Though considerably less outstanding than the previous year, 2001 was still very noteworthy in hydrological terms. Like its predecessor, 2001 was a warm year but river flow and aquifer recharge patterns were markedly different. The differences reflect a return to a spatially more typical distribution of rainfall during 2001, but combined with a very uneven distribution through the seasons. The early months were exceptionally wet in southern Britain but notable rainfall deficiencies developed in some western parts of the UK over the latter half of the year and, nationally, there was a very dry conclusion to 2001.
Throughout most of southern Britain the exceptionally wet phase that began during the latter half of 1997 culminated over the last four months of 2000 - but sustained and heavy rainfall continued well into the spring of 2001. Existing maximum recorded rainfall accumulations (over a wide range of months) were eclipsed for England and Wales as a whole. A major contributory factor was the unusually southerly tracks followed by many Atlantic frontal systems - producing outstanding rainfall totals over most of southern Britain whilst, concurrently, modest rainfall deficiencies developed in parts of Scotland. For many English catchments rainfall totals over periods of three to eight months (within the September 2000 - April 2001 timespan) were without precedent. Commonly they were considerably greater than previous maxima over the period for which river flow or groundwater level data are available.
Since 1987, the annual mean Central England Temperature (Manley, G., 1974) has fallen below the 1961-90 average only once (in 1996). 2001 added to the cluster of recent warm years. Partly as a consequence, potential evaporation losses for Britain as a whole were above average, notably so in parts of eastern England. Actual evaporation losses - which normally fall short of the corresponding potential evaporation losses due to the inhibiting effect of limited soil moisture in the late summer and early autumn - were generally in the normal range. Much of the English Lowlands provided an important exception - here sustained rainfall during July-September restricted the period over which substantial soil moisture deficits were maintained; as a result actual evaporation losses for 2001 were close to the highest on record in many areas.
Nationally, water resources were exceptionally high during the late winter of 2000/01 - unprecedented if reservoir and groundwater storage are considered together. During the early months of 2001, some drawdown of major reservoirs was necessary to provide increased flood alleviation capability. Nonetheless, overall reservoir stocks for England and Wales remained above the monthly average throughout the year (Figure 1). Stocks decreased briskly through the early summer but, more significantly for the water resources outlook, a further decline during November and October - when reservoir levels are normally rising - resulted in overall stocks falling close to the monthly average at the beginning of 2002 - having remained above average since the autumn of 1997 (Figure 2).
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 3; the percentage departures are based on outflows from representative networks of major river basins. For England and Wales as a whole, runoff exceeded the average for the fourth successive year; the 1997-2001 runoff total significantly exceeds that for any other four-year sequence in the 41-year national series. Well above average runoff also characterised Scotland over the 1998-2000 period but the runoff total for 2001 was considerably below average - a very uncommon occurrence over the last 25 years. In Northern Ireland, a province-wide gauging station network has operated only since the mid 1970s. In the context of the last 20 years, the 2001 runoff was around 30% below average, establishing a new annual minimum for Northern Ireland. The contrasting runoff patterns at the national scale were reflected in substantial regional and catchment variability in 2001 runoff totals.
Relative to 2000, the frequency of flood events was much reduced in 2001 - but overbank flows were still common during the late winter and early spring, particularly in permeable catchments across southern England. Over this period, the recorded range of flows in springs and groundwater-fed rivers was extended over wide areas. In permeable catchments, river flows remained at seasonally high levels throughout the summer. Flows in rivers draining impermeable catchments however were characterised by steep recessions from the late spring, as a result summer flows generally fell into the normal range. Following heavy early autumn rainfall, most catchments were again saturated by late October and many were vulnerable to further significant rainfall. The risk of a repetition of the widespread floodplain inundations in the autumn of 2000 remained high until mid-November. Thereafter a sustained dry spell, extending over 10 weeks in some areas, triggered lengthy recessions in the west and north, and stalled the seasonal recovery in spring-fed southern and eastern rivers. By year-end, depressed river flows characterised many responsive catchments in western and northern Britain, and parts of Northern Ireland also.
Available evidence, long-term rainfall records in particular, suggest that the magnitude of groundwater replenishment over the eight months beginning in September 2000 has no close parallel throughout the period over which routine groundwater level measurements have been taken (over 150 years in a few localities). Late-2000 and early-2001 rainfall was exceptional across most aquifer outcrop areas but the most outstanding anomalies were in the South-East where some areas recorded around twice the 1961-90 average rainfall for the winter half year - generally well above previous maxima. Correspondingly, dramatic groundwater level rises over the last 10 weeks of 2000 heralded an unprecedented period of sustained high groundwater levels. Levels remained above previous maxima for extended periods during the first five months of 2001 and sustained 'clearwater' flooding was very widely reported. The lowland stream network extended high into the headwaters of many 'dry' valleys and the extent of the groundwater flooding almost certainly any similar episode in the instrumented era. Groundwater levels remained well above average in the less fissured (and less responsive) aquifers for very lengthy periods - exceeding previous maxima for over four months in parts of the Chalk, and longer in many Permo-Triassic sandstones outcrops. Rapid drainage from high level springs produced a steep water-table decline through the late spring and summer of 2001 but groundwater levels remained above, or within, the normal range until the late autumn in most regions. Thereafter the very modest nature of the seasonal recovery in the autumn left groundwater levels in most Chalk wells and boreholes around the seasonal average at year-end; generally levels remained very high in the Permo-Triassic sandstones.
2001 saw a continuation of the hydrological volatility that has been a feature of the recent past. The protracted droughts of the early and mid-1990s have been succeeded by a sustained wet period that culminated in the exceptional flooding of 2000/01. The last 10 years have witnessed an extension in the range of recorded runoff and recharge rates in many parts of the UK. However, the significance of this extension needs to be considered alongside the limited record length (typically < 25 years) of most UK hydrometric datasets, and the broad stability that typifies long river flow and groundwater level series unaffected by artificial influences. Hydrological conditions in the recent past show some consistency with favoured climate change scenarios, e.g. the high temperatures and the cluster of wet winters. However, in a warmer world, hydrological patterns in the UK will continue to reflect the complex interaction of rainfall, evaporation and soil moisture conditions (as well as land use changes and other more direct artificial influences on runoff and recharge patterns). There remains considerable uncertainty regarding climate change impacts - future rainfall patterns especially - at the regional and catchment scales. As yet there is very limited evidence of any compelling long term hydrological trends in the UK - careful monitoring will continue to be essential to identify and interpret any significant climate-driven signals.
Reference
Manley, G. 1974. Central England Temperatures: monthly means 1659 to 1973. Quart. Jour. Royal Met. Soc., 100, 389-405.