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New monthly Hydrological summary published for April 2011 - a very remarkable month in hydrometeorological terms
12 May 2011
April was a very remarkable month in hydrometeorological terms: provisionally it was the warmest April in the 352-year Central England Temperature series, estimated (river) outflows from Britain were the lowest on record for the last week of April, and the end-of-month soil moisture deficits were the highest (for England & Wales) in a 50-year series.
The exceptional aridity of the early spring, following a relatively dry 2010, has resulted in agricultural and hydrological drought conditions affecting large parts of southern Britain. Currently, the primary impacts are on farmers and growers, an increased risk of forest and heath fires and, importantly, on river flows. Correspondingly, replenishment to most gravity-fed reservoirs was very meagre and overall stocks for England & Wales registered their second largest March/April decline since 1997.
The above analysis is contained within the latest monthly hydrological summary (for April 2011, PDF 2.31mb) produced by the National Hydrological Monitoring Programme, operated by the Centre for Ecology & Hydrology in conjunction with the British Geological Survey.
Where practical, water companies have been drawing from alternative sources to help conserve reservoir stocks (e.g. the transfer of River Severn water, via the Sharpness canal, to moderate pressure on the Mendip reservoirs) and early-May stocks in almost all index reservoirs remain above 80% of capacity. Nonetheless, stocks are well below the late-spring average in parts of the South West, Wales and Yorkshire. Groundwater levels are also seasonally depressed in a number of the most responsive aquifer units but levels across most major aquifers remain considerably above drought minima.
Terry Marsh from the Centre for Ecology & Hydrology said, “Historical rainfall figures indicate a tendency for dry spring periods to be followed by above average summer rainfall, but with evaporation rates increasing even average summer rainfall would imply very low late summer river flows – and an associated major contraction in the river network.”
Around the end of April, flows in a substantial number of rivers, including the Trent, Exe, Tone, Wye, Tawe and Ribble reported flows similar to, or below, the corresponding flows registered during the extreme drought of 1976.
The 1975-76 drought was considered at the time the most severe yet experienced across much of the UK. This week CEH published a new, 58-page report providing a contemporary and retrospective review of the events of 1975 and 1976. The report, co-authored by hydrologists Terry Marsh and John Rodda is available to download here (PDF, 12.9mb).
The Centre for Ecology & Hydrology jointly operates the National Hydrological Monitoring Programme (NHMP) in conjunction with the British Geological Survey. NHMP scientists produce the UK Monthly Hydrological Summary which assesses rainfall, river flows, groundwater and reservoir levels. They also operate the National River Flow archive. The NHMP has a remit to analyse major flood and drought events in the UK and analyse long term trends in UK hydrological data. The Programme was set up in 1988 and relies on the active co-operation of measuring authorities throughout the UK.
- Early-May stocks in almost all index reservoirs remain above 80% of capacity
- Active Atlantic low pressure systems brought substantial rainfall to western Scotland in the first week of April – Tyndrum reported 130mm in 48 hrs
- In Oxford, the Radcliffe Met. Station recorded an April total of 0.5mm
- The lowest March/April rainfall total since 1938 for England & Wales was recorded with a few regions (e.g. Anglian) eclipsing previous minima in series of >100 years
- Provisional data suggest that the combined March/April outflows (calculated using a set of index river flow gauging stations) from England & Wales are the lowest in the 50-yr national series
- April rainfall totals across the outcrop areas of almost all major aquifers was very modest, <15% of average in most areas.
Read the full April 2011 Hydrological Summary for the UK [PDF, 2.31mb]
Details of the National Hydrological Monitoring Programme
Media enquiries related to the Hydrological Summaries should be directed to the CEH Press Office. Our scientists can provide explanation and analysis of historic hydrological patterns, possible future scenarios under climate change and scientific understanding of the current situation. We are not able to comment on immediate operational issues.
If you wish to reproduce figures from the Monthly Hydrological Summary please respect the copyright credits contained within the document.