Catchment modelling of sediment, nitrogen and phosphorus nutrient loads with SedNet/ANNEX in the Tully-Murray basin

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.