An index-based modelling approach to evaluate nutrient loss risk at catchment-scales

This paper presents an approach to evaluate phosphorus loss risk to waterways using indexbased modelling at the large catchment scale. Index-based approaches are used to identify areas of greatest nutrient source strengths, and so called 'critical source areas' of nutrient loss risk. Compared with more sophisticated water quality modelling approaches, index-based approaches are less likely to be constrained by a lack of input data and provide good spatial representation of catchment source strengths, although they do not represent nutrient generation processes explicitly. In this research, soil regolith source and transport risk factors such as soil P concentrations, soil P sorption, soil map information, land use, erodibility, texture, and catchment-scale hydrology are used. The research is designed to help evaluate source strengths and transport risks to assist managers to improve the focus of remediation actions to reduce nutrient delivery to waterways. This paper presents a case study of index-based modelling in the 1810 km² Tuross River catchment of coastal southeast Australia. Predominant land uses are conservation and production forests, national parks, cattle grazing, dairy production and urban expansion. Knowledge of potential sediment and nutrient losses in the region is poor and so the index-based approach is a useful one to prioritise sources and resources. Development of the P index in the Tuross River catchment (Figure 1) shows the majority of the catchment (67%) area is considered to have a very low or low risk of P loss. A P index <2.4 is considered low risk, while a P index 2.4-5.4 is considered moderate risk. The proportion of the catchment area of moderate P loss risk was 32.9%, while the high risk area was 0.03%. Management should be directed firstly to areas of high risk to reduce losses, although this should be implemented with knowledge of the costs of managing different areas and source strengths. Nutrient indices such as the type developed and applied here are complementary to other modelling approaches. The index approach generally provides much more detailed spatial resolution than fully- or semi-distributed modelling approaches. Semi-distributed models such as CatchMODS and SedNet/ANNEX can be used to set priorities at a broader catchment scale and they assist in the quantification of pollutant fluxes and simulation of broad management changes. Such changes could include conversion of areas from forestry to intensive land use. Index approaches are however potentially more relevant to the spatial level at which management changes, and particularly changes in farming systems, are applied i.e. the paddock and small subcatchment scale. In this way semi-distributed models can be combined with index approaches to provide a set of powerful tools to influence management outcomes. Semi-distributed models can provide overall direction to set the broad focus of management and index approaches can then be used to refine on-the-ground investigations and investment priorities.