ABSTRACT: Hydrograph separation is one of the common tasks by hydrogeological research. Several methods for hydrograph separation have been developed. One of them, the Foster method is rather subjective, however widely used. The author modified the Foster method for computer use and proposes more objective way to separate the groundwater runoff based on the Foster method. The modified Foster method us es running minimums of discharge for past 30 days as a representative estimation of groundwater run off ratio. Described method is compared with other hydrograph separation means.