Decision Support Systems and Nutrient Models: Methodology Developed for the Geographe Bay Catchment

In Western Australia, where the natural environment has adapted to low nutrient conditions, detrimental ecological changes to rivers and receiving water bodies are widespread in regions of increased anthropogenic delivery of nutrients. Catchment nutrient delivery to receiving water bodies is highly dependent on climate, soil type, land use and hydrology. The quantity and/or concentration of nutrients can be reduced by using a suite of best management practices (BMPs). To decide on which BMPs to implement, managers need to know the effectiveness of these measures, and where in the catchment they should be located. In Geographe Bay (a coastal catchment in the south-west of Western Australia) the SQUARE (Stream Quality Affecting Rivers and Estuaries) model was selected as the tool used to guide these decisions. SQUARE is a semi-lumped process-based hydrological and nutrient model, which takes into account land use, geomorphology, and climatic inputs to produce nutrient concentrations and loads at a daily time-step. SQUARE takes into account the unique hydrological characteristics of the Swan-Coastal Plain, and can be used to predict the effect of a drying climate on the nutrient budgets of the waterways. Nutrient species modelled include total nitrogen, total phosphorus, filterable reactive phosphorus, ammonia/ammonium, nitrate/nitrite, dissolved organic nitrogen and total suspended solids. This paper describes the methodology used to guide management decisions in the Geographe Bay catchment, based on the application of the SQUARE model.