The determination of total nitrogen and total phosphorus concentrations in freshwaters from land use, stock headage and population data: testing of a model for use in conservation and water quality management

agua azote calidad del agua eau ensayo eutrophication eutrophisation fosforo gestion des eaux land use measurement mesure modele modelos models nitrogen nutrientes nutrients phosphore phosphorus pollution reino unido substance nutritive testage testing united kingdom utilisation des terres water water management water quality