The aim of the work is to present a new hydrological model, developed at the Institute of Technology and Life Sciences (Instytut Technologiczno-Przyrodniczy), which can be used to simulate and study the formation of total runoff and its components from a small lowland agricultural catchment, in various meteorological conditions, taking into account the different initial retention status of the catchment and the impact of urbanized areas on the said runoff. The study involved the necessity of choosing the type of the model, and the manner of describing hydrological processes. Based on studying subject literature, as well as on the results of previous research and observations conducted in the upper Mławka river catchment (left tributary of the Wkra river), it was assumed that the most appropriate tool for prognostic and simulation purposes would be a conceptual model, that – in its structure and description of hydrological processes – would take into account the theory of active areas (variable source areas). It was assumed that a simple model would be developed as far as possible, that would create the possibility of simulating hydrographs of total runoff, surface runoff, subsurface runoff, and ground water runoff from a natural catchment, and from catchments which have been urbanized to varying degrees. The work also includes the presentation of proposed principles for the classification of urbanized areas, taking into account their location within the catchment area and the way of draining rainwater, as well as a description of adequate modules of the model that would make it possible to simulate the runoff from the urbanized sub-catchments. The model is described by state-space representations (differential equations) and output equations. It works with a one-hour time interval. The input is total precipitation and reference evapotranspiration. On the output end, we receive the total runoff hydrograph, and the hydrographs of the components of this runoff. The model includes 10 optimized parameters, occurring in procedures simulating processes in the natural, non-transformed part of the catchment, and 10 measurable parameters used to describe partially urbanized catchments. The article presents the results of identifying the parameters for the model, and of its verification. For this purpose, 5 hydrographs of total runoff were used, between 192 and 432 hours long, including storm surges. They were recorded in the catchment of the upper-course Mławka river. The obtained values of the criterion functions lead us to conclude that the quality of the model is very good. The model has been used for simulation studies of the impact of urbanization of the agricultural catchment on the parameters of river runoff (Krężałek 2018).