An attempt has been made to provide a basis on which the amounts of deficit storage occurring in the natural hydrographs of small catchments can be estimated as a function of yield and the frequency of occurrence. The yield levels envisaged constitute only small proportions of the mean flow and the deficits arise due to seasonal fluctuations only. It is presumed that any deficits built up during the dry periods of the year are made up by the winter flows, so that there is no year to year carry over. The streamflow data of 26 catchments in Ireland are analysed to obtain the series of annual maxima storages on each catchment. Assuming that they are distributed according to the extreme value type 1 distribution, the relationship between the deficit storage and its frequency of occurrence is calculated for various levels of yield on each catchment. A simple model is described which expresses this relationship in terms of a few parameters. For use on ungauged catchments, those parameters are related to the physical characteristics of the catchments by multiple linear regression.