In this thesis, we established a runoff model of the Lonja watercourse sub-catchment for the purpose of determining the highly probable flood hazard in the catchment area. In the case of open watercourses, it is very important to know the distribution and dynamics of water flow in space and time in order to predict and timely prevent flood damage. A very important parameter in creating a runoff model is the runoff itself. We define runoff as the flow of water, rain, snow or other liquids on the ground, and it is a key part of the hydrological cycle. The runoff model is established in QGIS, which is a geographic information system that enables viewing, editing, printing and analysis of geospatial data. By using publicly available databases such as the digital elevation model (EU-DEM v1.1), we obtain the topography of the area on which we create the runoff model. We calibrated the model using the discharge with the annual exceedance probability of 1% at Lonjica most, a downstream hydrological station. Through the numerical model we obtain a spatial distribution of water depths inside the catchment area at the occurrence of a flow of 1% duration.