The FORGEX method of rainfall growth estimation - I: Review of requirement

A growth factor is the ratio of the T-year extreme value to an index extrem e value such as the mean of annual maxima. Whereas a record length of ten o r more years may suffice to estimate the index variable, it is generally ne cessary to blend data from several sites if estimates of exceptional extrem e values are to be obtained. Methods of rainfall growth estimation are revi ewed, including traditional methods which extend frequency curves to long r eturn period by a distributional assumption, and methods which study spatia l dependence in extreme rainfalls. It is desirable that estimates at neighb ouring sites, and across different durations and return periods, are intern ally consistent. The review concludes that rather special techniques may be required if this goal of estimating extreme rainfall depths consistently i s to be met. The motivation of the Focused Rainfall Growth Extension (FORGE X) method is presented.