Abstract.A long-term, annual-average catchment biophysical model (SedNet/ANNEX) was used to calculate sediment, nitrogen (N) and phosphorus (P) loads in the Tully-Murray catchment of north-eastern Australia. A total of 119 000 t year −1 of suspended sediment, equivalent to 430 kg ha −1 year −1 , was calculated to be exported to the Great Barrier Reef (GBR). Most of the sediment (64%) was generated from hill-slope erosion. The modelled load of dissolved inorganic N (1159 t year −1 or 4.2 kg N ha −1 year −1 ) was similar to that from other wet tropics catchments in Queensland with similar areas of sugarcane. Sugarcane produced 77% of this load. The annual loads of total N and total P were 2319 t and 244 t, respectively. Simulations (scenarios) were run to evaluate the impact of improved land management on pollutant loads to the GBR. A combination of improved cultivation and fertiliser management of sugarcane and bananas (99% of cropping land) and restoration of the most degraded riparian areas reduced sediment by 23 000 t year −1 (18%) and dissolved inorganic N by 286 t year −1 (25%). However, this reduction is much less than the reduction of 80% that may be needed in the catchment to meet target chlorophyll loads in the marine environment.

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The Great Barrier Reef (GBR) Water Quality Plan outlines a clear set of water quality and management practice targets (50% reduction in nutrients and photosystem II (PSII) herbicides by 2013, 20% reduction in TSS by 2020). Five lines of evidence have been used to report on progress towards reef plan targets due to investment into improved agricultural management practices. This paper provides an overview of the results from one line of evidence, catchment modelling.

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