An approach to assess and manage nutrient loads in two coastal catchments of the Eurobodalla region, NSW, Australia

Abstract

This paper describes a research programme to estimate nitrogen (N), phosphorus (P) and sediment event-based loads in the Moruya and Tuross River catchments of the New South Wales south coast. The research programme is designed within the context of an integrated catchment modelling framework (CatchMODS), to assess relative contributions from diffuse sources of nutrient and sediment export loads, and provide information for catchment management. In particular, the relative potential risk to water quality from dairying in the Eurobodalla region is being evaluated using a farm-scale nutrient budget approach. Predominant land uses in the Moruya and Tuross River catchments are conservation and production forests, national parks, cattle grazing, and dairy production. There is little information on the quality of water entering the catchment estuaries, particularly during storm events when the majority of sediment and nutrients is transported to estuaries. The use of catchment models is commonly required to assist catchment managers to investigate water quality impacts at a catchment scale due to cost restrictions and data availability. To assess nutrient and sediment loads and enable management to achieve sustainable practices, the CatchMODS model is linked with a field-based data collection programme including water quality sampling to estimate suspended sediment, and total and dissolved nutrient loads on an event basis. Potential sources of nutrients in the catchments are likely to include diffuse forest and agricultural inputs and gully erosion. Diffuse source pastoral agriculture has been linked to decreases in water quality and recreational use of surface waters. To assess the potential impact of dairying, an evaluation of nutrient inputs and outputs, including leaching/runoff losses using a nutrient budget approach for a typical dairy farm in the Eurobodalla region, was undertaken. The Overseer nutrient budget model was used. Farms were divided into relatively homogeneous management areas, namely irrigated-block, non-irrigated and effluent-application areas for use in the model. The model produces nutrient budget inputs and outputs for a range of nutrients for the farm as a whole and for each individual management block. Initial results in this paper indicate N fertiliser usage on the dairy farms in this region is relatively low. Results indicate whole-farm long-term dairy farm leaching losses were 11 kg N/ha/year, which are considered low relative to other published studies. The overall whole-farm long-term leaching/runoff losses for P were estimated at 1.4 kg P/ha/year. Predicted whole-farm N concentration in drainage water at current average fertiliser usage is 3 ppm. This concentration is less than the guideline maximum for drinking water, although environmental acceptability depends on the sensitivity of receiving waters. In contrast, the simulated drainage N concentrations are greater than the guideline for lowland rivers in southeast Australia. Further evaluation of soil information, nutrient management and subsequent implications for water quality in the catchments as a whole is being investigated. Show more