Modelling catchments, land use impacts, and management effectiveness
The establishment of agricultural and urban land use, and associated clearing of native vegetation, changes the hydrology and ecology of catchments and their waterways. These landuses can increase the amount of nutrients, non-nutrient contaminants, and organic matter entering waterways, which can change their ecology and in some cases cause serious degradation.
The Water Science Branch at Department of Water (DoW) has developed catchment models which quantify the amount of nutrients (nitrogen and phosphorus) that flows to our rivers and estuaries. Our modelling has concentrated on the southwest of Western Australia and models have been completed for the following areas: Swan- Canning, Peel-Harvey, Leschenault, Geographe Bay and Hardy Inlet focussing on Scott River. Reports for the Lower Blackwood and Avon river are currently being published.
The models are based on hydrological catchments and are thus driven by an underlying hydrological model. The land uses are represented in the model in terms of vegetation cover, fertilisation behaviour and management practices. These are coupled with the land characteristics (soils, slope etc), hydrology and nutrient export behaviour to quantify nutrient loads that flow to streams and rivers. The hydrological and nutrient models are stringently calibrated against observed river flow and nutrient concentration data.
The models quantify the sources of nutrients in terms of their location and land use. Scenario modelling is undertaken to determine the potential impacts of proposed land management and use changes. The effect of the drying climate in the southwest of Western Australia has also been estimated. Our catchment models have been used to support water qulaity improvement plans for many of our estuary catchments. The impact of management practice changes such as fertiliser application based on soil and tissue testing, application of soil amendments, dairy shed effluent treatment and riparian zone rehabilitation have been quantified. Land use changes, particularly urban development, and the benefit of water sensitive urban designs are also examined.
A recently completed project modelled the flow, nutrient (nitrogen and phosphorus) and sediment loads in the Avon Basin. A range of land-use and management scenarios based on extensive consultation with Department of Agriculture and Food staff, farmers and natural resource managers were included. This work showed that changed management of wheat and sheep farms, primarily based on treatment of soil acidity and application of nutrients to closely match crop demand would benefit both the farmers and the adjacent waterways. Riparian zone rehabilitation, as has been carried out for many years in the Avon Basin, was also shown to have a large benefit to waterways.