In order to simulate drainage and elements of hydrological evaluation in a monthly step, the mostly used methods are conceptual models of hydrological evaluation with concentrated parameters which simulate hydrological processes by means of basin simplification to a series of connected fictional reservoirs. The rainfall water enters them and the flow-rate issues in the final basin profile. The proposed thesis deals with the drainage simulation in the final basin profile while at the same time it explores the influence of determination of calculation methods for the potential evapotranspiration in the process of rainfall-drainage simulation. The thesis gives more detailed attention to the determination of evapotranspiration. It also describes factors which influence this characteristic and offers possibilities of determination of potential monthly evapotranspiration with regard to available climatic figures. The main aim of this thesis is the choice of the suitable method to determine the potential evapotranspiration at availability of climatic basin figures in order to achieve acceptable results with regard to the quality of drainage simulation in the final basin profile. The methodical procedure was applied on the basin of Myjava to as far as Šaštín-Stráže.