45-year record, by a substantial margin. Surface runoff and mudslides were widely reported. Many vehicles were trapped on the A85 near Lochearnhead when a combination of intense rainfall, saturated ground and steep slopes generated several damaging landslides; peak flows in Glen Ogle and Glen Ample were extreme - and had substantial geomorphological impacts. Despite the very modest flows at the beginning of the month, gauging stations which closely approaching, or exceeded, their August runoff maxima showed a wide distribution - from the Midlands to northern Scotland. Runoff totals for the Summer (June-August) were mostly in the normal range but 12-month accumulations remained well below average for many rivers across the UK.
The normal north-west/south-east contrast in runoff rates across the UK was strongly accentuated in September. Heavy rainfall and near-saturated catchments promoted spate conditions in western Scotland whilst limited rainfall, mostly dry catchments and (in most areas) declining baseflow contributions resulted in modest flows in much of eastern and southern England especially in southern catchments. Notable high flows were common in rivers draining the hills of northern Britain on the 19-21st; the Strae (where) eclipsed its previous maximum flow in a 26-year record and the Ness reported its highest September flow in a series from 1973. September runoff totals exceeded the average in most catchments from Wales to northern Scotland where both the Ness and Nevis eclipsed previous September maxima. In the English Lowlands September runoff totals reflected the balance between flows which were initially appreciably above average but ended the month appreciably below, albeit well within the normal range. Recessions were most persistent in the more southerly catchments where runoff totals were commonly less than 70% of the September average. The Otter registered its 2nd lowest September runoff in a series from 1962 whilst, more notably, the Medway (at Chafford) and the Ravensbourne (south London) both established new minima for any month. Accumulated runoff totals in the six-month timeframe broadly reflected the September pattern with April-September totals falling in the lower quartile for a few catchments in central, southern and south-west England (the Lower Bann in Northern Ireland also).
The brisk elimination of soil moisture deficits allowed flow recoveries to gain momentum in most catchments during October. As catchments approached saturation, the threat of flooding increased - this was mitigated somewhat by high windspeeds which ensured a relatively rapid passage of most frontal systems (moderating storm rainfall totals and the magnitude of the resulting spates). Nonetheless, moderate floodplain inundations were common (e.g. on the 4/5th in the west. In the Lake District, the River Ellen reported a new maximum flow (in a series from 1976) on the 4th and the flood threat culminated around the 23rd when more than 50 Flood Warnings were in operation across England and Wales. Around half related to Wales where severe flooding was experienced in the north (e.g. at Beaumaris, Carnarvon and on Anglesey). To the south, the estimated peak on the Tawe ranks 5th highest in a 47-year record and. In coastal areas of southern England the flood risk was exacerbated by a combination of high tides and strong south-westerly winds - a number of tidal defences were overtopped (e.g. at Bournemouth). Away from the English Lowlands, October runoff totals were mostly well above average; a number of index rivers (including the Tweed, Witham, Tawe and Clyde - each with records >45 years) registered their 2nd highest October runoff. By contrast, runoff was appreciably below average in a few southern catchments and seasonal recoveries had yet to begin in some spring-fed rivers in the South-East. The Lambourn reported its 18th successive month with below average flow but, more typically, runoff accumulations over periods of 3-9 months were generally above average.
Contrary to the normal seasonal pattern, flows in most rivers declined substantially through November. Most recessions were interrupted in mid month as spates triggered a few Flood Warnings (e.g. on the Great Ouse) but, thereafter, they continued well into December. Index gauging stations reporting above average monthly runoff totals were mainly confined to the East Midlands and East Anglia. Elsewhere, runoff was below normal, notably so in a number of impermeable catchments - mostly in the West. In Wales, a very steep recession on the Tawe produced its lowest November flow since 1970 and, in Northern Ireland, the Faughan registered its second lowest in the last 22 years. In many spring-fed rivers and streams, November flows were similar to those in late August and a sustained seasonal recovery was still awaited. Autumn (Sept-Nov) runoff totals - boosted by the very healthy October flows - were well above average in most western and northern catchments, the Ness reported its second highest on record, but moderately depressed in parts of southern England (e.g. the Mole). For the year to November, runoff totals showed wide spatial variability - mostly above average but runoff deficiencies exceeded 20% for a number of, mostly southern, catchments including the Otter and Wallington.
Unusually steep recessions characterised the first half of December, resulting in flows in a number of index rivers (including the Taw, Great Stour and Faughan) approaching their mid-December minima. Thereafter, a spatially uneven recovery gained momentum. By year-end, flows were generally within the normal range (in much of northern Britain this recovery heralded exceptionally high flows in early January 2005). The recoveries were sufficient to produce above average December flows in a few, mostly north-western, catchments but, generally, runoff totals were appreciably below average - typically in the 40-80% range across England. Flows were particularly depressed in some sheltered eastern catchments - the Whiteadder and Soar reported their second lowest December runoff in the last 30 years. Runoff deficiencies for the November-December period were also substantial in many responsive catchments across southern England; the healthy autumn runoff being succeeded by sustained below average flows - raising concern regarding the resources outlook for 2005. Flows in many rivers draining permeable catchments remained more than usually stable over the latter half of 2004, confirming the limited groundwater recharge. In contrast to the regional rainfall figures, catchment runoff totals for 2004 as a whole were - northern Scotland apart - mostly below average (notably so for the Medway) reflecting, in large part, the notably high actual evaporation losses - across the English Lowlands especially.