This paper presents the development of a practical tool to evaluate the required width of a hydrological buffer zone, in order to maintain the effects of subsurface drainage within reasonable limits. A simple mathematical formulation describing the lateral drawdown extent of a drained water table in unsteady flow conditions was used. Simulation results were worked out by introducing the concepts of “protection level” or “threshold of tolerance” for the vegetation of the protected area The latter are expressed in terms of an admissible drawdown combined with exceedance duration and/or frequency criteria. An application of this method to the protection of a peat bog system in the Swiss Jura region is presented.