A state model for infiltration based on a spatial discretization of partial differential equations of vertical water movement in unsaturated homogeneous soil is presented. A transformation of the spatial coordinate is made in the basic equtions, thus allowing the representation of a semi-infinite soil in the state form. The aim of the model is to describe the dynamical relationship between ponding conditions in the catchment and infiltration during highly variable rainfall rates so that the model can be used flexibly as a part of larger, rainfall-runoff models. The relation of the present model to the conventional use of Philip's equation is considered. An approximative way to combine the infiltration model with a surface runoff model including many subcatchments is presented, in which the number of state variables in the combined model is reduced. Some features of the state model are discussed by using numerical examples. The behaviour of the combined model including infiltration and surface runoff parts is also compared with simulation results in the literature.

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