After five successive years with above average rainfall in many parts of the country, 2003 registered well below average totals in all regions; for the UK as a whole it was the driest year since 1955. Large rainfall deficiencies developed through the spring and an exceptionally arid late summer/early autumn established severe and widespread drought conditions by late September. Significant water resources stress was experienced but the resilience of the UK, even to outstanding within-year rainfall deficiencies was well demonstrated. This resilience is underpinned by a range of water management strategies and drought response mechanisms but, in 2003, relatively wet conditions early and late in the year were also especially helpful. January was particularly unsettled with notable floods experienced in most of southern Britain. But the abundant runoff served to replenish reservoir stocks and groundwater resources, establishing a buffer against the impact of the subsequent drought conditions. Nonetheless, the wet end to the year was very welcome – helping to eliminate seasonally high soil moisture deficits and initiate a much delayed recovery in river flows and aquifer replenishment rates. After a notable deterioration in the water resources outlook over the nine months to October, surface and groundwater resources recovered smartly (in most areas) leaving overall resources well within the normal range by year end.
Record temperatures in August – a feature across much of Europe – provided a dramatic focus for another warm year. However, October was notably cool and average temperatures for 2003 as a whole were marginally below those of a number of recent years; provisionally, 2003 ranks 7th warmest in the Central England Temperature series which begins in 16591. Correspondingly, potential evaporation demands again exceeded the average across much of the UK. In most of northern and western Britain, this was true of actual evaporation losses also but 2003 totals were considerably below average throughout much of the English Lowlands where transpiration losses were restricted, over lengthy periods, by notably high soil moisture deficits. By the early autumn, exceptionally dry soil conditions were very widespread; concern about the prospective foreshortening of the 2003/04 aquifer recharge season was only allayed by sustained wet conditions from mid-October through into January 2004.
Apart from a few major reservoirs (e.g. Colliford in Devon) most impoundments began 2003 at, or close to, capacity. Drawdown of some large gravity-fed reservoirs was necessary to provide scope for flood alleviation during the very wet conditions in late December 2002 and early January 2003. Figure 1 shows the variation in overall stocks for a representative network of major reservoirs across England and Wales for the 1995-2003 period. The late winter and early spring of 2002/03 saw a brisk decline in stocks but above average May/June rainfall provided a modest, but important, seasonally late boost to reservoir stocks. Thereafter, a steep decline in stocks became established. This continued into the autumn and, by early October, overall stocks were only a little above 50% of capacity – their lowest since the autumn of 1995. Stocks were particularly depressed in southern England and the North-West – stocks were seasonally very low in parts of Scotland and Northern Ireland also. The sustained decline in surface water resources triggered calls for restraint in water use and a number of water conservation and management measures were introduced to conserve resources and help maintain dwindling river flows; however no Drought Orders were needed. The resources outlook remained rather fragile until a steep recovery in reservoir levels gained momentum through November. The increase in overall stocks for England and Wales during December was the greatest for any single month on record (in a 16-year series); this late surge allowed surface water resources to return to normal levels entering 2004.
A guide to the variation in annual runoff for England and Wales, Scotland and Northern Ireland – expressed as percentage departures from the long term mean – is shown on Figure 2. The runoff assessments are based on outflows from representative networks of gauging stations monitoring outflows from major river basins; the estimates for the first few years featured on each plot are less reliable due to the relatively sparse monitoring network at the time. Total outflows from England and Wales in 2003 were around 25% below average, markedly lower than for the previous five years and the 6th lowest runoff in the 43-year series. Runoff across Scotland was even more depressed with estimated total outflows eclipsing 1973 as the lowest on record, and in notable contrast with the above average runoff which has characterised most of the post-1980 period. In Northern Ireland, where a province-wide gauging station network has been in place only since the late 1970s, the 2003 runoff total was also notably low, closely approaching the lowest on record – registered in 2001.
Although catchment runoff totals for 2003 were notably low in many areas, river flows generally exhibited an abnormally large range particularly in southern Britain where the year began with widespread floodplain inundations. A significant number of new maximum daily flows were reported for gauging stations in the South-East and the Thames – in its middle reaches – recorded its highest flow since March 1947. Further spates occurred in February but these heralded very protracted river flow recessions which resulted in new minimum recorded flows for April in many catchments. Flows picked up in most rivers during the late spring and generally remained within the normal range through the early summer. However, the continuation of the seasonal recession well into the autumn resulted in flows approaching long term September/October minima over wide areas. In a few rivers (e.g. in the Tweed basin) new minimum daily mean flows were reported and, more widely, monthly flows amongst the lowest three or four on record (for any month) were recorded. A spatially uneven recovery began in mid October and runoff rates increased rapidly in the early winter. Nonetheless, many rivers across the UK registered 2003 runoff totals in the lowest decile with a significant minority (e.g. in eastern Scotland) establishing new annual minimum runoff totals.
Across most of the major aquifer outcrop areas – which are found principally in eastern and southern England – groundwater resources were exceptionally healthy over the 1998-2002 period; record groundwater levels were registered in early 2001 in many areas. Abundant recharge over the last three months of 2002 resulted in early 2003 groundwater levels also approaching the highest on record – particularly in the Chalk where, briefly, there was a high risk of a repetition of the sustained groundwater flooding experienced two years previously. A very dry late-winter banished these concerns and triggered an early start to the 2003 seasonal recession in groundwater levels. These were generally steep and protracted, but having mostly begun from near seasonal maxima, levels in most index wells and boreholes remained appreciably above average well into the summer. This underlines the value of groundwater in moderating the impact of rainfall deficiencies through the summer half-year but, unusually, the 2003 recessions persisted into the late autumn, and longer in the slowest-responding aquifer units. Consequently, groundwater levels in most index wells and boreholes had declined to well within the normal range by the late autumn and the delayed seasonal onset of recharge raised concerns about the outlook for 2004. Sustained rainfall over the last 10 weeks of the year – more than twice the average over some outcrop areas – allowed recoveries to be initiated and overall groundwater resources were around average entering 2004.
Temperatures and evaporation patterns during 2003 provided an insight into conditions which may become more familiar in a warmer world. The more distinctive partitioning of winter (Nov-April) and summer (May-Oct) rainfall for England and Wales (for 2003-04) is also consistent with a number of favoured climate change scenarios. Figure 3 illustrates winter and summer rainfall and temperature anomalies from England and Wales from 1845 – the plotting positions for the most recent 30-year period appear as red diamonds. 2002-03 adds to a cluster of mild, wet winters and dry, warm summers over the recent past. However, the UK climate exhibits large year-on-year variability and, over longer timespans, wet and dry periods can show considerable persistence. Any short-term climatic tendencies should therefore be treated with caution.
The last 15 years has seen a modest extension in the range of recorded variation in river flows for some seasons – the autumn particularly. This is largely a reflection of the notable flood episodes over the 1998-2003 period (and 1989-1994 across much of Scotland) and the impact of drought conditions in the early and mid-1990s. At the national scale, average runoff for the post-1990 period is considerably above average. This is almost entirely a consequence of enhanced winter half-year river flows. For the late spring and early summer months 1991- 2003 average flows have been closely comparable with the 1961-90 mean but considerably below average for August and September. Given the substantial year-on-year variation in monthly flows, appreciable departures from standard averages are to be expected when comparisons of this type are undertaken.
In water resources and river flow terms the evaporation rates in 2003 were particularly interesting. Unsurprisingly, evaporative demands were amongst the highest on record across most of the country. However, the exceptional dry soil conditions through the late summer and early autumn restricted actual evaporation losses substantially. Consequently, annual water balances for many of the driest catchments were not unusual, emphasising that increasing potential evaporation demands may not necessarily be associated with declining water resources. Certainly, notably dry autumn soils may delay the seasonal recovery in river flows and groundwater levels which, in the event of a dry winter, could jeopardise the water resources outlook. But at the national scale, positive trends in temperature and annual potential evaporation totals over the last 15 years have not been accompanied by any decline in mean annual catchment runoff totals or any reduction in mean groundwater levels.