It is proposed that estimates of the dependable yields of surface water reservoirs be based on probable minimum runoff and probable maximum net evaporation during drouth periods, rather than on the recurrence of specific drouths covered by existing hydrologic records. The probable limiting conditions, expressed as design drouth criteria, can be presented in the form of tables or curves showing average runoff and evaporation rates for various drouth durations. Methods for derivation and application of the design drouth criteria are presented and evaluated.