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A numerical model simulating water uptake by root systems is presented. This model can combine the locally measured root hydraulic conductances with data on the root system architecture to give a detailed description of water absorption, from the single root level to the entire root system. This is achieved by coupling a three-dimensional root system architecture model with laws describing water flow in roots. In addition to water absorption studies, the model has been developed so that it can be included in a soil water transfer simulator in order to analyse soil-plant interactions for water uptake. The use of the model in describing water absorption is illustrated for a specific case where the hydraulic conductances are considered uniform in the whole root system. In this way, analytical results of Landsberg and Fowkes (Annals of Botany 42: 493-508, 1978) are extended from the single root to the root system level. The influence of the type of root system architecture, axial conductance between crowns of maize nodal roots, transpiration in the course of the day, and non-homogeneous soil water potential on fluxes and water potentials in the root system are examined. The dynamics of the total conductance of the maize root system with plant growth is also shown for this case of uniform conductance in the root system. Cases which consider other distributions of the conductance in the root system are presented in an accompanying paper. (C) 1998 Annals of Botany Company.