Farm-scale stormwater losses of sediment and nutrients from a market garden near Sydney, Australia

Agricultural runoff contributes to water quality problems in the Hawkesburyââ"šÂ¬Ã¢â‚¬Å“Nepean River near Sydney, Australia. This paper presents a case study of sediment, nitrogen (N) and phosphorus (P) losses from a market garden in the Hawkesburyââ"šÂ¬Ã¢â‚¬Å“Nepean catchment. Event-based runoff sampling and flow and rainfall monitoring were carried out at the boundary of an 8-ha commercial market garden near Richmond, NSW, over two years (1995ââ"šÂ¬Ã¢â‚¬Å“1997). A record of fertiliser use and soil management was compiled during the runoff monitoring period. Soil management practices were similar to other market gardens on duplex soils in the area. Farm practices were consistent with ââ"šÂ¬Ã‹Å“traditionalââ"šÂ¬Ã¢"žÂ¢ industry practices. Over the 2-year period, stormwater runoff losses from the farm were equivalent to 19 t haâË" Ã¢â‚¬â„¢1 per year of suspended sediment, 11 kg of P haâË" Ã¢â‚¬â„¢1 per year, and 127 kg of N haâË" Ã¢â‚¬â„¢1 per year. Sixty percent of soil loss occurred in summer (December, January and February), reflecting both seasonal climatic changes and soil management practices. Soil erosion was the major mechanism for P mobilisation and transport; however, a significant increase in the event mean concentration (EMC) of soluble P was observed over the 2 years. During this period, we measured an increase in extractable soil P (Bray) in surface soil from 174 to 304 mg kgâË" Ã¢â‚¬â„¢1 and a concomitant decrease in P sorption. The highest concentrations of N in runoff occurred after applications of poultry manure and during an extensive fallow period (late spring through to the end of summer). The results show that, in the Sydney region, market gardening on duplex soils using traditional practices is environmentally unsustainable, and may provide economic incentive to bring about change.