As trade-off relationships between the agronomy and the environment are common over land uses within a catchment, one of major concerns of catchment management plans is how to balance the trade-offs over land uses at the catchment-scale. In this two-part paper, an integrated modelling and multicriteria analysis (MCA) methodology is presented which can be used to evaluate a set of land use alternatives and to identify an 'ideal' compromise between economic return and environmental pollution. This 'ideal' compromise here is achieved by land use optimisation of which the objective is to minimise the environmental pollution (nitrate leaching rate) and to maximise the economic return (agronomic gross margin), considering both environmental and economic potential across the catchment in a site-specific manner. The suggested methodology can be used to produce a site-specifically optimised land use scenario that is an 'ideal' compromise between nitrate diffuse pollution and agronomy at the catchment-scales. Some issues on the actual application of the methodology are also discussed.