Phosphorus export at the paddock, farm-section, and whole farm scale on an irrigated dairy farm in south-eastern Australia

Phosphorus ( P) exported from agricultural land contributes to the eutrophication of inland water systems. Although P export has been extensively researched at the paddock scale, our understanding of farm-scale export is limited. This paper presents the results of a 3-year monitoring project that investigated P export at the paddock, farm-section, and whole farm scales on an irrigated dairy farm in south-eastern Australia. Annual average concentrations of 2.2-11 mg P/L, and annual loads of 2.5-23 kg P/ha were measured at the paddock and farm-section scale over the 3 years, with the quality of irrigation water applied having no significant effect on P export in surface runoff. At the farm scale, effective management of the water reuse system significantly reduced phosphorus export by up to 98%. During the 3-year period, P concentrations and loads exported in surface runoff consistently decreased between the paddock and farm-section scales (e.g. P-28 exported 13.8 kg P/ha, whereas S-4 exported 6.7 kg/ha in 2001), with the decrease in P export described using a scaling factor. Our results suggest that data on paddock-scale P export can rarely be proportionally assigned to predict section- or farm-scale export, at least on irrigated dairy farms in south-eastern Australia.