Resource information
The effects of agricultural practices on runoff water quality are a major concern worldwide, particularly where there is the potential to cause harm to aquatic ecosystems receiving the runoff. Cropping land has been identified as a major source of pollutants in the Fitzroy Basin, the largest catchment discharging into the Great Barrier Reef (GBR) lagoon. However, knowledge of pollutant generation rates in runoff from the dryland cropping (grains) industry's best management practices (BMPs) on Vertisol soils is limited, partly due to the scarce distribution of these soils worldwide. During a 9 year study sediment movement in runoff from grains dryland cropping was measured at the outlet of 2 contour bays where the distance between contour banks, referred to as the slope length, was standard (single) (180m) and triple (450m) the recommended space. Additionally, nutrient and herbicide movement in runoff was quantified for a sorghum (Sorghum bicolour) crop grown with fertiliser and herbicide applied at planting for a single wet season. During this period a double spaced slope length (380m) was also monitored. All contour bays were farmed under a zero tilled–controlled traffic farming system. Average annual runoff and sediment yield at the standard slope length was 75mm and 1.2tha⁻¹ respectively. This was greater than losses from grazing land use (0.1tha⁻¹yr⁻¹) observed during the study, but less than losses from conventional tillage practices (4tha⁻¹yr⁻¹) previously reported from a long term study in the region. Total nitrogen concentrations (median 8.3mgL⁻¹) in runoff from the sorghum crop were higher than those reported for other agricultural land uses at similar scales, and high proportions of NOₓ–N in relation to total nitrogen were observed (69%). Estimated NOₓ–N lost in runoff from the double spaced slope length after fertiliser application was 7-8kgha⁻¹. Off farm movement of metolachlor (2-chloro-N-(6-ethyl-o-tolyl)-N-[(1RS)-2-methoxy-1-methylethyl]acetamide) (Strada®) was observed with a maximum event mean concentration of 360μgL⁻¹ detected 19 days after application, equivalent to 4.3% of the applied amount. Current dryland cropping BMPs have reduced sediment losses compared to conventional practices. However, observations from a single wet season indicate that high concentrations of nutrients and herbicides can be transported off farm in runoff where they have the potential to be transported to the GBR. Further monitoring and analysis of delivery ratios of pollutants from paddock to catchment is required in the Fitzroy Basin, as is the continued development and adoption of BMPs that reduce nutrient and herbicide movement off farm.
