Hydrological Summary for the United Kingdom

Dear Mr Rowe

On behalf of NERC, I am acknowledging receipt of your request dated 17/12/09.

Colin Pelton
FOI Office
NERC
02380 596288
01793 411730
________________________________________
From: Martin Rowe [[FOI #24819 email]]
Sent: 17 December 2009 17:09
To: foi-office, foi-office
Subject: Freedom of Information request - Hydrological Summary for the United Kingdom

Dear Natural Environment Research Council,

In the recently-published “Hydrological Summary for the United
Kingdom” it is stated that a new 24-hr maximum rainfall for the UK
was established in the Lake District. Seathwaite Farm in Borrowdale
experienced 316.4mm in just 24 hours - an event likely to occur
only once every two thousand years and the site also recorded
remarkable totals of 402mm over 37 hrs and 495mm over 4 raindays
(provisionally once in 4000 & once in 3000 years).

On what basis are those statistics derived?

Yours faithfully,

Martin Rowe

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Our Ref: FOI18/12/09-112230

Dear Mr Rowe

On behalf of NERC I am responding to your request for information dated 17/12/09.

NERC is handling this request under the Environmental Information Regulations 2004. These regulations provide broadly similar access rights to the Freedom of Information Act 2000, but relate specifically to environmental information.

Return period estimates of rainfall event at Seathwaite Farm on 20 November 2009

The rainfall estimates were published by the NERC Centre for Ecology and Hydrology (CEH) in the Hydrological Summary for the UK for November 2009. This publication is aimed at an audience which will generally be familiar with the ideas being discussed and hence detailed explanations are avoided in favour of brevity. One role of the publication is to present current water resource conditions and recent weather and river behaviour against the background of what has been observed in the past.

Interpretation

The figures quoted need to be considered separately for each duration mentioned. Considering the rainfall total of 402 mm in a period of 37 hours, the ‘return period’ of 4000 years quoted means that we estimate that in any given year there is only a 1 in 4000 chance that the largest rainfall total observed at this one particular site (Seathwaite Farm) in any 37-hour time interval will equal or exceed 402 mm. The results reported are a summary of an analysis where we estimated the return period of rainfall totals from this event for many different interval lengths (durations). The 37-hour duration was chosen because its estimated return period was the highest of those examined and we also reported results for some other durations for which rainfall amounts have been noted as remarkable in other forums (newspapers, web discussion groups, etc.). None of these return periods should be taken as indicating how often a similar event can be expected to occur, even if only this one site is considered, because the return periods are calculated on the basis of a single pre-selected duration. Thus, an event having equally damaging consequences might consist of more intense rainfall over a shorter time period. Consideration of the rainfall occurring over the whole of the river catchments upstream of Cockermouth and Workington brings in an even wider range of possibilities and is likely to be the subject of later research.

Data sources

For the immediate event, we were supplied by the Environment Agency (EA) with rainfall totals at a 1-hour resolution for 18 to 21 November and with daily rainfall totals for the month of November 2009. Our analysis also makes use of an extensive archive of rainfall totals at 1-hour and 1-day resolutions which consists of all known computerised records of UK rainfall for sites where systematic recording of rainfall amounts has taken place. These data, which we already held for a longer-term project, were provided by the Met Office, EA and the Scottish Environmental Protection Agency (SEPA).

Statistical analysis

The return periods published have been produced using a procedure developed during a project that has just been completed, the aim of which was to allow the estimation of rainfall amounts having return periods of over 100 years for any location in the UK. For Seathwaite, our archive contains 22 years providing data for past annual maxima. This would be a very short record on which to base estimates for long return periods, but the procedure is designed to work even for locations with no historical records. The basis of the methodology is to merge together information from any rainfall records in the immediate vicinity, and from increasingly more distant locations, in order to produce estimates of rainfall for long return periods. In this process, account is taken of the differences in the distribution of rainfall that is predictable from known factors such as average annual rainfall. The procedure used is the result of careful re-examination, re-formulation and re-structuring of the procedure set out in the Flood Estimation Handbook (Volume 2: ISBN 0-948540-90-7). Like the previous procedure, the statistical analysis is done on the basis that past experience of rainfall extremes is representative of the immediate future. The study leading to the revision of the previous procedure could not deploy substantial resources to re-examine the question of whether there is evidence that rainfall extremes are either increasing or decreasing. A detailed exposition of the revised method and its development will be available in a forthcoming report which will be available from the Defra website. An overview of the procedure is currently online in the form of a presentation made to a meeting of the British Dam Society in May 2009, at:
http://www.britishdams.org/meetings_even... .

I hope this provides you with all the information you require, however, if you are unhappy with the way the NERC has handled your request, or about any aspect of the procedure, you may ask for an internal review in the first instance. You should contact the NERC Complaints Officer at the above address.

If you are not content with the outcome of the internal review, you then have the right to apply directly to the Information Commissioner for a decision.

Yours sincerely

Colin Pelton
FOI Office
NERC
02380 596288
01793 411730

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This message (and any attachments) is for the recipient only. NERC
is subject to the Freedom of Information Act 2000 and the contents
of this email and any reply you make may be disclosed by NERC unless
it is exempt from release under the Act. Any material supplied to
NERC may be stored in an electronic records management system.