Precipitation is a very complex phenomenon, which is relatively difficult to predict and model. It also has a big influence on the flow rate of the catchment. In the past few years we are witnessing many floods all over Slovenia as a result of high precipitation rates, therefore appropriate hydrological models have to be selected in order to estimate the runoff from the investigated catchment. A small catchment that is located in Slovenia was modelled using different hydrological tools. In the first part of the thesis we presented the basic concepts that are important for the understanding of the runoff and methods for calculating all individual parameters that are required for the determination of the hydrological model, e.g. calculation of the precipitation losses and unit hydrograph. In the practical part of the thesis all the required steps were presented in order to construct a hydrological model of the upper part of the Velunja catchment and the input data was prepared.. The calibration and validation of the HEC-HMS model was conducted using the discharge data measured by the Slovenian Environment Agency (ARSO).. We used discharge data from the Gaberke station and the precipitation data from the Šmartno pri Slovenj Gradcu station.