Use of values of maximum rainfall for incomplete records. Project note to Defra, project WS194/2/39 (Reservoir Safety - Long Return Period Rainfall

The present project (Reservoir Safety - Long Return Period Rainfall) has continued with the approach used in the Flood Estimation Handbook extreme rainfall analysis that, in determining the set of annual maxima for a given raingauge, the value attributed to a given year is counted as valid only if at least 75% of the potential observations within the period are available. In this project the same rule is applied to seasonal maxima. This note examines the extent to which the annual and seasonal data extracted do include maxima for incomplete periods. It also considers the potential effect of replacing the current analyses (which effectively treat all the accepted maxima as if they had been obtained from complete periods) with parallel analyses which would make an allowance for the use of maxima from incomplete periods.