A conceptual model of nutrient mobilisation and transport is presented and coupled to a hydrological model, LASCAM. The model predicts catchment export of diffuse-source soluble and particulate phosphorus, particulate nitrogen, nitrate-nitrogen and ammonium-nitrogen. It explicitly includes the nutrient cycling processes of sorption, mineralisation, leaching, fixation, volatilisation, nitrification, denitrification, plant uptake and harvest losses. Soluble nutrients are assumed to be mobilised by surface runoff and by baseflow discharge, while particulate nutrients are associated with surface erosion and the stream sediment transport processes of deposition, re-entrainment, bed degradation and settling. The nutrient model includes 29 optimisable parameters, 11 for phosphorus and 18 for nitrogen, although several of these can be prescribed from measurements or from the literature. The model is applied to two large subcatchments of the 120,000 km(2) Avon River basin in Western Australia and shown to provide good predictions of daily nutrient export in a long-term simulation. (C) 2000 Elsevier Science B.V. All rights reserved.