Sediment yields and soil loss rates from different land uses on Triassic shales in western Sydney, NSW

Abstract

Sedimentation surveys of small dams demonstrate that land use is the dominant factor generating high sediment yields in the ungullied shale catchments of western Sydney where rainfall erosivity and soil erodibility are relatively constant. A single urban catchment produced 6.5 t/ha.year and cropped catchments an average of 6.7 ± 1.99 t/ha.year, whereas grazed woodland/forest and grazed pasture exported averages of only 2.5 ± 0.57 and 2.9 ± 1.02 t/ha.year, respectively. These yields are high by Australian standards and the farm dam sediments are enriched in both clay and phosphorus, in comparison to catchment topsoils. Empirical soil loss equations based on the Universal Soil Loss Equation (USLE) [Modified Universal Soil Loss Equation (MUSLE), Soiloss and Revised Universal Soil Loss Equation (RUSLE)] accurately predicted the measured sediment yields, with Soiloss being the most accurate. Although Soiloss is the only empirical equation to use Australian data, it is only marginally better than MUSLE, a simplified version of the USLE used for teaching. RUSLE predictions of soil loss rates were closely correlated with measured sediment yields but required inputs for poorly defined parameters. European land uses in the South Creek catchment, the largest shale catchment in western Sydney, have probably increased mean annual sediment yield by 4.4 times over that in 1788. Further increases are likely with increasing urbanisation.