Development of a catchment-wide nutrient model

Diffuse-source nutrient pollution remains a major problem within sustainable catchment management. The aim of this study is to provide a decision-support tool that can predict the impact of land-use changes on water quality (WQ) in receiving waters. A linked modelling approach was used to integrate land-use changes with a WQ model and a 2D numerical hydrodynamic model. The linked model was adapted to conditions in estuarine and coastal waters and applied to the catchments of Carmarthen Bay in West Wales, UK. A new set of biochemical reaction rates for nutrients in estuarine waters was found and presented. This approach took into account the assessment of nutrient production rates in the catchment for dry and wet weather conditions, as well as the reactions of constituents passing from the catchment into the coastal basin. The predominant types of land use in the catchment are arable land, improved grassland, rough grazing and woodland. Nutrient loads from these areas were estimated using Geographical Information System (GIS) data and export coefficients, which characterise the amount of nutrient losses. Furthermore, economical and ecological effects of land-use changes were integrated using an input–output analysis approach. A range of scenario simulations involving different river discharges and reduced nutrient input rates resulting from appropriate measures for land-use changes showed the nutrient pathways and concentration distributions over time throughout the bay. The outcome from these scenario simulations can be used to indicate catchment-wide distribution of diffuse pollution sources and to locate areas that are characterised by high nutrient concentrations and are prone to the occurrence of mass algal growth.