An approximative lumped parameter model for infiltration is introduced. The model is a spatial discretization of a basic non-linear partial differential equation of the infiltration. The model aims to describe the input-output relation between the rainfall rate and the infiltration rate. The model can be used as a part of a larger rainfall-runoff model, and it can be interpreted as a simple aggregation model of an infiltrating catchment. A great number of simulation runs were carried out with the model. Many of the conventional results of infiltration can be satisfactorily described by the present simple model, e.g. the commonly used Horton's equation.