2003 was notable for the range of flows experienced in many rivers and the sustained low flows caused by the drought which affected all parts of the UK. Relative to the seasonal average, the low flows were most exceptional in the late spring and early autumn, resulting in a modest extension in the range of recorded variability for those periods in many areas – parts of Scotland particularly. New minimum accumulated runoff totals over timespans of 8-10 months were also established for many gauging stations and a significant number of existing minimum annual runoff totals were eclipsed – particularly in northern Britain. By contrast, many rivers across southern Britain registered exceptionally high flows in January with widespread flooding; existing record January runoff totals were superseded in a number of rivers sustained primarily by groundwater. Thereafter, major floodplain inundations were rare but localised, mostly urban, events were relatively frequent occurrences – often causing severe, if short-lived, transport disruption. In some responsive catchments in the English Lowlands localised flooding was common during November whilst flow recoveries were still awaited in other lowland rivers (the co-existence of ongoing drought conditions and significant flood risk is not especially rare during the late autumn).
Annual minimum river flows in recent years have generally been considerably greater than those recorded over the 1989-1997 period. Despite the drought, 2003 followed this pattern across much of the UK; minimum flows were mostly well above those registered in other drought episodes over the last 30 years – this reflected the wetness of the early summer period and, in permeable catchments, groundwater contributions which helped sustain flows throughout most of the drought period. Importantly however, new minimum daily or, more commonly, monthly flows were recorded in a modest number of responsive catchments in parts of northern and western Britain – most notably in eastern Scotland where a new minimum monthly flow (for any month) was established for the Don (in August) in a 35-year flow record. In some cases, the depressed flow rates, combined with high water temperatures, produced considerable ecological stress (e.g. salmon fatalities were reported in the Aberdeenshire Dee during August).
Figure 1 shows 1999-2003 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. To emphasise the periods of low flow during 2003 a red infill is shown between the actual flow and the corresponding long term average; long term daily maximum and minimum flows are represented by the pink and blue envelopes. At the national scale, the hydrographs confirm the seasonally depressed flow rates which accompanied the most intense phases of the drought; extensions to the recorded range of daily outflows occur where the 2003 hydrograph trace overlaps the low flow envelope (see, for instance, the April outflows for Scotland and Northern Ireland).
The depressed flow rates during the drought period are reflected in Figure 2 which illustrates 2003 runoff totals (expressed as a percentage of the period-of-record average) for a network of index gauging stations throughout the UK. In much of southern and eastern England, annual runoff totals were within the normal range, and above average flows characterised a number of rivers draining permeable catchments. By contrast, flows in impermeable catchments, which predominate across northern and western regions of the UK, were notably below average – new annual minima runoff totals were established for a small proportion of index rivers (including the Spey, Yscir, Taw and, in Northern Ireland, the Faughan).
A more detailed regional breakdown of the river flow patterns in 2003 is provided by the hydrographs for 20 index gauging stations featured on Figure 3. The year began with high flows across most of southern Britain but very depressed runoff rates typified a number of rivers draining northern Scotland (see Figure 4). Common features of the 2003 river flow hydrographs are the very steep and protracted spring and autumn recessions together with the relative stability of flows throughout much of the summer half-year. Runoff rates in April and October were especially depressed relative to the monthly averages (see Figures 5 and 6). Regional rainfall patterns and contrasting catchment geology resulted in large spatial variations in the rate of seasonal river flow recoveries during the late autumn and early winter. However, flow rates across almost all of the UK were well within the normal range at the end of 2003.
In common with the preceding three years, river flow patterns in 2003 across the UK departed significantly from those experienced in a typical year. The unusual nature of flow regimes in 2003 is evident from the flow duration curves featured in Figures 7 and 8. These curves allow the proportion of time that river flows are above, or below, any given threshold to be identified – they also provide a means of comparing the regime for a particular year with that for the previous record. The 'national' duration curves (Figure 7) testify to the broad extent and persistence of drought conditions. All three curves depart substantially from the corresponding period of record traces (in blue) throughout almost the entire flow range. An important exception however is found in the lowest flow range where the flow exceeded 98% of the time is close to the long term average. This was beneficial from both a water resources and ecological perspective.
Flow duration curves for a representative network of rivers across the UK (Figure 8) mostly reflect the national picture but they also reveal significant regional departures from the general pattern. In eastern Scotland, the 95% exceedance flow was markedly below the long term average, and close to the lowest on record for some rivers (e.g. the Tweed and Dee). Conversely, rivers in western Scotland maintained relatively healthy low flows. This was also true of many spring-fed streams and rivers throughout the English Lowlands.
River flows displayed wide temporal variations during January which began with remarkably high flows – and extensive flooding – across much of southern Britain as many rivers exercised their natural right of dominion over their floodplains. In a few catchments – including the Thames where over 450 properties were flooded – flows were the highest since the very widespread flooding in March 1947. A number of gauging stations (e.g. in Cambridgeshire and Berkshire) reported flow rates above previous maxima – in records exceeding 30 years in some cases. Flood warnings (peaking at more than 300 on the 2nd) applied across much of the river network in southern Britain. Locally, flood risks were exacerbated by drainage problems and direct runoff from farmland; 'clearwater' flooding (the result of exceptionally high groundwater levels and spring outflows) was also reported, e.g. in the Kennet valley. However, given the antecedent rainfall and the magnitude of the peak flows, flooding of properties was relatively modest. Nonetheless, the onset of anticyclonic conditions provided a very welcome respite, heralding sustained recessions in most rivers. January runoff totals were well above average across the English Lowlands, particularly in East Anglia and the Thames basin; rivers establishing new maximum January flows included the Little Ouse and Kennet. By contrast, frozen upland catchments resulted in very low flows in many western and northern rivers during the first week and January runoff totals were appreciably below average, albeit well within the normal winter range.
Across most of the country the risk of flooding declined as recessions continued through much of February before steep flow recoveries occurred around month-end in many river basins. February runoff totals displayed a clear contrast between the responsive, impermeable catchments in western and northern regions, and baseflow-dominated rivers in the English Lowlands. In the former, February mean flows were well below normal – some rivers in Scotland and Wales registered only around 50% of average runoff – whilst flows in many rivers draining permeable catchments approached seasonal maxima. Relative to the long-term average there was a strong south-east/north-west gradient in runoff for the winter (December-February) as a whole. In western Scotland the River Nevis registered its second lowest winter runoff on record, whilst the Stringside, Little Ouse (in East Anglia) and Upper Kennet (Berkshire) recorded new maximum winter runoff totals. In many more chalk streams, the winter runoff ranks second only to 2000/01.
Substantial storm rainfall totals in late February and early March triggered significant spates in some western catchments. In Northern Ireland, the Annacloy reported its highest March peak flow on the 1st and flood warnings were issued in Wales during the second week. Thereafter, recessions were steep and protracted in many rivers draining impermeable catchments. The Luss and Tawe were among a significant number of index rivers where flows fell below previous late March/early April daily minima, and the Naver (in northern Scotland) registered its lowest March runoff in a 26-year record. In contrast, the continuing benefit of heavy groundwater recharge over the preceding winter kept flows in many spring-fed rivers in the English Lowlands above average. March runoff for the Mimram, for instance, was more than 50% above the monthly mean – the 30th successive month with above average flows; easily the longest such sequence in a 52-year record. Very healthy three- and six-month runoff totals in such rivers contrasted with modest accumulations in neighbouring clay catchments and, more starkly, with many rivers in north and west Scotland. Runoff for the Carron (Highland Region) over the eight months ending in March was considerably below any previous August-March total in a 24-yr record.
The sharp drought conditions during the spring resulted in notably low April river flows in many impermeable catchments whilst flows in rivers benefiting from natural groundwater support remained mostly above average. Such rivers aside, flow recessions had commonly persisted from early February through to the last week of April, albeit punctuated by a number of short-lived spates. In relation to low flows, the worst affected rivers were in northern Britain; gauging stations recording new April runoff minima displayed a wide distribution (including stations on the Ness, Tay, Tweed, and Carron). New daily minimum flows for April were also common – being reported from the Taw to the Nevis. In most western and northern catchments, the protracted recessions were smartly reversed following substantial rainfall around month-end but, due to the seasonally high soil moisture deficits, the rainfall was too late to be hydrologically effective in much of the English Lowlands.
May began with spate conditions characterising many responsive catchments and relatively high late-spring runoff continued throughout much of the month in Scotland and Northern Ireland. At the same time, flows in groundwater-fed rivers across the English Lowlands were also healthy but, by month-end, flows were substantially below average in many impermeable English catchments. The Warwickshire Avon reported its second lowest May daily flow since 1992 and May runoff totals fell below 60% of average for a number of index rivers. By contrast, runoff in many Scottish rivers was well above average – the second highest for May in a 21-year record for the Nevis, terminating a protracted low runoff episode in the Highlands. May runoff totals were high also in Northern Ireland and North Wales – the Welsh Dee reported its second highest May runoff since 1969. With the exception of some groundwater-fed rivers, spring (March-May) runoff totals were significantly below average, notably so in north-eastern England. Over the 12 months to the end of May however, most runoff accumulations were well within the normal range (northern Scotland excepted).
Away from north-west Scotland (where modest spates were common) protracted seasonal recessions continued in most rivers throughout much of June. Fluvial flood risk remained low but thunderstorms generated short-lived and very localised events some of which overwhelmed drainage networks causing significant flooding (e.g. at St Leonards, East Sussex where around 30 properties were flooded on the 2nd) and considerable transport disruption. These minor spates, and continuing baseflow support in permeable catchments, helped to keep river flows above drought minima in the great majority of catchments. In many responsive catchments however, flows were well below the June average by month end – the lowest since 1996 in a number of English catchments – triggering flow augmentation measures in some areas (e.g. in Dorset and Essex). June runoff totals, whilst generally in the normal range, displayed substantial spatial variability. Healthy June runoff totals typified catchments in north-western Britain and parts of Northern Ireland but, in other impermeable catchments, June runoff was generally well below the monthly average and notably low in a few areas. In the West Midlands a new daily minimum flow was recorded on the upper Stour (on the 16th) and in eastern Scotland, the Don (at Haughton) registered its lowest June runoff in a series from 1969. The sustained recessions are reflected in depressed February-June runoff totals: the South Tyne and Spey (at Boat of Garten) reported new minima in this timeframe and totals for many rivers were in the lowest quartile.
The first half of July saw further seasonal recessions in most catchments with relatively depressed flows characterising many rivers draining impermeable catchments late in the second week. Thereafter, spates generated steep but short-lived flow recoveries away from the English Lowlands (and northern Scotland). Notably high late-summer flows were reported for some western and northern catchments; localised urban flooding was also common. July runoff totals were depressed across northern Britain – commonly the lowest for any month since August 1995 – but in Northern Ireland, the Camowen reported its 2nd highest July flow and in the south of the UK most July totals were in the normal range. The unusual persistence of modest flow rates in Scotland was confirmed by the August 2002-July 2003 runoff accumulations. New minimum 12-month totals (for any start month) were established in a few catchments (e.g. the Carron). After an unprecedented sequence of above average monthly runoff totals, flows in many spring-fed streams in eastern England (including the Mimram) had fallen close to the late summer mean.
Some increases in river flow occurred in western catchments towards the end of August but generally the summer recessions continued and, with natural storage greatly depleted in permeable catchments, runoff rates were generally depressed during the second half of the month. Daily flows were commonly the lowest since the 1995/96 drought and new absolute minimum flows were established for a few rivers draining impermeable catchments e.g. the Livet (in the Spey basin) and the Callan in Northern Ireland. Abstraction restrictions and other flow support measures were activated to mitigate low flows but, more generally, the benefit of baseflows to lowland rivers was underlined in many spring-fed rivers where flows remained in the normal range, albeit appreciably below average. August runoff totals were particularly depressed across much of northern Britain – the Deveron registered its 2nd lowest August flow in a 42-year record. Flows were also very low in some impermeable southern catchments, but generally well above drought minima. This was broadly true for the summer (Jun-August) runoff totals also but the Spey, Dee and Whiteadder were among a small minority of rivers establishing new summer minimum runoff totals.
Many recent droughts (including 1995, 1984 and 1976) have seen significant flow recoveries in the early autumn. Given the parched condition of most catchments in September 2003, a repetition seemed very unlikely and notably low flows were expected to become very widespread in the early autumn. Aside from rivers fed by localised storm runoff and a few lowland rivers reliant on groundwater (e.g. the Ver in Bucks), flows were very depressed throughout the month. The Tweed, Whiteadder, South Tyne, Exe and Yscir were among a number of rivers where flows approached, or eclipsed, previous September daily minima. Despite a modest flow recovery over the latter half of the month in northern Britain, monthly runoff totals were also very depressed especially in impermeable catchments. Index gauging stations establishing new September minimum runoff totals showed a very wide distribution – from the Aberdeenshire Dee (at the Park gauging station) to the Exe. Evidence of the dramatic hydrological transformation since the floods of January 2003 was provided by the February-Sept runoff totals. These were mostly in the lowest quartile with new minima common – the Spey and South Tyne closely approached 8-month minima for any start month. The exceptional early autumn soil moisture deficits, together with declining groundwater levels (and the associated drying-up of high level springs) foreshadowed further low flows in baseflow-fed rivers later in the autumn.
With the drought entering its most severe phase, the extended seasonal river flow recessions continued through the first half of October. River flows declined below previous late-October minima in some rivers (e.g. the Tawe, Tay and Luss Water). Short-lived spates at month end provided some relief but October runoff totals were depressed across much of the UK. Rivers draining impermeable western and northern catchments were characterised by exceptional low October mean flows – the lowest since 1972 in much of northern Britain and Northern Ireland. Runoff totals were below 30% of average in many basins and, as in September, rivers registering new monthly runoff minima showed a very wide distribution (from the Dorset Stour to the Nevis in western Scotland) and the Churn (at Cirencester) was effectively dry for the first time in a 24-year record. A longer historical perspective is provided by the Severn where only in 1947 was the October flow lower (in an 83-year record). A measure of the hydrological severity of the drought in northern Britain is provided by the June-October runoff for the Aberdeenshire Dee which, remarkably, was the lowest for any 5-month sequence in a series from 1929. Generally the 5-month runoff totals were close to long term minima in impermeable catchments across much of the country. Flows were less severely depressed in the English Lowlands where some residual benefit of groundwater support could still be recognised.
Despite some moderate spates triggered by late-October rainfall, mid-November daily flows were depressed across most of the UK. Period-of-record minimum flows for November were closely approached in many rivers, and eclipsed in a few (including the Tay, in a 52-year record). Thereafter, recoveries were evident in most catchments – but their magnitude varied greatly. In the South-East, the Mimram peaked at its highest November flow; localised flooding and Flood Watches were common across the English Lowlands. Notable spates were more widespread in Scotland where the Cree registered a new November maximum flow on the 29th. Despite this very welcome upturn, November runoff totals were substantially below average in almost all index catchments. The Teme reported a new minimum runoff for November and some index rivers (including the Tone and Trent) reported their 9th successive month with below average runoff. The drought's hydrological severity (and the associated decline in water resources) is well captured in the runoff accumulations for the Jan-Nov period: the Spey, South Tyne, Exe and Yscir are amongst those river establishing new minimum 10-month runoff totals for any start month. Fortunately, the brisk recovery in flows during the late-autumn signalled that the 2003 drought had entered its terminal phase.
In most regions, river flows continued to recover, albeit erratically, through December and, approaching year-end, flood risk was increasing rapidly in some areas. On the 24th the Lochy recorded its second highest flow in a 26-year record (having recorded new daily minima for March, April and October earlier in the year). Nonetheless, the Naver was the only index river to register an above average December mean flow. Generally, runoff totals were between 50-80% of average – mostly well within the normal range and considerably greater than drought minima (for December). The water resources stress evident over the late summer and autumn was principally a reflection of the modest runoff since January 2003. This is emphasised by the runoff deficiencies for the Feb-Dec period which, for impermeable catchments, commonly exceeded 40%. Index gauging stations establishing new period-of-record minima over this timespan once again showed a wide distribution – including the Aberdeenshire Dee (in a 74-yr record), the Taw and, in Northern Ireland, the Annacloy. The importance of groundwater to river flows, in the English Lowlands especially, is emphasised by the above average runoff over the same period for a number of spring-fed streams (including the Mimram and Itchen). Baseflow contributions had generally declined greatly by early December but, with groundwater levels rising briskly, river flow recoveries were set to continue in early 2004.