The concept of the specific yield as it is applied to unconfined aquifers, is discussed, and it is shown that the value of the true specific yield is generally a function of both horizontal position and time. Because of the difficulties associated with the measurement of localised fluxes and localised water-table movements necessary to obtain values of the true specific yield, theaverage, thebulkand thevirtualspecific yields are defined, all of which are just time dependent and the last two of which require the measurement of average fluxes over the catchment area. The non-constancy of the specific yield is illustrated in experimental results obtained with intermittent rainfall over a drainage installation where negative values were obtained, reaching ±∞ when the water-table height was a maximum or minimum.