Explanation of the criteria for classification and numbering of components     I26.2.06
(MAY 99)
Explanation of the dates on the component's description




1.   Purpose and objectives

     To examine how environmental data discretization (such as averaging rainfall over particular time steps) affects risk assessment for extreme events.

2.   Description

     This report looks at the effect of data discretization on the estimation of period maxima. Particular reference is made to rainfall extremes where underestimation errors can be significant. A correction 
model is proposed for estimating maxima that would be derived if data were continuous.

     The report sets out the problem and gives an historical overview. The theory is examined and the methodology used in the study is described along with the proposed correction model. There is a detailed examination of hourly rainfall data leading to the derivation of correction factors for rainfall maxima occurring over periods of from one to 32 hours. A comparison between these and similar factors for wind speed and air temperature are given and a link to fractals is suggested.

     A comparison is made between results from hourly and non-hourly rainfall data. Other investigations incidental to the study which proved to be of interest are included. Appendices contain technical details and information about the data records used in the study.

     The report makes practical recommendations on correcting for discretization in data for rainfall and other environmental variables.

3.   Input

     Not applicable.

4.   Output

     Not applicable.

5.   Operational requirements and restrictions

     Not applicable.

6.   Form of presentation

     A report in English of 45 pages with 18 tables and 36 figures.

7.   Operational experience

     The methodology was applied to numerous sites in the UK, Australia and South Africa.

8.   Originator and technical support

     Institute of Hydrology, UK.

9.   Availability

     This report is available as a HOMS component free of charge from the Centre for Ecology and Hydrology, formerly the Institute of Hydrology, Wallingford, Oxon OX10 8BB, UK.

10.  Conditions on use



(First entered: DEC 96

Last updated: DEC 96)